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Biocatalysis & Multicomponent Reactions: The Ideal Synergy Znabet, A.
2012
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citation for published version (APA) Znabet, A. (2012). Biocatalysis & Multicomponent Reactions: The Ideal Synergy: Asymmetric Synthesis of Substituted Proline Derivatives.
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Download date: 07. Oct. 2021 Biocatalysis & Multicomponent Reactions: The Ideal Synergy
Asymmetric Synthesis of Substituted Proline Derivatives
Anass Znabet
2012 This Research was supported by the Netherlands Organisation for Scientific Research (NWO) under project number: 017.004.008
Printed by: Ridderprint BV, Ridderkerk, the Netherlands Lay out: Simone Vinke, Ridderprint BV, Ridderkerk, the Netherlands Cover Design: Nikki Vermeulen, Ridderprint BV, Ridderkerk, the Netherlands
ISBN: 978-90-5335-497-1 VRIJE UNIVERSITEIT
Biocatalysis & Multicomponent Reactions: The Ideal Synergy
Asymmetric Synthesis of Substituted Proline Derivatives
ACADEMISCH PROEFSCHRIFT
ter verkrijging van de graad Doctor aan de Vrije Universiteit Amsterdam, op gezag van de rector magnificus prof.dr. L.M. Bouter, in het openbaar te verdedigen ten overstaan van de promotiecommissie van de faculteit der Exacte Wetenschappen op donderdag 26 januari 2012 om 13.45 uur in de aula van de universiteit, De Boelelaan 1105
door Anass Znabet geboren te Amsterdam promotoren: prof.dr. ir. R.V.A. Orru prof.dr. M.B. Groen copromotor: dr. E. Ruijter
إ وادي
Table of Contents
Chapter 1 General Introduction: 9 Biocatalysis & Multicomponent Reactions
Chapter 2 Monoamine Oxidase N: 41 A Promising Biocatalyst for Asymmetric Synthesis
Chapter 3 Highly Stereoselective Synthesis of Substituted Prolyl Peptides 53 Using a Combination of Biocatalytic Desymmetrization and Multicomponent Reactions
Chapter 4 Asymmetric Synthesis of Synthetic Alkaloids by a Tandem 77 Biocatalysis/Ugi/Pictet–Spengler-Type Cyclization Sequence
Chapter 5 A Highly Efficient Synthesis of Telaprevir® by Strategic use of 99 Biocatalysis and Multicomponent Reactions
Chapter 6 Stereoselective Synthesis of Substituted N-Aryl Proline Amides 125 by Biotransformation/Ugi-Smiles Sequence
Chapter 7 Reflections & Outlook 145
Summary 159 Samenvatting (Summary in Dutch) 165 Dankwoord 173 List of Publications/Patents 179
Chapter 1 General Introduction: Biocatalysis & Multicomponent Reactions
General Introduction
1.1 Introduction
The chemical and pharmaceutical industry provides us with a myriad of useful products without which our standard of living would not be what it is now. However, the industry is also one of the major contributors to environmental pollution, due to the use of hazardous chemicals and in particular large amounts of flammable, volatile and often toxic organic solvents and reagents. For the production of fine chemicals, the waste/product ratio ranges between 5 and 50, while for pharmaceuticals this ratio may even be as high as 100.[1] The problems posed by this, including the inefficient use of resources, energy and capital, together with the risk to welfare and the environment are widely recognized throughout society. Although we have reaped many benefits from our fossil fuel-based economies, man faces an urgent environmental crisis. In recent decades, a growing consensus has risen about the negative influences of the increase of various gases on the global climate, such as CO2 and CH4. For example, since th the start of the industrial revolution in the 18 century, the CO2-concentration in the air has increased from roughly 100 ppm to more or less 400 ppm.[2-3] These gases, also called greenhouse gases, share a common feature that they tend to absorb heat and keep earth’s atmosphere at a comfortable average temperature of 15 °C. Without these greenhouse gases, the earth would lose too much heat to space and would be too cold to be habitable. But the increasing amount of these gases in the atmosphere will isolate the earth too much, resulting in elevated temperatures and the melting of vast amounts of ice on both poles and various high mountain ranges. Beside ecological destruction of these areas, the oceans will also rise and flood low-lying areas around the globe. Since many large cities, such as Amsterdam, New York City, harbors, such as Rotterdam, Singapore, and historical treasures, such as Venice, are situated at sea level, these will be lost if the sea level rises substantial due to melt water. These issues were emphasized when Al Gore’s documentary film “An inconvenient truth” was aired drawing the attention of politicians as well as that of the general public, which has put global warming and environmental issues high on the political and socio-economic agenda. In order to fight environmental decay, rising sea levels and increasing toxic waste piles development of new technologies for the production of energy, chemicals and products is vital. Among others, synthetic chemists are challenged to find solutions that maintain our standard of living but spare earth’s resources. The focus is set on developing novel, clean, atom-and step-efficient procedures for sustainable production for valuable fine chemicals and pharmaceuticals. The “ideal synthesis” should lead to the desired product from readily available starting materials in one or two reaction steps, in good overall yield and using environmentally benign reagents.[4] This minimizes energy consumption and waste
11 Chapter 1
production. A powerful strategy would be combining two methodologies which have proven to be efficient and environmentally benign: (i) biocatalysis and (ii) multicomponent reaction (MCR) methodology.
1.2 Biocatalysis
1.2.1 Enzymes as Catalysts In chemistry, a catalyst is a substance that decreases the activation energy of a chemical reaction without itself being changed at the end of the reaction. Catalysts participate in reactions but are neither reactants nor products of the reaction they catalyze (a strange ‘exception’ is the process of autocatalysis). They work by providing an alternative pathway for the reaction to occur, thus reducing the activation energy and increasing the reaction rate (Figure 1).
Figure 1: Generic graph showing the effect of a catalyst in a hypothetical exothermic chemical reaction. The catalyzed pathway, despite having a lower activation energy, produces the same final result.
In biocatalysis, natural catalysts, mostly enzymes, are used to perform chemical transformation of organic compounds. Biocatalysis is one of the oldest chemical transformations known to man; 6000 years ago it was already used for e.g. brewing beverages or cheese making. A brief historical background is depicted in Table 1.
12
General Introduction
Table 1: Brief history of enzyme engineering and their application.
Year Milestones Discoverer Chymosin from the stomach of cattle employed for the 6000 B. C. production of cheese 1783 Hydrolysis of meat by gastric juice (digestion) demonstrated Spallazani 1846 Invertase activity observed Dubonfout 1893 Definition of a catalyst including enzymes is postulated Ostwald Discovery of enzyme stereospecificity. “Lock-and-key” 1894 E. Fischer[5] model was proposed Cell free extract form yeast was employed for the 1897 Büchner[6] conversion of glucose to ethanol 1908 Application of pancreatic enzymes in the leather industry Röhm Application of pancreatic enzymes to clean laundry. 1913-1915 Röhm Commercialized as “Burnus” 1926 Enzymes are proven to be proteins Sumner[7] The first amino acid sequence of a protein (Insulin) 1953 Sanger[8] established, proving the chemical identity of proteins 1965 “Allosteric model” of enzyme was proposed Monod[9] Protein engineering developed for the improvement of After 1980 Many enzyme production and properties
Since the pioneering work of Büchner[6] (Table 1), it has been demonstrated that enzymes do not require the environment of a living cell to perform catalysis. From those findings, the use of enzymes has been increasing in importance and has been employed by the industry in several applications in food technology, for example in bread, beer, wine, cheese, yoghurt. Last but not least, also in the production of washing powder, textiles and paper.
Biocatalysis can offer several advantages over conventional chemical catalysis since enzymes show great chemoselectivity, regioselectivity and enantioselectivity. Furthermore, enzymes show higher substrate selectivity, milder reaction conditions, lower energy requirements and fewer side reactions, such as isomerization, racemization and rearrangement than conventional chemical catalysis. In addition, biocatalytic processes often provide products of high purity in one reaction step. As a result, biocatalysts can be used in both simple and complex transformations without the need for the tedious protecting and deprotecting steps that are common in enantio- and regioselective organic synthesis. Although biocatalysis offers a great number of advantages, there are some drawbacks that should be taken into consideration. In particular, the high cost of enzyme isolation and purification still discourages their use, especially in areas which currently have an established alternative procedure. The reaction rates for substrates that are not natural to an enzyme are usually slower than those observed for native substrates or conventional chemical catalysis.
13 Chapter 1
The generally unstable nature of enzymes, when removed from their natural environment, is also a major drawback to their more extensive use. Thus, high concentrations of reagents can negatively affect enzyme stability and enzymes are often unstable in organic or biphasic organic/aqueous media and can easily be inactivated. Another drawback is that some enzymes require cofactors in order to function, such as ATP or NAD(P)H (Figure 2).
Figure 2: Cofactors may be required for biocatalysis.
These cofactors are expensive and often require recycling, since stoichiometric amounts of ATP or NAD(P)H are extremely expensive. To circumvent these issues, often another set of enzymes are included that are used to regenerate these cofactors. In the case of NAD(P)H, these cofactor regeneration enzymes often use a cheap source of hydrogen to replenish the NAD(P)H, such as ethanol or isopropanol; or use molecular oxygen as an oxidant.
Different strategies exist to obtain enantiomerically pure compounds. As such, resolution procedures play an important role. A very old resolution procedure which is widely used is the kinetic resolution (KR). Kinetic resolution is defined as “The achievement of partial or complete resolution by virtue of unequal rates of reaction of the enantiomers in a racemate with a chiral agent (reagent, catalyst, solvent, etc.).” This means that two enantiomers (in a racemic mixture) show different reaction rates in a chemical reaction thereby creating an excess of the less reactive enantiomer. The maximum yield in kinetic resolutions is 50% (Scheme 1).
Scheme 1: A kinetic resolution (KR).
14 General Introduction
The obvious drawback of a kinetic resolution is that the maximum achievable yield is only 50% and the product has to be separated from the reactants. This has led to an important development called the dynamic kinetic resolution (DKR). Because the reactants form a chemical equilibrium and exchange, a theoretical conversion of 100% can be achieved. The enantioselective reaction is much slower than the racemization of the racemic substrate. This eventually leads to replenishment of the faster reacting enantiomer at the expense of the slower reacting one (Scheme 2).
Scheme 2: A dynamic kinetic resolution (DKR).
Finally, enzymes possess the ability to differentiate between enantiotopic groups of prochiral and meso compounds (Scheme 3). In contrast to a kinetic resolution, the theoretical yield of these conversions is 100%. The enzymatic desymmetrization of meso compounds has gained popularity in recent years and constitutes an elegant approach for the synthesis of enantiomerically pure compounds.
Scheme 3: An enzymatic desymmetrization (ED).
Although biocatalysis offers a great number of advantages, there are some objections that cannot be ignored. Consequently, these objections are currently being addressed and overcome by protein engineering.
1.2.2 Protein Engineering The “ideal enzyme” should have the desired activity, stability, specificity and so on in each individual case, i.e. should meet the demands of any user. One way to achieve this objective is via protein engineering.[10-16] This approach increases the basic understanding of how enzymes function and have evolved, and it is the key method of improving enzyme properties for applications in pharmaceuticals, green chemistry and biofuels. Protein
15 Chapter 1 engineering has already proven itself and has been employed industrially with great success.[17-20] Two main strategies are employed to adjust enzyme properties towards the desired application: rational design and directed (molecular) evolution.[21] Rational design[22-24] uses detailed knowledge of the structure and function for the prediction of changes in the protein structure in order to alter or induce the desired properties. This generally has the advantage of being economically favorable and technically easy, since site-directed mutagenesis techniques are well-developed.[25-27] The advances in computing have helped in creating better protein models to improve predictions for rational design. However, its major drawback is that detailed structural knowledge of a protein is often unavailable and structure-activity relationships are still not trivial. In directed evolution,[28-30] mutant libraries are created by random changes, and a selection regime is used to pick out variants that have the desired properties. The variants (showing improved properties) are then subjected to further rounds of evolution (mutation) until the mutant meets the desired necessities. This selection procedure mimics natural evolution. In contrast to rational design, prerequisite knowledge of the enzyme structure and how the structure determines function is not required when performing directed evolution. Not surprisingly, desired changes are often caused by mutations that were not anticipated. However, major concerns might arise when the libraries of mutants are not simple enough to screen for desired function and complex enough to contain rare, beneficial mutations and also if there is no rapid and cost-effective screen or selection that reflects the desired function. Nowadays, more researchers realize that rational design and directed evolution go hand in hand and both are applied in engineering new biocatalysts. Information available from related protein structures, families and mutants already identified is combined and then used for targeted randomization of certain areas of the protein. Table 2 provides a brief overview of enzymes engineered by rational design and/or directed evolution that are important for organic synthesis.
1.2.2 Optically Active Synthons generated by Biocatalysis As mentioned previously in this chapter, enzymes have been employed in numerous applications. Among others, enzymes have been employed in organic synthesis and especially in the synthesis of enantiopure compounds. The synthesis of chiral amines, alcohols, aldehydes/ketones and carboxylic acids is very important as these compounds are frequently used as synthons for various pharmaceutically active substances and agrochemicals. The selected examples given below will show some biocatalytic methods to generate these highly valued synthons.
16 General Introduction
Table 2: Brief overview of engineered enzymes. An extensive survey can be found in reference.[41]
Enzyme Property Method Milestone Ref T for enzymatic activity Dioxygenase Thermostability Rational Design max [31] increase by 15 ˚C Residual activity after 6 h Lipase Thermostability Rational Design [32] increased by 50% Improved stability at pH Endoglucanase General stability Rational Design [33] 3-4 Reaction rate for Monooxygenase Substrate specificity Rational Design 2-phenylethanol increased [34] 190-fold Directed Oxidation of secondary Galactose Oxidase Substrate specificity [35] Evolution alcohols Alcohol Improved preference for Cofactor specificity Rational Design [36] dehydrogenase NAD(P)H over NAD(H) E-value of 111 for the Directed Lipase Enantioselectivity kinetic resolution of e.g. an [37] Evolution axially chiral allene Phenylalanine Directed Reaction rate increased Increased activity [38] ammonium lyase Evolution 15-fold Preferences switched O-Glycosyltransferase Promiscuous activity Rational Design from O-Glycosylation to [39] N- Glycosylation Directed Allosteric regulation by β-Lactamase Miscellaneous [40] Evolution metal ions
1.2.2.1 Optically Active Amines In principle, various enzymatic routes can be employed to generate optically active amines using several different enzymes like hydrolases, oxidoreductases and transferases. Hydrolases are the most frequently used enzymes for the production of optically active amines by kinetic resolution of racemic starting material, for example, N-acyl amides by peptidases, amidases or lipases.[42] For instance, Burkholderia plantarii lipase has been used to generate optically active amines 4 via kinetic resolution (Scheme 1).[43] Most important is the use of highly activated methoxyacetate 2 as an acyl donor since this donor increases the carbonyl activity by a factor >100 compared to analogous reactions.
Scheme 4: Kinetic resolution of racemic amines with Burkholderia plantarii lipase.
17 Chapter 1
Although this process is very efficient, the yield of a kinetic resolution is limited to a maximum of 50% unless a racemization step is incorporated to improve the yields. However, racemization of amines has proven to be fairly complicated and harsh conditions are typically required.[44] Alternatively, transaminases can be used to synthesize chiral amines. Transaminases transfer an amino group from a donor substrate to an acceptor compound. However, it is essential to add stoichiometric quantities of an amine acceptor (e.g. pyruvate) which in addition to the cost can also result in inhibition of the transaminase. Turner and coworkers have designed an indigenous system to circumvent this problem by introducing a catalytic amount of amine acceptor (pyruvate; 8) together with an amino acid oxidase (Scheme 5) generating chiral amines 7 in very selective fashion.[45]
Scheme 5: Kinetic resolution of a racemic amine using a transaminase in combination with a catalytic amount of pyruvate and amino acid oxidase.
Transaminases can also be used in an asymmetric fashion (theoretical yield of 100%) generating chiral amines from ketones using a suitable amine donor (e.g. L-alanine). However, there is a significant disadvantage to these type of reactions; the unfavorable conversion of a ketone to an amine. The equilibrium of the reactions is mostly lies on the ketone substrate. Bornscheuer and coworkers developed a highly efficient methodology (Scheme 6) for the asymmetric synthesis of optically pure amines 11 by combining a transaminase and a pyruvate decarboxylase shifting the equilibrium to the amine.[46]
In principle, far more biocatalytic methods exist to synthesis optically active amines which were not discussed in the preceding part, including, asymmetric synthesis with amine hydrogenase,[47] asymmetric reduction of aryl amines[48] and monoamine oxidase (discussed in chapter 2).
18 General Introduction
Scheme 6: Asymmetric synthesis of a ketone using transaminase in combination with alanine and pyruvate-decarboxylase.
1.2.2.2 Optically Active Alcohols Optically pure alcohols are widely used in the agrochemical and pharmaceutical industry, flavors and fragrances, food applications and in material sciences, such as liquid crystals. Despite the fact that there are several methods to generate these optically pure alcohols chemically mostly by ruthenium catalysis,[49-52] biocatalysis offers a very efficient and green approach to obtain these synthons.[53] Racemic alcohols are attractive starting compounds, since they are often more easily accessible than the corresponding ketones, although ketones are frequently reduced in an asymmetric fashion. An example of both strategies will be discussed. Kroutil and coworkers have demonstrated an efficient deracemization of racemic alcohols 15 by the stereoinversion of one enantiomer in a tandem reaction sequence making use of an oxidase from Alcaligenes faecalis and an alcohol dehydrogenase (ADH) in which only glucose and molecular oxygen are required as co-factors (Scheme 4).[54]
Scheme 7: Tandem oxidation and reduction for the deracemization of secondary alcohols.
19 Chapter 1
ADH from Rhodococcus ruber DSM 44541 was also demonstrated to be a highly effective reducing agent of ketones (19). High concentrations of isopropanol (up to 50% v/v) using whole cells not only shift the equilibrium toward the product side, but also enhance the solubility of lipophilic substrates in the monophasic aqueous/organic medium. As a result, this proved to be a highly regio- and enantioselective method giving the desired products (20) in excellent yield (Scheme 8).[55]
Scheme 8. Biocatalytic oxidation/reduction of alcohols/ketones by ADH andd cosubstrate (20% (v/v) acetone/50% (v/v) isopropanol).
1.2.2.2 Optically Active Aldehydes/Ketones Needless to say, aldehydes and ketones present interesting classes of compounds in the synthesis of complex products, especially when these compounds are used as optically pure inputs. An appealing synthon in this case is an α-hydroxy ketone (acyloin). Its bifunctional nature and the presence of chiral information make it a highly valuable building block for further synthetic transformations. Because of its significance, chemical approaches aiming to prepare them have been reported.[56-57] Several representatives of thiamine diphosphate (ThDP)-dependent enzymes have proven highly efficient biocatalysts in the synthesis of acyloins, i.e. benzaldehyde lyase from Pseudomonas fluorescens (BAL) and benzoylformate decarboxylase from Pseudomonas putida (BFD). For example, Trauthwein and coworkers developed an efficient biphasic whole cell system to generate acyloins 23 in high yield and enantiomeric excess (Scheme 9). No external cofactors (ThDP and Mg2+) were necessary in combination with these whole cells. MTBE proved to be the best organic solvent for the whole cells but also for the isolated enzymes.[58]
20 General Introduction
Scheme 9: Acyloin formation using whole cells expressing BAL and BFD with a range of aldehydes in a biphasic system.
α-Substituted chiral aldehydes are widely used as versatile building blocks for the asymmetric synthesis of complex natural products and drug like molecules, therefore a wide range of methodologies has been developed for their asymmetric synthesis.[59-63] Unfortunately, few methods relating to biocatalysis have been developed. A rare example was reported by Smonou and coworkers where a kinetic resolution by a lipase of corresponding acylal 24 resulted in the corresponding optically active α-substituted aldehyde 26 in modest conversion and enantiomeric excess (Scheme 10).[64]
Scheme 10: Synthesis of chiral aldehydes through lipase-catalyzed resolution of acylal.
1.2.2.3 Optically Active Carboxylic Acids Chiral carboxylic acids can be derived from chiral aldehydes and alcohols (see previous sections) by classical oxidation chemistry. Nevertheless, they can also be formed from ester hydrolysis as demonstrated by the group of Kazlauskas (Scheme 11).[65] In this kinetic resolution the crude lipase from Candida rugosa (CRL) was first dissolved in 50% isopropanol followed by dialysis to remove the isopropanol. This treatment enhanced the activity and enantioselectivity dramatically. This led to 2-substituted carboxylic acids 29 in moderate yield and excellent enantiomeric excess.
Scheme 11: Kinetic resolution of 2-substituted carboxylic acids esters by Candida rugosa.
21 Chapter 1
Nitrilases are also capable of generating carboxylic acids by hydrolyzing the corresponding nitriles. This is illustrated by the group of Burk who performed a dynamic kinetic resolution (DKR) of aromatic α-hydroxy nitriles 32 and 33 (Scheme 12). A wide range of (R)-mandelic acid derivatives (35) and analogues were generated by nitrilase B and a series of (S)- phenyllactic acid (34) and other aryllactic acid derivatives were generated by nitrilase A in high yield and enantiomeric excess.[66]
Scheme 12: Nitrilase catalyzed DKR of α-hydroxy nitriles in production of valuable chiral hydroxy carboxylic acid derivatives.
1.2.2.4 Optically Active Nitriles Cyanohydrins have proven to be versatile building blocks in organic synthesis.[67-68] Therefore, the synthesis of chiral cyanohydrins 36 has attracted considerable attention (scheme 13).[69- 71] Nowadays, the majority of cyanohydrin syntheses is performed by hydroxynitrile lyases (HNLs) owing to their broad applicability, availability, excellent yield and enantiomeric excess (Scheme 13).[69-70, 72, 73] The HNLs from Prunus amygdalus (PaHNL), Sorghum bicolor (SbHNL), Manihot esculenta (MeHNL) and Linum usitatissimum (LuHNL) are currently the four major HNLs that are used in the synthesis of chiral cyanohydrins with ee’s up to >99%.[72] When starting with aldehydes, the equilibrium usually lies to the product side. Nevertheless, when ketones are employed, yields are lower due to the considerable steric bulk in the product, a tertiary alcohol with three other bulky substituents. This can be circumvented by using a large excess of HCN. Furthermore, chemical synthesis of these cyanohydrins shows lower yield and enantioselectivity compared to synthesis via HNLs.[70]
22 General Introduction
Scheme 13: HNL-catalyzed synthesis of cyanohydrins and possible applications.
1.3 Multicomponent Reactions
The field of organic chemistry has reached a high level of sophistication in the past few decades; the syntheses of complex natural products have become more and more efficient and the stereoselectivity has significantly improved. These improvements however, have not yet led to the ideal synthesis (Figure 3).[4, 74] This proposed ideal synthesis consists of different factors; both economical and environmental parameters are considered. An important factor, both economically and environmentally, is one-pot synthesis.
Figure 3: Ideal synthesis described by Wender and coworkers.[4]
23 Chapter 1
Multicomponent reactions (MCRs) combine essentially all the atoms of at least three components to form a single product in one pot for atom and process efficiency with reduced waste. MCRs are resource effective and environmentally acceptable and thus greener compared to classical multistep synthesis (Figure 4). Therefore, they contribute to different aspects of the ideal synthesis. MCRs are not only resource effective, but also more environmentally friendly than a linear synthesis. Also, because of the versatility and the simple experimental set-up of MCRs they have proven to be convenient tools for easy access to large collections of structurally and functionally diverse organic compounds.[75-79] Most notable, they play an important role in drug discovery.[80-81]
B B
MCR A + C A C
D D
D B B B C A A A C
Classical multistep synthesis Figure 4: Classical multistep synthesis versus multicomponent reactions.
In the ideal synthesis a compound should be obtained in 100% yield. Therefore controlling the stereochemistry of a reaction is of significant importance. Stereochemistry is an important factor in medicinal chemistry, since often only one enantiomer has the desired biological activity. Biologically active (natural) products often occur as a single enantiomer, while the other enantiomer is inactive or in some cases even harmful. A good example of this is thalidomide (tradename: Softenon®, Figure 5), a drug which was prescribed to many pregnant women in the 1950s and 1960s to prevent morning sickness. The drug, which contains one stereocenter, was dosed to the pregnant women as a racemic mixture, of which the (R)-enantiomer is effective against morning sickness the (S)-isomer is teratogenic.[82] Due to this drug, over ten thousand children were born with malformations. It is therefore highly desirable to synthesize enantiomerically pure compounds.
24 General Introduction
Figure 5: Structural representation of thalidomide.
In 1850, Strecker reported what is nowadays considered to be the first MCR: a reaction between an aldehyde or ketone, ammonium chloride and potassium cyanide to form an α-aminonitrile, which can be hydrolyzed to give an amino acid (Scheme 14).[83] First, iminium formation (38) takes place, followed by addition of the cyanide. The amino nitrile (39) is then hydrolyzed to give desired amino acid 40.
Scheme 14: Strecker amino acid synthesis.
In 1891, Biginelli reported a MCR between urea, an aldehyde and a 1,3-diketone, yielding dihydropyrimidone (Scheme 15).[84] The mechanism starts with condensation of the aldehyde and urea to form iminium species 43 as an intermediate. This is followed by nucleophilic addition of the ketoester (44), after which condensation takes place to give the product (46) containing a newly formed stereocenter. These heterocyclic systems are known to be of interest in the pharmaceutical industry, since the partly reduced pyrimidone derivatives show biological effects such as antiviral, antitumor, antibacterial and anti-inflammatory activity.[85]
Scheme 15: The Biginelli pyrimidone synthesis.
The Mannich reaction, discovered in 1912,[86] is a three-component reaction between a non- enolizable aldehyde, a primary or secondary amine and an enolizable carbonyl compound (Scheme 16), which has many useful applications in polymer chemistry and the agrochemical and pharmaceutical industry.[87] The reaction starts with formation of an iminium ion (49),
25 Chapter 1 which is then attacked by the enolized aldehyde (50) to form a β-amino-carbonyl (also known as a Mannich base; 51) with two stereocenters.
Scheme 16: The Mannich reaction.
In 1921, Passerini described the very first isocyanide-based MCR between a carboxylic acid (53), a carbonyl compound (52) and an isocyanide (53) giving α-hydroxy carboxamide 55 containing a newly formed stereocenter as a product (Scheme 17).[88] There have been disputes about the actual mechanism of the Passerini reaction.[89-90] Two different mechanisms have been postulated; the ionic and the concerted mechanism. In contrast to the Ugi reaction (see below), the Passerini reaction is accelerated in aprotic solvents indicating a concerted mechanism.[91-92] The concerted mechanism proceeds through intermediate 57, which rearranges to the desired α-hydroxy carboxamide 55 (Scheme 17).
Scheme 17: The Passerini reaction via the concerted the mechanism.
Although Passerini reported his isocyanide based MCR as early as 1921, it was not until 1959 that the popularity of isocyanide chemistry began with the first 4-component reaction (4- CR) with isocyanides reported by Ugi and coworkers (Scheme 18).[93-94] This reaction between a ketone or aldehyde (52), an amine (58), an isocyanide (54) and a carboxylic acid (53) is a very versatile reaction, and is widely used in the fields of modern combinatorial and medical chemistry.[75-81] The proposed mechanism starts with imine formation by condensation of the amine and the aldehyde/ketone, followed by formation of an iminium ion (59) by proton exchange with the carboxylic acid. Subsequent addition of the isocyanide to the iminium
26 General Introduction ion followed by nucleophilic addition of the carboxylic acid forms adduct 61. In the final step, an irreversible Mumm rearrangement takes place, transferring the acyl group from the oxygen to the nitrogen and generating α-aminoacyl amide derivatives 62 (Scheme 18).
Scheme 18. Proposed mechanism of the Ugi four-component reaction (U-4CR).
1.4 Biocatalysis & Multicomponent Reactions
In spite of the fact that MCRs are very efficient by their nature, the stereocontrol in these reactions is mostly not straightforward.[95-100] For most MCRs, catalytic asymmetric methods to control the stereochemical outcome of the reaction are so far not available. Since asymmetric induction is typically achieved by using optically pure inputs, it is important to utilize methods which can efficiently and environmentally friendly generate these inputs. The broad repertoire of stereospecific conversions by biocatalysts presents a unique opportunity to address the stereoselectivity issue of certain MCRs. The combination of (i) MCRs with (ii) biocatalysis presents an ideal approach (solution), regarding selectivity (stereocontrol), flexibility, and sustainability. Several combinations of biocatalysis with MCRs are conceivable (Figure 6):
(A) Kinetic resolution of a racemic MCR product. (B) Dynamic kinetic resolution in which the optically pure product reacts in a MCR. (C) Resolution of a racemic MCR product to generate a chiral input for a second MCR (combination of A and B). (D) Biotransformation to generate optically pure inputs for diastereoselective MCRs.
27 Chapter 1
Figure 6a: Combination (A-B) of MCRs and biocatalysis.
Figure 6b: Combination (C-D) of MCRs and biocatalysis.
Thus, combining MCRs and biocatalysis can result in enantioselective complex molecules from readily available compounds, possibly providing molecules having major potential in medicinal chemistry. However, the combination of these two methods also addresses more fundamental issues in the stereoselective construction of complex molecules. Despite the great potential of biocatalysis and MCRs, the combination of these two methodologies to generate optically pure complex compounds has hardly been described in literature (see next section).
1.5 Applications of Biocatalysis & Multicomponent Reactions
1.5.1 Multicomponent Reactions/Enzymatic Kinetic Resolution The first combination of multicomponent chemistry and biocatalysis was reported by Dordick and coworkers. They combined the selectivity of biocatalysis with the strength of multicomponent chemistry in the synthesis of a Ugi four-component condensation (U-4CR) library of α-(acylamino)amides of type 71 (Scheme 19).[101] β-hydroxy acid 63 and amino
28 General Introduction alcohol 66 underwent enzyme-catalyzed acylation prior to being employed in the U-4CR as these inputs would require protecting and deprotecting steps when chemically synthesized (Scheme 19). Porcine pancreatic lipase (PPL) was employed since lipases can catalyze the acylation of both hydroxy acids and amino alcohols.[102-103] PPL proved to be an efficient biocatalyst and gave acylated carboxylic acid 65 and amine 68 in yields over >90%. The yield for the acylated carboxylic acid (>90%) implies that the corresponding lipase was non- stereoselective. With these inputs in hand, the U-4CR was performed using acetaldehyde 69 and methyl isocyanoacetate 70 and resulted in a 9-membered library of α-(acylamino) amides 71. Unfortunately, no enantiomeric and diastereomeric ratios were reported.
Scheme 19: Lipase-catalyzed acylation of hydroxy acid 68 and amino alcohol 67 followed by a U-4CR generating α-(acylamino)amides 71.
In the same year, enantiomerically pure dihydropyrimidones (DHPMs) were generated by a Biginelli multicomponent reaction followed by a lipase-catalyzed kinetic resolution of the corresponding MCR products.[104] DHPMs play an important role as calcium channel modulators.[105-107] Moreover, different enantiomers of dihydropyrimidones show differentiation in pharmacological activity (calcium entry blocking versus activation). Access to enantiomerically pure DHPMs via chemical synthesis showed to be not that straightforward.[105, 107] So, introducing stereoselectivity using biocatalysis seems appealing. To show the feasibility of MCRs (Biginelli) and biocatalysis (lipase) as a combination, SQ 32926 (82) a potent orally active antihypertensive agent, was synthesized (Scheme 20).[105- 106] After generating DHPM scaffold 75 via a Biginelli MCR, a cleavable ester functionality at the N3 position was introduced allowing a kinetic resolution and obtaining the desired enantiomer (78) in excellent enantiomeric excess (98%). Compound 78 was subsequently manipulated providing the desired target compound (R)-SQ 32926 (82, Scheme 20).
29 Chapter 1
Scheme 20: Synthesis of (R)-SQ 32926 (82) via a Biginelli MCR and a lipase-catalyzed kinetic resolution.
A modified strategy in obtaining enantiomerically pure DHPMs was developed by Prasad and coworkers.[108] The Biginelli reaction was not performed under conventional heating but was microwave assisted. This method gave short reaction times and high yields for the resulting compounds (90, Scheme 21). Also, instead of having a cleavable ester functionality at the N3 position, they introduced an acetoxy group on the aldehyde‘s aromatic ring system (87, Scheme 21). This acetoxyl group was cleaved of by Novozyme 435 generating optically pure DHPMs 88 and 89. Compound 88 was subsequently deacetylated generating 90. Unfortunately, this group was not able to determine the ee’s of their compounds but just mentioned that the ee’s were moderate to high.
30 General Introduction
Scheme 21: Alternative strategy to enantiomerically pure DHPMs by Prasad and coworkers.[108]
Two groups have employed a similar strategy to generate enantiomerically pure α,α-dialkyl- α-hydroxylic acid esters 96 and 102 where a hydrolase selectively cleaves an ester moiety from a Passerini multicomponent product (94 and 100, Scheme 22 and 23). α,α-Dialkyl- α-hydroxylic acid esters are important intermediates in organic chemistry. For example, this moiety is present in the natural product clerodendrin A[109] but also found use as cyclooxygenase inhibitor.[110] Ostaszewski and coworkers obtained enantiomerically pure secondary alcohol 96 by first performing a Passerini MCR of acetic acid, phenylacetaldehyde and 4-methoxybenxyl isocyanide generating α-acetoxyamide 94 subsequently followed by a hydrolysis with Wheat Germ lipase obtaining the desired compound (96) in 47% yield and 93% ee (Scheme 22).[111]
31 Chapter 1
Scheme 22: Synthesis of optically pure secondary alcohol 96 using a Passerini MCR and enzyme-catalyzed hydrolysis.
Bornscheuer and coworkers obtained enantiomerically pure tertiary alcohol 102 by first executing a Passerini MCR using 1,1,1-trifluoro-2-butanone, acetic acid and tert-butyl isocyanide to obtain Passerini product 100 (Scheme 23). Subsequent hydrolysis using an esterase of metagenome origin (genetic material recovered directly from environmental samples, esterase 8) led to the desired product in a 46% yield and 85% ee.[112]
Scheme 23: Synthesis of optically pure tertiary alcohol 102 using a Passerini MCR and enzyme-catalyzed hydrolysis.
Another MCR combined with a kinetic resolution has been employed by Beller and coworkers.[113] This MCR generates O-acyl-substituted cyclohexenes 108 from anhydrides 103, aldehydes 104 and dienophiles 106 and involves a Diels-Alder reaction (Scheme 24).
32 General Introduction
The addition of the dienophile was very selective and only the endo-addition product was observed (with one exception). Since this MCR is performed at relatively high temperatures (110 °C), the development of an enantioselective variant of this MCR is difficult. Therefore, a lipase-catalyzed kinetic resolution was employed in order to obtain enantiomerically pure cyclohexenols. For each individual ester the optimal enzyme was determined giving high yields (48–50%) with ee’s ranging from 91–99%. The stereoselectivity could be varied by using different lipases.
Scheme 24: A MCR of anhydrides, aldehydes, and dienophiles involving a Diels-Alder reaction with a subsequent lipase-catalyzed kinetic resolution generating enantiomerically pure cyclohexanols.
Besides anhydrides, also acyl chlorides, alcohols or amides could be used as inputs in the generation of acylamino-1,3-butadiene 110 (Scheme 25).[114] Furthermore, various dienophiles could be used, which increases the versatility of this reaction quite significantly (Scheme 25).[114] The resulting compounds were obtained in comparable yields and diastereoselectivity as described previously.[113] Unfortunately, cleaving the ester moiety with a lipase-catalyzed kinetic resolution proved to be quite difficult. The ee’s obtained from the kinetic resolution of Diels-Alder product 111 and 114 varied significantly (no conversion-99% ee), whereas Diels-Alder products 112 and 113 did not give any conversion.
33 Chapter 1
Scheme 25: Synthesis of a wide range of enantiomerically pure cyclohexenols derivatives by combining MCRs and lipases.
1.5.2 Multicomponent Reactions/Enzymatic Dynamic Kinetic Resolution The first combination of a MCR and an enzymatic desymmetrization was reported by Ostaszewski and coworkers in 2005.[115] They developed a one-pot enzymatic desymmetrization of prochiral 3-phenylglutaric anhydrides 115 coupled to an Ugi 4CR generating a small library of chiral peptidomimetics (Scheme 26). Various lipases were screened and Chirazyme L was found to be the most suitable catalyst. Moderate to reasonable ee’s and excellent yields were obtained of desired carboxylic acids 116. The subsequent Ugi 4CR reaction tolerated aliphatic and aromatic aldehydes and amines, but also benzyl isocyanide and ethyl isocyanoacetate. Most notably, all compounds 120 were obtained as diastereoisomeric mixtures, in approximately 1:1 ratio. This is not very surprising as it is known that chiral carboxylic acid give poor induction.[76]
Scheme 26: Enzyme-catalyzed desymmetrization combined with an Ugi 4CR generating chiral peptidomimetics 120.
34 General Introduction
1.5.3 Enzymes as Catalysts for Multicomponent Reactions Besides using biocatalysis as a tool for introducing chirality into a compound, enzymes can also be used to make certain chemical conversions possible. These conversions might be virtually impossible using traditional methods which could be attributed to the fact that these conversions require, for example, an extremely expensive reactant, high stereo- or regio-selectivity, or have a very high energy barrier. Since substituted aromatic and aliphatic aldehydes lower the yield of the Biginelli multicomponent reaction considerably,[116-117] bakers’ yeast (Saccharomyces cerevisiae) has been employed as an efficient catalyst in the Biginelli reaction.[118] With only glucose as additive, several activated and deactivated aromatic aldehydes (121), β-keto esters 122 and urea/thiourea (123) were employed and gave excellent yield of the desired DHPMs (124, Scheme 27). However, no stereoselectivity could be observed when employing this strategy.
Scheme 27: Biginelli reaction catalyzed by bakers’ yeast (Saccharomyces cerevisiae).
Another interesting example of enzymes catalyzing MCRs was shown by Yu and coworkers, who have shown that lipases are able to catalyze Mannich reactions efficiently. [119-120] The Mannich reaction is one of most important reaction in generating C-C bonds. The resulting β-amino carbonyl compounds are important intermediates for several pharmaceuticals and natural products.[121] Unfortunately, direct Mannich reactions often suffer from selectivity problems since both the aldehyde and the CH-acidic substrate can often act as either nucleophile or electrophile. Therefore, preformed electrophiles, such as imines, iminium salts and hydrazones or nucleophiles such as enolates, enol ethers and enamines, or both are regularly used (also called an indirect Mannich). These pretreatments may lead to several drawbacks caused by preparation, isolation and purification. Yu and coworkers developed a one-pot CRL (Candida rugosa lipase) catalyzed direct Mannich reaction resulting in the corresponding β-amino carbonyl compounds 128 in high yields but disappointing diastereomeric ratios (Scheme 28). A wide range of aromatic aldehydes (125) were employed and electron-withdrawing substituents on the aldehyde were favored. Unfortunately, using aliphatic aldehydes as inputs did not lead to the desired β-amino carbonyl compounds.
35 Chapter 1
Scheme 28: Lipase-catalyzed direct Mannich reactions.
1.6 Conclusions
Multicomponent chemistry is a convenient tool for easy access to large collections of structurally and functionally diverse organic compounds that can be screened for pharmacological activity or as ligands for novel transition metal catalysis. In spite of the fact that MCRs are very efficient by their nature, the stereocontrol in these reactions is mostly not straightforward. So it is evident that the combination of MCRs and biocatalysis can be very useful in generating enantiomerically pure complex products. Most of the procedures that have appeared in the literature rely on an enzymatic kinetic resolution which inherently leads to 50% loss of the desired optically pure compound. To our knowledge only one precedent methodology combining an enzymatic dynamic resolution with an MCR is available (see section 1.5.2). This opens up possibilities to explore the combination of MCRs and DKR/desymmetrization to generate high added value compounds in excellent yields and enantiomeric excesses.
1.7 Scope and Outline of this Thesis
The research described in this thesis involves the generation of highly complex and optically pure molecules based on the combination of optically pure 3,4-disubstituted 1-pyrrolines and several multicomponent reactions. Chapter 2 will describe the main biocatalyst (monoamine oxidase N) used during this research and illustrate how to gain access to optically pure 3,4-disubstituted 1-pyrrolines efficiently. In Chapter 3 3,4-disubstituted 1-pyrrolines are employed in a Joullié-Ugi MCR generating pharmaceutically relevant optically active substituted prolyl peptides. These substituted prolyl peptides also showed to be useful as highly active organocatalysts for stereoselective conjugate addition of aldehydes to nitroolefins. In Chapter 4 the prolyl peptides scaffold could be further complexed and diversified
36 General Introduction by introducing cyclization reactions. Carefully selected prolyl peptides will undergo Pictet-Spengler cyclization to obtain highly complex alkaloid-like polycyclic compounds (2,5-diketopiperazines) with high diversity. To the best of our knowledge, it also constitutes the first example of MCR chemistry to synthesize 5-membered ring-fused 2,5-diketopiperazines. The efficiency and generality of the approach and the resulting molecular diversity and complexity makes our monoamine oxidase N and Joullié-Ugi MCR combination highly interesting for medicinal chemistry. Most notably, a very short and efficient synthesis of important hepatitis C drug Telaprevir (Incivek™), featuring a biocatalytic desymmetrization and two multicomponent reactions (Passerini and Joullié-Ugi 3CR) as the key steps is described in Chapter 5. In Chapter 6 3,4-disubstituted 1-pyrrolines are combined with a Ugi-Smiles MCR generating enantiomerically pure N-aryl proline (thio)amides. Remarkably, some Ugi-Smiles products show unusually high optical rotations. Finally, Chapter 7 summarizes the main conclusions of this work and provides a further outlook on the combination of biocatalysis and multicomponent chemistry.
1.8 References & Notes
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38 General Introduction
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39 Chapter 1
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40 Chapter 2 Monoamine Oxidase N: A Promising Biocatalyst for Asymmetric Synthesis
Partly Published in: Angew. Chem. 2010, 122, 2228-2230; Angew. Chem. Int. Ed. 2010, 49, 2182-2184
Monoamine Oxidase N
2.1 Introduction
Amines are ubiquitous in Nature. For example they play an important role in cell growth and differentiation,[1] neoplastic cell proliferation[2-3] and are also a very important source for carbon and nitrogen for a number of yeast and bacterial species. It is therefore not surprising that amine oxidation is an omnipresent process in biology, participating in the transformation of neurotransmitters via oxidative deamination[4] as well as the detoxification of xenobiotics.[5] Enzymes capable of oxidizing amines (amine oxidases; AO) are found in numerous organisms ranging from bacteria, yeasts, and plants to mammals and can be grouped into the flavoprotein and quinoprotein families. The mechanism of amine oxidation catalyzed by the quinoprotein amine oxidases which comprises e.g. diamine oxidases (E. C. 1.4.3.6) has been well characterized and occurs through the formation of an enzyme–substrate covalent adduct with either topaquinone (TPQ), tryptophan tryptophylquinone (TTQ), cysteine tryptophylquinone (CTQ) or lysine tyrosyl quinone (LTQ) as a cofactor (Scheme 1).[6-7]
Scheme 1: Mechanism for the oxidation of amines by a quinoprotein amine oxidase.
The intermediates in Scheme 1 have been characterized by a combination of chemical, kinetic and spectroscopic techniques. First, TPQox reacts with an amine substrate giving quinone ketimine A, which, after a formal proton shift, gives rise to quinolaldimine B. After hydrolysis and release of the aldehyde, TPQred is formed. In a mechanism which is not fully understood, the reduced cofactor is reoxidized by molecular oxygen back to TPQox to start a new catalytic cycle.
43 Chapter 2
Scheme 2: Proposed mechanism by of amine oxidation by flavoproteins.
In contrast to amine oxidation by the quinoprotein, the mechanism of amine oxidation by flavoproteins comprising e.g. monoamine oxidases (E. V. 1.4.3.4), amino acid oxidases (E. C. 1.4.3.2 and E. C. 1.4.3.3) and polyamine oxidases (EC 1.5.3.11) is still a topic of debate. The uncertainties regarding the mechanism of amine oxidation by flavoenzymes are due to the high chemical versatility of the isoalloxazine system. Because the fully oxidized cofactor is readily reduced by either one or two electrons, flavoproteins are capable of participating in several reactions resulting in several proposed mechanisms. While this range of reactivities is clearly of advantage to biological systems, it complicates the mechanistic analyses of
44 Monoamine Oxidase N flavoenzymes, in that several viable mechanisms must be considered (scheme 2). These include (i) formation of a substrate carbanion by loss of a proton due to an active site base,[8] (ii) single electron transfer reaction generating an aminium radical,[9] (iii) nucleophilic attack by the amine on the flavin C4α followed by proton abstraction by an active site base.[10] Despite these proposals, the mechanism of oxidation of amines by flavoproteins has been controversial for decades and a consensus view (although the single electron transfer mechanism is the one most widely accepted) has not emerged following a period of intensive research activity over several years.
2.2 Monoamine Oxidase
2.2.1 Mammalian Monoamine Oxidase In 1928, Mary Bernheim (née Mary Hare) was the first to describe a new oxidative pathway in the liver and named the enzyme involved tyramine oxidase.[11] This enzyme was eventually renamed monoamine oxidase (MAO). MAO oxidizes primary aliphatic and aromatic amines as well as some secondary and tertiary amines. The monoamine oxidases from Homo sapiens are located intracellularly in the outer membrane of mitochondria and comprise two isoforms, MAO-A and MAO-B. MAO-A preferentially oxidizes serotonin and norepinephrine,[12] while MAO-B has a higher affinity for phenylethylamine and benzylamine.[13] MAO-A and MAO-B are co-expressed in most human tissues and are mostly found in the intestines.[14] However, fibroblasts and placenta express MAO-A exclusively.[15] On the other hand, platelets and lymphocytes only express MAO-B.[16] In the human brain MAO-A is confined to catecholaminergic neurons whereas MAO-B is found in serotonergic neurons and astrocytes.[17] Although MAO-A and MAO-B are distinctly different in substrate preference and tissue and cell distribution, they share a covalently bound flavin and 70% sequence identity. [18] MAO-A and MAO-B started to attract the attention from the biochemical and pharmaceutical communities from the 1950s when Zeller and coworkers showed that these enzymes served as targets for antidepressant drugs.[19] Since those findings more than 14000 papers have been published relating to MAO inhibitors and structural and functional properties of MAO. MAO dysfunction (high or low levels of MAO) is thought to be responsible for a number of psychiatric and neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with depression,[20] schizophrenia,[21] attention deficit disorder (ADD),[22] and migraines.[23] Actually, MAO-A inhibitors (e.g. moclobemide) act as antidepressant and antianxiety agents (Table 1), while MAO-B inhibitors (e.g. selegiline, rasagiline) are used alone or in combination to treat Alzheimer’s and Parkinson’s diseases (Table 1).[24] Also, studies have shown that MAO-B is heavily depleted (40% decrease) by use of tobacco cigarettes.[25] In fact, MAO inhibitors have recently been reported to facilitate stopping smoking in highly dependent smokers.[26]
45 Chapter 2
Table 1: Clinically used specific inhibitors of MAO-A and MAO-B
Compound MAO Selectivity Application
A Antidepressant Moclobemide
A Antidepressant
Befloxatone
A Antidepressant Clorgyline
B Antiparkinson Selegiline
B Antiparkinson
Rasagiline
B Antiparkinson Lazabemide
2.2.2 Monoamine Oxidase N Amine oxidases are also found in lower eukaryotes. In the fungus Apsergillus niger, two types of amine oxidase have been described. Type I belongs to the quinoprotein family,[27] while Type II is a flavin-dependent MAO (designated MAO-N).[28] Unlike the mammalian MAOs, flavin-dependent MAO-N is soluble, peroxisomal, readily extractable, could be isolated extensively by conventional procedures and showed no tendency to aggregate. Most interestingly, the flavin cofactor FAD is non-covalently linked, in contrast to MAO-A and -B from mammalians. Despite the lack of a pentapeptide sequence which covalently anchors FAD to the mammalian MAO, MAO-N shows a significant sequence similarity (49%) and sequence identity (24%) with human MAO-A and -B. Also, the substrate specificity of MAO-N in some respects resembles that of MAO-A and in others that of MAO-B, while it also exhibits some unique properties.[28] All these observations have led to the suggestion that MAO-N may be an evolutionary precursor of vertebrate MAOs.
46 Monoamine Oxidase N
2.3 Directed Evolution of Monoamine Oxidase N
As shown previously, amine oxidases have been classified into two groups, i.e., Type I (quinoprotein family) and Type II (flavin-dependent family). In the catalytic cycle, Type II MAO generate “free” imines (scheme 2, ii), while in Type I MAO the imines stay covalently bound to the protein (Scheme 1). This has led to the conclusion that Type II MAOs were more suitable for generating optically active imines. The possibility of cloning and expressing of MAO-N from Aspergillus niger resulted in substrate-specificity and kinetic studies. These studies showed MAO-N to have high activity towards simple aliphatic amines but a much lower activity towards benzylamine.[28-29] Turner and coworkers chose α-methylbenzylamine as a feasible starting point substrate for the directed evolution of MAO-N due to the importance of this substrate as chiral amine[30] but also because L-α-methylbenzylamine although possessing very low activity for MAO-N, had a significantly higher activity compared to D-α-methylbenzylamine.[31] First, an effective high-throughput assay was developed were the hydrogen peroxide that is liberated during MAO-N oxidation is oxidized by a peroxidase in the presence of a substrate (3,3’-diaminobenzidine) that yields dark products (Figure 1).
Figure 1: High-throughput screening of libraries of MAO-N
A library of MAO-N mutants was generated by random mutagenesis. By using the colorimetric assay described in Figure 1, Turner and coworkers were able to identify clones that possessed higher activity towards L-α-methylbenzylamine compared to wild-type MAO-N. Eventually, a single mutant was identified which showed a 47-fold higher activity towards L -α-methylbenzylamine than the wild-type. Also, the selectivity towards L-α- methylbenzylamine versus D -α-methylbenzylamine was increased 5.8-fold relative to the wild-type.[31] This mutant showed to have a single point mutation where asparagine 336 is replaced by serine (Asn336Ser). Subsequently, an additional mutation was introduced (Met348Lys) resulting in higher specific activity and expression levels but leaving the substrate specificity unchanged.[32] This enzyme with two mutations showed the highest activity towards substrates (S enantiomer) containing a primary amine group flanked by a methyl group and a bulky alkyl/aryl group, for which the wild-type MAO-N showed no or very poor activity (Scheme 3).[32]
47 Chapter 2
Scheme 3: Schematic overview of substrate specificity of wild-type and mutant MAO-N.
Because the activity towards oxidation of secondary amines was still very poor, Turner and coworkers performed a new round of random mutagenesis to identify mutants capable of oxidizing secondary amines.[33] The starting point for the second round was a MAO-N mutant with four point mutations, i.e., Asn336Ser, Met348Lys (Scheme 3), Arg259Lys and Arg260Lys (improved expression). This mutant was subjected to a new round of random mutagenesis with 1-methyltetrahydroisoquinoline as the substrate. This resulted in a clone which showed significantly more activity (5-fold) than the parent. Interestingly, this mutant also showed enhanced activity and very high (S)-selectivity (E>100) towards a wide variety of secondary amines compared to the parent mutant. Analysis showed that this mutant possessed an additional point mutation (Ile246Met) and it was termed MAO-N-D3.[33] Turner and coworkers were also interested in mutants capable of oxidizing tertiary amines as these are difficult to obtain in enantiomerically pure form by alternative methods.[34-35] Another mutant, with an additional two point mutations from MAO-N-D3 (Thr384Asn and Asp385Ser, mutant designated MAO-N-D5) showed activity towards tertiary amines whereas the wild-type and MAO-N-D3 did not show any activity.[36-37] Recently, the structure of MAO-N was solved using X-ray crystallography and data showed that the Asn336Ser mutation reduces the steric bulk at the active site, Ile246Met gives more flexibility to the substrate binding site, and Thr384Asn and Asp385Ser seem to influence the tertiary structure of the enzyme resulting in change in the active-site environment.[37] This greatly improved catalyst shows that random mutagenesis with no prior knowledge of the structure is a powerful tool for the improvement of enzymes to achieve specific requirements (e.g. improved activity and selectivity) to obtain a highly attractive biocatalyst.
48 Monoamine Oxidase N
2.4 Application of Monoamine Oxidase N
Optically pure chiral amines have found widespread use in the agrochemical, pharmaceutical and chemical industries. Consequently, there is a need for efficient methods to obtain these compounds. Turner and coworkers designed an ingenious catalytic method for the preparation of chiral amines by deracemization of the corresponding racemic mixture (Scheme 4). The deracemization approach depends upon coupling of an enantioselective amine oxidase with a nonselective reducing agent (ammonia borane) where stereoinversion from the S to the R enantiomer via the achiral imine is achieved.[32-33, 36] Mutant monoamine oxidase N (Asn336Ser/Met348Lys) was used to generate a range of enantiomerically enriched primary R amines (substrate specificity is depicted in Scheme 3) in high yields and ee’s (scheme 4A, R3 = H).[32] A second-generation biocatalysts, MAO-N-D3, was used to produce a series of enantiomerically enriched secondary R amines in high yields and ee’s (scheme 4A, R3 ≠ H).[33] A B + + M M AO A N O m N u D5 N ta m H nt N u 3 H ta B 3 nt H B 3 H 3 + + 3 BH 3 BH H 3 N H 3 N Scheme 4: A. Deracemization of primary and secondary amines using mutant amine oxidases in combination with ammonia-borane. B. Deracemization of tertiary amines using mutant amine oxidases in combination with ammonia-borane.
In addition, MAO-N-D5 was used to obtain enantiomerically pure cyclic tertiary amines (scheme 4B) in good yields and excellent ee’s.[36] For example, the complete deracemization of N-methyl-2-phenylpyrrolidine was achieved within 24 h and yielded (R)-N-methyl-2- phenylpyrrolidine in 75% isolated yield and an ee of 99%.
49 Chapter 2
Pseudooxynicotine
N 3 H H B 5 3 D B 3 N H H N O 3 A M
()nicotine
nicotine
Scheme 5: Synthesis of (R)-nicotine from pseudooxynicotine via reductive amination.
This practical procedure is also useful as a means of enantioselective intramolecular reductive amination. Exemplary is the synthesis of (R)-nicotine from pseudooxynicotine (Scheme 5). As pseudooxynicotine predominantly exists as iminium ion rather than the enamine or hemiaminal in aqueous solution, it would be expected to be reduced much faster than the pseudooxynicotine and enamine and enter into the deracemization cycle. As expected, (R)-nicotine was formed after 24 h with no other detectable species in 99% ee. Moreover, MAO-N-D5 also exhibits high activity towards a wide range of substituted meso- pyrrolidines (a selection is presented in Figure 2) resulting in a method for the preparation of substituted 1-pyrrolines in high enantiomeric excess.[38]
50 Monoamine Oxidase N
Figure 2: A. Meso-pyrrolidines used as substrates for amine oxidation by MAO-N-D5. B. 1-pyrrolines resulting from oxidation with MAO-N-D5.
Unfortunately, not all imines (e.g. 1, 3 and 4) were readily isolated and had to be derivatized in situ to the trifluoroacetamide derivative from the corresponding amino nitrile, presumably due to decomposition of the corresponding imines. The applicability of these substituted 1-pyrrolines was demonstrated by addition of HCN to these imines to yield the corresponding amino nitriles (Scheme 6). Subsequent hydrolysis of the amino nitrile furnished the free amino acid in 94 % ee and 51% overall yield. Finally, the absolute configuration of the imines resulting from the oxidation by MAO-N-D5 was established. By reacting D,L-8 with D-amino acid oxidase and observing oxidation of D-8 assigned the product derived from the MAO- N-D5 oxidation as L-8.
Scheme 6: Diastereoselective synthesis of amino acid derivative 8.
2.5 Conclusion
Directed evolution can optimize specific requirements like substrate specificity and selectivity of enzymes in a highly effective way. Furthermore, the resulting mutants often possess a broader substrate specificity compared to the wild-type enzyme. Monoamine oxidase N from Aspergillus niger has proven particularly suitable for directed evolution. Mutant monoamine oxidases N tolerate primary, secondary, and tertiary amines as substrates. In addition, 3,4 substituted meso-pyrrolidines were shown to be excellent substrates for oxidation. Moreover, the stereoselectivity of the biocatalyst is very high (ee > 94%) and no additional cofactor is required. This makes MAO-N one of the leading biocatalysts for amine oxidation.
51 Chapter 2
2.5 References & Notes [1] T. Thomas and T. J. Thomas, Cell. Mol. Life Sci. 2001, 58, 244-258. [2] A. E. Pegg, Cancer Res. 1988, 48, 759-774. [3] N. Seiler, C. L. Atanassov and F. Raul, Int. J. Oncol. 1998, 13, 993-1006. [4] D. E. Edmondson, A. Mattevi, C. Binda, M. Li and F. Hubalek, Curr. Med. Chem. 2004, 11, 1983-1993. [5] W. Weyler, Y. P. P. Hsu and X. O. Breakefield, Pharmacol. Ther. 1990, 47, 391-417. [6] S. X. Wang, M. Mure, K. F. Medzihradszky, A. L. Burlingame, D. E. Brown, D. M. Dooley, A. J. Smith, H. M. Kagan and J. P. Klinman, Science 1996, 273, 1078-1084. [7] M. Mure, S. A. Mills and J. P. Klinman, Biochemistry 2002, 41, 9269-9278. [8] R. J. Rohlfs and R. Hille, J. Biol. Chem. 1994, 269, 30869-30879. [9] R. B. Silverman, Prog. Brain. Res. 1995, 106, 23-31. [10] J. M. Kim, M. A. Bogdan and P. S. Mariano, J. Am. Chem. Soc. 1993, 115, 10591-10595. [11] M. L. C. Hare, Biochem. J. 1928, 22, 968-979. [12] P. A. Shore, J. A. R. Mead, R. G. Kuntzman, S. Spector and B. B. Brodie, Science 1957, 126, 1063-1064. [13] Y. Ochiai, K. Itoh, E. Sakurai, M. Adachi and Y. Tanaka, Biol. Pharm. Bull. 2006, 29, 2362-2366. [14] P. Levitt, J. E. Pintar and X. O. Breakefield, Proc. Natl. Acad. Sci. U. S. A. Biol. Sci. 1982, 79, 6385-6389. [15] Q. S. Zhu, K. Chen and J. C. Shih, J. Neurosci. 1994, 14, 7393-7403. [16] W. K. Wong, K. Chen and J. C. Shih, J. Biol. Chem. 2003, 278, 36227-36235. [17] W. K. Wong, X. M. Ou, K. Chen and J. C. Shih, J. Biol. Chem. 2002, 277, 22222-22230. [18] A. W. J. Bach, N. C. Lan, D. L. Johnson, C. W. Abell, M. E. Bembenek, S. W. Kwan, P. H. Seeburg and J. C. Shih, Proc. Natl. Acad. Sci. U. S. A. 1988, 85, 4934-4938. [19] J. Barsky, W. L. Pacha, S. Sarkar and E. A. Zeller, J. Biol. Chem. 1959, 234, 389-391. [20] J. H. Meyer, N. Ginovart, A. Boovariwala, S. Sagrati, D. Hussey, A. Garcia, T. Young, N. Praschak- Rieder, A. A. Wilson and S. Houle, Arch. Gen. Psychiatry 2006, 63, 1209-1216. [21] J. J. Schildkraut, J. M. Herzog, P. J. Orsulak, S. E. Edelman, H. M. Shein and S. H. Frazier, Am. J. Psychiatry 1976, 133, 438-440. [22] S. D. Jiang, R. Xin, S. C. Lin, Y. P. Qian, G. M. Tang, D. X. Wang and X. D. Wu, Am. J. Med. Genet. 2001, 105, 783-788. [23] O. Meienberg and F. Amsler, Praxis (Bern 1994) 1997, 86, 1107-1112. [24] P. Riederer, L. Lachenmayer and G. Laux, Curr. Med. Chem. 2004, 11, 2033-2043. [25] J. S. Fowler, N. D. Volkow, G. J. Wang, N. Pappas, J. Logan, R. MacGregor, D. Alexoff, C. Shea, D. Schlyer, A. P. Wolf, D. Warner, I. Zezulkova and R. Cilento, Nature 1996, 379, 733-736. [26] I. Berlin, S. Said, O. Spreuxvaroquaux, J. M. Launay, R. Olivares, V. Millet, Y. Lecrubier and A. J. Puech, Clin. Pharmacol. Ther. 1995, 58, 444-452. [27] H. Yamada, O. Adachi and K. Ogata, Agric. Biol. Chem. 1965, 29, 117-123. [28] B. Schilling and K. Lerch, Biochim. Biophys. Acta 1995, 1243, 529-537. [29] S. O. Sablin, V. Yankovskaya, S. Bernard, C. N. Cronin and T. P. Singer, Eur. J. Biochem. 1998, 253, 270-279. [30] G. Hieber and K. Ditrich, Chim. Oggi 2001, 19, 16-20. [31] M. Alexeeva, A. Enright, M. J. Dawson, M. Mahmoudian and N. J. Turner, Angew. Chem. Int. Ed. 2002, 41, 3177-3180. [32] R. Carr, M. Alexeeva, A. Enright, T. S. C. Eve, M. J. Dawson and N. J. Turner, Angew. Chem. Int. Ed. 2003, 42, 4807-4810. [33] R. Carr, M. Alexeeva, M. J. Dawson, V. Gotor-Fernandez, C. E. Humphrey and N. J. Turner, ChemBioChem 2005, 6, 637-639. [34] N. E. Lee and S. L. Buchwald, J. Am. Chem. Soc. 1994, 116, 5985-5986. [35] S. Miyano, L. D. L. Lu, S. M. Viti and K. B. Sharpless, J. Org. Chem. 1985, 50, 4350-4360. [36] C. J. Dunsmore, R. Carr, T. Fleming and N. J. Turner, J. Am. Chem. Soc. 2006, 128, 2224-2225. [37] K. E. Atkin, R. Reiss, V. Koehler, K. R. Bailey, S. Hart, J. P. Turkenburg, N. J. Turner, A. M. Brzozowski and G. Grogan, J. Mol. Biol. 2008, 384, 1218-1231. [38] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184.
52 Chapter 3 Highly Stereoselective Synthesis of Substituted Prolyl Peptides Using a Combination of Biocatalytic Desymmetrization and Multicomponent Reactions
Published in: Angew. Chem. 2010, 122, 5417-5420; Angew. Chem. Int. Ed. 2010, 49, 5289-5292 Chapter 3
Abstract: Optically pure 3,4-disubstituted 1-pyrrolines, generated from the corresponding meso-pyrrolidines by biocatalytic desymmetrization (MAO-N=monoamine oxidase N), react with carboxylic acids and isocyanides in a highly diastereoselective Ugi-type multicomponent reaction to give pharmaceutically relevant substituted prolyl peptides that would require lengthy reaction sequences using other methods. The resulting prolyl peptides can also be used as highly active organocatalysts for stereoselective conjugate addition reactions.
54 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
3.1 Introduction
3.1.1 Biocatalytic amine oxidation combined with Ugi chemistry Multicomponent reactions (MCRs) offer the ability to rapidly and efficiently generate collections of structurally and functionally diverse organic compounds.[1-5] MCRs are important tools for both combinatorial chemistry and diversity-oriented synthesis, and thus play a significant role in the development of synthetic methodology for drug discovery.[6, 7] Although MCRs are very efficient by their nature, the stereocontrol in these reactions is mostly not trivial.[8-13] For most MCRs, catalytic asymmetric methods to control the stereochemical outcome of the reaction are so far not available. The Ugi reaction is undoubtedly one of the most widely applied MCRs.[14] It is of considerable interest owing to its exceptional synthetic efficiency and is widely used in the fields of modern combinatorial and medicinal chemistry.[1-7] The Ugi reaction involves a one-pot condensation of an aldehyde (1), an amine (2), a carboxylic acid (3) and an isocyanide (4) to produce chiral α-acylaminoamides 7 (Scheme 1). However, as in most MCRs, controlling the newly formed stereocenter is not straightforward.
Scheme 1: Proposed mechanism of the Ugi four-component reaction (U-4CR).
In 1982, Nutt and Joullié reported the use of an Ugi-type three-component reaction (U-3CR) that employed substituted 1-pyrrolines 10 instead of the amine and aldehyde components to produce substituted prolyl peptides 13 (scheme 2).[15] Commercially available 3-hydroxypyrrolidine 8 was converted into the corresponding aryl ether 9 via a Mitsunobu reaction. After deprotection of the Boc group, the N-chloro derivative was synthesized, which was dehydrohalogenated to form the imine species (10). The authors did not comment on the regioselectivity of the formed imine. Subsequently, a U-3CR was performed leading to the desired substituted 1-pyrrolines 13.
Scheme 2. First application of substituted 1-pyrrolines as input for U-3CR.
55 Chapter 3
In this case and in later applications,[16-23] the (dia)stereoselectivities were poor or unpredictable at best, and the routes to the required substituted 1-pyrrolines were tedious and/or low-yielding. A representative overview of the compounds (14-18) prepared in this way is given in Figure 1.
Figure 1: U-3CR with substituted 1-pyrrolines. OP = protected alcohol.
Recently, Turner and co-workers reported the biocatalytic desymmetrization of 3,4-substituted meso-pyrrolidines with engineered monoamine oxidase N (MAO-N) from Aspergillus niger[24, 25] to yield optically active 1-pyrrolines in excellent yields and ee values.[26, 27] As imines are intermediates for many common multicomponent reactions, the use of these optically active 1-pyrrolines in a MCR would be highly attractive given the excellent diastereoselectivity that can be achieved from the addition of nucleophiles to the imine, owing to its steric bulk (Scheme 3).
Scheme 3: Pyrrolidine derivatives accessible via amine oxidation/MCR strategy.
56 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
We envisioned that cyclic imines would give excellent selectivity in the Ugi-type reaction previously described by Nutt and Joullié due to their steric bulk. Also, these 3,4-substituted prolyl peptides, with generic structure 22, are of considerable interest to applications in organocatalysis and medicinal chemistry.
3.1.2 3,4-Disubstituted bisamide prolyl peptides in medicinal chemistry Compounds with a cis-3,4-substituted bisamide pyrrolidine scaffold (e.g. compounds with generic structure 22) show a broad range of interesting biological activities. In addition, this scaffold is quite common in several classes of natural products. For example, the echinocandins (Figure 2A), are a well-known class of 21-membered cyclic lipopeptides comprising a cis-4-methyl-3-hydroxy-pyrrolidine moiety. These macrocycles are isolated from Aspergillus rugulosus and Aspergillus nidulans and are characterized by their potent antifungal activity[28-30] as they effectively disrupt fungal cell wall development. Further, the astins A–C (Figure 2B), are a family of cyclopentapeptides isolated from the roots of Aster tataricus (Compositae) thatare characterized by a 16-membered ring containing a distinctive cis-3,4-dichloro- pyrrolidine moiety. These astins exhibit potent antitumour activity both in vitro and in vivo.[31-33] Noteworthy, astins A-C are significantly more potent than astins D-I which lack the dichloroproline showing the importance of the substituents on the proline ring. Also a designed fucodipeptide containing a cis-3,4-dihydroxy-pyrrolidine ring (Figure 2C) demonstrates excellent inhibitory effects on the binding of sialyl Lewis X (an essential carbohydrate for activity) to E-selectin.[34] Previous studies have shown that inhibition of this E-selectin result in anti-inflammatory activity in certain diseases such as reperfusion injury.[35-37]
57 Chapter 3
Figure 2: 3,4-subsituted bisamide pyrrolidines showing a broad range of biological activity. A. Echinocandins. B. Astins. C. Fucopeptide.
Finally, these 3,4-substituted prolyl peptides are also key structural elements of Telaprevir[38, 39] (Vertex and Johnson & Johnson) and Boceprevir[40, 41] (Merck & Co.), which inhibit the hepatitis C virus NS3 protease (Figure 3). Comprising a bicyclic proline core, these HCV protease inhibitors have passed clinical trials and have been approved for the treatment of HCV disease. These drugs are currently on the market under the names Incivek™ and Victrelis™.
Figure 3: HCV NS3 protease inhibitors.
3.1.3 Bisamide pyrrroldines in organocatalysis Organocatalysis has become an efficient tool to complement transition-metal complexes and enzymes for the synthesis of chiral building blocks. Recently, Wennemers and co-
58 Highly Stereoselective Synthesis of Substituted Prolyl Peptides workers showed that small peptides can be used as very efficient catalysts. The H-D-Pro-
Pro-Glu-NH2 tripeptide 27 showed to be a very efficient catalyst in 1,4-addition reactions of aldehydes 25 to nitro-olefins 26 (Scheme 4).[42-44]
Scheme 4: Conjugate addition reactions catalyzed by tripeptide H-D-Pro-Pro-Glu-NH2.
The generally accepted catalytic cycle of this reaction involves enamine formation (B) followed by reaction with the nitro-olefin and hydrolysis of the resulting imine (C) which generates the desired nitro-aldehyde and recycles the catalyst (scheme 5).
Scheme 5: Generally accepted catalytic cycle for the conjugate addition of aldehydes to nitro-olefins.
This H-D-Pro-Pro-Glu-NH2 tripeptide contains the same generic structure as these 3,4-substituted prolyl peptides 22 which are accessible via the MAO-N/U-3CR sequence making this combination of biocatalysis and multicomponent chemistry very interesting not only in the synthesis of biological interesting compounds but also in the synthesis of potential organocatalysts.
59 Chapter 3
3.2 Results & Discussion
3.2.1 Solvent study First, we turned our attention to finding the most suitable conditions for the Ugi-type MCR. As methanol is usually the solvent of choice in the Ugi reaction, we decided to do a solvent screen to determine if there would be any solvent effect on the diastereomeric ratio (d.r.). The reaction of racemic 3-azabicyclo[3.3.0]oct-2-ene (rac-29, synthesized according to a literature procedure),[45] benzoic acid, and tert-butyl isocyanide was selected as the model reaction. Various solvents were screened at room temperature (Table 1). Based on GC analysis, reactions performed in dichloromethane (Figure 4) and toluene gave the best diastereomeric ratios. In these solvents the influence of the temperature on the d.r. of the U-3CR was studied. Table 1 shows that dichloromethane at 4 °C gave the best diastereomeric ratio. The yields were comparable in dichloromethane and methanol in contrast to toluene where the yields of the U-3CR were lower (data not presented). Finally, we performed the MCR at -40 °C, but to our surprise the diastereomeric ratio decreased. Because of the only marginal improvement in diastereomeric ratio at 4 °C we decided to perform our MAO-N/ MCR sequence in dichloromethane at room temperature.
Table 1. Solvent and temperature dependence of the d.r. of the U-3CR of rac-29, benzoic acid, and tert- butyl isocyanide.[a]
Entry Solvent d.r. [b] (RT) d.r. (4 °C) d.r. (-40 °C)
[c] [c] 1 H2O 87:13 ------2 buffer[d] 87:13 ---[c] ---[c] 3 MeOH 90:10 92:8 91:9
4 CH2Cl2 92:8 93:7 90:10 5 DMSO 87:13 ---[c] ---[c] 6 DMF 89:11 ---[c] ---[c] 7 Toluene 92:8 93:7 ---[e] 8 TFE 90:10 ---[c] ---[c]
[a] All reactions were performed with 0.073 mmol of imine rac-29 and 0.1 mmol of benzoic acid and tert-butyl isocyanide. Reactions were stirred for 24 h at the appropriate temperature. [b] Based on GC analysis. [c] Not tested. [d] 100 mM KPO4 buffer, pH = 8.0. [e] Reaction too slow for accurate determination.
60 Highly Stereoselective Synthesis of Substituted Prolyl Peptides pA
min Figure 4: GC-FID trace of the U-3CR of rac-29, benzoic acid and tert-butyl isocyanide in CH2Cl2. Diastereomers are depicted as peak 1 and 2. 3.2.2 Two-pot MAO-N/ U-3CR sequence Enantiomerically enriched cyclic imine (3S,7R)-29 was prepared by MAO-N catalyzed desymmetrization of the corresponding commercially available pyrrolidine derivative[27, 46] in very good yield and ee (85%, 94 % ee). The ee could be improved to 97% by recrystallization during workup. With the chiral imine 29 in hand, we turned our attention to the Ugi-type 3CR. Different carboxylic acids and isocyanides were used to generate substituted prolyl peptides 30a-g in good yield and diastereoselectivity with very good ee (Table 2). Notably, as the ee’s of the desired compounds were >94 % we could conclude that no epimerization has occured during the U-3CR. In illustration of these high ee’s, a chiral HPLC trace (95% ee) of the benchmark reaction (compound 30b) is shown in Figure 5.
61 Chapter 3
Table 2. Scope of U-3CR using optically enriched 29.
Entry Product R1 R2 t (h) Yield (%) d.r.[a] ee (%)[b]
1 30a Me t-Bu 48 73 93:7 95[c] 2 30b Ph t-Bu 24 80 93:7 94 3 30c 2-Furyl i-Pr 48 75 92:8 94 4 30d Ph i-Pr 24 78 92:8 94 5 30e Me Bn 48 71 92:8 94
[c] 6 30f Ph Bn 24 81 92:8 97 7 30g i-Pr t-Bu 48 83 93:7 97[c] [a] d.r. determined by GC analysis. [b] ee determined by HPLC and GC analysis. [c] Partial crystallization of imine.
Figure 5: HPLC trace of prolyl peptide 30b with an ee of 95%. Enantiomers of the major diastereomer are depicted as peak 1 and 2.
Crystallographic analysis of 30f (Figure 6) allowed unambiguous assignment of the
absolute configuration (as the stereochemistry at the C3 and C4 positions, resulting from the biotransformation, has been reported previously [26]), and showed that attack by the isocyanide occured from the sterically less hindered face. The 2,3-trans relationship is
in agreement with the generally accepted mechanism of the Ugi reaction (scheme 1), in which the stereodetermining step is the direct nucleophilic attack of the isocyanide on the imine (or iminium) carbon. The extraordinarily high selectivity for the 2,3-trans isomer is in [16-20, 23] sharp contrast with other reports, where stereoinduction is poor or the 2,3-cis isomer [21-22] is preferentially formed. All other pyrrolidines 30 were assigned the same absolute stereochemistry as 30f, based on analogy of the 1H NMR spectroscopic data.
62 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
Figure 6. Molecular structure of 30f in the crystal. Displacement ellipsoids are drawn at 50% probability.
Subsequently, the sterically demanding imine 35 was prepared by MAO-N-catalyzed desymmetrization in 84% yield and >99% ee,[46] (Scheme 6). First cyclopentadiene 31 and maleimide 32 underwent a Diels-Alder reaction forming imide 33. Subsequent reduction with lithium aluminum hydride furnished the sterically demanding amine 34 which was used as substrate for the biotransformation with MAO-N and the corresponding imine (35) was applied in a series of Ugi-type 3CRs. To our delight, substituted prolyl peptides 36 were obtained as single diastereomers in >99% ee (Table 3, entries 1–8).
Scheme 6: Synthesis of bridged imine 35.
63 Chapter 3
Table 3. Scope of U-3CR using optically pure 35.
Entry Product R1 R2 t (h) Yield (%) d.r.[a] ee (%)[b]
1 36a Me t-Bu 48 83 >99:1 >99 2 36b Ph t-Bu 24 82 >99:1 >99 3 36c Furyl i-Pr 48 75 >99:1 >99 4 36d Ph i-Pr 24 78 >99:1 >99 5 36e Me Bn 48 78 >99:1 >99 6 36f Ph Bn 24 80 >99:1 >99 7 36g i-Pr t-Bu 48 81 >99:1 >99
[a] d.r. determined by GC analysis. [b] ee determined by HPLC and GC analysis.
To confirm that the stereochemical outcome of the reaction was solely determined by the starting chiral imine, we reacted 35 with tert-butyl isocyanide and either Fmoc-L-Pro- OH or Fmoc-D-Pro-OH to give 36i and 36j, respectively (Figure 7). In both cases, only one diastereomer was formed.[47] Likewise, we reacted 35 with benzoic acid and either (S)- or (R)- α-methylbenzyl isocyanide[48] to give 36k and 36l, respectively, as single diastereomers. 1H NMR analysis indicated that the shown 2,3-trans isomers were selectively formed in all cases.
Figure 7:. Substituted prolyl peptides from optically pure acid or isocyanide inputs.
3.2.3 One-pot MAO-N/U-3CR sequence With the MAO-N/U-3CR sequence established, attention was focused on combining it in a one-pot procedure. Earlier results (Table 1, entry 2) show that the U-3CR can be performed under the conditions of the enzymatic desymmetrization. First we executed the enzymatic desymmetrization to give (3S,7R)-29. The crude suspension was then centrifuged to discard the cells and benzoic acid and tert-butyl isocyanide were
64 Highly Stereoselective Synthesis of Substituted Prolyl Peptides added to the supernatant (Scheme 7). After full conversion (no imine was present any more based on GC analysis) a disappointing isolated yield for 30b of 19% was obtained with a d.r. of 88:12. This low yield could be due to the stability and solubility of the isocyanide under aqueous conditions. Therefore we decided to perform the subsequent U-3CR in a biphasic system. So, after discarding the cells, toluene was added to the supernatant followed by benzoic acid and tert-butyl isocyanide. Unfortunately, after full conversion of imine (3S,7R)- 29 still rather low yields of 30b were observed (28% based on GC analysis). These low yields could also be caused by the pH of the reaction mixture. The proposed mechanism of the Ugi reaction involves a protonation of the imine by the carboxylic acid (Scheme 1). This reactive iminium species undergoes α-addition by the carbenoid carbon of the isocyanide to finally rearrange into the desired α-acylaminoamide. However, the pH of the reaction is 8, implicating that most of the imine would not be protonated resulting in low conversion.
Scheme 7: One-pot MAO-N/U-3CR sequence.
3.2.4 3,4-substituted prolyl peptides as organocatalyst
Finally, we reacted imine 35 with Fmoc-d-Pro-OH 37 and methyl 3-isocyanopropionate 38, which, after treatment with NaOH in methanol/dichloromethane,[49] afforded 40 as a single
d stereoisomer (Scheme 8). Compound 40 strongly resembles H- -Pro-Pro-Glu-NH2 (27), which was described by Wennemers and co-workers (Scheme 4).
35 + 37 38 39 40 Scheme 8: Synthesis of prolyl peptide 40 as organocatalyst.
65 Chapter 3
To our delight, peptide 40 catalyzed the reaction between propanal and nitrostyrene to give 43 (Scheme 9) in 91% yield, 87:13 syn/anti ratio, and 86% ee (compared to 90:10 syn/anti
d [44] and 91% ee using H- -Pro-Pro-Glu-NH2 tripeptide ). Thus, our MAO-MCR sequence allows efficient asymmetric synthesis of proline derivatives containing all structural requirements for catalytic activity.
Scheme 9: Organocatalytic asymmetric conjugate addition using 40
3.3 Conclusions & Outlook
In conclusion, we have developed a highly efficient combination of MAO-N-catalyzed desymmetrization of cyclic meso-amines with the Ugi-type 3CR. This procedure is characterized by mild conditions, simple experiment procedures, and excellent yields and d.r. and ee values. We also demonstrated that a one-pot biotransformastion/MCR process is possible although the yield and diastereomeric ratios are superior in the two-pot procedure. Further optimization is recommended as access to less stable imines would be possible. We expect this methodology to be applicable to a wide variety of 3,4-cis-substituted 1-pyrrolines and therefore of considerable synthetic value in the construction of arrays of otherwise hard-to-access 3,4-substituted prolyl peptides, for example, as Wennemers- type organocatalysts. Moreover, our methodology holds great promise for applications in medicinal chemistry, especially in the synthesis of novel hepatitis C drugs.
3.4 Experimental section
General Information Starting materials and solvents were purchased from ABCR and Sigma-Aldrich and were used without further purification. 3-Azabicylo[3,3,0]octane hydrochloride was purchased from AK Scientific. (1R,2S,6R,7S)-4-methyl-4-azatricyclo[5.2.1.02.6]dec-8-ene was prepared according to literature procedure.[45] Column chromatography was performed on silica gel. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 (400.13 MHz for 1H and 100.61 13 1 13 MHz for C) or Bruker Avance 500 (500.23 MHz for H and 125.78 MHz for C) in CDCl3 and DMSO. Chemical shifts are reported in δ values (ppm) downfield from tetramethylsilane.
66 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
Electrospray Ionization (ESI) mass spectrometry was carried out using a Bruker micrOTOF-Q instrument in positive ion mode (capillary potential of 4500 V). GC-MS spectra were recorded on a Hewlett Packard HP 6890 equipped with a J & W Scientific; HP-1MS; 30 m × 0.32 mm × 0.25 μm column (injector temp. 300 °C, oven temp. 100 °C to 280 °C at 5 °C/min, hold for 10 min., He 1.6 ml/min. and detector temp. 275 °C) and a HP 5973 Mass Selective Detector. GC-FID analyses were performed on Agilent 6850 GC with a J & W Scientific; HP-1; 30 m × 0.32 mm × 0.25 μm (injector temp. 300 °C, oven temp. 100 °C to 280 °C at 5 °C/min, hold for 10 min., He 1.6 ml/min. and detector temp. 275 °C) and a Varian Chirasil-Dex CB; 25 m × 0.25 mm × 0.26 μm column (inj. 250 °C, oven temp. 100 °C to 180 °C at 5 °C/min, hold for 10 min., He 1.6 ml/min. and detector temp. 275 °C) equipped with a Gerstel Multipurpose sampler MPS2L. Normal phase HPLC was performed on Agilent systems. Normal phase HPLC system was equipped with a G1322A degasser, a G1311A quaternary pump, a G1329 autosampler unit, a G1315B diode array detector and a G1316A temperature controlled column compartment. Infrared (IR) spectra were recorded neat, and wavelengths are reported in cm-1. Optical 20 rotations were measured with a sodium lamp and are reported as follows: [α]D (c =g/100 mL, solvent). Methyl 3-isocyanopropionate was synthesized as reported previously.[50]
General Procedure 1: Preparation of optically active imines (3S,7R)-29 and 35. Unless stated otherwise: imines were synthesised according to literature procedure[46] with minor adjustments. 2.5 g of freeze-dried MAO-N D5 E.Coli were rehydrated for 30 min. in 20 ml of KPO4 buffer (100 mM, pH = 8.0) at 37 °C. Subsequently 1 mmol amine ((3S,7R)-29 or 35) in 30 ml of KPO4 buffer (100 mM, pH = 8.0) was prepared. The pH of the solution was adjusted to 8,0 by addition of NaOH and then added to the rehydrated cells. After 16-17 h the reaction was stopped (conversions were > 95 %) and worked up. For workup the reaction mixture was centrifuged at 4000 rpm and 4°C until the supernatant had clarified (40 – 60 min.). The pH of the supernatant was then adjusted to 10-11 by addition of aq. NaOH and the supernatant was subsequently extracted with t-butyl methyl ether or dichloromethane (4 x 70 mL). The combined organic phases were dried with Na2SO4 and concentrated at the rotary evaporator.
General procedure 2: Screening of reaction conditions Unless stated otherwise: 3-azabicyclo[3,3,0]oct-2-ene (3S,7R)-29 (0.073 mmol) was dissolved in 3 ml of the appropriate solvent followed by the addition of benzoic acid (0.1 mmol) and tert-butyl isocyanide (0.1 mmol). The reaction mixture was stirred for 16 h at RT. Samples were dried before analysis was carried out using GC-MS and GC-FID.
67 Chapter 3
General procedure 3: Preparation of optically active Ugi products 30a-g and 36a-g Unless stated otherwise: Imine (0.70 mmol) was dissolved in 2 ml of DCM followed by the addition of carboxylic acid (0.93 mmol) and isocyanide (0.93 mmol). The reaction mixture was stirred for 24 h at RT. DCM (8 mL) was added and the resulting mixture was washed with Na2CO3 (2x10 mL), dried (MgSO4), filtered, and concentrated. Note: rotamers could be observed in the NMR data.
General Procedure 4: Determination of enantiomeric excess (ee) and diastereomeric ratio (d.r.) Racemic imines were synthesized according to literature procedure.[46] Racemic Ugi- type products were prepared according to general procedure 3. Diastereomers could be separated by GC-MS and GC-FID. Enantiomers could be separated by normal phase HPLC and GC-FID.
Compound 30a: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct-2- ene ((3S,7R)-29, 76 mg, 0.70 mmol), acetic acid (55 mg, 52 μl, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 30a as a white solid, yield 73%. 93:7 d.r. [HP-1, t (major) = 14.852 min, t (minor) = 16.773 min]; 95% ee [CP Chirasil DEX CB, t (minor) = 20.449 min, t (major) = 20.860 min]; [α] 20 = -47.8 o (c = 0.34, MeCN). 1H D NMR (400.1 MHz, CDCl3): d 6.58 (bs, 1H), 4.28 (d, J = 2.1 Hz, 1H), 3.70 (dd, J = 8.3, 10.6 Hz, 1H), 3.24 (dd, J = 4.5, 10.6 Hz, 1H), 2.96-2.93 (m, 1H), 2.91-2.82 (m, 1H), 2.01 (s, 3H), 1.93-1.78 (m, 2H), 13 1.71-1.42 (m, 2H), 1.41-1.31 (m, 2H), 1.25 (s, 9H); C NMR (100.6 MHz, CDCl3) d 170.5, 170.0, 66.8, 54.4, 51.0, 45.0, 42.7, 32.5, 32.3, 28.7, 25.7, 22.6; IR (neat): νmax (cm-1) = 3277 (m), 2957 (m), 1668 (s), 1630 (s),
1549 (s), 1447 (s), 1420 (s), 1223 (s), 667 (m), 606 (m); HRMS (ESI+) calcd for C14H24N2O2 (MH+) 253.1916, found 253.1925.2
Compound 30b: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct- 2-ene ((3S,7R)-29, 76 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and tert- butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 30b as a white solid, yield 73%. 93:7 d.r. [HP-1, t (major) = 23.672 min, t (minor) = 25.601 min]; 95% ee [Daicel Chiralpak AD H, hexane/iso = 96/4, v = 1.0 mL.min1, λ = 254 nm, t (minor) = 10.698 20 o 1 min, t (major) = 11.620 min]; [α]D = -53.7 (c = 0.34, MeCN). H NMR (400.1 MHz,
CDCl3): d 7.49-7.38 (m, 5H), 6.66 (bs, 1H), 4.54 (d, J = 2.8 Hz), 3.72 (dd, J =m 11.4, 7.8 Hz, 1H), 3.23 (d, J = 11.0, 1H), 3.15-3.10 (m, 1H), 2.73-2.58 (m, 1H), 1.96-1.82 (m, 1H), 1.82-1.69 (m, 1H),
1.68-1.41 (m, 3H), 3.15-3.10 (m, 1H), 1.28 (s, 9H), 1.24-1.06 (m, 1H); 13C NMR (100.6 MHz, CDCl3) d 170.3, 170.1, 136.3, 130.1, 128.4, 126.9, 67.1, 60.4, 55.9, 51.1, 44.2, 43.3, 33.0, 32.7, 28.7, 26.2; ; IR (neat): νmax
(cm-1) = 3310 (m), 2961 (m), 1674 (s), 1618 (s), 1416 (s), 1223 (s), 698 (s); HRMS (ESI+) calcd for C19H26N2O2 (MH+) 315.2073, found 315.2077.
Compound 30c: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct-2- ene ((3S,7R)-29, 76 mg, 0.70 mmol), 3-furoic acid (102 mg, 0.91 mmol) and isopropyl isocyanide (63 mg, 86 μl, 0.91 mmol) giving 30c as a white solid, yield 75%. 92:8 d.r. [HP-1, t (major) = 21.290 min, t (minor) = 23.012 min] 94% ee [Daicel Chiralpak AD-H, hexane/isopropanol = 90/10, v = 1.0 mL.min1, λ = 254 nm, t (minor) = 7.417 min, t (major) = 12.039 min]; [α] 20 = -33.3 o (c = 0.30, MeCN). 1H NMR (400.1 D MHz, CDCl3): d 7.80 (bs, 1H), 7.43 (bs, 1H), 6.72 (bs, 1H), 6.51 (d, J = 6.3 Hz, 1H), 4.56 (d, J = 2.3 Hz, 1H), 4.03 (oct, J = 7.1 1H), 3.88 (dd, J = 10.4, 8.3 Hz, 1H), 3.53 (dd, J = 10.4, 3.8 Hz, 1H), 3.09- 3.01 (m, 1H), 2.95-2.84 (m, 1H), 2.00-1.84 (m, 2H), 1.74-1.65 (m, 1H), 1.64-1.54 (m, 1H), 1.53-1.43 (m, 1H),
68 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
13 1.43-1.33(m, 1H), 1.17 (d, J = 6.3 Hz, 3H) 1.13 (d, J = 6.3 Hz, 3H); C NMR (100.6 MHz, CDCl3) d 170.1, 163.2, 144.3, 142.8, 121.8, 110.4, 66.8, 54.8, 44.4, 43.3, 41.3, 32.4, 32.2, 25.6, 22.5, 22.4; IR (neat): νmax (cm-1) = 3281 (w), 2949 (w), 1647 (m), 1609 (s), 1547 (s), 1427 (s), 1159 (s), 737 (s), 598 (s); HRMS (ESI+) calcd for
C16H22N2O3 (MH+) 291.1709, found 291.1721.
Compound 30d: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct- 2-ene ((3S,7R)-29, 76 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and isopropyl isocyanide (63 mg, 86 μl, 0.91 mmol) giving 30d as a white solid, yield 78%. 92:8 d.r. [HP-1, t (major) = 23.809 min, t (minor) = 25.563 min]; 94% ee [Daicel Chiralpak AD-H, hexane/isopropanol = 96/4, v = 1.0 mL.min1, λ = 254 nm, t (minor) 20 o 1 = 16.613 min, t (major) = 21.363 min]; [α]D = -52.4 (c = 0.42, MeCN). H NMR (400.1
MHz, CDCl3): d 7.46-7.36 (m, 1H), 6.63 (bs, 1H), 4.59 (d, J = 2.0 Hz, 1H), 4.10-4.01 (m, 1H), 3.73 (dd, J = 11.4,. 7.8 Hz, 1H), 3.73 (dd, J = 11.4,. 7.8 Hz, 1H), 3.32-3.29 (m, 1H), 3.23-3.17(m, 1H), 2.76-2.71 (m, 1H), 2.02-1.94 (m, 1H), 1.88-1.78 (m, 1H), 1.75-1.63 (m, 1H), 1.63-1.50 (m, 1H), 1.16 (d, J = 6.6 Hz, 3H) 1.13 (d, J = 6.6 Hz, 13 3H); C NMR (100.6 MHz, CDCl3) d 170.4, 170.0, 136.2, 130.1, 128.4, 126.9, 126.6, 66.5, 55.9, 44.3, 43.3, 41.5, 32.9, 32.6, 26.1, 22.7, 22.6; IR (neat): νmax (cm-1) = 3300 (m), 2959 (m), 1615 (s), 1545 (s), 1416 (s),
1229 (m), 700 (m);HRMS (ESI+) calcd for C18H24N2O2 (MH+) 301.1916, found 301.1914.
Compound 30e: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct- 2-ene ((3S,7R)-29, 76 mg, 0.70 mmol), acetic acid (55 mg, 52 μl, 0.91 mmol) and benzyl isocyanide (107 mg, 111 μl, 0.91 mmol) giving 30e as a white solid, yield 71%. 92:8 d.r. [HP-1, t (major) = 25.098 min, t (minor) = 26.457 min]; 94% ee [Daicel Chiralpak OJ-H, hexane/isopropanol = 93/7, v = 1.0 mL.min1, λ = 254 nm, t (minor) 20 o 1 = 9.948 min, t (major) = 10.718 min]; [α]D = -18.8 (c = 0.32, MeCN). H NMR (400.1 MHz, CDCl3): d 7.36- 7.25 (m, 5H), 7.16 (bs, 1H), 4.47 (d, J = 2.0 Hz, 1H), 4.25 (dd, J = 15.2, 5.8 Hz, 2H), 3.73 (dd, J = 10.6, 8.3 Hz, 1H), 3.28 (dd, J = 10.5, 4.5 Hz, 1H), 3.10-3.03 (m, 1H), 2.96-2.88 (m, 1H), 2.10 (s, 3H), 2.01-1.85 (m, 1H), 13 1.82-1.55 (m, 2H), 1.52-1.39 (m, 2H); C NMR (100.6 MHz, CDCl3) d 171.3, 170.1, 138.3, 128.5, 128.4, 127.9, 127.4, 127.0, 65.9, 54.3, 45.4, 43.2, 42.7, 32.6, 32.2, 25.5, 22.1; IR (neat): νmax (cm-1) = 3267 (m), 2951 (w), 1626 (s), 1537 (m), 1418 (s), 1231 (s), 1030 (w), 748 (s); HRMS (ESI+) calcd for C17H22N2O2 (MH+) 287.1760, found 281.1765.
Compound 30f: General procedure 3 was followed using 3-azabicyclo[3,3,0] oct-2-ene ((3S,7R)-29, 76 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and benzyl isocyanide (107 mg, 111 μl, 0.91 mmol) giving 30f as a white solid, yield 81%. 92:8 d.r. [HP-1, t (major) = 33.333 min, t (minor) = 35.085 min]; 97% ee [Daicel Chiralpak AD-H, hexane/isopropanol = 96/4, v = 1.0 mL.min1, λ = 254 nm, t 20 o (minor) = 18.134 min, t (major) = 23.440 min]; [α]D = -52.6 (c = 0.38, MeCN). 1 H NMR (400.1 MHz, CDCl3): d 7.34-7.11 (m, 10H), 6.73 (bs, 1H), 4.61 (d, J = 2.8 Hz, 1H). 4.37 (dd, J = 5.3, 2.8 Hz, 2H), 3.66 (dd, J = 11.1, 7.6 Hz, 1H), 3.24 (dd, J = 10.9, 1.8 Hz, 1H), 3.18-3.11 (m, 1H), 2.72-2.64 (m, 13 1H), 1.92-1.82 (m, 1H), 1.82-1.62 (m, 1H), 1.25-1.13 (m, 1H); C NMR (100.6 MHz, CDCl3) d 171.1, 170.3, 138.3, 135.9, 132.6, 130.0, 129.7, 128.4, 128.21, 128.0, 127.7, 127.3, 127.0, 126.9, 126.4, 66.3, 55.8, 44.9, 43.2, 43.1, 32.8, 32.4, 25.9; IR (neat): νmax (cm-1) = 3262 (m), 2928 (m), 1674 (s), 1613 (s), 1545 (s), 1423 (s),
1223 (m), 698 (s), 669 (s); HRMS (ESI+) calcd for C22H24N2O2 (MH+) 349.1916, found 349.1924.
Compound 30g: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct-2- ene ((3S,7R)-29, 76 mg, 0.70 mmol), isobutyric acid (80 mg, 84 μl, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 30g as a white solid, yield 83%. 93:7 d.r. [HP-1, t (major) = 17.165 min, t (minor) = 18.750 min]; 97% ee [CP Chirasil- 20 o DEX CB, t (minor) = 21.439 min, t (major) = 21.846 min]; [α]D = -47.8 (c = 0.34,
69 Chapter 3
1 MeCN). H NMR (400.1 MHz, CDCl3): d 6.71 (bs, 1H), 4.37 (d, J = 1.8 Hz, 1H), 3.65 (dd, J = 10.6, 8.3 Hz, 1H), 3.34 (dd, J = 4.3, 10.9 Hz, 1H), 3.04-2.98 (m, 1H), 2.89-2.81 (m, 1H), 2.65 (sep, J = 6.8 Hz, 1H), 2.01-1.83 (m, 2H), 1.70-1.48 (m, 2H), 1.45-1.35 (m, 2H), 1.29(s, 9H), 1.13 (d, J = 6.6 Hz, 3H), 1.10 (d, J = 6.8 Hz, 3H); 13 C NMR (100.6 MHz, CDCl3) d 176.5, 170.4, 66.5, 53.0, 43.7, 42.7, 32.8, 32.3, 32.0, 30.8, 24.9, 18.9, 18.5; IR (neat): νmax (cm-1) = 3291 (m), 2963 (m), 2870 (w), 1684 (s), 1618 (s), 1551 (s), 1433 (s), 1225 (s), 1090 (m),
658 (m); HRMS (ESI+) calcd for C14H24N2O2 (MH+) 281.2229, found 281.2235.
Compound 36a: General procedure 3 was followed using (1R,2S,6R,7S)-4-methyl-4- azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), acetic acid (55 mg, 52 μl, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 36a as a white solid, yield 83%. >99:1 d.r. (t (major) = 18.179 min); >99% ee [Daicel Chiralpak ADH, hexane/ isopropanol = 92/8, v = 1.0 mL.min1, λ = 220 nm, t (major) = 5.319 min, t (minor) = 20 o 1 6.587 min]; [α]D = -24.0 (c = 0.25, MeCN). H NMR (400.1 MHz, CDCl3): d 6.65 (bs, 1H), 6.14-6.13 (m, 2H), 4.09 (d, J = 2.0 Hz, 1H), 3.47 (dd, J = 11.4, 8.6 Hz, 1H), 3.36-3.32 (m, 1H), 3.15 (dd, J = 11.4, 2.0 Hz, 1H), 2.98- 13 2.92 (m, 3H), 1.95 (s, 3H), 1.51-1.41 (m, 2H), 1.30 (s, 9H); C NMR (100.6 MHz, CDCl3) d 170.6, 169.0, 135.4, 134.0, 62.9, 51.7, 51.0, 50.3, 47.0, 46.6, 46.0, 45.1, 28.7, 22.8; IR (neat): νmax (cm-1) = 3283 (w), 2970 (w),
2942 (w), 1647 (s), 1634 (s), 1553 (s), 1414 (s), 1223 (s), 733 (s); HRMS (ESI+) calcd for C16H24N2O2 (MH+) 277.1916, found 277.1922.
Compound 36b: General procedure 3 was followed using 3-azabicyclo[3,3,0]oct- 2-ene (35, 76 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 36b as a white solid, yield 82%. >99:1 d.r. (t (major) = 26.830 min); HP-1, >99% ee [Daicel Chiralpak AD-H, hexane/ isopropanol = 96/4, v = 1.0 mL.min1, λ = 220 nm, t (minor) = 9.712 min, t (major) = 20 o 1 11.741 min]; [α]D = -43.1 (c = 0.33, MeCN). H NMR (400.1 MHz, CDCl3): d 7.43-7.34 (m, 1H), 6.63 (bs, 1H), 6.20 (dd, J = 5.8, 3.0 Hz, 1H), 5.91 (dd, J = 5.6, 2.6 Hz, 1H), 4.43 (d, J = 2.0 Hz, 1H), 3.52 (dd, J = 11.9, 8.6 Hz, 1H), 3.44-3.39 (m, 1H), 3.05-3.00 (m, 2H), 2.91-2.85 (m, 1H), 13 2.78-2.76 (m, 1H), 1.48-1.45 (m, 1H), 1.40-1.37 (m, 1H), 1.32 (s, 9H); C NMR (100.6 MHz, CDCl3) d 168.6, 168.0, 135.0, 133.2, 132.7, 128.4, 126.8, 124.9, 61.3, 50.4, 49.9, 49.5, 45.4, 44.9, 43.9, 43.3, 27.1; IR (neat): νmax (cm-1) = 3283 (m), 2970 (m), 2942 (m), 1647 (s), 1634 (s), 1553 (s), 1414 (s), 1223 (s), 733 (s); HRMS
(ESI+) calcd for C21H26N2O2 (MH+) 339.2073, found 339.2082.
Compound 36c: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl-4- azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), 3-furoic acid (102 mg, 0.91 mmol) and isopropyl isocyanide (63 mg, 86 μl, 0.91 mmol) giving 36c as a white solid, yield 75%. >99:1 d.r. (t (major) = 24.364 min); >99% ee [Daicel Chiralpak ADH, hexane/isopropanol = 90/10, v = 1.0 mL.min1, λ = 220 nm, t (minor) = 8.404 min, t 20 o 1 (major) = 9.968 min]; [α]D = -35.7 (c = 0.28, MeCN). H NMR (400.1 MHz, CDCl3): d 7.71 (dd, J = 1.5, 0.8 Hz, 1H), 7.42 (dd, J = 2.0, 1.5 Hz, 1H), 6.65 (dd, J = 1.8, 0.8 Hz, 1H), 6.50 (d, J = 6.6 Hz, 1H), 6.19-6.17 (m, 1H), 5.98-5.96 (m, 1H), 4.42 (d, J = 2.0 Hz, 1H), 4.06-3.94 (m, 1H), 3.63 (dd, J =11.4, 8.8 Hz, 1H), 3.43-3.39 (m, 2H), 3-06-3.02 (m, 2H), 2.90-2.88 (m, 1H), 1.51-1.43 (m, 2H), 1.13 (d, 13 J = 6.6 Hz, 3H), 1.10 (d, J = 6.6 Hz, 3H); C NMR (100.6 MHz, CDCl3) d 170.2, 162.7, 144.1, 142.9, 135.1, 134.4, 121.9, 110.4, 62.8, 51.6, 51.2, 47.1, 46.5, 45.8, 45.2, 41.5, 22.6, 22.7, 22.6; IR (neat): νmax (cm-1) = 3275 (m), 2970 (m), 2934 (m), 1678 (s), 1594 (s), 1545 (s), 1437 (s), 1219 (s), 1153 (m), 1018 (m)874 (m0,
754 (s); HRMS (ESI+) calcd for C18H22N2O2 (MH+) 315.1709, found 315.1725.
Compound 36d: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl- 4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and isopropyl isocyanide (63 mg, 86 μl, 0.91 mmol) giving 36d as a white solid, yield 78%. >99:1 d.r. (t (major) = 27.054 min); >99% ee [Daicel Chiralpak ADH, hexane/ isopropanol = 96/4, v = 1.0 mL.min1, λ = 220 nm, t (minor) = 17.354 min, t (major)
70 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
20 o 1 = 29.404 min]; [α]D = -38.7 (c = 0.31, MeCN). H NMR (400.1 MHz, CDCl3): d 7.43-7.35 (m, 5H), 6.59 (d, J = 7.6 Hz, 1H), 6.23-6.21 (m, 1H), 5.93-5.91 (m, 1H), 4.48 (d, J = 1.7 Hz, 1H), 4.11-3.98 (m, 1H), 3.55 (dd, J = 11.9, 8.8 Hz, 1H), 3.48-3.45 (m, 1H), 3.07-3.00 (m, 2H), 2.92-2.87 (m, 1H), 2.81-2.77 (m, 1H), 1.48- 13 1.39 (m, 2H), 1.14 (d, J = 6.6 Hz, 3H), 1.10 (d, J = 6.6 Hz, 3H); C NMR (100.6 MHz, CDCl3) d 170.1, 169.8, 136.5, 134.8, 134.4, 130.1, 128.4, 126.6, 62.3, 52.0, 51.5, 47.0, 46.5, 45.6, 44.9, 41.5, 22.7, 22.6; IR (neat): νmax (cm-1) = 3287 (m), 2967 (m), 2940 (m), 1682 (s), 1601 (s), 1539 (s), 1424 (s), 1217 (s), 739 (s), 664
(m);HRMS (ESI+) calcd for C20H24N2O2 (MH+) 325.1916, found 325.1919.
Compound 36e: General procedure 3 was followed using 1R,2S,6R,7S)-4- methyl-4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), acetic acid (55 mg, 52 μl, 0.91 mmol) and benzyl isocyanide (107 mg, 111 μl, 0.91 mmol) giving 36e as a white solid, yield 78%. >99:1 d.r. (t (major) = 28.213 min); >99% ee [Daicel Chiralpak OJ-H, hexane/ isopropanol = 92/8, v = 1.0 mL.min1, λ = 220 nm, t (minor) = 9.100 min, t (major) 20 o 1 = 10.760 min]; [α]D = -21.4 (c = 0.28, MeCN). H NMR (400.1 MHz, CDCl3): d 7.31-7.21 (m, 5H), 6.13-6.08 (m, 2H), 4.46 (dd, J = 14.9, 6.1 Hz, 1H), 4.32 (dd, J = 15.2, 5.8 Hz, 1H), 4.25 (d. J = 2.0 Hz, 1H), 3.47-3.43 (m, 2H), 3.18 (dd, J = 11.4, 2.0 Hz, 1H), 3.01-2.95 (m, 3H), 2.0 (s, 3H), 1.54-1.43 (m, 2H); 13C NMR (100.6 MHz,
CDCl3) d 171.3, 169.3, 138.3, 135.4, 134.1, 128.6, 127.4, 127.3, 62.2, 51.7, 50.3, 47.1, 46.7, 46.0, 45.1, 43.4, 22.7; IR (neat): νmax (cm-1) = 3314 (w), 3082 (w), 2970 (w), 2932 (w), 1553 (s), 1433 (s), 1360 (m), 1317 (m),
1233 (m), 745 (s), 696 (s); HRMS (ESI+) calcd for C19H22N2O2 (MH+) 311.1760, found 311.1745.
Compound 36f: General procedure 3 was followed using 31R,2S,6R,7S)-4- methyl-4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and benzyl isocyanide (107 mg, 111 μl, 0.91 mmol) giving 36f as a white solid, yield 80%. >99:1 d.r. (t (major) = 36.331 min); >99% ee [Daicel Chiralpak OD-H, hexane/ isopropanol = 92/8, v = 1.0 mL.min1, λ = 220 nm, t (major) = 11.489 min, t 20 o 1 (minor) = 13.626 min]; [α]D = -35.1 (c = 0.29, MeCN). H NMR (400.1 MHz,
CDCl3): d 7.44-7.17 (m, 10H), 6.24-6.18 (m, 1H), 5.95-5.93 (m, 1H), 4.60 (d, J = 1.8 Hz, 1H), 4.45 (d, J = 5.8 Hz, 2H), 3.58-3.50 (m, 2H), 3.10-3.04 (m, 2H), 2.95-2.89 (m, 1H), 2.83-2.79 (m, 1H), 1.50-1.41 (m, 2H); 13C NMR
(100.6 MHz, CDCl3) d 171.1, 169.9, 138.4, 136.4, 134.4, 130.1, 128.6, 128.4, 127.4, 127.3, 126.4, 62.2, 52.1, 51.6, 47.0, 46.6, 45.6, 45.0, 43.5; IR (neat): νmax (cm-1) = 3268 (m), 3077 (w), 2972 (w), 2872 (w), 1684 (s),
1597 (s), 1560 (s), 1495 (m), 1431 (s), 1221 (s), 731 (s), 696 (s); HRMS (ESI+) calcd for C24H24N2O2 (MH+) 373.1916, found 373.1901.
Compound 36g: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl-4- azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), isobutyric acid (80 mg, 84 μl, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 36g as a white solid, yield 81%. >99:1 d.r. (t (major) = 19.912 min); >99% ee [Daicel Chiralpak AD-H, hexane/ isopropanol = 95/5, v = 1.0 mL.min1, λ = 220 nm, t (minor) = 5.037 min, t (major) = 20 1 6.877 min]; [α]D = -35.3 o (c = 0.34, MeCN). H NMR (400.1 MHz, CDCl3): d 6.13-6.08 (m, 2H), 4.15 (d, J = 1.8 Hz, 1H), 3.43 (dd, J = 11.4, 8.6 Hz, 1H), 3.37-3.33 (m, 1H), 3.26 (dd, J = 11.4, 2.0 Hz, 1H), 2.99-2.91 (m, 3H), 2.50 (sep, J = 6.8 Hz, 1H), 1.54-1.38 (m, 2H), 1.27 (s, 9H), 1.06 (d, J = 6.8 Hz, 3H), 13 1.03(d, J = 6.8 Hz, 3H); C NMR (100.6 MHz, CDCl3) d 175.6, 170.7, 135.3, 134.2, 62.7, 51.8, 50.8, 49.1, 47.01, 46.5, 45.2, 32.2, 28.7, 19.2, 18.3; IR (neat): νmax (cm-1) = 3325 (m), 2966 (m), 1678 (m), 1624 (s), 1553 (s),
1435 (s), 1315 (w), 1231 (m), 1088 (w) ; HRMS (ESI+) calcd for C18H28N2O2 (MH+) 305.2229, found 305.222
71 Chapter 3
Compound 36i: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl-4- azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), Fmoc-D-Pro-OH (307 mg, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol). The crude product
36i was subjected using column chromatography (SiO2, EtOAc (1): cyclohexane (2)). Fmoc deprotection using 25 % piperidine in DMF followed by column chromatography (DCM/MeOH 9:1) gave 36i as a white solid in 66% yield over two steps. 20 o 1 [α]D = -75.0 (c = 0.16, MeCN). H NMR (500.2 MHz, CDCl3): d 6.75 (bs, 1H), 6.11 (d, J = 5.7 Hz, 2H), 5.01 (bs, 1H), 4.20 (bs, 1H), 3.92-3.83 (m, 1H), 3.45-3.41 (m, 1H), 3.24-3.22 (m, 1H), 3.20-3.10 (m, 2H), 3.03-2.98 (m, 2H), 2.95-2.89 (m, 1H), 2.19-2.09 (m, 1H), 1.94- 1.57 (m, 3H), 1.57-1.39 (m, 2H), 1.28 (s, 9H); 13C NMR
(125.8 MHz, CDCl3) d 170.5, 135.4, 134.4, 63.7, 61.4, 51.6, 51.0, 49.01, 47.2, 46.9, 46.5, 46.3, 45.4, 30.0, 28.7, 25.9; IR (neat): νmax (cm-1) = 2960 (w), 1668 (s), 1622 (s), 1566 (m), 1414 (s), 1234 (m), 1094 (w), 853 (s);
HRMS (ESI+) calcd for C19H29N3O2 (MH+) 332.2338, found 332.2342.
Compound 36j: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl- 4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), Fmoc-L-Pro-OH (307 mg, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol) giving 36j as a white solid, yield 76%. 20 o 1 [α]D = -6.7 (c = 0.60, MeCN). H NMR (500.2 MHz, CDCl3): d 7.76 (d, J = 7.6 Hz, 2H), d 7.59 (d, J = 7.5 Hz, 1H), 7.55 (d, J = 7.5 Hz, 1H), 7.40 (d, J = 7.4 Hz, 2H), 7.40 (d, J = 7.4 Hz, 2H), 7.31 (d, J = 7.4 Hz, 2H), ), 6.62 (bs, 1H), 6.19 (dd, J = 5.6, 2.9 Hz, 1H), 6.11 (dd, J = 5.6, 2.5 Hz, 1H), 4.35-4.33 (m, 1H), 4.30-4.28 (m, 2H), 4.24-4.22 (m, 1H), 4.20 (d, J = 1.9 Hz, 1H), 3.72-3.58 (m, 2H), 3.35-3.32 (m, 1H), 3.25-3.22 (m, 2H), 2.93-2.90 (m, 2H), 2.18-2.13 (m, 13 2H), 1.95-1.91 (m, 2H), 1.52-1.40 (m, 2H), 1.29 (s, 9H) ; ); C NMR (125.8 MHz, CDCl3) d 170.6, 170.4, 154.8, 143.9, 141.3, 135.9, 134.2, 127.8, 127.7, 127.1, 127.0, 125.1, 125.0, 120.0, 67.5, 63.3, 58.3, 51.8, 51.2, 49.4, 47.7, 47.4, 47.2, 47.1, 46.9, 44.4, 28.7, 28.6, 25.0; IR (neat): νmax (cm-1) = 2965 (w), 1643 (s), 1520 (w),
1449 (s), 1418 (s), 1358 (m), 1123 (m), 758 (m), 739 (s); HRMS (ESI+) calcd for C34H39N3O4 (MH+) 554.3019, found 554.3019.
Compound 36k: General procedure 3 was followed using 31R,2S,6R,7S)-4- methyl-4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and (R)-(+)-methylbenzyl isocyanide (119 mg, 123 μl, 0.91 mmol) giving 36k as a white solid, yield 73%. [α] 20 = -24.0 o (c = 0.25, MeCN). 1H NMR (500.2 MHz, CDCl ): d 7.45-7.19 (m, 11H), D 3 6.24 dd, J = 5.7, 2.9 Hz, 1H), 5.93 dd, J = 5.7, 2.9 Hz, 1H), 5.11-5.03 (m, 1H), 4.59 d, J = 1.8 Hz, 1H), 3.53-3.49 (m, 1H), 3.40-3.35 (m, 1H), 3.02-2.97 (m, 2H), 2.88-2.81 (m, 1H), 2.77-2.75 (m, 1H), 1.48-1.39 (m, 2H), 1.45 (d, J = 7.0 Hz, 2H); 13C NMR (125.8
MHz, CDCl3): d 169.8, 169.8, 144.0, 136.4, 134.8, 134.3, 130.0, 128.5, 128.4, 126.9, 126.4, 125.7, 62.0, 51.8, 51.5, 49.0, 47.0, 46.4, 45.5, 44.4, 22.7; IR (neat): νmax (cm-1) = 3302 (w), 3239 (w), 3059 (w), 2665 (w), 2929 (w), 1664 (s), 1559 (s), 1558 (s), 1427 (s), 1248 (m), 1020 (m), 698 (s), 667 (m); HRMS (ESI+) calcd for
C25H26N2O2 (MH+) 387.2073, found 387.2067.
Compound 36l: General procedure 3 was followed using 31R,2S,6R,7S)-4-methyl- 4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), benzoic acid (111 mg, 0.91 mmol) and (S)-(−)-methylbenzyl isocyanide (119 mg, 123 μl, 0.91 mmol) giving 36l as a white solid, yield 70%. 20 o 1 [α]D = -78.6 (c = 0.28, MeCN). H NMR (400.1 MHz, CDCl3): d 7.43-7.16 (m, 11H), 6.24-6.19 (m, 1H), 5.97-5.90 (m, 1H), 5.10-5.01 (m, 1H), 4.52 (bs, 1H), 3.61-3.56 (m, 1H), 3.47-3.44 (m, 1H), 3.09-3.06 (m, 1H), 2.99 (bs, 1H), 2.93-2.87 (m, 1H), 2.80(bs, 1H), 1.50-1.39 (m, 2H), 1.43 (d, J = 6.8 Hz, 3H); 13C NMR (125.8
MHz, CDCl3) d 170.1, 169.9, 143.3, 136.5, 134.8, 134.4, 130.1, 128.7, 128.5, 127.2, 126.5, 126.1, 62.4, 52.1, 51.6, 49.1, 47.0, 46.5, 45.6, 44.9, 22.4; IR (neat): νmax (cm-1) = 3281 (m), 3281 (m), 2955 (m), 1638 (s), 1528 (s), 1397 (s), 1343 (m), 1227 (m), 1117 (m), 700 (s); HRMS (ESI+) calcd for
C25H26N2O2 (MH+) 387.2073, found 387.2067
72 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
Compound 40: General procedure 3 was followed using 1R,2S,6R,7S)-4-methyl- 4-azatricyclo[5.2.1.02,6]dec-8-ene (35, 93 mg, 0.70 mmol), Fmoc-L-Pro-OH (307 mg, 0.91 mmol) and tert-butyl isocyanide (76 mg, 103 μl, 0.91 mmol). The crude product was purified by column chromatography (SiO2, EtOAc (1): cyclohexane (2)). Simultaneous Fmoc deprotection and saponification according to literature procedure[49] followed by addition of 1.1 eq. TFA and purification using reversed phase chromatography (C , H O (1): EtOH (1)) gave 40 as a 18 2 colorless solid, in 62% yield over two steps. 20 o 1 [α]D = -6.8 (c = 0.30, MeCN). H NMR (500.2 MHz, DMSO): d 12.43 (bs, 1H), 9.68-9.44 (m, 1H), 8.54 (bs, 1H), 8.19-8.14 (m, 1H), 6.29-6.25 (m, 1H), 6.13 (dd, J = 5.7, 2.9 Hz, 1H), 3.95 (bs, 1H), 3.66-3.59 (m, 1H), 3.37-3.18 (m, 5H), 2.96 (bs, 2H), 2.79-2.73(m, 1H), 2.50-2.38 (m, 4H), 1.97-1.85 (m, 2H), 1.69 1.57 (m, 2H), 1.46-1.35 (m, 2H); 13C NMR (125.8 MHz, DMSO) d 172.7, 171.2, 165.8, 158.1, 157.9, 135.6, 134.8, 117.9, 115.5, 62.9, 58.1, 50.9, 49.6, 49.0, 46.5, 46.4, 45.8, 44.1, 34.6, 33.8, 27.9, 23.6; IR (neat): νmax (cm-1) = 2949
(w), 1640 (s), 1175 (s) 1130 (s), 833 (m), 719 (m); HRMS (ESI+) calcd for C20H26F3N3O3 (MH+) 347.1845, found 348.1929.
X-ray crystal structure determination of 30f
3.5 References & Notes
[1] A. Domling and I. Ugi, Angew. Chem. Int. Ed. 2000, 39, 3169-3210. [2] J. Zhu and H. Bienayme, Multicomponent Reactions, Wiley-VCH, Weinheim, 2005. [3] R. V. A. Orru and M. de Greef, Synthesis 2003, 1471-1499. [4] A. Domling, Chem. Rev. 2006, 106, 17-89. [5] B. Ganem, Accounts Chem. Res. 2009, 42, 463-472. [6] C. Hulme and V. Gore, Curr. Med. Chem. 2003, 10, 51-80. [7] I. Akritopoulou-Zanze, Curr. Opin. Chem. Biol. 2008, 12, 324-331. [8] U. Kusebauch, B. Beck, K. Messer, E. Herdtweck and A. Domling, Org. Lett. 2003, 5, 4021-4024.
73 Chapter 3
[9] S. E. Denmark and Y. Fan, J. Am. Chem. Soc. 2003, 125, 7825-7827. [10] P. R. Andreana, C. C. Liu and S. L. Schreiber, Org. Lett. 2004, 6, 4231-4233. [11] S. C. Pan and B. List, Angew. Chem. Int. Ed. 2008, 47, 3622-3625. [12] T. Yue, M. X. Wang, D. X. Wang, G. Masson and J. P. Zhu, J. Org. Chem. 2009, 74, 8396-8399. [13] D. J. Ramon and M. Yus, Angew. Chem. Int. Ed. 2005, 44, 1602-1634. [14] I. Ugi, Angew. Chem. Int. Ed. 1959, 71, 386-386. [15] R. F. Nutt and M. M. Joullie, J. Am. Chem. Soc. 1982, 104, 5852-5853. [16] L. Banfi, A. Basso, G. Guanti, S. Merlo, C. Repetto and R. Riva, Tetrahedron 2008, 64, 1114-1134. [17] M. M. Bowers, P. Carroll and M. M. Joullie, J. Chem. Soc.-Perkin Trans. 1 1989, 857-865. [18] D. M. Flanagan and M. M. Joullie, Synth. Commun. 1989, 19, 1-12. [19] L. Banfi, A. Basso, G. Guanti and R. Riva, Tetrahedron Lett. 2004, 45, 6637-6640. [20] T. M. Chapman, I. G. Davies, B. Gu, T. M. Block, D. I. C. Scopes, P. A. Hay, S. M. Courtney, L. A. McNeill, C. J. Schofield and B. G. Davis, J. Am. Chem. Soc. 2005, 127, 506-507. [21] K. M. Bonger, T. Wennekes, S. V. P. de Lavoir, D. Esposito, R. den Berg, R. Litjens and G. A. van der Marel, Qsar Comb. Sci. 2006, 25, 491-503. [22] K. M. Bonger, T. Wennekes, D. V. Filippov, G. Lodder, G. A. van der Marel and H. S. Overkleeft, Eur. J. Org. Chem. 2008, 3678-3688. [23] T. Ngouansavanh and J. Zhu, Angew. Chem. Int. Ed. 2007, 46, 5775-5778. [24] M. Alexeeva, A. Enright, M. J. Dawson, M. Mahmoudian and N. J. Turner, Angew. Chem. Int. Ed. 2002, 41, 3177-3180. [25] R. Carr, M. Alexeeva, A. Enright, T. S. C. Eve, M. J. Dawson and N. J. Turner, Angew. Chem. Int. Ed. 2003, 42, 4807-4810. [26] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184. [27] M. D. Clay, J. Colbeck, J. M. Gruber, J. Lalonde, J. Liang, J. Mavinhalli, B. Mijts, S. Muley, J. D. Munger, L. M. Newman, R. Sheldon, X. Zhang, J. Zhu, J. Mavinahalli, L. Newman, M. Clay, J. Gruber and J. Munger in Vol. CODEXIS INC (CODE-Non-standard), 2009, pp. 2307419-A2307412:. [28] R. S. Gordee, D. J. Zeckner, L. F. Ellis, A. L. Thakkar and L. C. Howard, J. Antibiot. 1984, 37, 1054- 1065. [29] R. F. Hector, Clin. Microbiol. Rev. 1993, 6, 1-21. [30] M. I. Morris and M. Villmann, Am. J. Health-Syst. Pharm. 2006, 63, 1693-1703. [31] H. Morita, S. Nagashima, K. Takeya, H. Itokawa and Y. Iitaka, Tetrahedron 1995, 51, 1121-1132. [32] H. Morita, S. Nagashima, K. Takeya and H. Itokawa, Chem. Pharm. Bull. 1993, 41, 992-993. [33] R. Cozzolino, P. Palladino, F. Rossi, G. Cali, E. Benedetti and P. Laccetti, Carcinogenesis 2005, 26, 733-739. [34] C. C. Lin, T. Kimura, S. H. Wu, G. WeitzSchmidt and C. H. Wong, Bioorg. Med. Chem. Lett. 1996, 6, 2755-2760. [35] M. Buerke, A. S. Weyrich, Z. L. Zheng, F. C. A. Gaeta, M. J. Forrest and A. M. Lefer, J. Clin. Invest. 1994, 93, 1140-1148. [36] M. S. Mulligan, J. C. Paulson, S. Defrees, Z. L. Zheng, J. B. Lowe and P. A. Ward, Nature 1993, 364, 149-151. [37] T. Murohara, J. Margiotta, L. M. Phillips, J. C. Paulson, S. DeFrees, S. Zalipsky, L. S. S. Guo and A. M. Lefer, Cardiovasc. Res. 1995, 30, 965-974. [38] P. Revill, N. Serradell, J. Bolos and E. Rosa, Drugs Fut. 2007, 32, 788-798. [39] G. J. Tanoury, M. Chen, J. E. Cochran, V. Jurkauskas and A. Looker in Vol. Vertex Pharm Inc; 2007. [40] F. G. Njoroge, K. X. Chen, N. Y. Shih and J. J. Piwinski, Acc. Chem. Res. 2008, 41, 50-59. [41] G. Wu, F. X. Chen, P. Rashatasakhon, J. M. Eckert, G. S. Wong, H. Lee, N. C. Erickson, J. A. Vance, P. C. Nirchio, J. Weber, D. J. Tsai, Z. Nanfei, G. S. K. Wong and N. Zou in Vol. Schering Corp; Wu G; Chen F X, 2008.
74 Highly Stereoselective Synthesis of Substituted Prolyl Peptides
[42] M. Wiesner, G. Upert, G. Angelici and H. Wennemers, J. Am. Chem. Soc. 2010, 132, 6-7. [43] M. Wiesner, M. Neuburger and H. Wennemers, Chem.-Eur. J. 2009, 15, 10103-10109. [44] M. Wiesner, J. D. Revell and H. Wennemers, Angew. Chem. Int. Ed. 2008, 47, 1871-1874. [45] S. Michaelis and S. Blechert, Chem.-Eur. J. 2007, 13, 2358-2368. [46] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184. [47] In case of 36h, the presence of rotameric signals prevented straightforward determination of the d.r. After Fmoc deprotection, it became evident that a single diastereomer was formed. [48] J. L. Zhu, X. Y. Wu and S. J. Danishefsky, Tetrahedron Lett. 2009, 50, 577-579. [49] V. Theodorou, K. Skobridis, A. G. Tzakos and V. Ragoussis, Tetrahedron Lett. 2007, 48, 8230-8233. [50] N. Elders, E. Ruijter, F. J. J. de Kanter, E. Janssen, M. Lutz, A. L. Spek and R. V. A. Orru, Chem.-Eur. J. 2009, 15, 6096-6099.
75
Chapter 4 Asymmetric Synthesis of Synthetic Alkaloids by a Tandem Biocatalysis/Ugi/ Pictet–Spengler-Type Cyclization Sequence
Published in: Chem. Commun. 2010, 46, 7706-7708 Chapter 4
Abstract: A highly efficient combination of a biocatalytic desymmetrization of 3,4-cis- substituted meso-pyrrolidines with an Ugi-type multicomponent reaction followed by a Pictet–Spengler-type cyclization reaction was developed. This sequence generated highly complex alkaloid-like polycyclic compounds.
78 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
4.1 Introduction
Multicomponent reactions (MCRs)[1] are powerful tools for the synthesis of complex, biologically relevant molecules. The atom economy of MCRs, their convergent character, operational simplicity, and the structural diversity and complexity of the resulting molecules make this chemistry exceptionally useful for discovery and optimization processes in the pharmaceutical industry.[2-3] However, catalytic asymmetric methods to control the stereochemical outcome of MCRs are scarce.[4-6] We recently reported a novel method combining the biocatalytic desymmetrization of 3,4-cis-substituted meso-pyrrolidines[7] using engineered monoamine oxidase N (MAO-N) from Aspergillus niger and an Ugi-type three-component reaction (MAO-N oxidation/MCR sequence) to generate highly functionalized and optically pure 3,4-cis-substituted prolyl peptides.[8] These prolyl peptides are of considerable interest in organocatalysis[9-12] but also in medicinal chemistry specifically as key structural elements of clinically important hepatitis C virus NS3 protease inhibitors such as Telaprevir (Figure 1B). We demonstrated the utility of our method by the development of a highly efficient and convergent synthesis of this important drug (see Chapter 5).[13] We realized that combination of our MCR approach with cyclization reactions[14-15] could further increase the resulting molecular complexity and diversity considerably.
Figure 1: A. Biocatalytic desymmetrization and diastereoselective Ugi-type 3CR towards optically pure 3,4-substituted prolyl peptides; B. Hepatitis C NS3 protease inhibitor telaprevir with 3,4-substituted proline moiety highlighted.
Recently, El Kaim and coworkers described an interesting approach involving an Ugi four- component reaction (U-4CR) between amines, aldehydes, a-ketocarboxylic acids and homoveratryl isocyanide to afford the Ugi intermediate 5, which subsequently undergoes a Pictet–Spengler cyclization to obtain 2,5-diketopiperazines 6 (DKPs; Scheme 1).[16]
79 Chapter 4
Scheme 1: Ugi/Pictet–Spengler formation of 2,5-diketopiperazine 6 from aldehyde/ketone 1, amine 2, α-keto acid 3 and isocyanide 4a.
For medicinal purposes, DKPs (Figure 2) containing natural products are highly interesting. The six membered ring structure of DKPs provides an increased stability in enzymatic cleavage and allows the compound to overcome the weak metabolic properties of peptides. Their important and wide range of physiological activities makes them even more attractive as building block in medicinal chemistry. The broad range of bioactivities of these cyclic dipeptides includes antitumor,[17] antiviral,[18] antifungal[19] and antibacterial activities.[20] These scaffolds can also serve as potent oxytocin antagonists (involved in preterm birth). [21-23]
Figure 2: General structure of diketopiperazines (DKPs).
The Pictet-Spengler reaction (Scheme 2) in which β-arylethylamines undergo ringclosure after condensation with an aldehyde or ketone is the most important method for the synthesis of alkaloid scaffolds.[24]
Scheme 2: The mechanism of the Pictet-Spengler reaction.
80 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
The Pictet-Spengler reaction is a very efficient method which is emphasized by the fact that nature also uses this reaction to synthesize alkaloids.[25] The first Pictet–Spenglerase, named strictosidine synthase (STR; EC 4.3.3.2), was already discovered in 1970’s.[26-27] Different alkaloids synthesized biosynthetically via a Pictet-Spengler were extracted from nature and showed to exhibit therapeutically interesting properties such as ajmalicine (a hypertensive drug),[28] ajmaline (antiarrhythmic agent, Figure 3),[29] camptothecin (anticancer, Figure 3),[30] vinblastine (anticancer),[28] vincristine (anticancer),[28] and toxiferines (pesticide).[31] The great potential of the Pictet-Spengler reaction has been compellingly proven in the construction of stereochemically and structurally complex alkaloids.[24, 32-34]
Figure 3: Selection of alkaloids synthesized in nature by means of a Pictet-Spengler reaction.
We soon realized that a successive Pictet–Spengler-type cyclization could present an efficient approach to expand the structural diversity accessible through our MAO-N oxidation/MCR sequence.[8] Here we describe the stereoselective generation of polycyclic DKP derivatives 10 by a novel MAO-N oxidation/Ugi MCR/ Pictet–Spengler-type cyclization (MUPS) sequence (Scheme 3). We also envisioned using 3-(2-isocyanoethyl)-1H-indole as isocyanide input generating alkaloids with an indole moiety.
Scheme 3: General scheme of the MUPS sequence.
81 Chapter 4
The compounds (10) generated from the MUPs sequence might posses interesting biological activity, as the core structure resemble certain mycotoxins (e.g. fumitremorgin C). Fumitremorgin C (Figure 4) has shown to be a potent and specific inhibitor of the breast cancer resistance protein (BCRP/ABCG2) multidrug transporter.[35]
Figure 4: Fumitremorgin C.
4.2 Results & Discussion
First, suitable conditions for the Pictet–Spengler-type cyclization in the MUPS sequence towards 15 were developed. The cyclization of Ugi product 14, synthesized from bridged imine 11, phenylglyoxylic acid 12 and homoveratryl isocyanide 13 was selected as the benchmark reaction (Table 1). Based on the standard reaction conditions for the Pictet– Spengler cyclization reported by El Kaim and coworkers,[16] we started the screening of reaction conditions using 33% TFA in CH2Cl2. Under these conditions, no reaction was observed after stirring at room temperature for 3 h (entry 1, Table 1). Also when the reaction was performed in TFA as the solvent at RT (4 h; entry 2, Table 1) or under microwave conditions at elevated temperatures (entries 3 and 4, Table 1) no DKP 15 was observed and the starting material 14 was recovered. In another attempt, the reaction mixture was heated to reflux in pure TFA. After 16 h, conversion to 15 was observed, but was accompanied by significant side product formation (entry 5, Table 1).
82 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
Table 1: Optimization of the Pictet–Spengler-type cyclization of Ugi product 14 towards DKP 15a
T (°C)/Time Isolated yield Entry Solvent Additive Conversion (%) (min) (%) TFA/CH Cl 1 2 2 - RT/180 - - (1 : 2) 2 TFA - RT/240 - - 3 TFA - MW 150/5 - - 4 TFA - MW 200/5 - - 5 TFA - ∆/960 n.d.c, d n.d.c, d
b 6 CH2Cl2 TMSOTf 10/960 100 72 a Reactions were performed with 0.073 mmol of Ugi product 14; b 1.1 equivalent of TMSOTf was added in one portion at the appropriate temperature; c Not determined; d Unidentified side products detected.
This prompted us to use trimethylsilyl triflate (TMSOTf) instead of TFA. With TMSOTf as the Lewis acid mediator the reaction indeed took place. Stirring at -5°C did not lead to any conversion based on HPLC-analysis (A, Figure 5), but letting the mixture slowly warm up to room temperature led to conversion to the desired DKP 15 (B-E, Figure 5). After 23 hours of stirring HPLC-analysis showed that all of starting material was consumed and the formation of DKP 15 had taken place. DKP 15 could be isolated in 72% yield (entry 6, Table 1). We later determined that full conversion can be reached after 16 hours at 10 ˚C (data not presented). Remarkably, we could also observe the hemi-aminal formed prior to Pictet- Spengler cyclization on HPLC (F, Figure 5).
83 Chapter 4
1 °C 7 °C HA Ugi 2 h 3 h HA Ugi
PS PS A B
11 °C 15 °C 4 h HA 5 h Ugi HA
Ugi
PS C PS D
18 °C 23 h PS
E Figure 5: A-E. HPLC analysis of the Pictet-Spengler (PS) reaction progress with respect to time and temperature. F. Heminal formation of Ugi 14 prior to Pictet-Spengler cyclization. HA = Hemiaminal
With this procedure in hand, we prepared a small set of different substituted prolyl peptides using several α-keto acids, imines, and isocyanides (Figure 6). Thus, pyrrolidine derivatives 14, 16-21 were obtained in good yields and excellent diastereomeric ratios (Figure 6).
84 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
Figure 6: Overview of synthesized substituted prolyl peptides using optically enriched 11, α-keto acids and isocyanides.
Although these products were obtained relatively easy, not all of the desired Ugi-like products were collected (Figure 7). Prolyl peptide 23 was not obtained due to solubility issues of the α-keto acid and prolyl peptide 24 was not obtained probably due to side product formation (e.g. aldol like condensations).
Figure 7: Not obtainable prolyl peptides.
The synthesized prolyl peptides were then subjected to the optimized Pictet–Spengler cyclization conditions to provide, in most cases, the desired DKPs 15, 25-28 in moderate to excellent yields and diastereomeric ratios (Scheme 4A). DKP 29 could not be obtained probably due to the conjugation between the α-keto acid component and the indole group making the keto group less electrophilic (Scheme 4B).
85 Chapter 4
Scheme 4: A. Structures of fused tricyclic DKPs 15, 25–28. B. Possible explanation of failed synthesis of DKP 29.
Strikingly, using homoveratryl isocyanide as input in the synthesis of DKP 15 resulted in a single diastereomer whereas using 3-(2-isocyanoethyl)-1H-indole in the synthesis of DKP 25 as input resulted in a diastereomeric ratio of 57:43. These results may be explained by the fact that the mechanism of these compounds proceed via different pathways (Scheme 5). The mechanism of 15 proceeds via a direct attack on the N-acyliminium ion (Scheme 5A) in contrary to the mechanism of 25 which is thought to proceed via a spiro intermediate (Scheme 5B),[36-38] although it has been postulated that cyclization can occur by direct attack. [39] This may explain the difference in diastereomeric ratios.
Scheme 5: A. Proposed mechanism of the DKP 15. B. Proposed mechanism of DKP 25.
The optimized Pictet–Spengler conditions did not produce DKPs of prolyl peptides 17 and 18 (Figure 6). In these examples the starting Ugi products 17 and 18 were recovered. Such 5-membered ring-fused DKPs have rarely been described in literature,[40] which is not surprising since they result from a disfavored 5-endo-trig cyclization. This explains
86 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler why harsher conditions are required to obtain such 5-membered ring-fused DKPs. Full conversion of 17 was achieved using 1 eq. of trifluoroacetic anhydride in TFA/CH2Cl2 (1 : 1) allowing isolation of the desired Pictet–Spengler-type product 30 in 60% yield as a single diastereomer (Scheme 6A). Under identical conditions, however, reaction of 18 (Scheme 4) gave the condensation product 32 instead of the desired 31. In this case, the disfavored 5-endo-trig cyclization does not occur because loss of a proton to give 32 is available as an alternative reaction pathway for the intermediate N-acyliminium ion (Scheme 6B).
Scheme 6: A. 5-membered ring-fused DKP 30. B. Condensation product 32 forms in favor of DKP 31.
Interestingly, the use of phenylglyoxylic acid and homoveratryl isocyanide (15; Scheme 6A) as components for the Ugi reaction resulted in a diastereoselective Pictet–Spengler- type cyclization. However, when 2-oxoisocaproic acid was employed, the corresponding DKPs were obtained as mixtures of diastereoisomers (26-27, Scheme 6A). The X-ray crystal structure of 15 (Figure 8) allowed the relative stereochemistry of the six asymmetric carbon atoms to be determined. Since the absolute stereochemistry at C2, C3 and C4 of the substituted proline residue is known and was previously reported by us,[7-8] the absolute configuration of the molecule could be deduced.
87 Chapter 4
Figure 8: Molecular structure of 15 in the crystal. Displacement ellipsoids are drawn at 50% probability level.
Subsequently, the sterically less demanding optically active 3-azabicyclo-[3,3,0]oct-2-ene (also prepared by MAO-N catalyzed desymmetrization[7]) was used as an input for a series of Pictet–Spengler-type substrates 33-35 (Figure 9A). The desired DKPs 36–38 were obtained in moderate to very good yields and diastereomeric ratios (Figure 9B).
Figure 9: A. Structures of Ugi products 33–35. B. Fused tricyclic DKPs 36–38.
88 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
4.3 Conclusions & Outlook
In summary, a highly efficient synthesis of fused DKPs has been demonstrated. The combination of MAO-N catalyzed desymmetrization of meso-pyrrolidines with an Ugi-type 3CR followed by Pictet–Spengler-type cyclization led to highly complex structures with high diversity. In addition, the experimental procedure is simple and very efficient. To the best of our knowledge, it also constitutes the first example of MCR chemistry to synthesize 5-membered ring-fused DKPs. The efficiency and generality of the approach and the resulting molecular diversity and complexity makes our methodology highly interesting for medicinal chemistry.
4.4 Experimental Section
General Information Starting materials and solvents were purchased from ABCR and Sigma-Aldrich and were used without treatment. 3-Azabicylo[3,3,0]octane hydrochloride was purchased from AK Scientific. (1R,2S,6R,7S)-4-methyl-4-azatricyclo[5.2.1.02,6]dec-8-ene was prepared according to literature procedure.[41] Column chromatography was performed on silica gel. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 (400.13 MHz for 1H and 100.61 13 1 13 MHz for C) or Bruker Avance 500 (500.23 MHz for H and 125.78 MHz for C) in CDCl3. Chemical shifts are reported in d values (ppm) downfield from tetramethylsilane. Electrospray Ionisation (ESI) mass spectrometry was carried out using a Bruker micrOTOF-Q instrument in positive ion mode (capillary potential of 4500 V). . Infrared (IR) spectra were recorded neat, and wavelengths are reported in cm-1. Optical 20 rotations were measured with a sodium lamp and are reported as follows: [α]D (c = g/100 mL, solvent). 2-(2-isocyanoethyl)-1H-indole, 4-(isocyanomethyl)-1,2-dimethoxybenzene and 4-(2-isocyanoethyl)-1,2-dimethoxy-benzene were synthesized according to literature procedures.[42]
General Procedure 1: Preparation of optically active imines (3S,7R)-11 and azabicyclo-[3,3,0]oct-2-ene Unless stated otherwise: imines were synthesised according to literature procedure[7] with minor adjustments. 0.7 g of freeze-dried MAO-N D5 E. coli were rehydrated for 30 min. in 20 ml of KPO4 buffer (100 mM, pH = 8.0) at 37 °C. Subsequently 1 mmol amine ((3S,7R)-11 or azabicyclo-[3,3,0]oct-2-ene) in 30 ml of KPO4 buffer (100 mM, pH = 8.0) was prepared. The pH of the solution was adjusted to 8.0 by addition of NaOH and then added to the rehydrated cells. After 16-17 h the reaction was stopped (conversions were > 95 %) and worked up. For workup the reaction mixture was centrifuged at 4000 rpm and 4 °C until the supernatant
89 Chapter 4 had clarified (40 – 60 minutes). The pH of the supernatant was then adjusted to 10-11 by addition of aq. NaOH and the supernatant was subsequently extracted with t-butyl methyl ether or dichloromethane (4 x 70 mL). The combined organic phases were dried with Na2SO4 and concentrated at the rotary evaporator.
General procedure 2: Preparation of optically active Ugi derivatives 14, 16-22 and 34-36 Unless stated otherwise: Ugi derivatives were synthesised according to literature procedure.8
Imine (0.70 mmol) was dissolved in 2 ml of CH2Cl2 followed by the addition of carboxylic acid (0.93 mmol) and isocyanide (0.93 mmol). The reaction mixture was stirred for 24 h at RT.
CH2Cl2 (8 mL) was added and the resulting mixture was washed with Na2CO3 (2x10 mL), dried
(MgSO4), filtered, and concentrated in vacuo. Subsequently the crude product was subjected to column chromatography (SiO2, EtOAc (1): Cyclohexane (1)). After concentration in vacuo, the pure oily compound was dissolved in a CH2Cl2/hexane mixture and concentrated to give the product as a solid. Note: Rotamer formation and traces of hexane might be observed in the NMR data.
General procedure 3: Preparation of DKP derivatives 25-29 and 37-39 via a Pictet- Spengler cyclization Unless stated otherwise: A dry 500 ml flask with activated 4 Å molecular sieves was prepared.
Ugi derivative (0.25 mmol) was dissolved in 300 ml dry CH2Cl2 and cooled down to -10°C. 1.3 eq. (0.325 mmol) of TMSOTf was dissolved in 5 ml dry CH2Cl2 and dropwise added in 5 h to the mixture while stirring the flask. After complete addition of the TMSOTf the mixture was allowed to warm up to room temperature. The reaction mixture was stirred for another 11 h. The resulting mixture was filtered and washed with NaHCO3 (2x20 mL), dried (MgSO4), filtered, and concentrated in vacuo. Subsequently the crude product was subjected to column chromatography (SiO2, EtOAc (1): Cyclohexane (1)). After concentration in vacuo, the pure oily compound was dissolved in a CH2Cl2/hexane mixture and concentrated to give the product as a solid. Note: Traces of hexane might be observed in the NMR data.
General procedure 4: Preparation of DKP derivatives 31 and 33 via a Pictet-Spengler cyclization Unless stated otherwise: A dry 50 ml flask with activated 4 Å molecular sieves was prepared.
Ugi derivative (0.25 mmol) was dissolved in 10 ml dry CH2Cl2 and 10 ml TFA. 1.0 eq. (0.25 mmol) of trifluoroacetic anhydride was added in and subsequently stirred for 16 h. The resulting mixture was filtered and washed with NaHCO3 (2x20 mL), dried (MgSO4), filtered, and concentrated in vacuo. Subsequently the crude product was subjected to column chromatography (SiO2, EtOAc (1): Cyclohexane (1)). After concentration in vacuo, the pure oily compound was dissolved in a CH2Cl2/hexane mixture and concentrated to give the product as a solid. Note: Traces of hexane might be observed in the NMR data.
90 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
Compound 14: General procedure 2 was followed using imine (94.4 mg, 0.709 mmol), phenylglyoxylic acid (138.6 mg, 0.923 mmol) and 4-(2-isocyanoethyl)-1,2- dimethoxybenzene (160.9 mg, 0.842 mmol) giving 14 as a pale yellow solid, yield 20 1 72%. [α]D = -18.2 (c = 0.440, MeCN). H NMR (500.23 MHz, CDCl3), δ 7.91 (dd, J = 8.2, 1.1 Hz, 2H), 7.67-7.64 (m, 1H), 7.53-7.49 (m, 2H), 6.80 (d, J = 8.3 Hz, 1H), 6.75 (d, J = 1.9 Hz, 1H), 6.72-6.69 (m, 1H), 6.64 (bs, 1H), 6.25 (dd, J = 5.7, 3.0 Hz, 1H), 6.00 (dd, J = 5.7, 3.0 Hz, 1H), 4.32 (d, J = 2.1 Hz, 1H), 3.86 (s, 3H), δ 3.83 (s, 3H), 3.81-3.80 (m, 1H), 3.54-3.40 (m, 3H), 3.10 (m, 2H), 2.94-2.89 (m, 1H), 2.87-2.83 (m, 1H), 2.77 (t, J = 13 7.2 Hz, 2H), 1.53-1.51 (m, 1H), 1.43-1.40 (m, 1H); C NMR (125.78 MHz, CDCl3), δ = 191.0, 170.2, 164.8, 149.0, 147.7, 135.4, 134.9, 134.7, 132.5, 131.2, 129.9, 129.1, -1 120.7, 111.9, 111.4, 64.5, 62.2, 55.9, 55.8, 51.7, 49.8, 47.2, 46.6, 45.0, 41.2, 35.0; IR (neat): νmax (cm ) = 3320 (w), 2930 (w), 1676 (m), 1628 (s), 1514 (m), 1443 (m), 1260 (s), 1234 (s), 1140 (m), 1026 (s), 714 (s), 665 (s);
HRMS (ESI+) calcd for C28H31N2O5 (MH+) 475.2155, found 475.2213.
Compound 16: General procedure 2 was followed using imine (70.5 mg, 0.529 mmol), 4-methyl-2-oxopentanoic acid (89.6 mg, 85 µL, 0.688 mmol) and 4-(2-isocyanoethyl)-1,2-dimethoxybenzene (140.6 mg, 0.735 mmol) giving 16 as a yellow wax, yield 77%. 1 H NMR (500.23 MHz, CDCl3), δ 6.78 (d, J = 7.9 Hz, 1H), 6.72-6.67 (m, 2H), 6.46 (bs, 1H), 6.12 (dd, J = 5.6, 3.0 Hz, 1H), 6.08 (dd, J = 5.6, 2.7 Hz, 1H), 4.16 (d, J = 1.9 Hz, 1H), δ 3.87 (s, 3H), δ 3.85 (s, 3H), 3.49-3.41 (m, 3H), 3.35 (m, 2H), 3.03-2.98 (m, 1H), 2.95-2.90 (m, 2H), 2.72-2.54 (m, 4H), 2.14 (sep, J = 6.7 Hz, 1H), δ 1.52-1.49 (m, 1H), 13 δ 1.43-1.40 (m, 1H), 0.95 (d, J = 6.7 Hz, 3H), 0.94 (d, J = 6.7 Hz, 3H); C NMR (125.78
MHz, CDCl3), δ = 200.1, 170.2, 163.8, 148.9, 147.6, 134.8, 134.7, 131.2, 120.6, 111.8, - 111.2, 62.8, 55.9, 55.8, 51.7, 49.8, 48.0, 47.0, 46.6, 45.7, 45.3, 40.9. 35.2, 23.9, 22.6, 22.4; IR (neat): νmax (cm 1) = 3310 (w), 2957 (w), 1624 (s), 1530 (m), 1454 (m), 1341 (m), 1221 (m), 739 (s); HRMS (ESI+) calcd for
C26H35N2O5 (MH+) 455.2468, found 455.2537.
Compound 17: General procedure 2 was followed using imine (133.0 mg, 1.0 mmol), phenylglyoxylic acid (195 mg, 1.3 mmol) and 4-(2-isocyanomethyl)-1,2- dimethoxybenzene (230 mg, 1.3 mmol) giving 17 as a yellow solid, yield 79%. 20 1 [α]D = +16.5 (c = 0.121, MeCN). H NMR (500.23 MHz, CDCl3), δ 7.86-7.84 (m, 1H), 7.65-7.62 (m, 1H), 7.51-7.45 (m, 3H), 7.00 (bs, 1H), 6.83-6.79 (m, 3H), 6.27 (dd, J = 5.7, 2.9 Hz, 1H), 6.04 (dd, J = 5.5, 2.9 Hz, 1H), 4.45 (dd, J = 14.7, 6.0 Hz, 1H), 4.36 (dd, J = 14.7, 5.6 Hz, 1H), 4.40 (d, J = 1.7 Hz, 1H), 3.86 (s, 3H), 3.84 (s, 3H), 3.52-3.47 (m, 1H), 3.12 (dd, J = 11.9, 1.9 Hz, 1H), 3.09-3.06 (m, 1H), 3.01-2.94 (m, 2H), 2.89-2.86 13 (m, 1H), 1.54-1.51 (m, 1H), 1.46-1.43 (m, 1H); C NMR (125.78 MHz, CDCl3), δ 190.7, 170.0, 165.3, 149.1, 148.3, 135.4, 134.9, 134.7, 132.4, 130.6, 129.9, 129.0, 119.8, -1 111.2, 110.8, 62.3, 55.9, 55.9, 51.7, 49.8, 47.0, 46.6, 46.1, 45.2, 43.5, ; IR (neat): νmax (cm ) = 3306 (w), 2940 (w), 1632 (s), 1512 (s), 1443 (s), 1234 (s) 1138 (s), 1022 (s), 718 (s), 667 (m), 459 (m); HRMS (ESI+) calcd for
C27H29N2O5 (MH+) 461.2076, found 461.2067.
Compound 18: General procedure 2 was followed using imine (70.7 mg, 0.531 mmol), 4-methyl-2-oxopentanoic acid (89.8 mg, 85 µL, 0.690 mmol) and 4-(isocyanomethyl)-1,2-dimethoxybenzene (122.2 mg, 0.690 mmol) giving 18 as yellow solid, yield 44%. 1 H NMR (500.23 MHz, CDCl3): 6.84-6.75 (m, 3H), 6.13 (dd, J = 5.7, 3.0 Hz, 1H), 6.11 (dd, J = 5.5, 2.8 Hz, 1H), 4.34 (d, J = 5.8 Hz, 2H), 4.25 (d, J = 1.9 Hz, 1H), 3.86 (s, 3H), 3.85 (s, 3H), 3.55 (dd, J = 12.5, 8.8 Hz, 1H), 3.42-3.30 (m, 2H), 3.02-2.93 (m, 3H), 2.71-2.52 (m, 2H), 2.13-2.09 (m, 1H), 1.53-1.51 (m, 1H), 1.45-1.39 (m, 1H), 0.94 (d, J 13 = 6.9 Hz, 3H), 0.92 (d, J = 6.9 Hz, 3H); C NMR (125.78 MHz, CDCl3), δ = 200.0, 170.1, 163.9, 149.1, 148.3, 134.9, 134.8, 130.6, 119.6, 111.1, 110.7, 62.9, 55.9, 55.8, 51.7, 51.7, 49.9, 48.1, 47.0, -1 46.6, 45.6, 45.4, 43.3, 23.9, 22.6, 22.5; IR (neat): νmax (cm ) = 3308 (w), 2959 (w), 2928 (w), 1711 (w), 1630
91 Chapter 4
(s), 1451 (m), 1358 (m), 1261 (s), 1234 (s), 1138 (s), 1026 (s), 729 (m); HRMS (ESI+) calcd for C25H33N2O5 (MH+) 441.2311, found 441.2387.
Compound 19: General procedure 2 was followed using imine (66.8 mg, 0.501 mmol), phenylglyoxylic acid (98.0 mg, 0.653 mmol) and 2-(2-isocyanoethyl)- 1H-indole (110.8 mg, 0.651 mmol) giving 19 as a pale yellow solid, yield 77%. 20 1 [α]D = -18.00 (c = 0.445, MeCN). H NMR (500. 23MHz, CDCl3), δ 8.05 (bs, 1H), 7.89 (dd, J = 1.3, 8.3 Hz, 2H), 7.65-7.08 (m, 7H), 6.60 (bs, 1H), 6.23 (dd, J = 5.7, 3.0 Hz, 1H), 5.99 (dd, J = 5.7, 3.0 Hz, 1H), 4.31 (d, J = 2.1 Hz, 1H), 3.64-3.56 (m, 2H), 3.43-3.38 (m, 2H), 3.01-2.98 (m, 3H), 2.96-2.87 (m, 3H), 2.85-2.82 (m, 1H), 2.80- 13 2.78 (m, 1H), 1.51-1.48 (m, 1H), 1.41-1.39 (m, 1H); C NMR (125.78 MHz, CDCl3), δ = 191.0, 170.1, 165.2, 136.4, 135.3, 134.9, 134.7, 132.4, 129.9, 129.0, 127.2,
122.5, 122.1, 119.5, 118.7, 112.6, 111.2, 64.5, 62.2, 51.7, 49.8, 47.0, 46.6, 46.5, 45.0, 39.8, 25.2; IR (neat): νmax (cm-1) = 3312 (w), 2930 (w), 1624 (s), 1530 (m), 1447 (m), 1343 (m), 1215 (s), 739 (s), 714 (s), 667 (s); HRMS
(ESI+) calcd for C28H28N3O3 (MH+) 454.2052, found 454.2121.
Compound 20: General procedure 3 was followed using imine (70.2 mg, 0.527 mmol), 2-(furan-2-yl)-2-oxoacetic acid (96.0 mg, 0.685 mmol) and 2-(2-isocyanoethyl)- 1H-indole (116.6 mg, 0.685 mmol) giving 20 as a yellow solid, yield 71%. 20 1 [α]D = -12.5 (c = 0.320, MeCN). H NMR (500.23 MHz, CDCl3): δ 8.04 (bs, 1H), 7.64 (d, J = 1.0 Hz, 1H), 7.61 (d, J = 7.9 Hz, 1H), 7.34 (d, J = 8.1 Hz, 1H), 7.24 (dd, J = 0.5, 3.7 Hz, 1H), 7.19 (t, J = 7.0 Hz, 1H), 7.14-7.10 (m, 1H), 7.06 (d, J = 2.2 Hz, 1H), 6.53 (dd, J = 1.7, 3.7 Hz, 1H), 6.46 (bs, 1H), 6.17 (dd, J = 3.0, 5.7 Hz, 1H), 6.02 (dd, J = 2.6, 5.7 Hz, 1H), 4.29 (d, J = 2.1 Hz, 1H), 3.60-3.53 (m, 2H), 3.44-3.35 (m, 2H), 3.24 (dd, J = 1.3, 11.9 Hz, 2H), 3.19-3.15 (m, 1H), 3.03-2.93 (m, 3H), 2.88-2.85 (m, 2H) , 1.49 (m, 1H). 1.41 (m, 1H); 13C
NMR (125.78 MHz, CDCl3), δ = 177.6, 169.9, 163.5, 149.6, 149.1, 136.4, 135.0, 134.6, 127.2, 122.5, 122.1, 119.4, 118.6, 113.0, 112.6, 111.3, 62.5, 51.6, 49.9, 47.0, 46.7, 46.2, 45.0, 39.6, 25.1; IR -1 (neat): νmax (cm ) = 3314 (w), 2924 (w), 1628 (s), 1535 (w), 1452 (s), 1389 (m), 1011 (m), 741 (s), 590 (m);
HRMS (ESI+) calcd for C26H26N3O4 (MH+) 444.1845, found 444.1897.
Compound 21: General procedure 3 was followed using imine (74.1 mg, 0.556 mmol), 4-methyl-2-oxopentanoic acid (94.1 mg, 89 µL, 0.723 mmol) and 2-(2-isocyanoethyl)-1H-indole (124.0 mg, 0.729 mmol) giving 21 as a yellow solid, yield 48%. 20 1 [α]D = -21.3 (c = 0.470, MeCN). H NMR (500.23 MHz, CDCl3), δ 8.04 (bs, 1H), 7.59 (d, J = 9.7 Hz, 1H), 7.35 (d, J = 9.8 Hz, 1H), 7.22-7.10 (m, 2H), 7.01 (d, J = 2.3 Hz, 1H), 6.42 (bs, 1H), 6.11-6.08 (m, 2H), 4.16 (d, J = 1.9 Hz, 1H), 3.59-3.51 (m, 2H), 3.46-3.43 (m, 1H), 3.33-3.29 (m, 2H), 3.00-2.89 (m, 4H), 2.63-2.49 (m, 2H), 2.16-2.06 (m, 1H), 1.51-1.48 (m, 1H), 1.41-1.39 (m, 1H), 0.95 (d, J = 6.7 Hz, 3H), 0.94 (d, J = 6.7 Hz, 3H); 13C NMR (125.78
MHz, CDCl3), δ = 200.2, 170.1, 163.8, 136.3, 134.8, 127.2, 122.2, 122.1, 119.4, 118.7, -1 112.7, 111.2, 62.8, 51.7, 49.8, 48.0, 47.0, 46.6, 45.8, 45.2, 39.7, 25.1, 23.8, 22.6, 22.4; IR (neat): νmax (cm ) =;
HRMS (ESI+) calcd for C26H32N3O3 (MH+) 434.2365, found 434.2438.
Compound 22: General procedure 2 was followed using imine (68.3 mg, 0.513 mmol), 2-(1H-indol-2-yl)-2-oxoacetic acid (130.0 mg, 0.687 mmol) and 4-(2-isocyanomethyl)-1,2-dimethoxybenzene (135.1 mg, 0.707 mmol) giving 22 as a yellow solid, yield 67%. 1 H NMR (500.23 MHz, CDCl3), δ 9.43-9.27 (m, 1H), 8.40-8.33 (m, 1H), 7.69-7.67 (m, 1H), 7.48-7.41 (m, 1H), 7.37-7.29 (m, 2H), 6.83-6.67 (m, 3H), 6.65-6.59 (m, 1H), 6.17- 6.13 (m, 1H), 6.09-6.05 (m, 1H), 4.31 (d, J = 1.1 Hz, 1H), 3.86 (s, 3H), 3.78 (s, 3H), 3.59- 3.38 (m, 4H), 3.36-3.31 (m, 1H), 3.02 (bs, 1H), 2.96-2.83 (m, 2H), 2.83-2.69 (m, 2H), 13 1.50-1.46 (m, 1H), 1.40-1.37 (m, 1H); C NMR (125.78 MHz, CDCl3), δ 185.0, 170.9, 165.6, 148.9, 147.5, 136.4, 1361, 135.4, 134.3, 131.4, 125.4, 124.4, 122.2, 121.0, 62.4,
92 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
-1 55.9, 55.9, 51.5, 50.0, 47.0, 46.7, 46.4, 45.1, 42.1, 41.0, 35.0; IR (neat): νmax (cm ) = 3296 (w), 2926 (w), 1610
(s), 1514 (s), 1418 (s), 1236 (s) 1138 (s), 1026 (s), 752 (s), 711 (s), 642 (m); HRMS (ESI+) calcd for C30H31N3O5 (MH+) 513.2344, found 461.2067.
Compound 15: General procedure 3 was followed using 14 (138.3 mg, 0.292 mmol) and TMSOTf (71.3 mg, 58 µL, 0.321 mmol) giving 25 as a white solid, yield 72%. 20 1 [α]D = +292.0 (c = 0.210, MeCN). H NMR (500.23 MHz, CDCl3), δ 7.36-7.26 (m, 5H), 6.61 (s, 1H), 6.59 (s, 1H), 6.23 (dd, J = 5.7, 3.0 Hz, 1H), 5.90 (dd, J = 5.7, 3.0 Hz, 1H), 4.96 (ddd, J = 12.9, 2.5, 2.3 Hz, 1H), 3.86-3.82 (m, 1H), 3.84 (s, 3H), 3.65 (s, 3H), 3.53- 3.49 (m, 1H), 3.32 (ddd, J = 25.1, 12.5, 3.3 Hz, 1H), 3.10-3.07 (m, 1H), δ 3.05-3.03 (m, 2H), δ 3.01-2.96 (m, 2H), δ 2.87-2.83 (m, 1H), δ 2.79-2.73 (m, 1H), δ 1.70-1.67 (m, 13 1H), 1.61-1.58 (m, 1H); C NMR (125.78 MHz, CDCl3), δ = 169.5, 165.7, 148.3, 146.8, 141.6, 136.9, 136.7, 128.9, 128.3, 127.2, 126.3, 125.4, 115.1, 110.3, 69.6, 61.5, 55.9, -1 55.8, 52.6, 50.0, 48.7, 45.7, 45.6, 43.8, 40.2, 29.0; IR (neat): νmax (cm ) = (w), 1665 (s), 1542 (m), 1398 (s),
1261 (s), 1219 (m), 743 (s), 706 (m); HRMS (ESI+) calcd for C28H29N2O4 (MH+) 457.2049, found 457.2107.
Compound 25: General procedure 4 was followed using 19 (137.8 mg, 0.304 mmol) and TMSOTf (92.3 mg, 75 µL, 0.415 mmol) giving 25 as a pale yellow solid, yield 92%. (Note: Minor diastereomer is given in italic). 1H NMR (500.23 MHz, CDCl ): δ 9.28 (bs, 3 1H), 7.51-7.08 (m, 9H), 6.32-6.28 (m, 1H), 6.24 (dd, J = 5.7, 3.0 Hz, 1H), δ 4.79 (dd, J = 12.5, 5.8 Hz, 1H), 4.14-4.02 (m, 1H), 3.75 (d, J = 8.9 Hz, 1H), 3.39-3.34 (m, 1H), 3.23-3.20 (m, 1H), 3.04-2.87 (m, 3H), 2.80-2.58 (m, 3H), 1.89-1.86 (m, 1H), 1.76-1.73 (m, 1H); 1H
NMR (500.23 MHz, CDCl3): δ 9.70 (bs, 1H), 7.51-7.08 (m, 9H), 6.34 (dd, J = 5.7, 3.0 Hz, 1H), 6.32-6.28 (m, 1H), δ 4.89 (dd, J = 12.6, 5.6 Hz, 1H), 4.14-4.02 (m, 1H), 3.86 (d, J = 7.9 Hz, 1H), 3.39-3.34 (m, 1H), 3.18-3.15 (m, 1H), 3.04-2.87 (m, 4H), 2.80-2.58 (m, 2H), 1.84-1.81 13 (m, 1H), 1.68-1.65 (m, 1H); C NMR (125.78 MHz, CDCl3), δ = 164.5, 163.4, 142.5, 137.7, 137.3, 126.6, 126.4, 126.2, 122.7, 119.8, 118.6, 111.4, 110.3, 60.9, 53.5, 53.1, 47.4, 45.0, 44.3, 44.0, 37.2, 13 20.7; C NMR (125.78 MHz, CDCl3), δ = 165.6, 164.0, 143.3, 137.6, 137.4, 126.4, 126.2, 126.0, 122.7, 119.6, -1 118.5, 111.6, 111.0, 60.6, 53.4, 53.0, 48.0, 45.3, 44.7, 43.3, 37.5, 20.6; IR (neat): νmax (cm ) = 3318 (w), 2928 (w),
1651 (s), 1422 (s), 1300 (m), 1233 (m), 733 (s), 505 (s); HRMS (ESI+) calcd for C28H26N3O2 (MH+) 436.1947, found 436.2005.
Compound 26: General procedure 3 was followed using 16 (109.6 mg, 0.241 mmol) and TMSOTf (70.1 mg, 57 µL, 0.315 mmol) giving 26 as a white solid, yield 86%. (Note: Minor diastereomer is given in italic). 1H NMR (400.13 MHz, CDCl ), δ 7.44 (s, 1H), 3 6.47 (s, 1H), 6.18 (dd, J = 5.7, 3.2 Hz, 1H), 6.13 (dd, J = 5.1, 2.7 Hz, 1H), 4.74 (ddd, J = 20.4, 13.3, 7.2 Hz, 1H), 4.13 (dd, J = 12.3, 9.0 Hz, 1H), 3.78 (s, 3H), 3.76 (s, 3H), 3.43 (d, J = 8.2 Hz, 1H), 3.18-3.10 (m, 1H), 3.08-2.98 (m, 3H), 2.91-2.82 (m, 2H), 2.57-2.46 (m, 3H), 1.86 (dd, J = 4.9, 14.8 Hz, 1H), 1.76-1.74 (m, 1H), 1.62-1.60 (m, 1H), 1.50 (sep, J = 6.4 Hz, 1H), 0.82 (d, J = 6.7 Hz, 3H), 0.78 (d, J = 6.6 Hz, 3H); 1H NMR (400.13 MHz, CDCl ): δ 7.53 3 (s, 1H), 6.45 (s, 1H), 6.27 (dd, J = 5.7, 3.0 Hz, 1H), 6.22 (dd, J = 4.8, 2.0 Hz, 1H), 4.85-4.81 (m, 1H), 3.91-3.86 (m, 1H), 3.84 (s, 3H), 3.77 (s, 3H), 3.62 (d, J = 7.6 Hz, 1H), 3.25-3.21 (m, 1H), 3.10-3.06 (m, 1H), 2.87-2.79 (m, 4H), 2.65 (dd, J = 12.3, 7.0 Hz, 1H), 2.56-2.50 (m, 1H), 2.11 (dd, J = 14.8, 6.3 Hz, 1H), 2.00-1.94 (m, 1H), 1.77-1.72 (m, 1H), 1.62-1.53 (m, 2H), 0.84 (d, J = 6.7 Hz, 3H), 0.74 (d, J = 6.6 Hz, 3H); 13 C NMR (100.61 MHz, CDCl3), δ = 167.5, 154.0, 147.3, 137.5, 137.2, 129.8, 124.9, 111.9, 108.8, 66.5, 60.0, 13 55.9, 55.8, 53.5, 53.4, 49.4, 45.8, 45.3, 44.5, 43.2, 36.9, 27.2, 25.2, 23.6, 21.8; C NMR (100.61 MHz, CDCl3), δ = 166.9, 166.2, 148.2, 146.8, 137.6, 137.3, 127.7, 127.3, 113.1, 110.9, 67.0, 60.4, 56.1, 55.8, 53.4, 52.6, 49.3, -1 48.3, 45.3, 44.8, 43.3, 37.8, 29.0, 24.8, 24.3, 23.7; IR (neat): νmax (cm ) = 2953 (w), 1651 (s), 1514 (m), 1408 (s),
1256 (s), 1219 (s), 1096 (m), 737 (m); HRMS (ESI+) calcd for C26H33N2O4 (MH+) 437.2362, found 437.2419.
93 Chapter 4
Compound 27: General procedure 3 was followed using 21 (110.0 mg, 0.254 mmol) and TMSOTf (73.4 mg, 60 µL, 0.330 mmol) giving 27 as a white solid, yield 90%. (Note: Minor diastereomer is given in italic).1H NMR (400.13 MHz, CDCl ), δ 9.33 (bs, 1H), 3 7.46 (d, J = 7.6 Hz, 1H), 7.33 (d, J = 8.1 Hz, 1H), 7.08 (t, J = 7.17 Hz, 1H), 7.06 (t, J = 7.8 Hz, 1H), 6.35 (dd, J = 5.7, 3.0 Hz, 1H), 6.31-6.29 (m, 1H), 5.03 (dd, J = 13.0, 4.5 Hz, 1H), 4.04 (dd, J = 12.4, 9.0 Hz, 1H), 3.72 (d, J = 8.0 Hz, 1H), 3.28-3.21 (m, 1H), 3.19-3.15 (m, 2H), 2.92- 2.88 (3H), 2.80-2.70 (m, 2H), 2.34 (dd, J = 14.8, 6.2 Hz, 1H), δ 2.10 (dd, J = 14.7, 5.6 Hz, 1H), 1 1.85-1.83 (m, 1H), 1.73-1.66 (m, 2H); H NMR (400.13 MHz, CDCl3), δ 8.91 (bs, 1H), 7.46-7.08 (m, 1H), 6.35 (dd, J = 5.7, 3.0 Hz, 1H), 6.21 (dd, J = 5.6, 2.8 Hz, 1H), 5.03 (dd, J = 13.0, 4.5 Hz, 1H), 4.21 (dd, J = 12.3, 9.0 Hz, 1H), 3.65 (d, J = 8.5 Hz, 1H), 3.28-3.21 (m, 1H), 3.19-3.15 (m, 2H), 2.92-2.88 (3H), 2.80-2.70 (m, 2H), 2.59 (dd, J = 12.3, 8.1 Hz, 1H), δ 2.49 (dd, J = 14.7, 6.5 Hz, 1H), 1.85-1.83 13 (m, 1H), 1.73-1.66 (m, 2H); C NMR (100.61 MHz, CDCl3), δ = 165.8, 165.3, 137.6, 137.5, 136.1, 133.5, 126.3, 122.4, 119.5, 118.3, 111.5, 108.6, 65.2, 60.4, 53.5, 52.9, 48.8, 47.9, 45.2, 44.7, 43.3, 37.0, 25.0, 23.9, 23.5, 13 20.6; (ESI+) calcd for C26H30N3O2 (MH+) 416.2360, found 416.2313. C NMR (100.61 MHz, CDCl3), δ = 165.6, 163.9, 137.7, 137.1, 135.6, 133.1, 126.6, 122.4, 119.6, 118.3, 111.3, 107.3, 64.5, 60.8, 53.5, 53.1, 48.8, 47.2, 45.0, -1 44.3, 43.9, 36.8, 25.1, 23.6, 22.8, 20.6; IR (neat): νmax (cm ) = 3329 (w), 2953 (w), 1649 (s), 1416 (s), 1300 (m),
1233 (m), 733 (s); HRMS (ESI+) calcd for C26H30N3O2 (MH+) 416.2360, found 416.2313.
Compound 28: General procedure 4 was followed using 20 (137.8 mg, 0.304 mmol) and TMSOTf (92.3 mg, 75 µL, 0.415 mmol) giving 28 as a white solid, yield 92%. (Note: Minor diastereomer is given in italic). 1H NMR (500.23 MHz, CDCl ): δ 9.57, 7.54- 3 7.12 (m, 5H), 6.36-6.33 (m, 1H), 6.27-6.24 (m, 2H), 6.08 (dd, J = 3.3, 0.72 Hz, 1H), 4.84- 4.81 (m, 1H), 4.27 (dd, J = 12.2, 9.0 Hz, 1H), 3.75 (d, J = 9.0 Hz, 1H), 3.40-3.33 (m, 1H), 3.21-3.19 (m, 1H), 3.10-2.75 (m, 3H), 2.61 (dd, J = 12.2, 8.3 Hz, 1H), 1.91-1.89 (m, 1H), 1 1.77-1.75 (m, 1H); H NMR (500.23 MHz, CDCl3): δ 9.28, 7.54-7.12 (m, 5H), 6.39 (dd, J = 5.8, 3.1 Hz, 1H), 6.36-6.33 (m, 1H), 6.27-6.24 (m, 1H), 5.99 (dd, J = 3.3, 0.72 Hz, 1H), 4.97- 4.92 (m, 1H), 4.02-3.97 (m, 2H), 3.40-3.33 (m, 1H), 3.21-3.19 (m, 1H), 3.10-2.75 (m, 4H), 13 1.86-1.84 (m, 1H), 1.71-1.69 (m, 1H); C NMR (125.78 MHz, CDCl3): δ = 164.7, 161.4, 152.5, 142.9, 137.7, 137.1, 135.817, 127.8, 126.3, 122.8, 119.7, 118.6, 111.6, 111.3, 110.5, 110.3, 61.2, 53.6, 13 52.7, 47.6, 44.8, 44.2, 44.2, 36.8, 29.7, 20.6; C NMR (125.78 MHz, CDCl3), δ =166.9, 162.4, 151.7, 143.3, 137.6, 137.4, 136.4, 128.3, 125.9, 122.8, 119.5, 118.6, 111.7, 111.4, 110.9, 110.4, 60.6, 53.4, 52.5, 48.2, 45.4, 44.9, 43.4, -1 37.2, 31.6, 22.7; IR (neat): νmax (cm ) = 3318 (w), 2928 (w), 1651 (s), 1422 (s), 1300 (m), 1233 (m), 733 (s), 505
(s); HRMS (ESI+) calcd for C26H24N3O3 (MH+) 427.1739, found 427.1904.
Compound 30: General procedure 4 was followed using 17 (110.0 mg, 0.255 mmol) and trifluoroacetic anhydride (33.7 mg, 21 µL, 0.255 mmol) giving 30 as a white solid, 60% yield. 20 1 [α]D = +247.6 (c = 0.210, MeCN). H NMR (500.23 MHz, CDCl3), δ 7.36-7.32 (m, 5H), 7.22 (s, 1H), 6.76 (s, 1H), 6.23 (dd, J = 7.1, 3.0 Hz, 1H), 5.94 (dd, J = 5.0, 2.6 Hz, 1H), 4.97 (d, J = 15.0 Hz, 1H), 4.80 (d, J = 15.0 Hz, 1H), 3.86 (s, 3H), 3.85 (s, 3H), 3.85-3.82 (m, 1H), 3.58-3.54 (m, 1H), 3.28 (d, J = 5.9 Hz, 1H), 3.09-3.03 (m, 1H), 3.00-2.97 (m, 1H), 2.94-2.91 (m, 1H), 2.87 (dd, J = 6.1, 12.5 Hz, 1H), 1.72-1.68 (m, 1H), 1.63-1.58 (m, 13 1H); C NMR (125.78 MHz, CDCl3), δ 167.7, 165.9, 150.0, 149.3, 139.8, 137.2, 136.6, 132.0, 129.2, 128.3, 125.9, 124.6, 108.3, 104.8, 75.5, 61.7, 56.1, 56.1 , 52.8, 50.9, 49.1, 48.3, 45.7, 45.5, 44.3; -1 ; IR (neat): νmax (cm ) = 2924 (w), 2855 (w), 1667 (s), 1408 (s), 1327 (m), 849 (m), 741 (s), 702 (m), 606 (m),
467 (w); HRMS (ESI+) calcd for C27H27N2O4 (MH+) 443.1971, found 443.1972.
94 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
Compound 32: General procedure 4 was followed using 18 (69.9 mg, 0.159 mmol) and trifluoroacetic anhydride (34.4 mg, 21 µL, 0.260 mmol) giving 32 as a pale yellow solid, 60% yield. 20 1 [α]D = -126.8 (c = 0.205, MeCN). H NMR (500.23MHz, CDCl3): δ = 6.80-6.70 (m, 3H), 6.31-6.28 (m, 2H), 5.42 (d, J = 10.0 Hz, 1H), 4.90 (d, J = 15.4, 1H), 4.74 (d, J = 15.4, 1H), 3.90 (dd, J = 9.6, 12.6 Hz, 1H), 3.67 (d, J = 6.5 Hz, 1H), 3.52-3.45 (m, 1H), 3.12- 3.09 (m, 1H), 3.08-3.02 (m, 1), 2.98-2.94 (m, 1H), 2.84 (dd, J = 6.5, 12.7 Hz, 1H), 1.79-1.77 (m, 1H), 1.68- 1.66 (m, 1H), 0.98 (d, J = 6.6, 3H), 0.90 (d, J = 6.6 Hz, 13 3H); C NMR (125.78 MHz, CDCl3), δ = 167.0, 159.2, 149.1, 148.3, 137.6, 137.0, 134.7, 129.6, 129.1, 119.4, 111.2, 110.6, 60.8, 55.9, 55.9, 53.1, 50.7, 47.7, 47.7, -1 45.5, 45.4, 43.8, 26.7, 23.2, 23.0; IR (neat): νmax (cm ) = 2963 (w), 1674 (s), 1624 (s), 1516 (s), 1452 (m), 1393
(s), 1258 (s), 1138 (s), 1026 (s), 733 (s); HRMS (ESI+) calcd for C25H31N2O4 (MH+) 423.2206, found 423.2269.
Compound 33: General procedure 2 was followed using imine (54.4 mg, 0.498 mmol), phenylglyoxylic acid (123.9 mg, 0.648 mmol) and 4-(2-isocyanoethyl)-1,2- dimethoxybenzene (97.3 mg, 0.648 mmol) giving 33 as a white solid, yield 75%. 20 1 [α]D = +25.0 (c = 0.240, MeCN). H NMR (500 .23MHz, CDCl3), δ 7.96 (d, J = 7.4 Hz, 2H), δ 7.64 (t, J = 7.0 Hz, 1H), δ 7.51 (t, J = 7.6 Hz, 2H), δ 6.78-6.67 (m, 3H), δ 6.59- 6.55 (m, 1H), δ 4.44 (s, 1H), δ 3.85 (s, 3H), δ 3.83 (s, 3H), δ 3.65-3.51 (m, 3H), δ 3.26 (dd, J = 2.7, 11.0 Hz, 2H), δ 3.02-2.96 (m, 1H), δ 2.81-2.71 (m, 3H), δ 2.01-1.88 (m, 1H), 1.87-1.77 (m, 1H), 1.76-1.65 (m, 1H), 1.63-1.45 (m, 2H), 1.36-1.27 (m, 1H); 13C NMR (125.78 MHz, CDCl3), δ = 190.9, 170.2, 165.9, 148.9, 147.6, 134.9, 132.6, 131.2, 129.8, 129.1, 120.7, 111.9, 111.3, 66.1, 55.8, 55.8, 53.5, 45.7, 42.8, 41.1, 40.7, 35.3, 32.8, 32.1, 25.8; IR (neat): -1 νmax (cm ) = 3312 (w), 2930 (w), 1676 (m), 1632 (s), 1514 (s), 1443 (s), 1260 (s), 1234 (s), 1140 (s), 1024 (s),
802 (m), 718 (s); HRMS (ESI+) calcd for C26H31N2O5 (MH+) 451.2155, found 451.2217.
Compound 34: General procedure 2 was followed using imine (54.2 mg, 0.496 mmol), phenylglyoxylic acid (97.2 mg, 0.647 mmol) and 2-(2-isocyanoethyl)-1H- indole (110.3 mg, 0.648 mmol) giving 34 as a yellow solid, yield 75%. 20 1 [α]D = -9.3 (c = 0.215, MeCN). H NMR (500.23 MHz, CDCl3), δ 8.07 (bs, 1H), 7.96- 7.93 (m, 2H), 7.65-7.62 (m, 2H), 7.48-7.45 (m, 2H), 7.38-7.35 (m, 1H), 7.22-7.12 (m, 3H), 6.49 (bs, 1H), 4.44 (d, J = 2.6 Hz, 1H), 3.78-3.58 (m, 3H), 3.25 (dd, J = 11.2, 3.6 Hz, 1H) 3.05-3.01 (m, 2H), 2.88-2.84 (m, 1H), 2.77-2.74 (m, 1H), 1.99-1.23 (m, 6H); 13C
NMR (125.78 MHz, CDCl3), δ = 191.1, 170.2, 166.0, 136.4, 134.9, 132.7, 129.8, 129.1, 127.3, 122.6, 122.1, 119.4, 118.7, 112.6, 111.3, 66.2, 53.5, 45.7, 42.8, 39.9, 32.9, 32.2, -1 25.9, 25.2; IR (neat): νmax (cm ) = 3304 (w), 2945 (w), 1670 (m), 1628 (s), 1530 (m), 1447 (s), 1227 (s), 741
(s), 718 (s), 665 (s); HRMS (ESI+) calcd for C26H28N3O3 (MH+) 430.2052, found 430.2116.
Compound 35: General procedure 2 was followed using imine (74.7 mg, 0.684 mmol), 4-methyl-2-oxopentanoic acid (115.8 mg, 110 µL, 0.890 mmol) and 2-(2-isocyanoethyl)-1H-indole (151.4 mg, 0.890 mmol) giving 36 as an orange solid, yield 56%. [α] 20 = -19.7 (c = 0.305, MeCN). 1H NMR (500.23MHz, CDCl ): δ = 8.10 (t, J = 21.7 D 3 Hz, 1H), 7.60 (d, J = 7.8 Hz, 1H), 7.34 (q, J = 8.0 Hz, 1H), 7.19 (p, J = 7.7 Hz, 1H), 7.12 (q, J = 7.3 Hz, 1H), 7.06-7.04 (m, 1H), 6.43-6.33 (m, 1H), 4.31 (d, J = 2.2 Hz, 1H), 3.76- 3.68 (m, 1H), 3.62-3.49 (m, 2H), 3.47-3.44 (m, 1H), 3.01-2.93 (m, 3H), 2.81-2.73 (m, 1H), 2.64- 2.58 (m, 2H), 2.20-1.29 (m, 7H), 0.97-0.90 (m, 6H); 13C NMR (125.78 MHz,
CDCl3), δ = 199.6, 169.2, 163.3, 135.3, 126.3, 121.2, 121.1, 118.4, 117.7, 111.7, 110.2, -1 65.9, 52.8, 48.4, 47.1, 43.9, 42.0, 38.7, 31.7, 31.3, 24.8, 24.1, 23.0, 21.6, 21.4. IR (neat): νmax (cm ) = 3314 (w), 2955 (w), 1624 (s), 1532 (w), 1454 (m), 1358 (m), 1227 (m), 741 (s), 424 (s).; HRMS (ESI+) calcd for
C24H32N3O3 (MH+) 410.2438, found 410.2431.
95 Chapter 4
Compound 36: General procedure 3 was followed using 33 (90.0 mg, 0.200 mmol) and TMSOTf (57.8 mg, 47 µL, 0.260 mmol) giving 36 as a yellow solid, yield 62%. 20 1 (Note: Minor diastereomer is given in italic). [α]D = -186.7 (c = 0.225, MeCN). H NMR (500.23 MHz, CDCl ), δ 7.30-7.20 (m, 5H), 6.73 (s, 1H), 6.53 (s, 1H), 4.88 (dq, J = 2.4 Hz, 3 1H), 4.01 (dd, J = 8.6, 12.4 Hz, 1H), 3.78 (s, 3H), 3.63 (s, 3H), 3.24 (td, J = 3.3, 12.3 Hz, 1H), 3.19 (d, J = 7.7 Hz), 3.13 (dd, J = 5.2 Hz, 1H), 3.04-2.98 (m, 1H), 2.87-2.80 (m, 1H), 2.69 (dt, J = 2.7, 15.6 Hz, 1H), 2.64-2.57 (m, 1H), 1.85-1.71 (m, 2H), 1.63-1.49 (m, 2H), 1 1.36-1.28 (m, 1H), 1.25-1.16 (m, 1H); H NMR (500.23 MHz, CDCl3), δ 7.55 (s, 1H), 7.31-7.26 (m, 3H), 7.06-7.03 (m, 2H), 6.65 (s, 1H), 4.48 (ddd, J = 2.0, 7.3, 20.5 Hz, 1H), 4.42 (dd, J = 9.0, 12.3 Hz, 1H), 3.89 (s, 3H), 3.88 (s, 3H), 3.65 (d, J = 9.0 Hz, 1H), 3.15-3.07 (m, 1H), 3.07-3.00 (m, 1H), 2.83 (dd, J = 6.9, 12.4 Hz, 1H), 2.80-2.72 (m, 2H), 2.47 (ddd, J = 1.7, 5.5, 16.5 Hz, 1H), 2.03-1.97 (m, 1H), 1.91-1.82 (m, 1H), 13 1.79-1.71 (m, 2H), 1.69-1.59 (m, 1H), 1.53-1.46 (m, 1H); C NMR (125.78 MHz, CDCl3), δ = 169.2, 165.4, 148.4, 146.8, 142.0, 129.0, 128.3, 127.4, 126.3, 125.5, 115.0, 110.4, 69.5, 64.0, 55.9, 55.8, 51.6, 47.5, 40.5, 40.0, 13 32.1, 31.4, 29.0, 25.0; C NMR (125.78 MHz, CDCl3), δ = 167.2, 163.8, 148.9, 146.9, 143.2, 128.4, 127.9, 127.3,
127.2, 125.8, 112.1, 110.4, 70.2, 62.6, 56.1, 55.9, 51.4, 49.9, 39.9, 37.9, 32.1, 31.5, 31.0, 26.7, 24.7; IR (neat): νmax (cm-1) = 2934 (w), 1655 (s), 1512 (m), 1400 (s), 1258 (s), 1221 (s), 1028 (m), 750 (m), 714 (m), 698 (m); HRMS
(ESI+) calcd for C26H29N2O4 (MH+) 433.2049, found 433.2109
Compound 37: General procedure 3 was followed using 34 (75.0 mg, 0.175 mmol) and TMSOTf (50.5 mg, 41 µL, 0.227 mmol) giving 37 as a white solid, yield 83%. 1 (Note: Minor diastereomer is given in italic). H NMR (500.23 MHz, CDCl3): δ 9.32 (bs, 1H), δ 7.54-7.12 (m, 9H), 4.82 (dd, J = 12.6, 6.0 Hz, 1H), 4.40 (dd, J = 12.2, 9.1 Hz, 2H), 3.80 1 (d, J = 6.7 Hz, 1H), 3.11-2.65 (m, 6H), δ 2.16-1.50 (m, 6H); H NMR (500.23 MHz, CDCl3): δ 9.74 (bs, 1H), 7.54-7.12 (m, 9H), 4.92 (dd, J = 13.1, 5.1 Hz, 1H), 4.31 (dd, J = 12.7, 9.3 Hz, 1H), 3.97 (d, J = 8.9 Hz, 1H), 3.11-2.65 (m, 6H), δ 2.16-1.50 (m, 6H). 13C NMR (125.78 MHz,
CDCl3), δ = 164.4, 163.4, 142.3, 135.7, 129.4, 128.7, 126.6, 126.3, 122.8, 119.8, 118.6, 111.5, 110.3, 67.1, 62.9, 50.8, 49.9, 40.5, 37.2, 31.8, 31.2, 24.6, 20.7; 13C NMR (125.78
MHz, CDCl3), δ =165.4, 163.9, 140.6, 136.2, 130.3, 129.0, 128.6, 126.4, 126.2, 119.7, 118.6, 111.6, 111.1, 67.1, -1 63.1, 51.4, 49.4, 39.8, 37.4, 32.2, 31.6, 24.9, 20.6; IR (neat): νmax (cm ) = 3331 (w), 2922 (w), 1649 (s), 1422 (s),
1298 (m), 1234 (m), 739 (s), 694 (s), 577 (m), 509 (m); HRMS (ESI+) calcd for C26H26N3O2 (MH+) 412.1947, found 412.2008.
Compound 38: General procedure 3 was followed using 35 (96.8 mg, 0.236 mmol) and TMSOTf (68.3 mg, 56 µL, 0.307 mmol) giving 38 as a white solid, yield 83%. 1 (Note: Minor diastereomer is given in italic). H NMR (500.23 MHz, CDCl3), δ 9.35 (bs, 1H), 7.48-7.09 (m, 4H), δ 5.06 (dd, J = 13.1, 4.7 Hz, 1H), 4.27 (dd, J = 9.3, 12.5 Hz, 1H), 3.76 (d, J = 9.0 Hz, 1H), 3.21-3.12 (m, 1H), 3.00-2.60 (m, 4H), 2.50 (dd, J = 14.7, 6.2, 1H), 2.38 (dd, J = 14.8, 6.1 Hz, 1H), 2.13 (dd, J = 14.8, 5.7 Hz, 1H), 2.00-1.48 (m, 6H), 0.94 (d, J = 1 4.7 Hz, 3H), 0.93 (d, J = 4.7 Hz, 3H); H NMR (500.23 MHz, CDCl3), δ 8.97 (bs, 1H), 7.48-7.09 (m, 4H), 5.06 (dd, J = 13.1, 4.7 Hz, 1H), 4.46 (dd, J = 12.0, 8.8 Hz, 1H), 3.67 (d, J = 9.6 Hz, 1H), 3.21-3.12 (m, 1H), ), 3.00-2.60 (m, 4H), 2.50 (dd, J = 14.7, 6.2, 1H), 2.08 (dd, J = 14.7, 5.7 Hz, 13 1H), 2.00-1.48 (m, 6H), 0.91 (d, J = 6.4 Hz, 3H), 0.89 (d, J = 3.6 Hz, 3H); C NMR (125.78 MHz, CDCl3), δ = 165.6, 165.0, 136.0, 134.0, 126.2, 122.4, 119.5, 118.3, 111.4, 108.5, 65.1, 62.8, 51.2, 49.3, 48.8, 39.6, 36.9, 32.2, 13 31.5, 25.0, 24.8, 23.9, 23.6, 20.6; C NMR (125.78 MHz, CDCl3), δ = 165.4, 163.8, 135.6, 133.1, 126.6, 122.4, 119.6, 118.3, 111.3, 107.4, 64.5, 62.7, 50.6, 49.6, 48.6, 40.4, 36.7, 31.8, 31.2, 25.0, 24.6, 23.6, 23.0, 20.6; IR (neat): -1 νmax (cm ) = 3349 (w), 2953 (w), 1651 (s), 1418 (s), 1298 (m), 1235 (m), 737 (s), 511 (m); HRMS (ESI+) calcd for C24H30N3O2 (MH+) 392.2260, found 392.2327.
96 Synthetic Alkaloids by Tandem Biocatalysis/Ugi/Pictet-Spengler
X-ray crystal structure determination of 15
4.5 References & Notes
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97 Chapter 4
[16] L. El Kaim, M. Gageat, L. Gaultier and L. Grimaud, Synlett 2007, 500-502. [17] B. Nicholson, G. K. Lloyd, B. R. Miller, M. A. Palladino, Y. Kiso, Y. Hayashi and S. T. C. Neuteboom, Anti-Cancer Drugs 2006, 17, 25-31. [18] S. Sinha, R. Srivastava, E. De Clereq and R. K. Singh, Nucleos Nucleot Nucl. 2004, 23, 1815-1824. [19] D. R. Houston, B. Synstad, V. G. H. Eijsink, M. J. R. Stark, I. M. Eggleston and D. M. F. van Aalten, J. Med. Chem. 2004, 47, 5713-5720. [20] O. S. Kwon, S. H. Park, B. S. Yun, Y. R. Pyun and C. J. Kim, J. Antibiot. 2000, 53, 954-958. [21] P. G. Wyatt, M. J. Allen, A. D. Borthwick, D. E. Davies, A. M. Exall, R. J. D. Hatley, W. R. Irving, D. G. Livermore, N. D. Miller, F. Nerozzi, S. L. Sollis and A. K. Szardenings, Bioorg. Med. Chem. Lett. 2005, 15, 2579-2582. [22] A. D. Borthwick, D. E. Davies, A. M. Exall, R. J. D. Hatley, J. A. Hughes, W. R. Irving, D. G. Livermore, S. L. Sollis, F. Nerozzi, K. L. Valko, M. J. Allen, M. Perren, S. S. Shabbir, P. M. Woollard and M. A. Price, J. Med. Chem. 2006, 49, 4159-4170. [23] A. D. Borthwick and J. Liddle, Med. Res. Rev. 2011, 31, 576-604. [24] W. M. Whaley and T. R. Govindachari, Org. React. 1951, 6, 151-190. [25] J. Stöckigt, A. P. Antonchick, F. Wu and H. Waldmann, Angew. Chem. Int. Ed. 2011, 50, 8538-8564. [26] J. Stockigt and M. H. Zenk, FEBS Lett. 1977, 79, 233-237. [27] J. Stockigt and M. H. Zenk, J. Chem. Soc., Chem. Commun. 1977, 646-648. [28] R. van der Heijden, D. I. Jacobs, W. Snoeijer, D. Hallared and R. Verpoorte, Curr. Med. Chem. 2004, 11, 607-628. [29] D. L. D. Keefe, R. E. Kates and D. C. Harrison, Drugs 1981, 22, 363-400. [30] Y. H. Hsiang, M. G. Lihou and L. F. Liu, Cancer Res. 1989, 49, 5077-5082. [31] W. D. M. Paton and W. L. M. Perry, Br. J. Pharmacol. Chemother. 1951, 6, 299-310. [32] M. Chrzanowska and M. D. Rozwadowska, Chem. Rev. 2004, 104, 3341-3370. [33] E. D. Cox and J. M. Cook, Chem. Rev. 1995, 95, 1797-1842. [34] B. E. Maryanoff, H. C. Zhang, J. H. Cohen, I. J. Turchi and C. A. Maryanoff, Chem. Rev. 2004, 104, 1431-1628. [35] J. D. Allen, A. van Loevezijn, J. M. Lakhai, M. van der Valk, O. van Tellingen, G. Reid, J. H. M. Schellens, G. J. Koomen and A. H. Schinkel, Mol. Cancer Ther. 2002, 1, 417-425. [36] P. D. Bailey, S. P. Hollinshead, N. R. McLay, K. Morgan, S. J. Palmer, S. N. Prince, C. D. Reynolds and S. D. Wood, J. Chem. Soc.-Perkin Trans. 1 1993, 431-439. [37] F. Ungemach and J. M. Cook, Heterocycles 1978, 9, 1089-1119. [38] A. H. Jackson, B. Naidoo and P. Smith, Tetrahedron 1968, 24, 6119-6129. [39] G. Casnati, A. Dossena and A. Pochini, Tetrahedron Lett. 1972, 5277-5280. [40] H. C. J. Ottenheijm, J. D. M. Herscheid, G. P. C. Kerkhoff and T. F. Spande, J. Org. Chem. 1976, 41, 3433-3438. [41] S. Michaelis and S. Blechert, Chem.-Eur. J. 2007, 13, 2358-2368. [42] N. Elders, E. Ruijter, F. J. J. de Kanter, E. Janssen, M. Lutz, A. L. Spek and R. V. A. Orru, Chem.-Eur. J. 2009, 15, 6096-6099.
98 Chapter 5 A Highly Effi cient Synthesis of Telaprevir® by Strategic use of Biocatalysis and Multicomponent Reactions
Published in: Chem. Commun. 2010, 46, 7918-7920 Patents: WO2011/103932A1, WO2011/103933A1 Chapter 5
Abstract: A very short and efficient synthesis of the important drug Telaprevir, featuring a biocatalytic desymmetrization and two multicomponent reactions as the key steps, is presented. The classical issue of lack of stereoselectivity in Ugi- and Passerini-type reactions is circumvented. The atom economic and convergent nature of the synthetic strategy require only very limited use of protective groups.
100 Telaprevir©
5.1 Introduction
5.1.1 Hepatitis C worldwide Hepatitis C is one of the major diseases in the world; about 3% of the world’s population is infected by the Hepatitis C virus (HCV), over 170 million people.
Figure 1: Global prevalence of Hepatitis C according to the world health organization (WHO).
Hepatitis C (discovered in 1989) is a virus infection spread through contact with infected blood. Common routes of infection include sharing of needles and blood transfusions. Consequently, from 1992 blood transfusions are always checked for HCV. Because of its slow progression and mild symptoms, hepatitis C is hard to detect and most cases remain unnoticed. HCV can cause liver infections, which in turn can lead to cirrhosis, liver failure and even liver cancer. Most of the infected patients never come to this phase, but suffer from illness and jaundice.[2] The HCV is a member of the Flaviviridea, which are a family of viruses that are mainly spread by arthropod vectors (like mosquitoes and ticks). The Flaviviridea family comprises many different viruses, which can be subdivided in three groups: Genus Flavivirus, Genus Hepacivirus and Genus Pestivirus. The Genus Flavivirus contains many identified human and animal viruses, like yellow fever, West Nile virus and Dengue fever. Genus Pestivirus contains viruses that infect non-human mammals. Bovine virus diarrhea is classified in this genus. HCV is the single member of the Genus Hepacivirus, which infects mammals, including humans. HCV is not a single organism, but is in fact a range of viruses, which are divided in different genotypes. At present, at least six genotypes are known, which have evolved over a period of several thousands of years. This evolution would explain the current general global patterns of the different genotypes (Figure 2). Genotype 1 is the major genotype of the HCV. It can be found in North & South America, Europe, Asia and Australia. Genotype 2 can also be
101 Chapter 5 found in these parts of the world. Genotype 3 is most common in Australia and South Asia. Genotypes 4 and 5 are highly prevalent in Africa and genotype 6 can be found in different countries in Asia. These genotypes differ in their genome. The different genotypes are mostly identified by the sequence which codes for the envelope protein (E1) and the non-structural protein (NS5).[7] These differences can have huge influence on the sensitivity to treatment. Global distribution of HCV genotypes
key
Genotype 1 Genotype 2 Genotype 3 Genotype 4 The boundaries and names shown and the designations used on this map do not imply the expression of any opinion whatsoever on the part of the World Health Organization Genotype 5 concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted lines on maps represent approximate border lines for which there may not yet be full agreement. Genotype 6 © WHO 2009. All rights reserved
Figure 2: Global distribution of different genotypes of HCV, confined to genotypes 1 to 6.
5.1.2 Life cycle of Hepatitis C HCV is a small enveloped positive stranded RNA virus comprising of a genome of 9600 nucleotides, which encodes a single polyprotein that is co- and post-translationally cleaved to produce different viral proteins. The N-terminal part of the polyprotein contains the structural proteins, the core protein (C) a highly basic, nonglycosylated nucleocapsid protein and two envelope E1 and E2 transmembrane glycoproteins. The nonstructural part of the RNA comprises of the p7 protein which forms the ion channel, followed by the non- structural proteins NS2, NS3, NS4A, NS4B, NS5A, and NS5B (Figure 3).
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Figure 3: The structure of the viral genome of HCV. Figure courtesy of reference [8].
A viral particle needs to undergo a multistep entry process to infect a target cell.[9-11] During this progression every step is tightly regulated in time and space. The first step in this process is virus entry which is initiated by the binding of a protein present at the surface of the virion to the cell surface of the host cell (step 1, Figure 4). After initial attachment, the viral particle is then internalized by endocytosis (step 2, Figure 4) which leads to release of the viral genome into the cytosol (step 3, Figure 4). Then decapsidation of viral nucleocapsids liberates free positive-strand genomic RNAs in the cell cytoplasm, where they are translated and processed to generate the HCV polyprotein (step 4, Figure 4). The polyprotein is then processed to generate viral proteins (step 5, Figure 4), like non-structural proteins which assemble the replication complex (step 6, Figure 4). Subsequently, new genomic RNA is formed and (step 7-9, Figure 4) and is released from the cell through the secretory pathway (step 10-12, Figure 4).
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Figure 4: Schematic representation of the HCV replication cycle. Figure courtesy of reference [9].
5.1.2 HCV treatment and recovery Until very recently, the standard treatment for Hepatitis C is a cocktail of pegylated interferon-α-2a or pegylated interferon-α-2b and the antiviral drug ribavirin for a period of 24 or 48 weeks, depending on hepatitis C virus genotype.[12] In 1986, interferon-α gave the first effects on patients with Hepatitis C, even without the discovery of HCV. Further research showed that pegylated interferon, in which a large molecule of poly(ethylene glycol) (PEG) is covalently attached to interferon-α, resulted in an active molecule with a longer half-life, better pharmacokinetic profile and higher rate of virological response.[13-14] There are three general patterns concerning the response to antiviral therapy of Hepatitis C (Figure 5).[12] These three patterns are: (1) sustained virological response (SVR), (2) end-of
104 Telaprevir© treatment response and relapse and (3) non-response. First, when a SVR is reached, no HCV RNA is detected during the treatment and for at least 6 months after the treatment. The SVR state continues in 95% of the patients after those 6 months.[15] In 10-25% of the patients with a perfect lifestyle, a temporarily response and relapse occurs. These patients will not experience a long-term cure of Hepatitis C. The relapse is dependent on short treatments and inadequate or absent doses. Patients with a relapse can be treated to reach SVR, but will need a longer treatment with higher doses. Last, non-response to treatment is shown in one-third of the patients with Hepatitis C. These patients will finally never reach the SVR state, and will always have HCV RNA in their bodies.
Figure 5: Virological responses during Hepatitis C therapy using pegylated interferon and ribavirin.
A 6-month course with Interferon-α leads to sustained response rates of 6-12%, and extending treatment to 12 months raised this rate to only 16-20%.[16] Better response rates were obtained, up to 35-40%, by combining the Interferon-α treatment with ribavirin, which is a broad-spectrum antiviral agent.[17] Pegylated interferon-α in combination with ribavirin gives response rates of 54-56%, which is around 15% higher than the response rates obtained previously.[18] These results are quite encouraging, but they also mean that 40-50% of the chronic Hepatitis C patients are not cured over the years. The current therapy is very expensive and the side effects, like depressions, make this not the ideal treatment of patients with Hepatitis C. For many different groups of patients the results are even lower, for example Africans, patients co-infected with HIV and patients with HCV genotype 1.
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5.1.3 NS3 serine protease The NS3 protein is an essential virally encoded protease/helicase which is responsible for the degeneration of the nonstructural part of the protein (Figure 3 and 4). The protein that is degenerated in this case takes part in the replication of the RNA of HCV. The NS3 protease is a typical serine protease, similar to chrymotrypsin (Figure 6). The typical chrymotrypsin-like fold has three catalytic active site residues positioned. The active site of the NS3 protease is located at the N-terminus. The main function of this protease is the cleavage of the NS4A- NS4B, NS4B-NS5A and NS5A-NS5B junctions (Figure 3).[8] These junctions self-assemble on the endoplasmatic reticulum to generate a replicative complex. Nowadays, focus has been set on finding potent inhibitors blocking NS3 serine protease activity, NS protein processing and HCV RNA replication.
Figure 6: Structural representation of the NS3 serine protease.
5.1.4 NS3 serine protease inhibitors In the past few years, a lot of research has been done to identify possible inhibitors of the NS3 protease. During extensive research efforts different and structurally diverse compounds have been considered. Because the NS3 protease structure contains a β-barrel serine protease (including a canonical Asp-His-Ser catalytic triad), substrate specificity is exclusive because of an extended polydentate substrate binding cleft, which is unique in
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NS3 protease. The important protease-substrate interactions are hydrogen bonds with the substrate backbone and electrostatic or hydrophobic contacts along the binding site. In the last decade, several NS3 inhibitors have reached clinical trials with mixed success (Figure 7).
Figure 7: Different inhibitors of NS3 protease; Telaprevir (Vertex), Boceprevir (Merck) and BILN 2061 (Boehringer-Ingelheim).
5.1.4.1 BILN2061 BILN 2061 was developed by Boehringer Ingelheim GmbH via an approach involving classical medicinal chemistry, parallel synthesis and structural data for the optimization of binding properties.[19] This was on account of the observation of an inhibitory effect on NS3 by hexameric N-terminal cleavage products of dodecapeptide substrates.[20] BILN 2061 was the first compound of its class to reach clinical trials. It has shown oral bioavailability and antiviral effect on patients with HCV.[21] In October 2003, Boehringer Ingelheim GmbH released a statement on their website regarding BILN 2061: “Routine chronic safety testing of high, supra-therapeutic doses in animals did, however, show relevant side effects which need further analysis. Boehringer Ingelheim is currently studying the available pre-clinical data in order to decide on their impact on the clinical development of this substance. There are currently no trials ongoing with BILN 2061 and decisions about future trials will be made after thorough evaluation of toxicity findings in animal studies.”[22] Probably the clinical trials of BILN 2061 are suspended due to cardiac issues shown by monkeys. Boehringer Ingelheim GmbH stopped reporting on BILN 2061 and the clinical trials after 2003.
5.1.4.2 Boceprevir (Victrelis™) Schering-Plough (nowadays Merck & Co.) also developed a very interesting NS3 protease inhibitor, Boceprevir. This drug shows great activity as an anti HCV drug in combination with ribavirin and pegylated interferon-α with SVR response rates of 63-66% in 24-48 weeks. The search for this compound started with the screening of several compound libraries, which did not lead to a potential lead candidate. Nevertheless, Schering-Plough switched their strategy and focused on structure-based drug design, starting from the
107 Chapter 5 three dimensional structure of the NS3 protease.[23] The focus was on trapping the catalytic serine of the NS3 protein with conventional electrophiles such as aldehydes, ketones and ketoamides. With most of these traps, the desired activity was not acquired. However, a ketoamide containing an undecapeptide showed excellent activity in the inhibition of the NS3 protease and was designated as the starting point for further drug development. After extensive modifications, a Boceprevir-like compound was developed which did not have the appreciable oral bioavailability in rats, dogs, and monkeys.[24] Optimization and modifications of this compound resulted in boceprevir. This drug was FDA (Food and Drug Administration) approved on May 13th, 2011 and will be marketed as Victrelis™.
5.1.4.3 Telaprevir (Incivek™) Telaprevir was developed by Vertex® pharmaceuticals and Eli Lilly & Co.[25] Telaprevir in combination with ribavirin and pegylated interferon-α shows a SVR response rate of 69- 75% in 8-12 weeks. Like with Boceprevir, Telaprevir started from a lead compound which was modified several times to obtain a compound with very good in vitro potency, toxicity profiles and adequate liver exposure upon oral administration. Further optimization led to Telaprevir which showed to be the best candidate. This drug was FDA approved on May 23th, 2011 and will be market as Incivek™. The reported synthesis of Telaprevir involves a lengthy, highly linear strategy relying heavily on standard peptide chemistry. Yip and coworkers[25] have reported a synthesis of Telaprevir, based on the fact that Telaprevir can be split up in three parts. These three parts can be made independently, and combined at the end via peptide coupling chemistry to furnish Telaprevir (Scheme 1).
Scheme 1: Reported synthesis of Telaprevir.
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Dipeptidyl acid unit 2 is readily available by standard peptide chemistry (Scheme 2).[25] The synthesis starts with the coupling of Boc-protected L-cyclohexylglycine 5 and L-tert- leucine methyl ester 6 to afford 7. Subsequent N-Boc deprotection and coupling with pyrazinecarboxylic acid yielded the desired peptide 2. The overall yield of 2 over four steps was 11%.
Scheme 2: Reported synthesis of Dipeptidyl acid 2.
The central bicyclic proline derivative was synthesized via a ten-step sequence (Scheme 3).[26-27] The synthesis starts with a tandem cycloaddition-cyclization of thiazolium ylide 11 (prepared in situ from thiazolium bromide and triethylamine) with 2-cyclopentenone, giving a diastereomeric mixture of tetracycles 12a and 12b (d.r. 87:13). Seperation of the diastereomers was not needed, in view of the fact that the 3-thiotetrahydrofuranyl portions of 12a and 12b are deleted from the remainder of the molecule. Subsequent reductive cleavage of the thiazoline C-S bond with tri-n-butyltin hydride followed by in situ hydrolysis of the resulting hemiaminal and Cbz protection furnished 13. Chiral HPLC separation was performed to obtain the desired enantiomer 14. Reduction of the ketone moiety with NaBH4 in ethanol afforded hydroxyl intermediate 15. Finally, removal of the hydroxy functionality was achieved using Barton-McCombie deoxygenation reaction affording the desired bicylic proline core 3. Because the yield for the last 2 steps (reduction of the carbonyl and the Barton-McCombie deoxygenation) in the synthesis of this proline derivative was not reported we made an assessment of 80% yield per step so we could for comparison reasons,
109 Chapter 5 establish the overall yield. We determined the overall yield of the bicyclic proline derivative 3 synthesis to be 12% over ten steps.
Scheme 3: Reported synthesis of bicyclic proline derivative 3.
β-Amino-α-hydroxy amide 4 was synthesized[28] (Scheme 4) starting from trans-2-hexanoic acid 17, which undergoes amide formation with cyclopropylamine in the presence of isobutyl chloroformate and N-methyl morpholine to form compound 18. Subsequent epoxidation of 18 with carbamide hydrogen peroxide and regiospecific ring-opening with sodium azide led to intermediate 20. Reduction of the azide in the presence of palladium on carbon, gave the racemic mixture of β-Amino-α-hydroxy amide 21. Chiral resolution of amide 21 with deoxycholic acid followed by anion-exchange with hydrochloric acid, afforded enantiomerically pure key intermediate 4. The overall yield of the synthesis of β-Amino-α-hydroxy amide 4 was 12% over 6 steps.
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Scheme 4: Reported synthesis of hydroxyl amine 4.
Finally, condensation of dipeptidyl acid unit 2 and bicyclic proline derivative 3 furnished tripeptide ester 23, which, after hydrolysis, underwent a coupling with amino alcohol 4 yielding the expected adduct 24. Ultimately, oxidation with Dess–Martin periodinane afforded Telaprevir (Scheme 5).
Scheme 5: Assembly of dipeptidyl acid 2, bicyclic proline ester 3 and amino alcohol 4 into Telaprevir.
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To summarize, the reported synthesis of Telaprevir is a 24 step synthesis with the longest sequence comprising nine steps with an overall yield of about 6%. Optimization of the synthesis of Telaprevir could significantly lower the production costs, thereby making this drug available to an increased proportion of the world population in the future.
5.1.4.4 Biocatalysis & Multicomponent Chemistry Multicomponent reactions (MCRs)[29-32] have become essential tools in the efficient generation of molecular diversity and complexity. Although MCRs have been used in medicinal chemistry,[33] issues related to the infamously poor stereoselectivity seriously hamper wider application of this promising methodology. The broad repertoire of stereospecific conversions by biocatalysts[34-35] presents a unique opportunity to address the stereoselectivity issue of certain MCRs. We recently reported the enzymatic desymmetrization of meso-pyrrolidines by means of a monoamine oxidase N (MAO-N) from Aspergillus niger optimized by directed evolution[36] and its combination with a highly diastereoselective Ugi-type three-component reaction (3CR).[37] We soon realized that this methodology could represent an efficient approach to the synthesis of Telaprevir. A retrosynthetic analysis of 1 exploiting this biocatalysis/MCR sequence is presented in Scheme 6. The required inputs for the key Ugi-type 3CR are carboxylic acid 2, cyclic imine 25, and isocyanide 26. The known acid 2[25] is readily available by standard peptide chemistry. Imine 25 can be generated in situ from commercially available 27 by MAO-N catalyzed oxidation.[36] Isocyanide 26 is accessible via β-formamido-α-ketoamide 28 which is available via oxidative cleavage of 29 followed by in situ trapping by cyclopropyl amine. [38-39] Cyanoketophosphorane derivative 29 is prepared by coupling of the corresponding carboxylic acid 30 which in turn is prepared from commercially available L-norvaline 31.
5.2 Results & Discussion
Thus, coupling of L-cyclohexylglycine methyl ester and pyrazinecarboxylic acid and subsequent saponification afforded 34 in excellent yield (Scheme 7). Subsequent coupling with L-tert-leucine methyl ester and saponification furnished the required optically pure acid 2. Compared to the reported synthesis of 2,[25] we have significantly increased both the atom and step economy and the overall yield (74% vs. 11% over four steps). The use of carbamate protective groups is avoided by essentially using the N-terminal pyrazine carboxylate as an amine protective group.
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Scheme 6: Retrosynthetic analysis of Telaprevir.
Scheme 7: Synthesis of dipeptidyl acid fragment 2.
We then turned our attention to the construction of the isocyanide fragment 26a (Scheme 8). Commercial L-norvaline 35 was N-formylated to compound 36 in 90% yield. Subsequently, coupling of carboxylic acid 36 and (cyanomethylene)phosphorane did not result in the formation of acylcyanomethylene(tripehenyl)phosphorane 37. We presumed the
113 Chapter 5 formamide functionality interfered with the coupling of carboxylic acid 36, so we decided to protect the amine part of L-norvaline 35 with a Boc-group to give compound 38 in 86% yield. The coupling of carboxylic acid 38 and (cyanomethylene)phosphorane went smoothly and resulted in Boc-protected cyanoketophosphorane 39 in a 84% yield. Subsequently, we attempted the oxidative cleavage of Boc-protected cyanoketophosphorane 40 and subsequent trapping with cyclopropylamine to form Boc-protected ketoamide 41. Unfortunately, the desired product was not formed and above all, we could not determine whether the oxidative cleavage or the nucleophilic trapping was the impeding step.
Scheme 8: Synthesis of isocyanide fragment 26a.
Therefore, an alternative procedure for the construction of isocyanide 26 was required. We envisioned a strategy making isocyanide 26b accessible via a Passerini three-component reaction (P-3CR)[30, 40-41] of 43, cyclopropyl isocyanide and acetic acid. Formamido aldehyde 43 can be derived from commercial (S)-2-amino-1-pentanol 44 (Scheme 9).
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Scheme 9: Retrosynthetic analysis of isocyanide fragment 26.
Commercial (S)-2-amino-1-pentanol 44 was transformed to aldehyde 43 by N-formylation and subsequent Dess–Martin oxidation (Scheme 10). A P-3CR between 43, cyclopropyl isocyanide and acetic acid afforded 42 as a 78 : 22 mixture of diastereomers. However, aldehyde 43 proved difficult to isolate due to partial dimerization and the P-3CR proceeded in disappointing yield. We then realized that these two steps might be combined in a one- pot process. Ngouansavanh and Zhu have previously shown that the P-3CR is compatible with hypervalent iodine oxidants such as IBX.[42] In our case, both the Dess–Martin oxidation and the Passerini reaction are performed in CH2Cl2, and the acetic acid formed as a by- product in the Dess–Martin oxidation can be used as the carboxylic acid input in the P-3CR. Thus, one-pot Dess– Martin oxidation/Passerini reaction of 45 furnished 42 (60% compared to 46% in the two-step procedure). Dehydration then afforded the required isocyanide 26 in very good yield (87%). No racemization of the C3 stereocenter was observed.[43] This crucial fragment is thus accessible in only three steps from commercial starting materials. Although the P-3CR has been used to construct Telaprevir fragments similar to 26,[44] our approach offers significant advantages compared to other approaches in terms of atom and step economy. We replaced the carbamate groups by a formamide that is actually incorporated in the final product.
Scheme 10: Synthesis of isocyanide fragment 26b via a P-3CR.
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The next task at hand was the convergent three-component Ugi-type coupling (Scheme 11). Thus, commercial amine 27 was oxidized to imine 25 (94% ee) by MAO-N as described previously,[36] and then combined with dipeptidyl acid 2 and isocyanide 26 to give 46. Finally, cleavage of the acetate followed by Dess–Martin oxidation[25] gave Telaprevir (1) as a 83 : 13 : 4 mixture of diastereomers, with one minor diastereomer derived from the incomplete stereoinduction of the Ugi-type 3CR, and the other from the minor enantiomer of imine 25. Flash chromatography allowed straightforward separation of the diastereomers to afford pure Telaprevir (1) in 80% yield over the last two steps.
Scheme 11: Multicomponent coupling of the fragments and completion of the synthesis of Telaprevir.
5.3 Conclusions & Outlook
We have developed a very short and efficient synthesis of the important drug candidate Telaprevir using a biotransformation and two multicomponent reactions as the key steps. The classical issue of lack of stereoselectivity in Ugi- and Passerini-type reactions is circumvented. The stereogenic carbon formed in the P-3CR is later oxidized to the corresponding ketone and the stereoselectivity of the Ugi-type 3CR is controlled by the absolute configuration of the cyclic imine, which is in turn derived from the highly stereoselective biotransformation.
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The use of protective groups is limited to two intermediate methyl esters and an acetate. The use of carbamate protective groups is avoided altogether. The synthesis comprises only eleven steps in total (seven steps in the longest linear sequence) compared to twenty-four in the originally reported procedure. The total yield (over the longest linear sequence) is 45% starting from L-cyclohexylglycine methyl ester. Our approach is general and will be applicable to many other HCV NS3 protease inhibitors that can be derived from meso- pyrrolidines, such as e.g. boceprevir[45-46] and narlaprevir.[44] The combination of synthetic efficiency and convergence in our approach allows both faster development of second generation inhibitors and a more economical production of Telaprevir. We strongly believe that biocatalysis and MCRs will be increasingly important tools for the improvement of atom and step economy towards the sustainable production of fine chemicals and pharmaceuticals.
5.4 Experimental section
General Information
Commercially available starting materials and solvents were used as received. Dry CH2Cl2 was dried and distilled from CaH prior to use. 2 1H and 13C NMR spectra measured at 250.13, 400.13, or 500.23 MHz for 1H and at 62.90, 13 100.61, or 125.78 MHz for C in CDCl3 or DMSO-d6. Chemical shifts are reported in d values (ppm) downfield from tetramethylsilane. Column chromatography was performed on silica gel. Chromatographic purification refers to flash chromatography using the indicated solvent (mixture) and silica (40-63 μm, 60 Å). Thin Layer Chromatography was performed using silica plates (silica on aluminum with fluorescence indicator). Compounds on TLC were visualized by UV detection unless stated otherwise. Electrospray Ionisation (ESI) mass spectrometry was carried out using a TOF-Quadrupole instrument in positive ion mode (capillary potential of 4500 V). Infrared (IR) spectra were recorded neat, and wavelengths are reported in cm-1. Optical rotations were measured with 20 a sodium lamp and are reported as follows: [α]D (c = g/100 mL, solvent).
Bicyclic imine 25. Imine 25 was synthesized according to literature procedure[36] with minor adjustments as follows: 2.5 g of freeze-dried MAO-N D5 E. coli were rehydrated for 30 min. in 20
ml of KPO4 buffer (100 mM, pH = 8.0) at 37 °C. Subsequently 1 mmol amine 27 in 30 ml of KPO4 buffer (100 mM, pH = 8.0) was prepared. The pH of the solution was adjusted to 8.0 by addition of NaOH and then added to the rehydrated cells. After 16-17 h the reaction was stopped (conversion was > 95 %) and worked up. For workup the reaction mixture was centrifuged at 4000 rpm and 4°C until the supernatant had clarified (40 – 60 min.). The pH of the supernatant was then adjusted to 10-11 by addition of aq. NaOH and the supernatant was subsequently extracted with t-butyl methyl ether (4 x 70 mL). The combined organic phases were dried with Na2SO4 and concentrated at the rotary evaporator.
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(S)-Methyl 2-cyclohexyl-2-(pyrazine-2-carboxamido)acetate (33). Pyrazinecarboxylic acid (2.72 g, 21.9 mmol) was added to a solution of l-cyclohexylglycine methyl ester (4.13 g, 19.9 mmol) in CH Cl (100 ml) at room 2 2 temperature under N , forming a white suspension. Triethylamine (6.33 ml, 4.62 2 g, 45.8 mmol) was added, followed by Benzotriazol-1-yloxy-tris-(dimethylamino)- phosphonium hexafluorophosphate (BOP; 9.69 g, 21.9 mmol), which turned the reaction mixture from purple to an orange solution. After two days of stirring at room temperature the reaction mixture was washed two times with 50 ml saturated Na2CO3, followed by the washing of the aqueous layers with CH2Cl2 (2 x 50 ml). The organic layers were collected and dried with MgSO4, followed by concentration in vacuo. Purification by silica gel flash chromatography (c-Hex:EtOAc = 2:1 with 0.5% triethylamine) afforded 33 (5.28 g, 19.03 mmol, 96%) as a yellow oil (that solidified upon standing to give a white solid). 20 = +42.5 (c= 1.13, CHCl ); 1H NMR (250.13 MHz, CDCl ) = 9.39 (d, J= 1.25 Hz, 1H), 8.76 (d, J = 2.5 Hz, [α]D 3 3 d 1H), 8.57 (t, J = 1.5 Hz, 1H), 8.25 (d, J = 8.8 Hz, 1H), 4.74 (dd, J = 5.5, 9.3 Hz, 1H), 3.78 (s, 3H), 1.96 (m, 1H), 13 1.77 (m, 5H), 1.24 (m, 5H); C NMR (62.90 MHz, CDCl3): d= 172.0 (C), 147.4 (CH), 144.6 (CH), 142.7(CH), -1 57.0 (CH), 52.3 (CH3), 41.2 (CH), 29.7 (CH2), 28.4 (CH2), 26.0 (CH2); IR (neat): νmax (cm ) = 3374 (m), 2920 (s), + + 2845 (w), 1740 (s), 1665 (s); HRMS (ESI, 4500 V): m/z calcd. for C14H19N3O3Na ([M + Na] ) 300.1319, found 300.1319.
(S)-2-cyclohexyl-2-(pyrazine-2-carboxamido)acetic acid (34). A solution of 1 M NaOH (12 ml, 12 mmol) was added to a solution of 33 (2.77 g, 10 mmol) in THF (25 ml) at 0˚C. MeOH was added to the formed suspension, to give a clear, colorless solution. The reaction mixture was stirred overnight at room temperature, followed by concentration in vacuo. The pH of the aqueous layer was set on 3.5 with a 1 M
KHSO4 solution and was extracted with EtOAc (2 x 25 ml). The mixture was dried with Na2SO4, filtered, and concentrated in vacuo, to give 34 (2.49 g, 9.45 mmol, 95%) as a white solid. 20 = +50.9 (c= 1.06, CHCl ); 1H NMR (250.13 MHz, CDCl ): = 9.38 (d, J = 1.5 Hz, 1H), 8.78 (d, J = 2.5 Hz, [α]D 3 3 d 1H), 8.58 (dd, J = 1.5, 2.5 Hz, 1H), 8.27 (d, J = 9.0, 1H), 4.77 (dd, J = 4.3, 5.0 Hz, 1H), 2.00 (m, 1H), 1.76 (m, 5H), 1.37 (m, 5H); 13C NMR (62.90 MHz, CDCl3): d = 175.7 (C), 163.0 (C), 147.2 (CH), 144.3 (CH), 144.2 (C), -1 142.0 (CH), 56.9 (CH), 40.9 (CH), 29.7 (CH2), 28.1 (CH2), 25.9 (CH2); IR (neat): νmax (cm ) = 3383 (m), 2928 + + (s), 2852 (w), 1713 (m), 1676 (s), 1518 (s); HRMS (ESI, 4500 V): m/z calcd. For C13H17N3O3Na ([M + Na] ) 286.1162, found 286.1158.
(S)-methyl-2-((S)-2-cyclohexyl-2-(pyrazine-2-carboxamido)- acetamido)-3,3-dimethyl-butanoate (9). 34 (0.653 g, 4.5 mmol) was added to a solution of H-Tle-OMe (0.653 g, 4.5 mmol) in DMF (40 ml). 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide-HCl (EDC•HCl; 0.919 g, 6.75 mmol) was added to this colorless solution followed by 1-hydroxy-7- azabenzotriazole (HOAt; 1.035 g, 5.4 mmol) giving a bright yellow solution. The reaction mixture was stirred for 3 days and afterwards concentrated in vacuo. The formed yellow solid was dissolved in EtOAc, washed with 40 ml saturated aqueous ammonium chloride solution and
40 ml of saturated aqueous NaHCO3 solution. The organic layers were collected, dried with MgSO4 and concentrated in vacuo to give 35 (1.48 g, 3.78 mmol, 84%) as a white solid. 20 1 [α]D = -2.0 (c= 1.0, CHCl3); H NMR (250.13 MHz, CDCl3): d = 9.39 (d, J = 1.5 Hz, 1H), 8.76 (d, J = 2.3 Hz, 1H), 8.55 (dd, J = 2.4, 1.8 Hz, 1H), 8.29 (d, J = 8.1, 1H), 6.40 (d, J = 9.3 Hz, 1H), 4.46 (m, 2H), 3.74 (s, 3H), 1.81 (m, 13 1H), 1.76 (m, 4H), 1.24 (m, 6H), 0.96 (s, 12H); C NMR (62.90 MHz, CDCl3): d = 171.7 (C) , 170.4 (C), 163.0 (C),
147.5 (CH), 144.5 (CH), 144.2 (C), 142.7 (CH), 60.2 (CH3), 58.4 (CH), 51.9 (CH), 40.5 (CH), 31.7 (C), 29.7 (CH2), -1 28.7 (CH2), 26.6 (CH3), 25.9 (CH2); IR (neat): νmax (cm ) = 3350 (m), 2928 (m), 2853 (w), 1738 (s), 1686 (s), + + 1640 (s), 1520 (s); HRMS (ESI, 4500 V): m/z calcd. for C20H30N4O4Na ([M + Na] ) 413.2159, found 413.2169.
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(S)-2-((S)-2-cyclohexyl-2-(pyrazine-2-carboxamido)acetamido)-3,3-
dimethylbutanoic acid (2). A solution of 1 M NaOH (0.94 ml, 0.94 mmol) was added to a solution of 9 (0.31 g, 0.78 mmol) in THF (3 ml) at 0˚C. MeOH was added to the formed suspension, to give a clear and colourless solution. The reaction mixture was stirred overnight at room temperature, followed by concentration in vacuo. The pH of this aqueous layer was set to 3.5 with 1
M KHSO4 and subsequently extracted with EtOAc (2 x 10ml). The mixture was dried with Na2SO4, filtered, and concentrated in vacuo, to give 2 (0.28 g, 0.75 mmol, 95%) as a white solid. 20 = +21.7 (c= 1.015, CHCl ); 1H NMR (250.13 MHz, CDCl ): = 9.39 (d, J = 1.3 Hz, 1H), 8.77 (d, J = 2.5 [α]D 3 3 d Hz, 1H), 8.57 (dd, J = 1.5, 2.5 Hz, 1H), 8.35 (d, J = 9 Hz, 1H), 6.70 (d, J = 9.0 Hz, 1H), 4.45 (t, J = 8.8 Hz, 1H), 13 4.42 (d, J = 9.2 Hz, 1H), 1.94 (m, 1H), 1.71 (m, 5H), 1.20 (m, 5H), 1.01 (s, 9H); C NMR (62.90 MHz, CDCl3): d = 173.4 (C), 170.5 (C), 163.3 (C), 147.4 (CH), 144.4 (CH), 144.2 (C), 142.8 (CH), 58.4 (CH), 51.9 (CH), 40.4 (CH), -1 34.7 (C), 29.8 (CH2), 28.6 (CH2), 26.6 (CH3), 25.8 (CH2); IR (neat): νmax (cm ) = 3335 (w), 2930 (m), 1726 (m), + + 1663 (s), 1514 (s); HRMS (ESI, 4500 V): m/z calc. for C19H29N4O4Na ([M + Na] ) 399.2003, found 399.2013.
(S)-2-formamidopentanoic acid (36). Acetic anhydride (33 ml, 325 mmol) was added dropwise added to a solution of L-norvaline (1.17 g, 10 mmol) in formic acid (100 ml) at room temperature. After 2 hours of stirring, the formic acid was evaporated, followed by co-evaporation with toluene (twice). After that, the reaction mixture was
washed with saturated Na2CO3 solution. After drying with Na2SO4 and concentration in 1 vacuo, 37 was obtained in quantative yield. H-NMR (250.13 MHz, DMSO-d6): d = 8.33 (d, J = 9.0 Hz, 1H), 8.04 (s, 1H), 4.26 (m, 1H), 1.64 (m, 2H), 1.40 (m, 2H), 0.88 (t, J = 7.4 Hz, 3H).
(S)-2-(tert-butoxycarbonylamino)pentanoic acid (38). L-norvaline (1.17 g, 10 mmol) was dissolved in a mixture of 1 M NaOH aqueous solution (10 ml) and MeCN (10 ml), resulting a white suspension. Boc2O (3.6 g, 16 mmol) in a solution of MCN (10 ml) was added dropwise to the basic reaction mixture. The MeCN was evaporated and the
solution acidified to pH 2 with KHSO4 solution. The product was extracted with EtOAc
(2 x 50 ml), the organic layers were collected and after drying with MgSO4 concentrated in vacuo gave 1 37 as a colourless oil in a yield of 86%. H-NMR (250.13 MHz,CDCl3): d = 6.41 (m, 1H), 5.08 (m, 1H), 1.76- 1.44 (m, 4H), 1.48 (s, 9H), 0.98 (t, J = 7.3 Hz, 3H).
Compound 39. EDC•HCl (0.21 g, 1.1 mmol) and DMAP (0.012 g, 0.1 mmol)) were added to a solution of 39 (0.22 g, 1 mmol) in CH2Cl2 (70 ml) at room temperature under N2 atmosphere. Next, cyanomethylene)phosphorane (0.60 g, 2 mmol) was added and a light yellow solution was formed. The reaction mixture was stirred overnight at room temperature, followed by concentration in vacuo. After silica gel 1 flash chromatography (c-hex:EtOAc = 1:1) a yield of 84%. H-NMR (250.13 MHz,CDCl3): d = 7.71 – 7.53 (m, 15H), 4.88 (m, 1H), 1.74-1.41 (m, 4H), 1.49 (s, 9H), 1.01 (t, J = 7.3 Hz, 3H); 13C-NMR (62.90 MHz, CDCl3): d= 190.0 (C), 150.0 (C), 133.6 (CH), 129.2 (CH), 123.1 (C), 122.3 (C), 120.9 (C), 85.4 (C), 66.0 (CH), 46.0 (C), 31.3 (CH2), 27.7 (CH3), 19.8 (CH2), 13.7 (CH3).
(S)-N-(1-hydroxypentan-2-yl)formamide (45). (S)-2-amino-1-pentanol (1.00 g, 9.7 mmol) was dissolved in ethylformate (7.84 ml, 7.19 g, 97 mmol). This reaction mixture was refluxed at 80 ˚C for 4 hours, followed by stirring overnight at room temperature. The colourless solution was concentrated in vacuo and stirred for 1
hour in a 10 mol% K2CO3 in MeOH (25 ml). Afterwards, the pH was set to 7 with DOWEX 50wx8, followed by filtration and concentration in vacuo, which gave 45 (1.26 g, 9.61 mmol, 99%). 20 = -29.6 (c= 1.15, CHCl ); 1H NMR (250.13 MHz, CDCl ): = 8.20 (s, 1H), 5.81 (b, 1H), 4.04 (m, 1H), 2.11 [α]D 3 3 d 13 (b, 1H), 1.47 (m, 4H), 0.94 (t, J = 7.0 Hz, 3H); C NMR (62.90 MHz, CDCl3): 161.8 (C), 65.1 (CH2), 50.6 (CH), -1 33.2 (CH2), 19.2 (CH2), 13.9 (CH3); IR (neat): νmax (cm ) = 3248 (s), 2957 (m), 1651 (s), 1528 (m), 1381 (m); + + HRMS (ESI, 4500 V): m/z calcd. for C6H13NO2Na ([M + Na] ) 154.0838, found 154.0835.
119 Chapter 5
(S)-N-(1-oxopentan-2-yl)formamide (43). Dess-Martin periodinane (5.514 g, 13 mmol) was added to a solution of 45 (1.31 g, 10 mmol) in CH Cl (100 ml) at room 2 2 temperature. The white suspension was stirred for 2 days and subsequently 35 ml MeOH was added and stirred for 30 minutes. The resulting suspension was filtrated and the filtrate was concentrated in vacuo. The crude was purified by silica gel flash chromatography (c-hex:EtOAc = 1:4) to give 43 (1.08 g, 8.29 mmol, 83%) as a white solid. NMR analysis indicates that 43 is in equilibrium with its cyclic dimer. 20 = +37.6 (c= 0.745, CHCl ); 1H NMR assigned to the monomer (250.13 MHz, CDCl ): = 8.22 (s, 1H), [α]D 3 3 d 7.84 (s, 1H), 7.10 (m, 1H), 5.31 (m, 1H), 1.52 (m, 4H), 0.95 (m, 3H); 13C NMR assigned to the monomer 1 (100.61 MHz, CDCl3): 198.8 (CH), 161.7 (CH), 57.4 (CH), 30.8 (CH2), 18.4 (CH2), 13.7 (CH3); H NMR assigned 13 to the dimer (400.13 MHz, CDCl3) 8.22 (s, 2H), 5.26 (m, 2H), 3.72 (m, 2H) 1.52 (m, 8H), 0.95 (m, 6H;) C
NMR (100.61 MHz, CDCl3) assigned to the dimer: 161.7 (CH), 89.8 (CH), 63.1 (CH), 30.8 (CH2), 18.4 (CH2), -1 13.7 (CH3); IR (neat): νmax (cm ): 3325 (s), 2959 (s), 1649 (s), 1530 (s), 1381 (m), 1123 (w); HRMS (ESI, 4500 + + V): m/z calc. for C6H12NO2 ([M + H] ) 130.0863, found 130.0858.
(S)-1-(cyclopropylamino)-3-formamido-1-oxohexan-2-yl acetate (42). From 43: (0.892 g, 6.91 mmol) was added to a solution of cyclopropyl isocyanide (0.410 g, 6.12 mmol) in CH Cl (110 ml) and stirred for 5 minutes at room 2 2 temperature. Acetic acid (0.711 ml, 0.747 g, 12.44 mmol) was added and the yellow reaction was stirred for 3 days at room temperature. The reaction
mixture was washed twice with 100 ml saturated Na2CO3, followed by drying
with Na2SO4 and concentration in vacuo. The crude was purified by silica gel flash chromatography (5% MeOH in CH2Cl2, 1% triethylamine). (S)-1-(Cyclopropylamino)-3-formamido- 1-oxohexan-2-yl acetate 42 (0.99 g, 3.87 mmol, 56%) was obtained as a white solid as a 78:22 mixture of diastereomers. From 45: Dess Martin periodinane (5.66 g, 12.3 mmol) was added to a solution of (S)-N-(1-hydroxypentan-
2-yl)formamide 45 (1.15 g, 8.8 mmol) in CH2Cl2 (12 ml) at room temperature. The white suspension was stirred for 60 minutes and subsequently cyclopropyl isocyanide (0.74 g, 10.0 mmol) was added and stirred for 48 hours. The resulting suspension was filtrated and washed twice with 10 ml saturated
Na2CO3, followed by drying with Na2SO4 and concentration in vacuo. The crude product was purified by silica gel flash chromatography (5% MeOH in CH2Cl2, 1% triethylamine) to give 42 (1.34 g, 5.22 mmol, 60%) as a pale yellow solid as a 78:22 mixture of diastereomers. 1 H NMR (130 °C, 400.13 MHz, DMSO-d6): d = 8.03 (s, 1H), 7.52 (m, 1H), 7.30 (m, 1H), 4.89 (d, J = 4.4, 1H), 4.28 (m, 1H), 2.65 (m, 1H), 2.17(s, 3H), 1.27-1.47 (m, 4H), 0.89(t, J = 7.2, 3H), 0.63 (m, 2H), 0.48 (m, 2H); 13C NMR
(125.78 MHz, DMSO-d6): d = 169.8 (C), 168.5 (C), 160.6 (CH), 74.4 (CH), 47.5 (CH), 22.2 (CH), 18.4 (CH3), 13.6 -1 (CH3), 5.7 (CH2); IR (neat): νmax (cm ) 3283 (s), 2961 (w), 1744 (m), 1661 (s), 1530 (s), 1238 (s); HRMS (ESI, + + 4500 V): m/z calcd. for C12H20N2O4Na ([M + Na] ) 279.1315, found 279.1325.
(S)-1-(cyclopropylamino)-3-isocyano-1-oxohexan-2-yl acetate (26b). N-methylmorpholine (0.66 ml, 0.607 g, 6.00 mmol) was added to a solution of (S)- 1-(cyclopropylamino)-3-formamido-1-oxohexan-2-yl acetate 42 (0.769 g, 3.00 mmol) in CH2Cl2 (40 ml) at room temperature. The reaction mixture was cooled to -78 ˚C and triphosgene (0.312 g, 1.05 mmol) was quickly added and stirred for 5 minutes at this temperature. The resulting yellow solution was warmed up to -30 ˚C and was stirred for another 3 h. Subsequently, the reaction was quenched with water and extracted twice with CH2Cl2 (40 ml). The organic layers were collected, dried with Na2SO4 and concentrated in vacuo. The crude product was purified by silica gel flash chromatography (2% MeOH in DCM) to give 26b (0.23 g, 0.96 mmol, 69%) as a white solid. 1 H NMR (250.13 MHz, CDCl3): d= 6.28 (s, 1H), 5.25 (d, J = 2.5 Hz, 1H), 4.2 (m, 1H), 2.74 (m, 1H), 2.24 (s, 3H), 13 1.55 (m, 4H), 0.96 (m, 3H), 0.84 (m, 2H), 0.60 (m, 2H); C NMR (62.90 MHz, CDCl3): d= 169.7 (C), 168.3 (C),
120 Telaprevir©
-1 74.4 (CH), 47.5 (CH), 22.0 (CH), 20.6 (CH3), 18.5 (CH2), 13.5 (CH3), 5.5 (CH2); IR (neat): νmax (cm ): 3267 (s), + 2959 (m), 1745 (m), 1643 (s), 1512 (m), 1221 (s); HRMS (ESI, 4500 V): m/z calcd. for C12H18N2O3Na ([M + Na]+) 261.1210, found 261.1214.
Compound 46. Isocyanide 26b (0.119 g, 0.50 mmol) was dropwise added to a solution of imine 25 (0.0949 g, 0.65 mmol) and carboxylic acid 2 (0.245 g, 0.65 mmol) in CH Cl (5 ml) at room 2 2 temperature. This yellow solution was stirred overnight and afterwards diluted with 5 ml CH Cl . 2 2 The reaction mixture was washed twice with saturated Na2CO3 solution (10 ml) and twice with saturated NH4Cl. The organic layers were collected, dried with MgSO4 and concentrated in vacuo. The crude product was purified by silica gel flash chromatography (5% MeOH in DCM) to give 46 (0.18 g, 0.25 mmol, 50%) as a mixture of diastereomers. 1 H NMR (500.23 MHz, CDCl3): d= 9.50 (s, 1H), 8.75 (d, J = 2.5, 1H), 8.59 (s, 1H), 8.35 (d, J = 9.0, 1H), 6.84 (d, J = 9.0, 1H), 6.44 (s, 1H), 5.20 (d, J = 3.0, 1H), 4.74 (d, J= 9.5, 1H), 4.58 (t, J = 7.5, 1H), 4.38 (m, 1H), 3.37 (d, J= 6.0, 1H), 2.82 (m, 1H), 2.69 (m, 1H), 2.11 (s, 3H), 1.26 (s, 2H), 0.97 (s, 9H), 0.86 (m, 3H), 0.84-2.00 (m, 21H), 13 0.76 (m, 2H), 0.51 (m, 2H); C NMR (125.78 MHz, CDCl3): d = 170.5 (C), 169.3 (C), 162.9 (C), 147.4 (CH),
144.6 (CH), 144.2 (C), 142.8 (CH), 74.4 (CH), 66.6 (CH), 58.3 (CH), 56.6 (CH), 54.5 (CH2), 44.9 (CH), 43.0 (CH), -1 41.3 (CH), 35.5 (C), 26.4 (CH3), 20.8 (CH3), 19.1 (CH2), 13.8 (CH3), 6.6 (CH2); νmax (cm ): 3306 (m), 2928 (m), + 2931 (m), 1743 (w), 1655 (s), 1520 (m), 1219 (m); HRMS (ESI, 4500 V): m/z calcd. for C38H57N7O7Na ([M + Na]+) 746.4212, found 746.4107.
Telaprevir (1). K2CO3 (0.003 g, 0.023 mmol) was added to a solution of 46 (0.166 g, 0.23 mmol) in MeOH (2 ml) at room temperature. The reaction mixture was stirred for 2 hours at room temperature resulting in a pale yellow suspension. After full conversion, the reaction mixture was washed with 2 ml Brine, the aqueous layer was washed again with
3 ml CH2Cl2 (2x). The organic layers were collected, dried with MgSO4 and concentrated in vacuo, to yield a pale yellow solid. The yellow solid was dissolved in CH2Cl2 (2 ml) and Dess-Martin periodinane (0.105 g, 0.25 mmol) was added at room temperature. The reaction mixture was stirred overnight before adding saturated NaHCO3 solution (2 ml) and saturated
Na2S2O3 solution (2 ml). This mixture was stirred for 10 minutes, separated and the aqueous layers were washed with EtOAc (2 x 3 ml). The organic layers were collected, dried with MgSO4 and concentrated in vacuo to give the crude product as an 83:13:4 mixture of diastereomers. After silica gel flash chromatography (1% MeOH in DCM), 1 (65 mg, 0.01 mmol, 80%) was obtained as a white solid. 1 H-NMR (500.23 MHz, DMSO-d6): d = 9.19 (d, J = 1.4 Hz, 1H), 8.91 (d, J = 24.5 Hz, 1H), 8.76 (dd, J = 1.5, 2.5 Hz, 1H), 8.71 (d, J = 5.3 Hz, 1H), 8.49 (d, J = 9.2 Hz, 1H), 8.25 (d, J = 6.8 Hz, 1H), 8.21 (d, J = 8.9 Hz, 1H), 4.94 (m, 1H), 4.68 (dd, J = 6.5, 9.0 Hz, 1H), 4.53 (d, J = 9.0 Hz, 1H), 4.27 (d, J = 3.5 Hz, 1H), 3.74 (dd, J = 8.0, 10 Hz, 1H), 2.74 (m, 1H), 3.64 (d, J = 3.5 Hz, 1H), 0.92 (s, 9H), 0.87 (t, 3H), 0.84-1.40 (m, 23H), 0.65 (m, 2H), 13 0.56 (m, 2H); C NMR (125.78 MHz, CDCl3): d = 197.0 (C), 171.8 (C), 170.4 (C), 169.0 (C), 162.1 (C), 161.9 (C), 147.9 (CH), 144.0 (C), 143.4 (CH), 56.4 (CH), 56.3 (CH), 54.2 (CH), 53.4 (CH), 42.3 (CH), 41.3 (CH), 32.1 -1 (CH), 31.8 (CH), 31.6 (CH), 29.1 (CH), 28.0 (CH), 26.4 (CH3); νmax (cm ): 3302 (m), 2928 (m), 2858 (w), 1658 + + (s), 1620 (s), 1561 (s), 1442 (m); HRMS (ESI, 4500 V): m/z calcd. for C36H53N7O6Na ([M + Na] ) 702.3950, found 702.3941.
121 Chapter 5
5.5 References & Notes
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[27] Y. Yip, F. Victor, J. Lamar, R. Johnson, Q. M. Wang, D. Barket, J. Glass, L. Jin, L. F. Liu, D. Venable, M. Wakulchik, C. P. Xie, B. Heinz, E. Villarreal, J. Colacino, N. Yumibe, M. Tebbe, J. Munroe and S. H. Chen, Bioorg. Med. Chem. Lett. 2004, 14, 251-256. [28] F. Maltais, Y. C. Jung, M. Z. Chen, J. Tanoury, R. B. Perni, N. Mani, L. Laitinen, H. Huang, S. K. Liao, H. Y. Gao, H. Tsao, E. Block, C. Ma, R. S. Shawgo, C. Town, C. L. Brummel, D. Howe, S. Pazhanisamy, S. Raybuck, M. Namchuk and Y. L. Bennani, J. Med. Chem. 2009, 52, 7993-8001. [29] A. Domling, Chem. Rev. 2006, 106, 17-89. [30] C. Hulme and V. Gore, Curr. Med. Chem. 2003, 10, 51-80. [31] R. V. A. Orru and M. de Greef, Synthesis 2003, 1471-1499. [32] D. J. Ramon and M. Yus, Angew. Chem. Int. Ed. 2005, 44, 1602-1634. [33] I. Akritopoulou-Zanze, Curr. Opin. Chem. Biol. 2008, 12, 324-331. [34] N. J. Turner, Nat. Chem. Biol. 2009, 5, 568-574. [35] K. Faber, Biotransformations in Organic Chemistry, Springer, Berlin, Heidelberg, New York, 1997. [36] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184. [37] A. Znabet, E. Ruijter, F. J. J. de Kanter, V. Kohler, M. Helliwell, N. J. Turner and R. V. A. Orru, Angew. Chem. Int. Ed. 2010, 49, 5289-5292. [38] H. H. Wasserman and W. B. Ho, J. Org. Chem. 1994, 59, 4364-4366. [39] M. K. Wong, C. W. Yu, W. H. Yuen and D. Yang, J. Org. Chem. 2001, 66, 3606-3609. [40] A. Domling, Curr. Opin. Chem. Biol. 2002, 6, 306-313. [41] A. Domling and I. Ugi, Angew. Chem. Int. Ed. 2000, 39, 3169-3210. [42] T. Ngouansavanh and J. P. Zhu, Angew. Chem. Int. Ed. 2006, 45, 3495-3497. [43] J. L. Zhu, X. Y. Wu and S. J. Danishefsky, Tetrahedron Lett. 2009, 50, 577-579. [44] A. Arasappan, F. Bennett, S. L. Bogen, S. Venkatraman, M. Blackman, K. X. Chen, S. Hendrata, Y. H. Huang, R. M. Huelgas, L. Nair, A. I. Padilla, W. D. Pan, R. Pike, P. Pinto, S. M. Ruan, M. Sannigrahi, F. Velazquez, B. Vibulbhan, W. L. Wu, W. Y. Yang, A. K. Saksena, V. Girijavallabhan, N. Y. Shih, J. S. Kong, T. Meng, Y. Jin, J. Wong, P. McNamara, A. Prongay, V. Madison, J. J. Piwinski, K. C. Cheng, R. Morrison, B. Malcolm, X. A. Tong, R. Ralston and F. G. Njoroge, ACS Med. Chem. Lett. 2010, 1, 64-69. [45] F. G. Njoroge, K. X. Chen, N. Y. Shih and J. J. Piwinski, Acc. Chem. Res. 2008, 41, 50-59. [46] G. Wu, J. Xie, P. Rashatasakhon, F. X. Chen, M. Poirier and V. M. Sprague in Vol. US2007149459-A1 Schering Corp, 2007.
123
Chapter 6 Stereoselective Synthesis of Substituted N-Aryl Proline Amides by Biotransformation/Ugi-Smiles Sequence
Published in: Org. Biomol. Chem. accepted Chapter 6
Abstract: An efficient combination of MAO-N-catalyzed desymmetrization of cyclicmeso - amines with Ugi-Smiles multicomponent chemistry produced optically pure N-aryl proline amides. This method represents the first report of a fully asymmetric Ugi-Smiles process.
126 Stereoselective Synthesis of N-aryl Amides
6.1 Introduction
The synthesis of complex and challenging target molecules requires sophisticated synthetic concepts and methodologies. Owing to their high flexibility, selectivity, and convergence, multicomponent reactions[1-5] (MCRs) have emerged as powerful tools in the synthesis of complex, biologically relevant molecules.[6] Undoubtedly the most widely used MCR is the Ugi four-component reaction (U-4CR). This reaction between amines, aldehydes, carbox- ylic acids and isocyanides (Scheme 1) affords a wide range of peptidic and peptidomimetic products.
Scheme 1. Proposed mechanism of the Ugi four-component reaction (U-4CR).
However, no catalytic asymmetric version of the U-4CR has been described to date and, as many MCRs, it typically suffers from very poor or unpredictable diastereoselectivity.[7-12] Recently, we developed a highly stereoselective synthesis of substituted prolyl peptides 1 (see Chapter 3) by combining the biocatalytic desymmetrization of 3,4-cis-substituted meso- pyrrolidines by using engineered monoamine oxidase N (MAO-N) from Aspergillus niger[13] and an Ugi–type three-component reaction (MAO-N oxidation/MCR sequence, Scheme 2).[14]
Scheme 2: Desymmetrization and diastereoselective Ugi-type 3CR towards optically pure 3,4-substituted prolyl peptides.[14]
These prolyl peptides are of considerable interest in medicinal chemistry and organocataly- sis. Also, this method has proven very efficient in the synthesis of alkaloid-type compounds (3, Figure 1A, Chapter 4)[15] and Hepatitis C drug Telaprevir (Figure 1B, Chapter 5).[16]
127 Chapter 6
Figure 1: A. Diastereoselective Ugi-type 3CR followed by a Pictet-Spengler reaction resulting in synthetic alkaloids.[15] B. Hepatitis C NS3 protease inhibitor Telaprevir with 3,4-substituted proline moiety highlighted.[16]
We then decided to investigate the possibility of extending this methodology to other MCRs. The Ugi-Smiles reaction, a variation of the Ugi reaction in which the carboxylic acid component is substituted by an electron-deficient phenol derivative (or a heteroaromatic analog),[17] is a promising candidate for combination with our biocatalytic oxidation due to its mechanistic similarity with the Ugi reaction. The Smiles rearrangement[18] represents a class of intramolecular nucleophilic aromatic sub- stitution reaction discovered in the 1930’s incorporating a heteroatom X as the nucleophilic component and displacing an aromatic electrophile, Y, forming an intermediate spirocyclic anion also known as the Meisenheimer complex[19] (Scheme 3). A limitation of the Smiles rearrangement is the requirement of an electron-deficient centre.
Scheme 3: The Smiles rearrangement.
The mechanism of the Ugi-Smiles reaction is very similar to the classic Ugi reaction. After formation of the iminium species, which is induced by the acidic nitrophenols (pKa values from 7 to 9) the isocyanide and nitrophenol react to form the intermediate imidate 4. This imidate then undergoes a Smiles rearangement to form N-aryl amine 5 (Scheme 4).
128 Stereoselective Synthesis of N-aryl Amides
Scheme 4: Proposed mechanism for the 4-nitrophenol Ugi–Smiles reaction.
Recently, El Kaïm and coworkers described a three-component Ugi–Smiles coupling of race- mic five- and six-membered cyclic imines.[20] Because the optically active 1-pyrrolines generated by MAO-N oxidation are versatile building blocks for a diverse set of MCRs, we envisioned that combining these substituted 1-pyrrolines with the Ugi-Smiles MCR would generate highly functionalized, optically pure 3,4-substituted N-aryl proline (thio)amides (Scheme 5).
Scheme 5: Asymmetric synthesis N-aryl proline amides by MAO-N oxidation and Ugi-Smiles 3CR.
Compounds of this type have been reported to be potent VLA-4 antagonists (very late antigen, α4β1, CD49d/CD29). This cell adhesion molecule is thought to be involved in cell trafficking, activation, and development during normal and/or pathophysiological processes. Therefore, these antagonists maybe useful in the treatment of diseases like asthma, atherosclerosis, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis.[21-23]
129 Chapter 6
6.2 Results & Discussion
We started our investigation by finding the most suitable conditions for the Ugi-Smiles MCR. The reaction of bridged imine 10, 2-nitrophenol (11) and tert-butyl isocyanide (12) was des- ignated as benchmark reaction (Table 1). We began by using the conditions reported by El Kaïm et al.[24] Unfortunately, these conditions afforded 13 in moderate yield at best (entries 1 and 2, Table 1). Therefore, we opted to extend the reaction time and raise the temperature. However, this did not have any effect and the isolated yield remained more or less the same (entries 3-6, Table 1). Neither microwave irradiation of the reaction mixture (entries 7-9, Ta- ble 1) nor the use of Sc(OTf)3 as additive (entries 10 and 11, Table 1) led to an improved yield. In a final attempt to improve the yield we used a two-fold excess of the imine (entries 12-14, Table 1). Performing the reaction at room temperature afforded the product in low yield (entry 12, Table 1). To our delight increasing the reaction temperature to 40 oC increased the yield to a satisfactory 73% (entry 13, Table 1). Elevating the temperature to 60 oC led to a significant drop in yield most likely due to the instability of the imine (entry 14, Table 1) at this temperature.[25]
Table 1: Optimization of the Ugi-Smiles 3CR of bridged imine 10, 2-nitrophenol (11) and tert-butyl isocyanide (12).
Temperature Reaction Entry Solvent Additives 10 : 11 : 12a Yield (%) (oC) Time 1 MeOH 60 24 h - 1.0 : 1.3 : 1.3 53b 2 Toluene 80 24 h - 1.0 : 1.3 : 1.3 26b 3 MeOH 60 48 h - 1.0 : 1.3 : 1.3 55b 4 Toluene 80 48 h - 1.0 : 1.3 : 1.3 25b 5 MeOH RT 72 h - 1.0 : 1.3 : 1.3 39b 6 MeOH 40 24 h - 1.0 : 1.3 : 1.3 43b 7 MeOH 125 30 mind - 1.0 : 1.3 : 1.3 40c 8 MeOH 150 30 mind - 1.0 : 1.3 : 1.3 40c 9 TFE 135 30 mind - 1.0 : 1.3 : 1.3 <10c b 10 MeOH RT 24 h Sc(OTf)3 1.0 : 1.3 : 1.3 34 b 11 MeOH 60 24 h Sc(OTf)3 1.0 : 1.3 : 1.3 24 12 MeOH RT 24 h - 2.0 : 1.0 : 1.5 33b 13 MeOH 40 24 h - 2.0 : 1.0 : 1.5 73b 14 MeOH 60 24 h - 2.0 : 1.0 : 1.5 44b a Molar ratios of the reactants; b Isolated yield; c Yield estimated by HPLC analysis; d Microwave irradiation in closed vessel; MeOH = methanol; TFE = 2,2,2-trifluoroethanol; Sc(OTf)3 = scandium triflate; RT = room temperature.
130 Stereoselective Synthesis of N-aryl Amides
With this procedure in hand we turned our attention to the synthesis of a small set of Ugi- Smiles products. Starting from bridged imine 10, various phenols and isocyanides were em- ployed to generate the corresponding Ugi-Smiles products 14-24 (Figure 2). As reported in Figure 1, the combination of ortho-nitrophenol and para-nitrophenol with different isocyanides gave the desired Ugi-Smiles products in moderate to good yields as single diastereomers (13-17, Figure 2). 3-Nitro-2-pyridinol and 2-mercaptopyrimidine were tolerated as inputs and gave the desired products in moderate to good yields as single dia- stereomers as well (18-21, Figure 2). 2-Pyridylmethyl isocyanide was also tolerated as the isocyanide input affording the corresponding Ugi-Smiles product 22 in fair yield (Figure 2). In addition, the use of an optically pure chiral isocyanide input ((R)-α-methylbenzyl isocya- nide led to the formation of a single diastereomer with a 2,3-trans configuration, confirming that the starting chiral imine is responsible for the diastereoselectivity of the reaction (23, Figure 2).
Figure 2: Scope of Ugi-Smiles 3CR using optically pure 8.
131 Chapter 6
X-Ray crystallographic analysis of 13 (Figure 3) unequivocally established the absolute con- figuration since the absolute stereochemistry at C3 and C4 positions, resulting from the bio- transformation, has been reported previously.[13]
Figure 3. Single-crystal structure of 13. Displacement ellipsoids are drawn at the 50% probability level.
Next, the sterically less demanding enantiomerically enriched (1S,4R)-3-azabicyclo[3.3.0] oct-2-ene (34)[13] was used in a series of Ugi-Smiles 3CRs (24-33, Figure 4). Very good diaste- reomeric ratios were obtained for compounds 24-33 and considerably higher yields were observed compared to reactions using the bulky bridged imine 10 as an input (cf. Figure 2). This observation can be rationalized by steric considerations or by the difference in stability between the used cyclic imines.[25]
132 Stereoselective Synthesis of N-aryl Amides
Figure 4. Scope of Ugi-Smiles 3CR using enantiomerically enriched 3-azabicyclo[3.3.0]oct-2-ene 34.
Subsequently, we synthesized a set of Ugi-Smiles products (Figure 5) that can undergo additional complexity-generating reactions, e.g. olefin metathesis,[26] cycloaddition reactions (Diels–Alder[27] or azide-alkyne cycloaddition[28]) or Pd-catalyzed cross-coupling reactions[29-30] to provide rapid access to highly complex scaffolds with pharmaceutically interesting properties. We chose to introduce an azide and allyl functionality into our Ugi-Smiles products by us- ing 2-azidoethyl isocyanide[31] and commercially available allyl isocyanide as an input. The presence of the azide and allyl functionalities in the final adducts allows interesting applica- tions to heterocycle synthesis. The desired azide compound 35 was obtained in good yield and diastereoselectivity (72%, d.r. = 88:12). Next, we secured Ugi-Smiles adducts 36 (55%, d.r. = 89:11) and 37 (34%, d.r. > 99:1) with the desired allyl functionality. In the latter case, we opted to use the saturated bridged imine 38[13] as the double bond in 10 could possibly interfere in subsequent follow-up reactions.
133 Chapter 6
Figure 5. Ugi-Smiles products with reactivity handles.
Remarkably, Ugi-Smiles products derived from 2-nitrophenol or 3-nitro-2-pyridinol as inputs showed unusually high αD values (see the Experimental Section). For example, compound 13 has a specific rotation of -1043.1˚. This is the highest specific rotation ever reported for a multicomponent product. This phenomenon was only observed for compounds with the nitro substituent in ortho position to the newly formed N-aryl bond. These compounds are likely highly rigid due to restricted rotation of the N-aryl bond. Further research regarding factors influencing these extraordinarily high specific rotations is ongoing.
6.3 Conclusions & Outlook
In conclusion, we have developed an efficient combination of MAO-N-catalyzed desym- metrization of cyclic meso-amines with Ugi-Smiles multicomponent chemistry to generate optically pure N-aryl proline amides. This method represents the first report of a fully asym- metric Ugi-Smiles process. The simple procedure, broad substrate scope and the presence of diverse (heterocyclic) ring systems make these compounds highly attractive. Especially the possibility to add more diversity and complexity to these N-aryl proline amides by introduc- ing strategic functional groups makes these products appealing for the design of synthesis of combinatorial libraries (e.g. better coverage of chemical space) on highly functionalized heterocyclic small molecules for the pharmaceutical industry.
134 Stereoselective Synthesis of N-aryl Amides
6.4 Experimental section
General Information Starting materials and solvents were purchased from ABCR and Sigma-Aldrich and were used without treatment. 3-Azabicylo[3,3,0]octane hydrochloride was purchased from AK Scientific. (1R,2S,6R,7S)-4-methyl-4-azatricyclo[5.2.1.02,6]dec-8-ene was prepared according to literature procedure.[32] Column chromatography was performed on silica gel. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 (400.13 MHz for 1H and 100.61 13 1 13 MHz for C) or Bruker Avance 500 (500.23 MHz for H and 125.78 MHz for C) in CDCl3. Chemical shifts are reported in d values (ppm) downfield from tetramethylsilane. Electrospray Ionisation (ESI) mass spectrometry was carried out using a Bruker micrOTOF-Q instrument in positive ion mode (capillary potential of 4500 V).
Thin Layer Chromatography was performed using TLC plates from Merck (SiO2, Kieselgel 60) and was visualized by UV detection. Flash chromatography was performed using the indi- cated solvents and Silicycle Silia-P Flash Silica Gel (particle size 40-63 μm, pore diameter 60 Å). Infrared (IR) spectra were recorded neat, and wavelengths are reported in cm-1. Optical rota- 20 tions were measured with a sodium lamp and are reported as follows: [α]D (c = g/100 mL, solvent). Optical rotations were measured on an Optical Activity AA-10 and a Perkin Elmer 241 polarimeter with a sodium lamp. 1-Azido-2-isocyanoethane was synthesized according to literature procedure,[31] (isocyanomethyl)benzene and 3-(isocyanomethyl)pyridine were synthesized according to literature procedure[33] and 2-Bromo-4-nitrophenol was synthe- sized according to literature procedure.[34]
General Procedure 1: Preparation of optically active imines (3S,7R)-10, azabicyclo-[3,3,0]oct-2-ene 34 and (3S,7R)-38 Unless stated otherwise: imines were synthesized according to literature procedure[13] with minor adjustments. 0.7 g of freeze-dried MAO-N D5 E. coli were rehydrated for 30 min. in 20 ml of KPO4 buffer (100 mM, pH = 8,0) at 37 °C. Subsequently 1 mmol of bridged amine or azabicyclo-[3,3,0]oct-2-ene) in 30 ml of KPO4 buffer (100 mM, pH = 8.0) was prepared. The pH of the solution was adjusted to 8.0 by addition of NaOH and then added to the rehydrated cells. After 16-17 h the reaction was stopped (conversions were > 95 %) and worked up. For workup the reaction mixture was centrifuged at 4000 rpm and 4 °C until the supernatant had clarified (40 – 60 minutes). The pH of the supernatant was then adjusted to 10-11 by addition of aq. NaOH and the supernatant was subsequently extracted with t-butyl methyl ether or dichloromethane (4 × 70 mL). The combined organic phases were dried with Na2SO4 and concentrated at the rotary evaporator.
135 Chapter 6
General procedure 2: Preparation of optically active Ugi-Smiles derivatives 13-37 Unless stated otherwise: To a solution of optically imine (0.5 mmol, 2.0 equiv.) in 2 ml methanol nitrophenol or thiol (0.25 mmol, 1.0 equiv.) was added, followed by the isocyanide (0.375 mmol, 1.5 equiv.). The reaction mixture was stirred at 40°C for 24 h. The mixture was concentrated in vacuo and the crude product was purified by column chromatography (SiO2,
CHCl2). After concentration in vacuo a bright colored oil or solid was obtained. Diastereomeric ratios were determined by 1H NMR.
Compound 13: General procedure 2 was followed using imine (3S,7R)-10 (60 mg, 0.45 mmol, 2.0 equiv.), 2-nitrophenol (31 mg, 0.22 mmol, 1.0 equiv.) and tert- butyl isocyanide (28 mg, 0.038 ml, 0.34 mmol, 1.5 equiv.) giving a yield of 57 mg (0.16 mmol, 73%) of a bright orange solid. 1 = -1043.1° (c = 0.51, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 7.65 (dd, J = 8.5 Hz, J = 1.5 Hz, 1H), 7.29 (m, 1H), 6.91 (d, J = 8.0 Hz), 6.78 (m 1H), 6.67 (bs, 1H), 6.14 (m, 1H), 5.79 (m, 1H) 4.00 (d, J = 3.0 Hz, 1H), 3.52 (dd, J = 10.8 Hz, J = 9.0, 1H), 3.17 (m, 1H), 3.02 (m, 1H), 2.89 (m, 1H), 2.79 (m, 1H) 2.20 (dd, J = 3.0 Hz, J = 11.0 Hz, 1H), 1.49 (d, J = 8.5 13 Hz, 1H), 1.37 (d, J = 8.5 Hz, 1H), 1.19 (s, 9H); C NMR (125.78 MHz, CDCl3): δ = 171.8, 141.3, 139.3, 135.5, 135.3, 133.3, 126.4, 118.4, 117.4, 65.9, 54.7, 51.9, 51.1, 51.0, 50.8, 48.2, 47.7, 46.6, 54.4, 45.2, 45.0, 28.5, -1 28.5, 28.5; IR (neat): νmax (cm ) = 3383, 2959, 2868, 1668, 1504, 852, 737; HR-MS (ESI, 4500 V): m/z calcd + for C20H26N3O3 ([M+H] ): 356.1969, found 356.1959;
Compound 14: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and isopropyl isocyanide (26 mg, 0.035 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 51 mg (0.15 mmol, 60%) of a bright sticky orange oil. 1 = -828.8° (c = 0.56, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 7.65 (dd, J = 3.5 Hz, J = 9.75 Hz, 1H), 7.29 (m, 1H), 6.91 (d, J = 9.0 Hz, 1H), 6.78 (m, 2H), 6.14 (m, 1H), 5.79 (m, 1H), 4.10 (m, 1H), 3.86 (m, 1H), 3.48 (m, 1H), 3.19 (m, 1H), 3.03 (m, 1H), 2.86 (m, 1H), 2.79 (m, 1H), 2.20 (dd, J = 4.0 Hz, J = 11.0 Hz, 1H), 1.48 (m, 1H), 1.36 (m, 1H), 1.07 (dd, J = 2.0 Hz, 13 J = 6.5 Hz, 3H), 0.89 (dd, J = 2.0 Hz, J = 6.5 Hz, 3H); C NMR (125.78 MHz, CDCl3): δ = 171.6, 141.2, 139.4, 135.6, 135.2, 133.3, 126.4, 118.5, 117.7, 65.4, 54.6, 51.8, 50.6, 47.7, 46.6, 45.1, 41.3, 22.4, 22.4; IR (neat): -1 νmax (cm ) = 3369, 2968, 2868, 1657, 1508, 741, 633, 493; HR-MS (ESI, 4500 V): m/z calcd for C19H24N3O3 ([M+H]+): 342.1812, found 342.1799.
Compound 15: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mmol, 0.25 mmol, 1.0 equiv.) and benzyl isocyanide (44 mg, 0.046 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 49 mg (01.26 mmol, 51%) of a sticky orange oil. 1 = -942.3° (c = 0.52, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 7.61 (dd, J = 8.0 Hz, J = 1.5 Hz, 1H), 7.30 (m, 1H), 7.16 (m, 4H), 7.14 (m, 2H), 6.94 (m, 1H), 6.17 (m, 1H), 5.84 (m, 1H), 4.35 (d, J = 6.0 Hz, 1H), 4.28 (d, 5.5 Hz, 1H), 4.17 (d, J = 3.0 Hz, 1H), 3.52 (dd, J = 10.8 J = 8.5, 1H), 3.22 (m, 1H), 3.07 (m, 1H), 2.90 (m, 1H), 2.80 (m, 1H), 2.22 (dd, J = 11.0 Hz J 13 = 3.0, 1H), 1.51 (d, J = 8.5 Hz, 1H), 1.38 (d, J = 8.5 Hz, 1H); C NMR (125.78 MHz, CDCl3): δ = 172.7, 141.1, 141.1, 137.9, 135.8, 135.3, 133.3, 128.6, 127.2, 126.4, 118.8, 117.7, 65.6, 54.6, 51.9, 50.9, 47.6, 46.6, 45.2, -1 43.2; IR (neat): νmax (cm ) = 3377, 2962, 2866, 1664, 1506, 1259, 1014, 796; HR-MS (ESI, 4500 V): m/z calcd + for C23H24N3O3 ([M+H] ): 390.1812, found 390.1794.
136 Stereoselective Synthesis of N-aryl Amides
Compound 16: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 4-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol 1.5 equiv.) giving a yield of 45 mg (0.13 mmol, 51%) of a yellow solid. 1 = -42.7° (c = 0.52, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 8.03 (d, J = 9.0 Hz, 2H), 6.34 (d, J = 9.0 Hz, 2H), 6.08 (t, J = 4.0 Hz, 2H), 5.36 (bs, 1H), 3.56 (t, J = 2.0 Hz, 1H), 3.53 (s, 1H), 3.15 (s, 1H), 3.03 (m, 3H), 2.95 (s, 1H), 1.58 (d, J = 8.5 Hz, 1H), 1.44 (d, J = 13 8.5 Hz, 1H) 1.19 (s, 9H); C NMR (100.61 MHz, CDCl3): δ = 171.6, 150.5, 138.3, 136.0, 135.9, 126.1, 126.1, 111.6, 111.6, 67.4, 52.7, 52.4, 52.0, 51.3, 47.4, 46.7, 44.1, 28.6, 28.6, 28.6; IR (neat): -1 νmax (cm ) = 3298, 2966, 2858, 1595, 1302, 1109, 823, 656; HR-MS (ESI, 4500 V): m/z calcd for C20H26N3O3 ([M+H]+): 356.1969, found 356.1956.
Compound 17: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 4-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and benzyl isocyanide (44 mg, 0.046 ml, 0.375 mmol and 1.5 equiv.) giving a yield of 56 mg (0.14 mmol, 58%) of a yellow solid. 1 = -49.0° (c = 0.49, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 8.00 (d, J = 9.5 Hz, 2H), 7.18 (m, 3H), 7.02 (m, 2H), 6.34 (d, J = 9.0 Hz, 2H), 6.10 (m, 2H), 5.92 (s, 1H), 4.33 (d, J = 6.0 Hz, 2H), 3.74 (d, J = 3.0 Hz, 1H), 3.58 (m, 1H), 3.19 (s, 1H), 3.06 (m, 3H), 2.96 13 (s, 1H), 1.59 (d, J = 8.5 Hz, 1H), 1.45 (d, J = 8.5 Hz, 1H); C NMR (125.78 MHz, CDCl3): δ =172.4, 150.4, 138.4, 137.7, 136.0, 135.9, 128.7, 128.7, 127.7, 127.5, 127.5, 126.1, -1 126.1, 111.7, 111.7 66.8, 52.8, 52.5, 52.0, 47.4, 45.8, 44.1, 43.3; IR (neat): νmax (cm ) = 3312, 2962, 2869, + 1651, 1597, 1300, 1109, 825, 690; HR-MS (ESI, 4500 V): m/z calcd for C23H24N3O3 ([M+H] ): 390.1812, found 390.1794.
Compound 18: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 3-nitropyridin-2-ol (35 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 36 mg (0.10 mmol, 40%) of a dark yellow solid. = -474.6° (c = 0.51, CHCl ); 1H NMR (500.23 MHz, CDCl ): δ = 8.23 (dd, J = 2.0 Hz, J 3 3 = 4.5 Hz, 1H), 8.03 (dd, J = 1.5 Hz, J = 8.0 1H), 6.69 (dd, J = 4.5 Hz, J = 8.5 1H), 6.33 (bs, 1H), 5.94 (m, 1H), 5.56 (m, 1H), 5.03 (s, 1H), 3.49 (dd, J = 8.5, J = 12 Hz, 1H), 3.13 (m, 1H), 2.76 (m, 1H), 2.16 (dd, J = 2.0, J = 12.0 Hz, 1H), 1.39 (d, J = 8.0 Hz, 1H), 1.31 (d, J = 8.5 Hz, 1H), 1.22 13 (s, 9H); C NMR (125.78 MHz, CDCl3): δ = 171.5, 151.7, 150.1, 150.1, 135.8, 134.7, 133.6, 113.1, 63.6, 53.8, -1 51.3, 51.0, 48.5, 47.8, 46.8, 44.8, 28.6; IR (neat): νmax (cm ) = 3398, 2959, 2872, 1668, 1448, 716, 557; HR-MS + (ESI, 4500 V): m/z calcd for C19H24N4O3 ([M+H] ): 357.1921, found 357.1905.
Compound 19: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 50 mg (0.15 mmol, 61%) of a light yellow solid. = -80.8° (c = 0.495, CHCl ); 1H NMR (500.23 MHz, CDCl ): δ = 8.23 (d, J = 4.5 Hz, 2H), 3 3 7.99 (bs, 1H), 6.48 (t, J = 5.0 Hz, 1H), 5.98 (t, J = 7.0 Hz, 2H), 4.39 (d, J = 2.5 Hz, 1H), 3.60 (dd, J = 3.5 Hz, J = 11.5 Hz, 1H), 3.50 (dd, J = 8.5 Hz, J = 11.8 Hz, 1H), 3.37 (m, 1H), 3.10 (m, 1H), 2.92 (m, 1H), 2.88 (m, 1H), 1.46 (d, J = 8.5 Hz, 1H), 1.38 (m, 10H); 13C NMR (125.78 MHz,
CDCl3): δ = 202.6, 159.8, 157.7, 157.7, 135.6, 135.4, 110.6, 72.9, 55.0, 53.4, 51.7, 50.1, 47.5), 46.9, 44.2, 27.6, -1 27.6, 27.6; IR (neat): νmax (cm ) = 3303, 2964, 2856, 1680, 1377, 797, 719, 627; HR-MS (ESI, 4500 V): m/z + calcd for C18H25N4S([M+H] ): 329.1794, found 329.1783.
137 Chapter 6
Compound 20: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and benzyl isocyanide (44 mg, 0.046 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 57 mg (0.16 mmol, 63%) of a off-white solid. = -60.0° (c = 0.6, CHCl ); 1H NMR (500.23 MHz, CDCl ): δ = 8.38 (bs, 1H), 8.17 (d, J 3 3 = 5.0 Hz, 2H), 7.21 (m, 3H), 7.10 (t, J = 5.5 Hz, 2H) 6.45 (t, J = 5.0 Hz, 1H), 5.98 (s, 1H), 4.82 (m, 1H), 4.67 (m, 1H), 4.61 (s, 1H), 3.64 (m, 1H), 3.46 (m, 2H), 3.13 (s, 1H), 13 2.96 (m, 1H), 2.89 (s, 1H), 1.49 (d, J = 8.5 Hz, 1H), 1.41 (d, J = 8.0 Hz, 1H); C NMR (125.78 MHz, CDCl3): δ = 203.7, 158.8, 156.7, 156.7, 135.2, 134.5, 134.4, 134.4, 127.7, 127.7, 127.7, 126.7, 126.7, 109.7, 70.2, 52.5, -1 50.6, 49.0, 48.4, 46.4, 45.9, 43.2; IR (neat): νmax (cm ) = 3175, 2966, 2870, 1585, 1493, 795, 696; HR-MS (ESI, + 4500 V): m/z calcd for C21H23N4S([M+H] ): 363.1638, found 363.1632.
Compound 21: General procedure 2 was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and isopropyl isocyanide (26 mg, 0.035 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 54 mg (0.17 mmol, 68%) of light orange solid. 1 = -173.7° (c = 0.50, CHCl3); H NMR (500.23 MHz, CDCl3): δ = 8.51 (dd, J = 5.0 Hz, J = 10.0 Hz, 1 H), 8.22 (d, J = 5.0 Hz, 2H), 7.92 (bs, 1H), 6.47 (t, J = 4.5 Hz, 1H), 5.98 (m, 2H), 4.53 (m, 1H), 4.49 (d, J = 2.5 Hz, 1H), 3.64 (dd, J = 3.0 Hz, J = 12 Hz, 1H), 3.48 (dd, J = 3.0 Hz, J = 12 Hz, 1H), 3.40 (m, 1H), 3.11 (m, 1H), 2.94 (m, 1H), 2.89 (m, 1H), 1.47 (d, J = 8.5 13 Hz, 1H), 1.39 (d, J = 8.5 H, 1H), 1.07 (m, 6H); C NMR (125.78 MHz, CDCl3): δ = 202.5, 159.9, 157.7, 157.7, -1 135.4, 135.4, 110.6, 71.4, 53.3, 51.7, 50.0, 47.4, 46.9, 46.7, 44.2, 21.3, 21.1; IR (neat): νmax (cm ) = 3329, + 2966, 2868, 1666, 1580, 1445, 804, 721; HR-MS (ESI, 4500 V): m/z calcd for C17H23N4S([M+H] ): 315.1638, found 315.1634.
Compound 22: General procedure A was followed using imine (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and SBL086 (44 mg, 0.375 mmol, 1.5 equiv.) giving a yield of 34 mg (0.09 mmol, 37%) of an orange solid. = -106.1° (c = 0.51, CHCl ); 1H NMR (400.16 MHz, CDCl ): δ = 8.59 (bs, 1H), 8.41 (m, 3 3 2H), 8.16 (d, J = 4.4 Hz, 2H), 7.47 (d, J = 7.6 Hz, 1H), 7.11 (dd, J = 4.8 Hz, J = 7.8 Hz, 1H), 6.46 (t, J = 4.8 Hz, 1H), 5.98 (m, 2H), 4.89 (dd, J = 6.0 Hz, J = 15.2 Hz, 1H), 4.70 (dd, J = 6.0 Hz, J = 15.2 Hz, 1H), 4.58 (d, J = 2.4 Hz, 1H), 3.63 (m, 1H), 3.42 (m, 2H), 3.11 (m, 1H), 2.94 (m, 13 2H), 1.48 (d, J = 8.4 Hz, 1H), 1.40 (d, J = 8.4 Hz, 1H); C NMR (100.62 MHz, CDCl3): δ = 204.7, 158.9, 156.7, -1 148.1, 134.5, 134.3, 131.1, 122.5, 109.8, 70.3, 52.6, 50.7, 49.1, 46.4, 46.0, 43.2, 41.2; IR (neat): νmax (cm ) = + 3173, 2976, 2204, 1499, 922, 725, 642; HR-MS (ESI, 4500 V): m/z calcd for C20H22N5S ([M+H] ): 364.1590, found 364.1575.
Compound 23: General procedure A was followed using (3S,7R)-10 (67 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and (S)-(-)- methylbenzyl isocyanide (49 mg, 0.051 ml, 0.375 mmol, 1.5 equiv giving a yield of 53 mg (0.13 mmol, 53%) of an orange solid. 1 = -756.4° (c = 0.55, CHCl3); H NMR (400.13 MHz, CDCl3): δ = 7.63 (d, J = 8.0 Hz, 1H), 7.26 (m, 2H), 7.02 (m, 3H), 6.88 (m, 3H), 6.79 (t, J = 7.5 Hz, 1H), 6.15 (m, 1H), 5.84 (m, 1H), 4.92 (m, 1H), 4.09 (s, 1H), 3.56 (dd, J = 8.5 Hz, J = 10.5 Hz, 1H), 3.20 (s, 1H), 3.09 (m, 1H), 2.95 (m, 1H), 2.81 (s, 1H), 2.26 (dd, J = 3.0, J = 10.8, 1H), 1.51 (m, 1H), 1.39 (m, 13 1H), 1.37 (d, J = 7.0 Hz); C NMR (100.61 MHz, CDCl3): δ = 171.7, 143.4, 141.1, 139.7, 135.8, 135.3, 133.3, 128.3 , 128.3, 126.9, 126.3, 125.7, 125.7, 118.9, 117.9, 65.5, 54.5, 52.0, 50.7, 48.8, 47.6, 46.6, 45.2, 22.5; IR -1 (neat): νmax (cm ) = 3379, 3312, 2966, 1655, 1502, 739, 698; HR-MS (ESI, 4500 V): m/z calcd for C20H26N3O3 ([M+H]+): 404.1969, found 404.1949.
138 Stereoselective Synthesis of N-aryl Amides
Compound 24: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct- 2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 61 mg (0.18 mmol, 72%) of a bright sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -734° (c = 0.52, CHCl3); H NMR
(500.23 MHz, CDCl3): δ = 7.67 (m, 2H), 7.33 (m, 1H), 7.00 (d, J = 8.0 Hz, 1H), 6.86 (m, 1H), 6.56 (bs, 1H), 3.88 (d, J = 6.0 Hz, 1H), 3.71 (m, 1H), 2.73 (m, 1H), 2.62 (m, 1H), 2.49 (dd, J = 3.0 Hz, J = 10.3 Hz, 1H), 1.88 (m, 2H), 1.70 (m, 2H), 1.66 (m, 1H), 1.21 (m, 1H), 1.18 (s, 9H); 13 C NMR (125.78 MHz, CDCl3): δ = 171.7, 142.1, 140.4, 135.5, 126.1, 119.5, 117.8, 69.6), 58.4, 50.8), 49.8), 1 43.2, 32.2, 31.7, 28.5, 28.5, 28.5, 24.9; H NMR (500.23 MHz, CDCl3): δ = 7.67 (m, 2H), 7.33 (m, 1H), 7.00 (d, J = 8.0 Hz, 1H), 6.86 (m, 1H), 6.48 (bs, 1H), 4.36 (d, J = 7.6 Hz, 1H), 4.00 (m, 1H), 2.73 (m, 1H), 2.62 (m, 1H), 2.49 (dd, J = 3.0 Hz, J = 10.3 Hz, 1H), 1.88 (m, 2H), 1.70 (m, 2H), 1.66 (m, 1H), 1.21 (m, 1H), 1.18 (s, 9H); 13C NMR (125.78
MHz, CDCl3): δ = 171.7, 142.1, 140.4, 135.2, 125.8, 119.8, 117.7, 70.1, 59.2), 50.8, 46.9, 42.8, 34.6, 29.7, 28.5, -1 28.5, 28.5, 25.9; IR (neat): νmax (cm ) = 3689, 3352, 2957, 1655, 1506, 1265, 746, 710; HR-MS (ESI, 4500 V): + m/z calcd for C18H26N3O3 ([M+H] ): 332.1969, found 332.1958;
Compound 25: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct- 2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and isopropyl isocyanide (26 mg, 0.035 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 56 mg (0.18 mmol, 71%) of a sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -722.9° (c = 0.55, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 7.62 (dd, J = 1.5 Hz, J = 8.0 Hz, 1H), 7.33 (m, 1H), 7.10 (m, 4H), 7.02 (d, J = 8.5 Hz, 1H), 6.96 (t, J = 3.8 Hz, 2H), 6.87 (t, J = 7.5 Hz, 1H), 4.30 (d, J = 6.0 Hz, 2H), 4.05 (d, J = 6.5 Hz, 1H), 3.72 (dd, J = 7.5 Hz, J = 10.3 Hz, 1H), 2.77 (m, 1H), 2.63 (m, 1H), 2.51 (dd, J = 3.5 Hz, J = 10.0 Hz, 1H), 1.91 (m, 2H), 1.72 (m, 2H), 1.61 (m, 1H), 1.21 (m, 1H); 13C NMR (100.62
MHz, CDCl3): δ = 170.5, 141.0, 139.7, 132.5, 125.0, 119.1, 117.1, 68.2, 57.5, 48.7, 42.2, 40.0, 31.2, 30.8, 24.0, 1 21.4, 21.3; H NMR (400.13 MHz, CDCl3): δ = 7.66 (dd, J = 1.5 Hz, J = 8.0 Hz, 1H), 7.33 (m, 1H), 7.00 (d, J = 8.5 Hz, 1H), 6.86 (t, J = 7.5 Hz, 1H), 6.50 (bs, 1H), 4.45 (d, J = 7.6 Hz, 2H), 3.88 (m, 1H), 3.70 (dd, J = 7.0 Hz, J = 10.0 Hz, 1H), 3.49 (m, 1H), 3.72 (m, 1H), 2.99 (m, 1H), 2.81 (m, 2H), 2.50 (dd, J = 3.5 Hz, J = 10.0 Hz, 1H), 1.88 (m, 1H), 13 1.62 (m, 1H), 1.21 (m, 1H), 1.12 (m, 3H), 0.79 (m, 3H); C NMR (100.62 MHz, CDCl3): δ = 170.5, 141.0, 139.7, -1 132.2, 124.7, 118.7, 117.0, 65.6, 58.3, 45.8, 42.0, 40.0, 33.6, 29.0, 25.0, 21.7, 21.3; IR (neat): νmax (cm ) = 3371, + 2962, 2860, 1651, 1506, 1277, 773, 741; HR-MS (ESI, 4500 V): m/z calcd for C17H24N3O3 ([M+H] ): 318.1812, found 318.1799.
Compound 26: General procedure 2 was followed using 3-azabicyclo[3.3.0] oct-2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and benzylisocyanide (44 mg, 0.046 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 69 mg (0.19 mmol, 76%) of a sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -650.9° (c = 0.55, CHCl3); H NMR
(500.23 MHz, CDCl3): δ = 7.62 (dd, J = 1.5 Hz, J = 8.0 Hz, 1H), 7.33 (m, 1H), 7.10 (m, 4H), 7.02 (d, J = 8.5 Hz, 1H), 6.96 (t, J = 3.8 Hz, 2H), 6.87 (t, J = 7.5 Hz, 1H), 4.30 (d, J = 6.0 Hz, 2H), 4.05 (d, J = 6.5 Hz, 1H), 3.72 (dd, J = 7.5 Hz, J = 10.3 Hz, 1H), 2.77 (m, 1H), 2.63 (m, 1H), 2.51 (dd, J = 3.5 Hz, J = 10.0 Hz, 1H), 1.91 (m, 2H), 1.72 (m, 2H), 1.61 (m, 1H), 1.21 (m, 1H); 13C NMR (125.78
MHz, CDCl3): δ = 171.5, 140.9, 139.7, 137.0, 132.5, 127.5, 127.5, 126.3, 126.3, 126.2, 125.1, 118.9, 117.1), 1 68.2, 57.4, 48.9, 42.3, 41.9, 30.9, 30.6, 23.8; H NMR (500.23 MHz, CDCl3): δ = 7.62 (dd, J = 1.5 Hz, J = 8.0 Hz, 1H), 7.33 (m, 1H), 7.10 (m, 4H), 7.02 (d, J = 8.5 Hz, 1H), 6.96 (t, J = 3.8 Hz, 2H), 6.87 (t, J = 7.5 Hz, 1H), 4.55 (d, J = 7.6 Hz, 2H), 4.05 (d, J = 6.5 Hz, 1H), 3.72 (dd, J = 7.5 Hz, J = 10.3 Hz, 1H), 3.17 (m, 1H), 3.01 (m, 1H), 2.51 (dd, J = 13 3.5 Hz, J = 10.0 Hz, 1H), 1.91 (m, 2H), 1.72 (m, 2H), 1.61 (m, 1H), 1.21 (m, 1H); C NMR (125.78 MHz, CDCl3): δ = 169.7, 141.1, 139.7, 136.8, 132.3, 127.4, 127.4, 126.2, 126.2, 126.1, 124.8, 119.2, 116.9, 65.7, 58.3, 45.9, 42.3, -1 33.6, 28.9, 26.7, 24.9; IR (neat): νmax (cm ) = 3373, 2945, 2868, 1655, 1508, 1277, 740, 598; HR-MS (ESI, 4500 + V): m/z calcd for C21H24N3O3 ([M+H] ): 366.1812, found 366.1801.
139 Chapter 6
Compound 27: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct-2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 4-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and tert- butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 80 mg (0.23 mmol, 90%) of a yellow solid. [35] 1 (Note: Minor diastereomer is given in italic). = -67.4° (c = 0.48, CHCl3); H NMR (400.13
MHz, CDCl3): δ = 8.04 (m, 2H), 6.46 (m, 2H), 5.61 (bs, 1H), 3.81 (m, 2H), 3.12 (dd, J = 6.8 Hz, J = 10.4 Hz, 1H), 2.83 (m, 1H), 2.73 (m, 1H), 2.01 (m, 1H), 1.84 (m, 1H), 1.73 (m, 1H), 13 1.63 (m, 1H), 1.53 (m, 2H), 1.21 (s, 9H); C NMR (100.62 MHz, CDCl3): δ = 170.3, 150.6, 137.6, 124.8, 124.8, 110.7, 110.7, 69.9, 54.7, 50.4, 49.2, 40.7, 31.6, 30.5, 27.6, 27.6, 23.8; 1H NMR (400.13
MHz, CDCl3): δ = 8.04 (m, 2H), 6.87 (m, 2H), 5.61 (bs, 1H), 4.12 (m, 2H), 3.34 (dd, J = 6.8 Hz, J = 10.4 Hz, 1H), 3.66 (m, 1H), 3.35 (m, 1H), 2.01 (m, 1H), 1.84 (m, 1H), 1.73 (m, 1H), 1.63 (m, 1H), 1.53 (m, 2H), 1.21 (s, 9H); 13C NMR
(100.62 MHz, CDCl3): δ = 168.3, 150.6, 137.6, 124.6, 124.6, 111.4, 111.4, 66.7, 55.4, 50.4, 46.4, 41.5, 30.0, 28.7, -1 26.9, 26.9, 25.2; IR (neat): νmax (cm ) = 3298, 2957, 2864, 1597, 1296, 1109, 825, 752; HR-MS (ESI, 4500 V): + m/z calcd for C18H26N3O3 ([M+H] ): 332.1969, found 332.1957.
Compound 28: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct-2- ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 4-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and benzylisocyanide (44 mg, 0.046 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 69 mg (0.19 mmol, 76%) of a sticky yellow oil. [35] 1 (Note: Minor diastereomer is given in italic). = -90.9° (c = 0.55, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 7.97 (d, J = 2.0 Hz, 2H), 7.19 (m, 3H), 7.06 (t, J = 5.6 Hz, 2H), 6.42 (m, 2H), 6.25 (bs, 1H), 4.34 (m, 2H), 3.99 (d, J = 2.4 Hz, 1H), 3.80 (dd, J = 8.4 Hz, J = 10.0 Hz, 1H), 3.11 (dd, J = 6.8 Hz, J = 10.0 Hz, 1H), 2.80 (m, 1H), 2.04 (m, 1H), 1.83 (m, 13 1H), 1.75 (m, 1H), 1.64 (m, 1H), 1.55 (m, 3H); C NMR (100.62 MHz, CDCl3): δ = 172.0, 151.5, 138.6, 137.9, 128.7, 128.7, 127.9, 127.9, 127.7, 126.2, 126.2, 112.1, 112.1, 70.2, 55.6, 50.3, 43.3, 38.6, 32.4, 30.8, 24.8; 1H
NMR (400.13 MHz, CDCl3): δ = 7.97 (d, J = 2.0 Hz, 2H), 7.19 (m, 3H), 7.06 (t, J = 5.6 Hz, 2H), 6.79 (m, 2H), 6.25 (bs, 1H), 4.34 (m, 2H), 3.99 (d, J = 2.4 Hz, 1H), 3.64 (dd, J = 8.4 Hz, J = 10.0 Hz, 1H), 3.32 (dd, J = 6.8 Hz, J = 10.0 Hz, 1H), 3.46 (m, 1H), 2.04 (m, 1H), 1.83 (m, 1H), 1.75 (m, 1H), 1.64 (m, 1H), 1.55 (m, 3H); 13C NMR (100.62 MHz,
CDCl3): δ = 170.4, 152.0, 138.6, 137.7, 128.7, 127.7, 127.7, 127.5, 126.0, 126.0, 112.4, 112.4, 67.3, 56.3, 47.4), -1 43.4, 42.7, 30.8, 26.1, 24.9; IR (neat): νmax (cm ) = 3283, 2949, 2866, 1653, 1597, 1294, 1109, 750; HR-MS + (ESI, 4500 V): m/z calcd for C21H24N3O3 ([M+H] ): 366.1812, found 366.1804.
Compound 29: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct- 2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 3-nitropyridin-2-ol (35 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 61 mg (0.18 mmol, 73%) of a sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -652.4° (c = 0.52, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 8.25 (dd, J = 2.0 Hz, J = 4.6, 1H), 8.04 (dd, J = 1.6 Hz, J = 8.0 Hz, 1H), 6.24 (bs, 1H), 4.86 (d, J = 4.4 Hz, 1H), 3.70 (dd, J = 7.6 Hz, J = 11.2 Hz, 1H), 2.82 (m, 1H), 2.68 (m, 1H), 2.54 (dd, J = 2.8 Hz, J = 11.2 Hz, 1H), 1.94 (m, 1H), 1.72 (m, 2H), 1.57 (m, 2H), 13 1.23 (s, 9H), 1.14 (m, 1H); C NMR (100.62 MHz, CDCl3): δ = 170.5, 150.9, 149.8, 133.7, 131.5, 112.5, 67.0), 1 56.1, 49.3, 46.7, 42.3, 31.7, 31.4, 31.1, 24.1, 24.1, 24.1; H NMR (400.13 MHz, CDCl3): δ = 8.25 (dd, J = 2.0 Hz, J = 4.6, 1H), 8.04 (dd, J = 1.6 Hz, J = 8.0 Hz, 1H), 6.07 (bs, 1H), 5.04 (d, J = 7.6 Hz, 1H), 3.70 (dd, J = 7.6 Hz, J = 11.2 Hz, 1H), 3.24 (m, 1H), 2.95 (m, 1H), 2.54 (dd, J = 2.8 Hz, J = 11.2 Hz, 1H), 1.94 (m, 1H), 1.72 (m, 2H), 1.57 (m, 2H), 13 1.23 (s, 9H), 1.14 (m, 1H); C NMR (100.62 MHz, CDCl3): δ = 169.0, 150.9, 149.8, 133.4, 131.5, 112.7, 64.6, 56.2, -1 49.3, 45.1, 42.6, 31.4, 28.7, 25.9, 25.9, 25.9, 24.32; IR (neat): νmax (cm ) = 3396, 2953, 2868, 1664, 1456, 763, + 540; HR-MS (ESI, 4500 V): m/z calcd for C17H25N4O3 ([M+H] ): 333.1921, found 333.1907.
140 Stereoselective Synthesis of N-aryl Amides
Compound 30: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct-2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and tert-butyl isocyanide (31 mg, 0.042 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 61 mg (0.20 mmol, 80%) of a off-white solid. [35] 1 (Note: Minor diastereomer is given in italic). = -129.4° (c = 0.51, CHCl3); H NMR (400.13 MHz, CDCl3): δ = 8.28 (d, J = 4.8 Hz, 2H), 8.07 (bs, 1H), 6.53 (t, J = 4.8 Hz, 1H), 4.67 (d, J = 2.8 Hz, 1H), 3.97 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 3.58 (dd, J = 6.4 Hz, J = 11.4 Hz, 1H) 3.03 (m, 1H), 2.84 (m, 1H) 2.05 (m, 1H), 1.85 (m, 1H), 1.73 (m, 1H), 1.60 (m, 2H), 1.51 (m, 1H), 1.24 (s, 9H); 13C
NMR (100.62 MHz, CDCl3): δ = 202.3, 160.9, 157.7, 157.7, 111.1, 46.5, 54.1, 51.7, 41.6, 32.8, 31.5, 27.6, 27.6, 1 27.6, 25.1; H NMR (400.13 MHz, CDCl3): δ = 8.28 (d, J = 4.8 Hz, 2H), 7.50 (bs, 1H), 7.08 (t, J = 4.8 Hz, 1H), 4.97 (d, J = 2.8 Hz, 1H), 4.28 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 3.58 (dd, J = 6.4 Hz, J = 11.4 Hz, 1H) 3.30 (m, 1H), 3.16 (m, 1H) 2.05 (m, 1H), 1.85 (m, 1H), 1.73 (m, 1H), 1.60 (m, 2H), 1.51 (m, 1H), 1.24 (s, 9H); 13C NMR (100.62 MHz,
CDCl3): δ = 199.5, 160.9, 157.7, 157.7, 111.7, 75.3, 54.9, 49.5, 41.7, 29.7, 27.2, 27.6, 27.6, 27.6, 25.8; IR (neat): -1 νmax (cm ) = 3312, 2961, 2872, 1580, 1460, 1076, 798, 635; HR-MS (ESI, 4500 V): m/z calcd for C16H25N4S ([M+H]+): 305.1794, found 305.1780.
Compound 31: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct-2- ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and benzyl isocyanide (44 mg, 0.046 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 49 mg (0.15 mmol, 58%) of a white solid. (Note: Minor diastereomer is given in italic). = -66.7° (c = 0.48, CHCl );[35] 1H NMR 3 (400.13 MHz, CDCl3): δ = 8.46 (bs, 1H), 8.21 (d, J = 4.8 Hz, 2H), 7.18 (m, 3H), 7.11 (m, 2H), 6.50 (t, J = 4.4 Hz, 1H), 4.83 (m, 2H), 4.70 (dd, J = 5.2 Hz, J = 15.2, 1H), 3.79 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 3.58 (dd, J = 6.4 Hz, J = 11.4 Hz, 1H) 3.04 (m, 1H), 2.81 (m, 1H) 2.05 (m, 13 1H), 1.80 (m, 1H), 1.59 (m, 2H), 1.50 (m, 2H); C NMR (100.62 MHz, CDCl3): δ = 204.4, 160.9, 157.8, 157.8, 1 136.4, 128.7, 128.0, 127.8, 111.1, 74.8, 54.1, 51.8, 49.3, 41.7, 32.7, 31.4, 25.1; H NMR (400.13 MHz, CDCl3): δ = 7.87 (bs, 1H), 8.27 (d, J = 4.8 Hz, 2H), 7.18 (m, 3H), 7.11 (m, 2H), 6.50 (t, J = 4.4 Hz, 1H), 5.15 (d, 1H), 4.83 (m, 1H), 4.98 (dd, J = 5.2 Hz, J = 15.2, 1H), 4.21 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 4.04 (dd, J = 6.4 Hz, J = 11.4 Hz, 1H) 13 3.16 (m, 1H), 2.67 (m, 1H) 2.05 (m, 1H), 1.80 (m, 1H), 1.59 (m, 2H), 1.50 (m, 2H); C NMR (100.62 MHz, CDCl3): δ = 202.0, 161.5, 157.7, 157.7, 136.4, 128.6, 127.8, 127.8, 111.7, 73.9, 54.8, 49.6, 48.8, 42.1, 29.5, 27.4, 25.7; IR -1 (neat): νmax (cm ) = 3196, 2947, 2862, 1491, 964, 744, 696; HR-MS (ESI, 4500 V): m/z calcd for C19H23N4S ([M+H]+): 339.1638, found 339.1626.
Compound 32: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct-2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-mercaptopyrimidine (28 mg, 0.25 mmol, 1.0 equiv.) and isopropyl isocyanide (26 mg, 0.035 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 62 mg (0.21 mmol, 85%) of an off-white sticky oil. (Note: Minor diastereomer is given in italic). = -102.9° (c = 0.51, CHCl );[35] 1H NMR (400.13 3 MHz, CDCl3): δ = 8.26 (d, J = 4.8 Hz, 2H), 7.95 (bs, 1H), 6.53 (t, J = 4.8 Hz, 1H), 4.69 (d, J = 2.8 Hz, 1H), 4.57 (m, 1H), 3.82 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 3.52 (dd, J = 6.0 Hz, J = 11.4 Hz, 1H) 2.98 (m, 1H), 2.79 (m, 1H) 2.00 (m, 1H), 1.79 (m, 1H), 1.57 (m, 2H), 1.50 (m, 2H), 1.05 (m, 6H); 13 C NMR (100.62 MHz, CDCl3): δ = 202.2, 160.9, 157.7, 157.7, 111.1, 75.0, 54.1, 51.6, 46.5, 41.7, 32.7, 31.4), 1 25.1, 20.9; H NMR (400.13 MHz, CDCl3): δ = 8.26 (d, J = 4.8 Hz, 2H), 7.39 (bs, 1H), 7.02 (t, J = 4.8 Hz, 1H), 4.69 (d, J = 2.8 Hz, 1H), 4.24 (m, 1H), 3.82 (dd, J = 8.4 Hz, J = 11.4 Hz, 1H), 3.52 (dd, J = 6.0 Hz, J = 11.4 Hz, 1H) 3.24 (m, 1H), 3.11 (m, 1H) 2.00 (m, 1H), 1.79 (m, 1H), 1.57 (m, 2H), 1.50 (m, 2H), 1.05 (m, 6H); 13C NMR (100.62 MHz,
CDCl3): δ = 199.6, 161.5, 157.9, 157.9, 111.7, 73.8, 54.9, 49.5, 46.2, 41.9, 29.6, 27.1, 25.7, 21.5, 21.1; IR (neat): -1 + νmax (cm ) ν = 3190, 2961, 2866, 1583, 1499, 795; HR-MS (ESI, 4500 V): m/z calcd for C15H23N4S ([M+H] ): 291.1638, found 291.1628.
141 Chapter 6
Compound 33: General procedure 2 was followed using 3-azabicyclo[3.3.0] oct-2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 2-nitrophenol (35 mg, 0.25 mmol, 1.0 equiv.) and (S)-(-)-a -methylbenzyl isocyanide (49 mg, 0.051 ml, 0.375 mmol, 1.5 equiv.) giving a yield of 80 mg (0.23 mmol, 90%) of a sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -650.0° (c = 0.72, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 7.61 (d, J = 1.6 Hz, 1H), 7.22 (m, 2H), 6.98 (m, 1H), 6.85 (m, 5H), 4.91 (m, 1H), 3.96 (d, J = 6.0 Hz, 1H) 3.73 (dd, J = 7.2 Hz, J = 10.0 Hz, 1H), 2.77 (m, 1H), 2.65 (m, 1H), 2.51 (dd, J = 3.6 Hz, J = 10.0 Hz, 1H), 1.88 (m, 2H), 1.66 (m, 3H), 1.34 (d, J = 7.2 Hz, 3H), 1.21 (m, 2H); 13 C NMR (100.62 MHz, CDCl3): δ = 170.6, 142.3, 140.8, 140.1, 132.5, 127.2, 125.8, 124.8, 124.6, 119.0, 117.4, 1 68.2, 57.4, 48.8, 47.4, 42.3, 31.0, 30.7, 23.9, 21.2; H NMR (400.13 MHz, CDCl3): δ = 7.61 (d, J = 1.6 Hz, 1H), 7.36 (m, 2H), 6.98 (m, 1H), 6.85 (m, 5H), 4.91 (m, 1H), 4.49 (d, J = 6.0 Hz, 1H) 3.73 (dd, J = 7.2 Hz, J = 10.0 Hz, 1H), 3.16 (m, 1H), 2.94 (m, 1H), 2.51 (dd, J = 3.6 Hz, J = 10.0 Hz, 1H), 1.88 (m, 2H), 1.66 (m, 3H), 1.34 (d, J = 7.2 13 Hz, 3H), 1.21 (m, 2H); C NMR (100.62 MHz, CDCl3): δ = 168.7, 142.3, 140.8, 140.1, 132.4, 127.7, 126.3, 125.1, -1 124.7, 119.3, 117.1, 65.6, 58.3, 47.4, 45.9, 42.0, 29.0, 28.7, 26.5, 20.1; IR (neat): νmax (cm ) = 3367, 2941, 2866, + 1657, 1508, 741, 698; HR-MS (ESI, 4500 V): m/z calcd for C22H26N3O3 ([M+H] ): 380.1969, found 380.1953.
Compound 35: General procedure 2 was followed using 3-azabicyclo[3.3.0] oct-2-ene 34 (55 mg, 0.5 mmol, 1.3 equiv.), 2-nitrophenol (70 mg, 0.5 mmol, 1.3 equiv.) and 1-Azido-2-isocyanoethane (36 mg, 0.375 mmol, 1.0 equiv.) giving a yield of 93 mg (0.27 mmol, 72%) of an orange solid. [35] 1 (Note: Minor diastereomer is given in italic). = -575.8° (c = 0.66, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 7.66 (d, J = 8.0 Hz, 1H), 7.33 (m, 1H), 7.02 (m, 1H), 6.87 (m, 1H), 4.04 (d, J = 5.6 Hz, 1H), 3.74 (m, 1H), 3.25 (m, 4H), 2.75 (m, 1H), 2.65 (m, 1H), 2.52 (dd, J = 3.6 Hz, J = 10.2 Hz, 1H), 1.89 (m, 2H), 1.65 (m, 3H), 1.26 (m, 2H); 13C NMR (100.62 MHz,
CDCl3): δ = 172.1, 140.9, 139.9, 132.6, 125.2, 119.1, 117.0, 68.2, 57.6, 49.6, 48.8, 42.5, 37.7, 30.9, 30.5, 24.7; 1 H NMR (400.13 MHz, CDCl3): δ = 7.66 (d, J = 8.0 Hz, 1H), 7.21 (m, 1H), 7.02 (m, 1H), 6.87 (m, 1H), 4.52 (d, J = 5.6 Hz, 1H), 3.74 (m, 1H), 3.25 (m, 4H), 3.00 (m, 1H), 2.65 (m, 1H), 2.52 (dd, J = 3.6 Hz, J = 10.2 Hz, 1H), 1.89 13 (m, 2H), 1.65 (m, 3H), 1.26 (m, 2H); C NMR (100.62 MHz, CDCl3): δ = 170.3, 141.0, 139.9, 132.3, 124.9, 119.4, -1 116.8, 65.6, 58.5), 49.7, 45.9, 42.0, 37.3, 28.9, 26.7, 25.0; IR (neat): νmax (cm ) = 3367, 2951, 2870, 2098, 1664, + 1510, 1277, 729, 541; HR-MS (ESI, 4500 V): m/z calcd for C16H21N6O3 ([M+H] ): 345.1670, found 345.1654.
Compound 36: General procedure 2 was followed using 3-azabicyclo[3.3.0]oct- 2-ene 34 (55 mg, 0.5 mmol, 2.0 equiv.), 4-nitro-3-bromophenol (54 mg, 0.25 mmol, 1.0 equiv.) and allyl isocyanide (27 mg, 0.4 mmol, 1.6 equiv.) giving a yield of 54 mg (0.14 mmol, 55%) of a sticky orange oil. [35] 1 (Note: Minor diastereomer is given in italic). = -122.1° (c = 0.48, CHCl3); H NMR
(400.13 MHz, CDCl3): δ = 7.91 (d, J = 9.2 Hz, 1H), 6.75 (d, J = 2.8 Hz, 1H), 6.40 (dd, J = 2.8 Hz, J = 9.2 Hz, 1H), 6.03 (bs, 1H), 5.69 (m, 1H), 5.03 (m, 2H), 3.94 (d, J = 2.4 Hz, 1H), 3.80 (m, 3H), 3.12 (dd, J = 6.8 Hz, J = 10.2 Hz, 1H), 2.75 (m, 2H), 2.04 (m, 13 1H), 1.75 (m, 2H), 1.60 (m, 3H); C NMR (100.62 MHz, CDCl3): δ = 170.5, 149.5, 137.7, 132.5), 127.4, 117.2, 1 116.9, 115.7, 110.3, 69.1, 54.6, 49.3, 40.7, 40.7, 31.4, 30.5, 23.8; H NMR (400.13 MHz, CDCl3): δ = 7.85 (d, J = 9.2 Hz, 1H), 7.09 (d, J = 2.8 Hz, 1H), 6.40 (dd, J = 2.8 Hz, J = 9.2 Hz, 1H), 7.30 (bs, 1H), 5.69 (m, 1H), 5.03 (m, 2H), 4.27 (d, J = 2.4 Hz, 1H), 3.66 (m, 3H), 3.12 (dd, J = 6.8 Hz, J = 10.2 Hz, 1H), 3.33 (m, 2H), 2.04 (m, 1H), 1.75 (m, 13 2H), 1.60 (m, 3H); C NMR (100.62 MHz, CDCl3): δ = 169.0, 149.9, 137.7, 132.6, 127.3, 117.6, 116.7, 116.0, 110.5, -1 66.0, 55.4, 46.4, 41.7, 33.6, 29.8, 27.1, 24.5; IR (neat): νmax (cm ) = 3286, 2951, 2866, 1589, 1302, 727, 646; + HR-MS (ESI, 4500 V): m/z calcd for C17H21BrN3O3 ([M+H] ): 394.0761, found 394.0754.
142 Stereoselective Synthesis of N-aryl Amides
Compound 37: General procedure 2 was followed using saturated bridged (3S,7R)-38 (68 mg, 0.5 mmol, 2.0 equiv.), 3-bromo-4-nitrophenol (54 mg, 0.25 mmol, 1.0 equiv.) and allyl isocyanide (27 mg, 0.040 ml, 0.4 mmol, 1.5 equiv.), giving a yield of 35 mg (0.08 mmol, 34%) of a yellow solid. 1 = -90.6° (c = 0.53, CHCl3); H NMR (400.13 MHz, CDCl3): δ = 7.95 (d, J = 9.2 Hz, 1H), 6.83 (d, J = 2.8 Hz, 1H), 6.49 (dd, J = 2.8 Hz, J = 9.2 Hz, 1H), 5.70 (m, 2H), 5.03 (m, 3H), 4.14 (s, 1H), 3.78 (m, 2H), 3.56 (d, J = 1.6 Hz, 1H), 3.47 (m, 1H), 2.74 (m, 1H), 13 2.41 (m, 1H), 2.24 (m, 1H), 1.51 (m, 2H), 1.14 (m, 4H); C NMR (100.62 MHz, CDCl3):
δ = 172.0, 149.6, 138.3, 133.6, 128.9, 118.3, 117.8, 116.6, 110.9, 63.5, 50.5, 49.3, 42.5, 42.0, 41.82), 41.8, -1 41.2, 22.8, 22.1; IR (neat): νmax (cm ) = 3283, 2953, 2869, 1651, 1589, 1304, 837, 744; HR-MS (ESI, 4500 V): + m/z calcd for C19H23BrN3O3 ([M+H] ): 420.0917, found 420.0897.
6.5 References & Notes
[1] A. Domling, Chem. Rev. 2006, 106, 17-89. [2] A. Domling and I. Ugi, Angew. Chem. Int. Ed. 2000, 39, 3169-3210. [3] B. Ganem, Acc. Chem. Res. 2009, 42, 463-472. [4] R. V. A. Orru and M. de Greef, Synthesis 2003, 1471-1499. [5] J. Zhu and H. Bienayme, Multicomponent Reactions, Wiley-VCH, Weinheim, 2005. [6] I. Akritopoulou-Zanze, Curr. Opin. Chem. Biol. 2008, 12, 324-331. [7] P. R. Andreana, C. C. Liu and S. L. Schreiber, Org. Lett. 2004, 6, 4231-4233. [8] S. E. Denmark and Y. Fan, J. Am. Chem. Soc. 2003, 125, 7825-7827. [9] U. Kusebauch, B. Beck, K. Messer, E. Herdtweck and A. Domling, Org. Lett. 2003, 5, 4021-4024. [10] S. C. Pan and B. List, Angew. Chem. Int. Ed. 2008, 47, 3622-3625. [11] D. J. Ramon and M. Yus, Angew. Chem. Int. Ed. 2005, 44, 1602-1634. [12] T. Yue, M. X. Wang, D. X. Wang, G. Masson and J. P. Zhu, J. Org. Chem. 2009, 74, 8396-8399. [13] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184. [14] A. Znabet, E. Ruijter, F. J. J. de Kanter, V. Kohler, M. Helliwell, N. J. Turner and R. V. A. Orru, Angew. Chem. Int. Ed. 2010, 49, 5289-5292. [15] A. Znabet, J. Zonneveld, E. Janssen, F. J. J. De Kanter, M. Helliwell, N. J. Turner, E. Ruijter and R. V. A. Orru, Chem. Commun. 2010, 46, 7706-7708. [16] A. Znabet, M. M. Polak, E. Janssen, F. J. J. de Kanter, N. J. Turner, R. V. A. Orru and E. Ruijter, Chem. Commun. 2010, 46, 7918-7920. [17] L. El Kaim, L. Grimaud and J. Oble, Angew. Chem. Int. Ed. 2005, 44, 7961-7964. [18] W. J. Evans and S. Smiles, J. Chem. Soc. 1935, 181-188. [19] F. Terrier, Chem. Rev. 1982, 82, 77-152. [20] L. El Kaim, L. Grimaud, J. Oble and S. Wagschal, Tetrahedron Lett. 2009, 50, 1741-1743. [21] S. P. Adams and R. R. Lobb, Annu. Rep. Med. Chem. 1999, 34, 179-188. [22] L. Chen, J. W. Tilley, R. W. Guthrie, F. Mennona, T. N. Huang, G. Kaplan, R. Trilles, D. Miklowski, N. Huby, V. Schwinge, B. Wolitzky and K. Rowan, Bioorg. Med. Chem. Lett. 2000, 10, 729-733. [23] L. Chen, J. W. Tilley, T. N. Huang, D. Miklowski, R. Trilles, R. W. Guthrie, K. Luk, A. Hanglow, K. Rowan, V. Schwinge and B. Wolitzky, Bioorg. Med. Chem. Lett. 2000, 10, 725-727. [24] L. El Kaim, M. Gizolme, L. Grimaud and J. Oble, Org. Lett. 2006, 8, 4019-4021. [25] Compound 10 can (theoretically) decompose via a retro-Diels-Alder reaction. We experimentally observed that 10 partially decomposed during the reaction, especially at elevated temperature. [26] A. H. Hoveyda and A. R. Zhugralin, Nature 2007, 450, 243-251.
143 Chapter 6
[27] K. C. Nicolaou, S. A. Snyder, T. Montagnon and G. Vassilikogiannakis, Angew. Chem. Int. Ed. 2002, 41, 1668-1698. [28] S. Dedola, S. A. Nepogodiev and R. A. Field, Org. Biomol. Chem. 2007, 5, 1006-1017. [29] J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and M. Lemaire, Chem. Rev. 2002, 102, 1359-1469. [30] T. Vlaar, E. Ruijter and R. V. A. Orru, Adv. Synth. Catal. 2011, 353, 809-841. [31] F. E. Hahn, V. Langenhahn and T. Pape, Chem. Commun. 2005, 5390-5392. [32] S. Michaelis and S. Blechert, Chem.-Eur. J. 2007, 13, 2358-2368. [33] J. L. Zhu, X. Y. Wu and S. J. Danishefsky, Tetrahedron Lett. 2009, 50, 577-579. [34] K. Hornberger, M. Cheung, M. A. Pobanz, K. A. Emmitte, K. W. Kuntz and J. G. Badiang in Vol. Smithkline Beecham Corp, 2007. [35] Optical rotation determined for diastereomeric mixture. Value is reported for illustrative purposes.
144 Chapter 7 Refl ections & Outlook
Reflections & Outlook
7.1 MAO-N & Ugi-type MCRs
The demand for fast, clean, atom and step efficient chemical processes is increasing by the day. Sustainable synthesis is not only a key feature for the chemical industry but also for the pharmaceutical world. The key objective of the research in this thesis is to combine two methodologies that have proven to be efficient and environmentally benign: (i) multicomponent reactions (MCRs) and (ii) biocatalysis. These methods offer significant advantages by reducing time, saving money, energy and raw materials, thus resulting in both economical and environmental benefits. The structurally complex products formed in MCRs often contain new stereocenters which are difficult to control.[1-6] For most MCRs, catalytic asymmetric methods to control the stereochemical outcome of the reaction are so far not available. The broad repertoire of stereospecific conversions by biocatalysts[7-8] presents a unique opportunity to address the stereoselectivity issue of certain MCRs. We have developed several methods (Scheme 1) making use of the enzymatic desymmetrization of meso-pyrrolidines by means of a monoamine oxidase N (MAO-N) from Aspergillus niger optimized by directed evolution[9] and its combination with highly diastereoselective Ugi-type three-component (Chapter 3) and Ugi-Smiles reactions (Chapter 6).[10-12] We were able to synthesize two distinct scaffolds; N-aryl proline amides 2 and 3,4-disubstituted prolyl peptides 3. The latter showed to be a very versatile scaffold providing access to organocatalyst of type 4. This catalyst possessed the ability to efficiently catalyze asymmetric 1,4-addition reactions of aldehydes to nitro-olefins.[10] Also, synthetic alkaloids 5 could be obtained by introducing a subsequent Pictet-Spengler cyclization (Chapter 4). It also constitutes the first example of MCR chemistry to synthesize 5-membered ring-fused diketopiperazines.[11] The efficiency and generality of the approach and the resulting molecular diversity and complexity makes our methodology highly interesting for medicinal chemistry.
147 Chapter 7
Scheme 1: Access to different scaffolds and follow-up chemistry using MAO-N and MCRs.
Most notably, a very short and efficient synthesis of important drug Telaprevir (Incivek™) 6, featuring a biocatalytic desymmetrization and two multicomponent reactions as the key steps was developed. The classical issue of lack of stereoselectivity in Ugi- and Passerini- type reactions was overcome. The atom economic and convergent nature of the synthetic strategy requires only very limited use of protective groups.[12] These results show the unrivaled complexity generation of MCRs is very well complemented by the (stereo)selectivity of enzymes. That is why the combination of biocatalysis and MCRs is a powerful tool leading to innovative synthetic strategies of high added value compounds, especially in a pharmaceutical context. Future directions in this field will focus on exploring diverse combinations of biocatalysis and complexity generating reactions and application of this methodology to make significant contributions to future developments in combinatorial chemistry and drug discovery.
7.2 Future Directions
7.2.1 Cyanopyrrolidine derivatives Protein activity and lifetime in organisms are extremely dependent on their processing by proteolytic enzymes known as proteases. These proteases are responsible for various tasks including post-translational modifications, regulation of peptide functions and digestion
148 Reflections & Outlook of proteins in smaller peptides. Most proteases execute highly specific processes and thus, cleave a limited number of substrates. Also, some substrates are only processed by a very small number of proteases. Over the past decades, researchers have linked proteases to certain diseases. For example, dipeptidyl peptidase IV has been linked to type 2 diabetes,[13-15] prolyl oligopeptidase has been linked to neurological disorders[16-18] and cathepsin K has been linked to osteoporosis.[19] Several compound classes are known to exert an inhibitory effect on these proteases, one of the most prominent being the cyanopyrrolidines 7 (Figure 1A).[20-22] These cyanopyrrolidines form a reversible covalent bond with the active site cysteine or serine moiety of the corresponding protease and thus, inhibit the active enzyme (Figure 1B).
Figure 1. A. General structure of cyanopyrrolidines. B. Inhibitory effect of cyanopyrrolidines is the results of reversible covalent bonding with the active site serine or cysteine of the protease.
We envision synthesizing these potential protease inhibitors by making use of the enzymatic desymmetrization of meso-pyrrolidines by means of MAO-N followed by a Strecker-type reaction[9] and in situ addition to acyl chlorides (Scheme 2).
Scheme 2: Approach towards asymmetric synthesis of cyanopyrrolidines 11.
To demonstrate the feasibility of this combination we synthesized cyanopyrrolidines 11a-e in high yield and selectivity.
149 Chapter 7
Figure 2: Cyanopyrrolidines compounds 11a-e obtained using optically pure 12.
The use of acid chlorides derived from amino acids as inputs would give access to amino acyl pyrrolidine 17 (Scheme 3) and these compounds show even higher inhibitory effects.[23] We envision starting with commercially available Boc-protected amino acid 13 which is converted under neutral conditions to the corresponding acyl chloride 14 using Ghosez’s reagent.[24] These acid chlorides would be added to the in situ generated amino nitrile 10 (Scheme 3) giving Boc-amino acyl pyrrolidine 15.
Scheme 3: Approach towards asymmetric synthesis of amino acyl pyrrolidines.
7.2.2 Amino acid derivatives It is evident that imine 1 has great potential in the combination with MCRs or complexity generating reactions. However, amino acid derivatives 16[9] opens up new possibilities for innovative novel synthetic strategies of pharmaceutically interesting compounds (Scheme 4).
150 Reflections & Outlook
Scheme 4: Access to new scaffolds via amino acid derivative 18. PG: protecting group.
Amino acid derivative 16 may be used as a starting point for a Passerini reaction.[25] The resulting compound (17) may be pharmaceutically very interesting as it resembles several of the Dolastatin 15 peptides.[26] Dolastatin 15 depsipeptides (e.g. Figure 3) are isolated from numerous marine cyanobacteria[27] and exhibit interesting anticancer activity.[26, 28] In addition, diastereoselectivity in this particular Passerini reaction is expected to be considerable as previously been reported when chiral carboxylic acids were involved.[29]
Figure 3: Dolastatin 15 isolated from extracts of the Indian Ocean sea hare Dolabella auricularia.
Furthermore, an intramolecular Ugi reaction (Scheme 4, 18) would also be attractive as this Ugi-5-centre-4-component reaction (U-5C-4CR)[30] would result in compounds (18) with potentially interesting biological properties like antiprion[31] (Creutzfeldt–Jakob disease, Gerstmann–Sträussler–Scheinker syndrome) and antidiabetic activity.[32] Because it is known
151 Chapter 7 that α-amino acid derivatives of general formula 19 are capable of significantly controlling the diastereoselectivity (Scheme 5),[30, 33] we expect this to also be the case when amino acid derivative 16 is employed in an U-5C-4CR and so, to be most likely highly diastereoselective.
Scheme 5: α-Amino acid derivative 19 employed in U-5C-4CRs resulting in compound 20.
As organocatalysis has become an efficient tool for the synthesis of chiral building blocks, amino acid 16 due to its versatility, might not only be used as input for a MCR but may well serve as catalyst mimicking the well-established catalyst; proline. The first example of proline as a catalyst was developed in the 1970s and is now known as the Hajos–Parrish reaction.[34] This asymmetric aldol reaction employs achiral triketone 21 and requires just 3% of L-proline to obtain the optically pure reaction product (22) in high yield and enantiomeric excess (Scheme 6).
Scheme 6: First example of an asymmetric aldol reaction employing chiral proline.
Later, List and coworkers developed the first proline-catalyzed asymmetric intermolecular aldol reaction.[35] This reaction between acetone and various aldehydes resulted in moderate to good yields and enantiomeric excess. A representative example is depicted in scheme 7.
Scheme 7: Proline-catalyzed asymmetric intermolecular aldol condensation between acetone and 4-nitrobenzaldehyde.
152 Reflections & Outlook
Therefore, determining if amino acid derivative 24 (Figure 4) due to its steric bulk, would give better selectivity in an asymmetric intermolecular aldol condensation (compared to L-proline) would be highly recommended. More interesting would be to establish the selectivity of amino acid derivative 25 (Figure 4) since List and coworkers have reported that trans-4-Hydroxy-L-proline shows slightly improved enantioselectivity in relation to L-proline.[35] It seems that electronic properties or hydrogen bonding influence selectivity of these kinds of catalysts.
Figure 4: Potential catalysts in asymmetric aldol condensations.
7.2.3 Alcohol Dehydrogenase This thesis has focused entirely on the use of MAO-N as biocatalyst. Nevertheless, numerous possibilities exist in combing different biocatalysts with multicomponent chemistry. Alcohol dehydrogenase (ADH, EC 1.1.1.1) for example, is an attractive target enzyme. This biocatalyst catalyzes the interconversion between alcohols and aldehydes or ketones and is mostly NAD(P)H-dependent. Since reduction of a carbonyl group requires stoichiometrical amounts of expensive cofactor NAD(P)H, regeneration through in situ reduction or whole cells systems are usually applied. An alcohol dehydrogenase employed as a whole-cell preparation for the oxidation/reduction with regeneration of the cofactor with acetone and i-propanol, respectively is ADH from Rhodococcus ruber 44541 (Scheme 8).[36-38] Advantages of this ADH are its ability to tolerate co-solvents, primarily acetone (25% v/v) and isopropanol (50% v/v), which, in addition enhance the solubility of lipophilic substrates. Moreover, these co-solvents can act as co-substrates for ADH activity in the reductive and oxidative directions, respectively, providing a means for in situ recycling of the requisite nicotinamide cofactor.
Scheme 8: Oxidation/Reduction of alcohol/ketones utilizing Rhodococcus ruber DSM 44541.
153 Chapter 7
Whereas Ugi and Passerini reactions typically suffer from lack of stereochemical induction, the Petasis (or borono-Mannich) MCR (Scheme 9) generally displays excellent diastereoselectivities with respect to both the amine and aldehyde input. Thus, enantiomerically pure aldehydes can also be considered valuable inputs for the Petasis MCR. Especially α-hydroxy aldehydes have been reported to give excellent induction.[39]
Scheme 9: The Petasis multicomponent reaction employing amines, aldehydes and boronic acids.
The chemical synthesis of enantiomerically pure α-hydroxy aldehydes is not straightforward. Therefore, synthesis of these aldehydes via enzymatic reduction of substituted keto- aldehyde 25 using alcohol dehydrogenase would generate α-hydroxy aldehyde 26 in high enantioselectivity (Scheme 10) which in turn is employed as input for a Petasis MCR to generate chiral compound 27. Moreover, similar cross-coupling reactions have been reported using aqueous media[40] and thus, possibilities of performing the biocatalysis and MCR sequence in a one-pot fashion arise.
Scheme 10: Enzymatic reduction of substituted keto-aldehyde 25 generating α-hydroxy aldehyde 26 which in turn is employed as input for a Petasis MCR to generate chiral compound 27.
The synthesis of optically active compound 27 by means of a Petasis reaction can lead to attractive pharmaceutical targets like antidiabetic,[41] antifungal[42], anticancer[43] and immunosuppressant[44] agents.
154 Reflections & Outlook
7.3 Conclusions & Outlook
Despite the exciting results obtained with various biocatalysis/MCR sequences presented in this thesis, the combination of these two types of methodologies has hardly been described in literature. Still there is a vast potential of feasible combinations between MCRs with biocatalysis as described above. Emphasis should be placed on full stereochemical control, one-pot procedures, further optimizing time, resource and energy efficiency. Ideally, medicinally relevant classes of compounds should be targeted, thereby contributing significantly to future developments in combinatorial chemistry and drug discovery as well as process chemistry.
7.4 Experimental Section
General Information Starting materials and solvents were purchased from Sigma-Aldrich and were used without further purification. (1R,2S,6R,7S)-4-methyl-4-azatricyclo[5.2.1.02.6]dec-8-ene was prepared according to literature procedure.[45] Column chromatography was performed on silica gel. 1H and 13C NMR spectra were recorded on a Bruker Avance 400 (400.13 MHz for 1H and 100.61 13 1 13 MHz for C) or Bruker Avance 500 (500.23 MHz for H and 125.78 MHz for C) in CDCl3 and DMSO. Chemical shifts are reported in δ values (ppm) downfield from tetramethylsilane. Electrospray Ionization (ESI) mass spectrometry was carried out using a Bruker micrOTOF-Q instrument in positive ion mode (capillary potential of 4500 V). Infrared (IR) spectra were recorded neat, and wavelengths are reported in cm-1.
General Procedure 1: Preparation of optically active imine 12 Unless stated otherwise: 12 was synthesised according to literature procedure[9] with minor adjustments. 2.5 g of freeze-dried MAO-N D5 E.Coli were rehydrated for 30 min. in 20 ml of KPO4 buffer (100 mM, pH = 8.0) at 37 °C. Subsequently 1 mmol amine in 30 ml of KPO4 buffer (100 mM, pH = 8.0) was prepared. The pH of the solution was adjusted to 8,0 by addition of NaOH and then added to the rehydrated cells. After 16-17 h the reaction was stopped (conversions were > 95 %) and worked up. For workup the reaction mixture was centrifuged at 4000 rpm and 4°C until the supernatant had clarified (40 – 60 min.). The pH of the supernatant was then adjusted to 10-11 by addition of aq. NaOH and the supernatant was subsequently extracted with t-butyl methyl ether or dichloromethane (4 x 70 mL). The combined organic phases were dried with Na2SO4 and concentrated at the rotary evaporator.
General procedure 2: Preparation of optically active amino nitriles 11a-e Unless stated otherwise: Imine (0.50 mmol, 1.0 eq.) was dissolved in 2 ml of DCM followed by the addition trimethylsilyl cyanide (0.65 mmol, 1.3 eq.). After stirring the reaction mixture
155 Chapter 7 for 1 h at RT, the acid chloride (0.65 mmol, 1.3 eq.)was added and the reaction mixture was stirred for another 16 h at RT. The resulting mixture was concentrated in vacuo and purified by silica gel flash chromatography (EtOAc (1): cyclohexane (2)). Note: Two sets of rotameric signals may be observed in the NMR data.
Compound 11a: General procedure 2 was followed using imine 12, (63 mg, 0.47 mmol), trimethylsilyl cyanide (69 mg, 88 µl, 0.70 mmol) and benzoyl chloride (86 mg, 71 µl, 0.61 mmol) giving 11a as a yellow solid, yield 90%. 1 H NMR (400.13 MHz, CDCl3): δ 7.43–7.35 (m, 5H), 6.18 (bs, 1H), 5.97 (bs, 1H), 4.91 (s, 1H), 3.68–3.63 (m, 1H), 3.17–3.13 (m, 2H), 2.98 (bs, 2H), 2.86 (bs, 1H), 1.53–1.51 (m, 1H), 1.38- 13 1.36 (m, 1H); C NMR (100.61 MHz, CDCl3): δ 169.0, 135.4, 135.2 133.7, 130.5, 130.5, -1 128.3, 127.1, 119,0, 51.4, 51.2, 49.5, 47.7, 46.9, 46.8, 45.5;IR (neat): νmax (cm ) = 1624 (m), + 1393 (m), 718 (m).HRMS (ESI+): calcd for C17H16N2O ([M + H] ) 265.1263, found 265.1341.
Compound 11b: General procedure 2 was followed using imine 12, (60 mg, 0.45 mmol), trimethylsilyl cyanide (61 mg, 78 μl, 0.62 mmol) and p-Toluoyl chloride (90 mg, 77 μl, 0.58 mmol) giving 11b as a white solid, yield 88%. 1 H NMR (500.23 MHz, CDCl3): δ 7.31–7.30 (m, 2H), 7.18–7.17 (m, 2H), 6.14 (bs, 1H), 5.92 (bs, 1H), 4.90 (s, 1H), 3.66–3.62 (m, 1H), 3.17–3.09 (m, 2H), 2.95 (bs, 2H), 2.82 (bs, 13 1H), 2.35 (s, 3H), 1.49–1.47 (m, 1H), 1.35–1.33 (m, 1H); C NMR (125.78 MHz, CDCl3): δ 168.9, 140.8, 135.1, 133.5, 132.2, 129.3, 129.3, 127.2, 127.2, 119.0, 51.2, 51.2, 49.3, -1 47.6, 46.8, 46.6, 45.3, 21.3; IR (neat) νmax (cm ) = 1636 (s), 1397 (m), 1300 (m), 833 (s), 733 (s); HRMS (ESI+): + calcd for C18H18N2O ([M + H] ) 279.1419, found 279.1491.
Compound 11c: General procedure 2 was followed using imine 12, (51 mg, 0.38 mmol), trimethylsilyl cyanide (52 mg, 66 μl, 0.53 mmol) and 2-quinoxaloyl chloride (96 mg, 0.50 mmol) giving 11c as a yellow solid, yield 92%. 1 H NMR (400.13 MHz, CDCl3): δ 9.45–9.35 (m, 1h), 8.20–8.10 (m, 2H), 7.89–7.83 (m, 2H), 6.31–6.21 (m, 1H), 6.13–5.98 (m, 1H), 4.81 (s, 1H), 4.14–3.99 (m, 1H), 3.61–3.54 (m, 1H), 3.36–3.25 (m, 1H), 3.18–2.98 (m, 3H), 1.60–1.56 (m, 1H), 1.47–1.44 (m, 13 1H); C NMR (100.61 MHz, CDCl3): δ 163.9, 146.5, 145.5, 143.0, 140.2, 136.4, 133.9, -1 132.1, 130.9, 129.8, 129,51, 128.7, 53.2, 51.6, 49.3, 48.1, 47.2, 46.3, 42.0; IR (neat) νmax (cm ) = 1620 (m), + 1427 (m), 1385 (b), 737 (m); HRMS (ESI+): calcd for C19H16N4O ([M + H] ) 317.1324, found 317.1398.
Compound 11d: General procedure 2 was followed using imine 12, (68 mg, 0.51 mmol), trimethylsilyl cyanide (69 mg, 88 µl, 0.70 mmol) and diphenylacetyl chloride (154 mg, 0.67 mmol) giving 11d as a white solid, yield 33%. 1 H NMR (500.23 MHz, CDCl3): δ 7.35-7.20 (m, 10H), 6.01 (dd, J = 5.7, 3.0 Hz, 1H), 5.51 (dd, J = 5.7, 3.0 Hz, 1H), 4.87 (s, 1H), 4.47 (d, J = 2.3 Hz, 1H), 3.48–3.40 (m, 1H), 3.24– 3.21 (m, 1H), 3.17-3.13 (m, 1H), 3.05–2.85 (m, 3H), 1.51–1.43 (m, 1H), 1.36–1.34 (m, 13 1H); C NMR (125.78 MHz, CDCl3): δ 169.8, 138.4, 137.9, 136.0, 134.2, 129.3, 129.3, 129.0, 129.0, 128.8, 128.8, 127.6, 127.6, 127.4, 127.4, 118.9, 56.7, 51.6, 49.9, 49.0, -1 48.8, 45.6, 43.3, 28.1; IR (neat) νmax (cm ) = 1647 (m), 1393 (m), 702 (m); HRMS (ESI+): calcd for C24H22N2O ([M + H]+) 355.1732, found 355.1790.
156 Reflections & Outlook
Compound 11e: General procedure 2 was followed using imine 12, (66 mg, 0.50 mmol), trimethylsilyl cyanide (67 mg, 86 µl, 0.69 mmol) and crotonoyl chloride (65 mg, 62 µl, 0.62 mmol) giving 11e as a yellow solid, yield 81%. 1 H-NMR (400.13 MHz, CDCl3): δ 6.92 (dq, J = 21.9, 6.9 Hz, 1H), 6.15–5.99 (m, 2H), 5,90 (dq, J = 15.0, 1.6 Hz, 1H), 4.41 (d, J = 2.3 Hz, 1H), 3.56–3.51 (m, 1H), 3.28–3.25 (m, 1H), 3.11–3.04 (m, 3H), 2.93 (bs, 1H), 1.81–1,80 (d, J = 10 Hz, 3H), 1.51–1.49 (m, 1H), 1.37– 13 1.35 (m, 1H); C NMR (125.78 MHz, CDCl3): δ 167.6, 143.6, 135.6, 134.2, 121.5, 118.8, 52.6, 49.8, 48.3, 48.2, -1 46.8, 46.5, 45.4, 18.2; IR (neat) νmax (cm ) = 1659 (m), 1609 (m), 1393 (m), 968 (m), 729 (s); HRMS (ESI+): + calcd for C14H16N2O ([M + H] ) 229.1263, found 229.1335.
7.5 References & Notes
[1] U. Kusebauch, B. Beck, K. Messer, E. Herdtweck and A. Domling, Org. Lett. 2003, 5, 4021-4024. [2] P. R. Andreana, C. C. Liu and S. L. Schreiber, Org. Lett. 2004, 6, 4231-4233. [3] S. E. Denmark and Y. Fan, J. Am. Chem. Soc. 2003, 125, 7825-7827. [4] S. C. Pan and B. List, Angew. Chem. Int. Ed. 2008, 47, 3622-3625. [5] D. J. Ramon and M. Yus, Angew. Chem. Int. Ed. 2005, 44, 1602-1634. [6] T. Yue, M. X. Wang, D. X. Wang, G. Masson and J. P. Zhu, J. Org. Chem. 2009, 74, 8396-8399. [7] N. J. Turner, Nat. Chem. Biol. 2009, 5, 568-574. [8] K. Faber, Biotransformations in Organic Chemistry, Springer, Berlin, Heidelberg, New York, 1997. [9] V. Kohler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell and N. J. Turner, Angew. Chem. Int. Ed. 2010, 49, 2182-2184. [10] A. Znabet, E. Ruijter, F. J. J. de Kanter, V. Kohler, M. Helliwell, N. J. Turner and R. V. A. Orru, Angew. Chem. Int. Ed. 2010, 49, 5289-5292. [11] A. Znabet, J. Zonneveld, E. Janssen, F. J. J. De Kanter, M. Helliwell, N. J. Turner, E. Ruijter and R. V. A. Orru, Chem. Commun. 2010, 46, 7706-7708. [12] A. Znabet, M. M. Polak, E. Janssen, F. J. J. de Kanter, N. J. Turner, R. V. A. Orru and E. Ruijter, Chem. Commun. 2010, 46, 7918-7920. [13] A. E. Weber, J. Med. Chem. 2004, 47, 4135-4141. [14] K. Augustyns, P. Van der Veken, K. Senten and A. Haemers, Curr. Med. Chem. 2005, 12, 971-998. [15] D. Hunziker, M. Hennig and J. U. Peters, Curr. Top. Med. Chem. 2005, 5, 1623-1637. [16] Z. Szeltner and L. Polgar, Curr. Protein Pept. Sci. 2008, 9, 96-107. [17] P. T. Maennisto, J. Vanaelaeinen, A. Jalkanen and J. A. Garcia-Horsman, Drug News Perspect. 2007, 20, 293-305. [18] I. Brandt, S. Scharpe and A.-M. Lambeir, Clin. Chim. Acta 2007, 377, 50-61. [19] B. R. Troen, Drug News Perspect. 2004, 17, 19-28. [20] J. R. Li, E. Wilk and S. Wilk, J. Neurochem. 1996, 66, 2105-2112. [21] E. B. Villhauer, J. A. Brinkman, G. B. Naderi, B. E. Dunning, B. L. Mangold, M. D. Mone, M. E. Russell, S. C. Weldon and T. E. Hughes, J. Med. Chem. 2002, 45, 2362-2365. [22] R. P. Hanzlik, J. Zygmunt and J. B. Moon, Biochim. Biophys. Acta 1990, 1035, 62-70. [23] J.-U. Peters, Curr. Top. Med. Chem. 2007, 7, 579-595. [24] A. Devos, J. Remion, A. M. Frisquehesbain, A. Colens and L. Ghosez, J. Chem. Soc., Chem. Commun. 1979, 1180-1181. [25] J. Zhu and H. Bienayme, Multicomponent Reactions, Wiley-VCH, Weinheim, 2005. [26] G. R. Pettit, Y. Kamano, C. Dufresne, R. L. Cerny, C. L. Herald and J. M. Schmidt, J. Org. Chem. 1989, 54, 6005-6006. [27] W. H. Gerwick, L. T. Tan and N. Sitachitta, Nitrogen-containing metabolites from marine cyanobacteria, Academic Press, San Diego, 2001.
157 Chapter 7
[28] Z. Cruz-Monserrate, J. T. Mullaney, P. G. Harran, G. R. Pettit and E. Hamel, Eur. J. Biochem. 2003, 270, 3822-3828. [29] R. Frey, S. G. Galbraith, S. Guelfi, C. Lamberth and M. Zeller, Synlett 2003, 1536-1538. [30] A. Basso, L. Banfi, R. Riva and G. Guanti, Tetrahedron Lett. 2004, 45, 587-590. [31] T. Kimura, J. Hosokawa-Muto, Y. O. Kamatari and K. Kuwata, Bioorg. Med. Chem. Lett. 2011, 21, 1502-1507. [32] A. S. Bell, M. P. Deninno, M. J. Palmer and V. M. S. in Vol. US2004/220186 Pfizer Inc,2004 . [33] A. Basso, L. Banfi, R. Riva and G. Guanti, J. Org. Chem. 2005, 70, 575-579. [34] Z. G. Hajos and D. R. Parrish, J. Org. Chem. 1974, 39, 1615-1621. [35] B. List, R. A. Lerner and C. F. Barbas, J. Am. Chem. Soc. 2000, 122, 2395-2396. [36] W. Stampfer, B. Kosjek, C. Moitzi, W. Kroutil and K. Faber, Angew. Chem. Int. Ed. 2002, 41, 1014- 1017. [37] W. Stampfer, B. Kosjek, W. Kroutil and K. Faber, Biotechnol. Bioeng. 2003, 81, 865-869. [38] G. de Gonzalo, I. Lavandera, K. Faber and W. Kroutil, Org. Lett. 2007, 9, 2163-2166. [39] N. Kumagai, G. Muncipinto and S. L. Schreiber, Angew. Chem. Int. Ed. 2006, 45, 3635-3638. [40] N. Miyaura, T. Ishiyama, H. Sasaki, M. Ishikawa, M. Satoh and A. Suzuki, J. Am. Chem. Soc. 1989, 111, 314-321. [41] A. S. Davis, S. G. Pyne, B. W. Skelton and A. H. White, J. Org. Chem. 2004, 69, 3139-3143. [42] N. A. Petasis and I. Akritopoulou, Tetrahedron Lett. 1993, 34, 583-586. [43] D. R. Li, Y. Q. Tu, G. Q. Lin and W. S. Zhou, Tetrahedron Lett. 2003, 44, 8729-8732. [44] S. Sugiyama, S. Arai and K. Ishii, Tetrahedron: Asymmetry 2004, 15, 3149-3153. [45] S. Michaelis and S. Blechert, Chem.-Eur. J. 2007, 13, 2358-2368.
158 Summary
Summary
Biocatalysis & Multicomponent Reactions: The Ideal Synergy
Asymmetric Synthesis of Substituted Proline Derivatives
The demand for fast, clean, atom and step efficient chemical processes is increasing by the day. Sustainable synthesis is not only a key feature for the chemical industry but also for the pharmaceutical world. The key objective of the research in this thesis is to combine two methodologies that have proven to be efficient and environmentally benign: (i) multicomponent reactions (MCRs) and (ii) biocatalysis. These methods offer significant advantages by reducing time, saving money, energy and raw materials, thus resulting in both economical and environmental benefits. The structurally complex products formed in MCRs often contain new stereocenters which are difficult to control. For most MCRs, catalytic asymmetric methods to control the stereochemical outcome of the reaction are so far not available. The broad repertoire of stereospecific conversions by biocatalysts presents a unique opportunity to address the stereoselectivity issue of certain MCRs. We have developed several methods (Scheme 1) making use of the enzymatic desymmetrization of meso-pyrrolidines by means of a monoamine oxidase N (MAO-N) from Aspergillus niger optimized by directed evolution and its combination with multicomponent chemistry.
Scheme 1: Access to different scaffolds and follow-up chemistry using MAO-N and MCRs.
161 Summary
Multicomponent reactions (MCRs) combine essentially all the atoms of at least three We envisioned that combining the diastereoselective Joullié-Ugi 3CR approach with components in one pot for atom and process efficiency with reduced waste. MCRs are cyclization reactions could further increase the resulting molecular complexity and diversity significantly more efficient than classical multistep synthesis in terms of time and resources. considerably. Reacting our 3,4-disubstituted 1-pyrrolines with a-ketocarboxylic acids and Biocatalysis employs enzymes and microorganisms in the chemical or food industry, making homoveratryl isocyanide afforded the ketoamide intermediate 5 which subsequently manufacturing processes more environmentally friendly. They can produce high yields of undergoes a Pictet–Spengler cyclization to afford highly complex alkaloid-like polycyclic specific products with low energy use and minimal waste generation. Despite the great compounds (2,5-diketopiperazines; DKPs) 6 with high diversity (Scheme 3). In addition, the potential of biocatalysis and MCRs, the combination of these two types of methodologies experimental procedure is simple and very efficient. To the best of our knowledge, it also to generate optically pure complex compounds has hardly been described in literature constitutes the first example of MCR chemistry to synthesize 5-membered ring-fused DKPs (Chapter 1). (Chapter 4).
The main biocatalyst described in this thesis is monoamine oxidase N (MAO-N) from Aspergillus niger. This enzyme has proven to be particularly suitable for directed evolution. Mutant MAO-N tolerates primary, secondary, and tertiary amines as substrates. In addition, 3,4-substituted meso-pyrrolidines were shown to be excellent substrates for biocatalytic oxidation. Moreover, the stereoselectivity of the biocatalyst is very high (ee > 94%) and no additional cofactor is required. This makes MAO-N one of the leading biocatalysts for amine oxidation (Chapter 2). Since 3,4-substituted meso-pyrrolidines 1 proved excellent substrates for MAO-N, Scheme 3: Diastereoselective Ugi-type 3CR followed by a Pictet-Spengler reaction resulting in DKPs 6. different optically pure 3,4-disubstituted 1-pyrrolines 2 were generated (Scheme 2). These 3,4-disubstituted 1-pyrrolines offer the possibility to do a subsequent MCR since imines are The efficiency and generality of the approach and the resulting molecular diversity and intermediates for several of these MCRs. 3,4-Disubstituted 1-pyrrolines were reacted with complexity makes our methodology highly interesting for medicinal chemistry. Most carboxylic acids and isocyanides in a highly diastereoselective Ugi-type multicomponent notably, a very short and efficient synthesis of important hepatitis C drug Telaprevir reaction (Joullié-Ugi 3CR, Scheme 2) to give pharmaceutically relevant substituted prolyl (Incivek™), featuring a biocatalytic desymmetrization and two multicomponent reactions peptides 3 that would require lengthy reaction sequences using other methods. The resulting (Passerini and Joullié-Ugi 3CR) as the key steps was developed. The classical issue of lack of prolyl peptides can also be used as highly active organocatalysts (4) for stereoselective stereoselectivity in Ugi- and Passerini-type reactions was circumvented. The atom economic conjugate addition of aldehydes to nitroolefins (Chapter 3). and convergent nature of the synthetic strategy requires only very limited use of protective groups. The use of protective groups is limited to two intermediate methyl esters and an
acetate. The use of carbamate protective groups is avoided altogether. The synthesis comprises only eleven steps in total (seven steps in the longest linear sequence) compared to twenty-four in the originally reported procedure. The total yield (over the longest linear sequence) is 45% starting from L-cyclohexylglycine methyl ester. Our approach is general and will be applicable to many other HCV NS3 protease inhibitors that can be derived from meso-pyrrolidines, such as e.g. Boceprevir (Victrelis™) and Narlaprevir. The combination of synthetic efficiency and convergence in our approach allows both faster development of Scheme 2: Desymmetrization and diastereoselective Ugi-type 3CR towards optically pure 3,4-substituted prolyl peptides second generation inhibitors and a more economical production of Telaprevir (Chapter 5).
162 Summary
We envisioned that combining the diastereoselective Joullié-Ugi 3CR approach with cyclization reactions could further increase the resulting molecular complexity and diversity considerably. Reacting our 3,4-disubstituted 1-pyrrolines with a-ketocarboxylic acids and homoveratryl isocyanide afforded the ketoamide intermediate 5 which subsequently undergoes a Pictet–Spengler cyclization to afford highly complex alkaloid-like polycyclic compounds (2,5-diketopiperazines; DKPs) 6 with high diversity (Scheme 3). In addition, the experimental procedure is simple and very efficient. To the best of our knowledge, it also constitutes the first example of MCR chemistry to synthesize 5-membered ring-fused DKPs (Chapter 4).
Scheme 3: Diastereoselective Ugi-type 3CR followed by a Pictet-Spengler reaction resulting in DKPs 6.
The efficiency and generality of the approach and the resulting molecular diversity and complexity makes our methodology highly interesting for medicinal chemistry. Most notably, a very short and efficient synthesis of important hepatitis C drug Telaprevir (Incivek™), featuring a biocatalytic desymmetrization and two multicomponent reactions (Passerini and Joullié-Ugi 3CR) as the key steps was developed. The classical issue of lack of stereoselectivity in Ugi- and Passerini-type reactions was circumvented. The atom economic and convergent nature of the synthetic strategy requires only very limited use of protective groups. The use of protective groups is limited to two intermediate methyl esters and an acetate. The use of carbamate protective groups is avoided altogether. The synthesis comprises only eleven steps in total (seven steps in the longest linear sequence) compared to twenty-four in the originally reported procedure. The total yield (over the longest linear sequence) is 45% starting from L-cyclohexylglycine methyl ester. Our approach is general and will be applicable to many other HCV NS3 protease inhibitors that can be derived from meso-pyrrolidines, such as e.g. Boceprevir (Victrelis™) and Narlaprevir. The combination of synthetic efficiency and convergence in our approach allows both faster development of second generation inhibitors and a more economical production of Telaprevir (Chapter 5).
163 Summary
Scheme 4: Synthesis of hepatitis C NS3 protease inhibitor Telaprevir (Incivek™).
We then decided to investigate the possibility of reacting our 3,4-disubstituted 1-pyrrolines in other MCRs. The Ugi-Smiles reaction, a variation of the Ugi reaction in which the carboxylic acid component is substituted by an electron-deficient phenol derivative (or a heteroaromatic analog) is a promising candidate for combination with our biocatalytic oxidation due to its mechanistic similarity with the Ugi reaction. Reacting our 3,4-disubstituted 1-pyrrolines with electron-deficient phenols and different isocyanides afforded the N-aryl proline (thio) amides 7 in high diastereoslectivity (Scheme 4). These 3,4-substituted N-aryl proline (thio) amides are pharmaceutically relevant since similar compounds have been reported to be potent VLA-4 antagonists. Therefore, these compounds may be useful in the treatment of diseases like asthma, atherosclerosis, rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. This method represents the first report of a fully asymmetric Ugi-Smiles process (Chapter 6).
Scheme 5: Asymmetric synthesis N-aryl proline amides by MAO-N oxidation and Ugi-Smiles 3CR.
In summary, the results described in this thesis demonstrate that biocatalysis and MCRs is a powerful combination for the production of optically pure complex compounds. We were able to synthesize two distinct scaffolds; N-aryl proline amides and 3,4-disubstituted prolyl peptides. The latter showed to be a very versatile scaffold providing access to organocatalysts, synthetic alkaloids and even to the important Hepatitis C drug Telaprevir (Incivek™).
164 Samenvatting
(Summary in Dutch)
Samenvatting
Biokatalyse & Multicomponentenreacties: De Ideale Synergie
Asymmetrische Synthese van Gesubstitueerde Proline Derivaten
De vraag naar snelle, schone, atoom- en stap- efficiënte chemische processen stijgt met de dag. Duurzame synthese is niet alleen belangrijke voor de chemische industrie, maar ook voor de farmaceutische wereld. De belangrijkste doelstelling van het onderzoek in dit proefschrift is het combineren van twee methodes die hebben bewezen efficiënt en milieuvriendelijk te zijn: (i) multicomponentreacties (MCRs) and (ii) biokatalyse. Deze methoden bieden aanzienlijke voordelen door het verkorten van de reactietijd, geld te besparen, efficiënter gebruik van energie en grondstoffen, hetgeen resulteert in zowel economische als milieuvoordelen. De structureel complexe producten die in MCRs gevormd worden bevatten vaak nieuwe stereocentra die moeilijk te controleren zijn. Voor de meeste MCRs zijn katalytische asymmetrische methoden om de stereochemische uitkomst van de reactie te sturen tot nu toe niet beschikbaar. Het brede repertoire van stereospecifieke conversies door biokatalysatoren biedt een unieke gelegenheid om de stereoselectiviteit van MCRs aan te pakken. We hebben verschillende methoden (Schema 1) ontwikkeld die gebruik maken van de enzymatische desymmetrisering van meso-pyrrolidines door middel van een monoamine oxidase N (MAO-N) uit Aspergillus niger geoptimaliseerd door directed evolution in combinatie met multicomponentchemie.
Schema 1: Toegang tot verschillende bouwstenen en vervolgchemie gebruikmakend van MAO-N en MCRs.
167 Samenvatting
MCRs combineren nagenoeg al de atomen van ten minste drie componenten in één pot voor atoom- en procesefficiëntie met een lagere afvalproductie. MCRs zijn aanzienlijk efficiënter dan de klassieke multistapsynthese in termen van tijd en middelen. Biokatalyse maakt gebruik van enzymen en micro-organismen en wordt vaak gebruikt in de chemische of voedingsmiddelenindustrie, waardoor productieprocessen milieuvriendelijker efficiënter en milieuvriendelijker worden gemaakt. Zij kunnen hoge opbrengsten van specifieke producten genereren met een laag energieverbruik en een minimale afvalproductie. Ondanks de potentie die biokatalyse en MCRs bezitten, is de combinatie van deze twee methoden om optisch zuivere complexe verbindingen te maken nauwelijks beschreven in de literatuur (Hoofdstuk 1).
De belangrijkste biokatalysator beschreven in dit proefschrift is monoamine oxidase N (MAO-N) uit Aspergillus niger. Dit enzym heeft bewezen bijzonder geschikt te zijn voor directed evolution. Mutante MAO-N tolereert primaire, secundaire en tertiaire amines als substraten. Daarnaast bleken 3,4-digesubstitueerde meso-pyrrolidines uitstekend substraten te zijn voor biokatalytische oxidatie. Bovendien is de stereoselectiviteit van de biokatalysator zeer hoog (enantiomere overmaat > 94%) en is geen extra co-factor is vereist. Dit maakt MAO-N een van de belangrijkste biokatalysatoren voor amine-oxidatie (Hoofdstuk 2).
Aangezien 3,4-digesubstitueerde meso-pyrrolidines 1 uitstekende substraten bleken te zijn voor MAO-N, werden verschillende optisch zuiver 3,4-digesubstitueerde 1-pyrrolines 2 gegenereerd (Schema 2). Deze 3,4-digesubstitueerde 1-pyrrolines bieden de mogelijkheid om vervolgens in een navolgend MCR te verrichten aangezien iminen intermediairen zijn voor een groot aantal van deze MCRs. 3,4-digesubstitueerde 1-pyrrolines gingen een reactie aan met carbonzuren en isocyaniden in een zeer diastereoselectieve Ugi-type multicomponentreactie (Joullié-Ugi 3CR, Schema 2) hetgeen resulteerde in farmaceutisch relevante prolylpeptiden 3. Deze verbindingen zouden met behulp van andere methoden vele reactie stappen vereisen. De resulterende prolyl peptiden kunnen ook gebruikt worden als zeer actieve organokatalysatoren (4) voor stereoselectieve geconjugeerde addities van aldehyden aan nitro-olefines (Hoofdstuk 3).
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Schema 2: Desymmetrisatie en diastereoselectieve Ugi-type 3CR naar optisch zuiver 3,4-gesubstitueerde prolyl peptiden
Wij voorzagen dat het combineren van de diastereoselectieve Joullié-Ugi 3CR aanpak met cyclisatiereacties de voortvloeiende moleculaire complexiteit en diversiteit aanzienlijk zou doen toenemen. Onze 3,4-digesubstitueerde 1-pyrrolines gingen een reactie aan met a-ketocarbonzuren en homoveratryl isocyanide hetgeen resulteerde in ketoamide tussenproduct 5 dat vervolgens een Pictet-Spengler cyclisatie onderging en een zeer complexe alkaloïde-achtige polycyclische verbindingen (2,5-diketopiperazines; DKPs) 6 met een hoge diversiteit (Schema 3) opleverde. Daarnaast is de experimentele procedure van deze aanpak eenvoudig en zeer efficiënt. Naar ons weten vormt dit ook het eerste voorbeeld van MCR chemie in de synthese van vijf ring gefuseerde DKPs (Hoofdstuk 4).
Schema 3: Diastereoselectieve Ugi-type 3CR, gevolgd door een Pictet-Spengler reactie resulterende in DKPs 6.
De efficiëntie en de algemeenheid van de benadering en de daaruit voortvloeiende moleculaire diversiteit en complexiteit maakt onze methodiek zeer interessant voor medicinale chemie. Het meest opvallend was de ontwikkeling van een zeer korte en efficiënte synthese van het belangrijke hepatitis C- medicijn Telaprevir (Incivek™), met een biokatalytische desymmetrisering en twee multicomponentreacties (Passerini en Joullié-Ugi 3CR) als de belangrijkste stappen. Het klassieke probleem van een gebrek aan stereoselectiviteit in de Ugi en Passerini reactie werd omzeild. Het atoom- economische
169 Samenvatting en convergente karakter van de synthetische strategie maakt gebruik van slechts een zeer beperkt aantal beschermgroepen. Het gebruik van beschermgroepen is beperkt tot twee methylesters en een acetaat. Het gebruik van carbamaatbeschermgroepen wordt helemaal vermeden. De synthese omvat slechts elf stappen in totaal (zeven stappen in de langste lineaire sequentie) in vergelijking met vierentwintig in de oorspronkelijk gerapporteerde procedure. De totale opbrengst (over de langste lineaire sequentie) is 45% uitgaande van L-cyclohexylglycinemethylester. Onze aanpak is van algemeen toepassing aangezien vele andere HCV-NS3 proteaseremmers, zoals Boceprevir (Victrelis™) en Narlaprevir kunnen worden afgeleid van meso-pyrrolidines. De combinatie van synthetische efficiëntie en convergentie in onze aanpak maakt het mogelijk om zowel snellere ontwikkeling van tweede generatie-remmers als een efficiëntere productie van Telaprevir te realiseren (Hoofdstuk 5).
Schema 4: Synthese van hepatitis C NS3 proteaseinhibitor Telaprevir (Incivek™).
We hebben toen besloten om de mogelijkheid te onderzoeken of we onze 3,4-digesubstitueerde 1-pyrrolines zouden kunnen laten reageren in andere MCRs. De Ugi-Smiles reactie, een variant van de Ugi reactie waarbij de carbonzuurcomponent wordt vervangen door een elektronenarme fenolderivaat (of een hetero-analoog) is een veelbelovende kandidaat voor de combinatie met onze biokatalytische oxidatie vanwege de mechanistische gelijkenis met de Ugi reactie. De reactie van onze 3,4-digesubstitueerde 1-pyrrolines met fenolen en verschillende isocyanides resulteert in de vorming van N-aryl proline(thio)amides 7 in hoge diastereoslectiviteit (Schema 4). Deze 3,4-gesubstitueerde N-aryl proline(thio)amides zijn farmaceutisch relevant, omdat soortgelijke verbindingen zijn gerapporteerd als potente VLA-4-antagonisten. Vandaar dat deze verbindingen misschien bruikbaar zouden kunnen zijn bij de behandeling van ziekten zoals astma, atherosclerose, reumatoïde artritis, inflammatoire darmziekten, en multiple sclerose. Deze methode is dan ook het eerste voorbeeld van een volledig asymmetrisch Ugi-Smiles proces (Hoofdstuk 6).
170 Samenvatting
Schema 5: Asymmetrische synthese van N-aryl proline amides via MAO-N oxidatie en Ugi-Smiles 3CR.
Kortom, de resultaten beschreven in dit proefschrift tonen aan dat biokatalyse en MCRs een krachtige combinatie zijn voor het verkrijgen van optisch zuivere complexe verbindingen. We waren in staat om twee verschillende kernstructuren te synthetiseren, namelijk N-aryl proline-amides en 3,4-digesubstitueerde prolylpeptiden. De 3,4-digesubstitueerde prolylpeptiden bleken veelzijdige bouwstenen te zijn en verschaften toegang tot organokatalysatoren, synthetische alkaloïden en zelfs de belangrijke Hepatitis C medicijn Telaprevir (Incivek ™).
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Dankwoord
Dankwoord
Het is niet voor niets dat het dankwoord het meeste gelezen stuk is van een proefschrift. Promoveren kun je niet alleen en het is daarom zeer evident om waardering uit te spreken richting de mensen die hun bijdrage hebben geleverd aan het tot stand komen van dit proefschrift. Ik ben dan ook bang dat op het moment dat dit proefschrift van de drukker komt, ik erachter kom dat ik iemand vergeten ben te vermelden in mijn dankwoord. Bij deze; mocht ik iemand vergeten zijn te noemen, alsnog bedankt!
Allereerst wil ik mijn promotor, prof. dr. Romano Orru bedanken. Romano, jij hebt mij de mogelijkheid geboden om aan dit promotietraject te beginnen. Jouw steun, vertrouwen, positieve feedback, tijd en hulp hebben ertoe geleid dat mijn project vrij soepel en zeer aangenaam is verlopen. Verder was je hulp bij mijn mozaïek aanvraag onmisbaar. Mijn tweede promotor, prof. dr. Marinus Groen, wil ik ook bedanken. Alhoewel we elkaar niet veel hebben meegemaakt aangezien u met pensioen bent gegaan, ben ik u zeer dankbaar voor de feedback die u leverde tijdens de werkbesprekingen en voor het nauwkeurig doorlezen van mijn proefschrift. Ook dr. Eelco Ruijter, mijn copromotor ben ik zeer erkentelijk. Wat eigenlijk voor Romano geldt, geldt ook voor jou. Jouw enorme vakkennis en kundigheid maken je tot een groot voorbeeld.
Prof. dr. Isabel Arends, prof. dr. Pedro Hermkens, prof. dr. Rob Leurs, dr. Tommi Meulemans en prof. dr. Adri Minnaard wil ik bedanken voor hun bereidheid om hun tijd en expertise ter beschikking te stellen en zitting te nemen in de lees- en/of promotiecommisie. I am grateful to prof. dr. Nicholas Turner for reading and correcting this thesis. In addition, I want to thank you for giving me the opportunity of staying six months at the Manchester Interdisciplinary Centre. Without you, this thesis would not be what it is now.
Tijdens mijn promotietijd heb ik het genoegen gehad een aantal studenten te mogen begeleiden. Karl, you were my first student and supervising you has taught me a lot about how to deal with students. Marloes, jij hebt je Bachelor- en Master- stage bij mij gelopen en dat heeft ons geen windeieren gelegd. Mede door jou heeft dit geresulteerd in een publicatie en twee patenten. Job, jij was mijn eerste HLO student. We hebben heel wat afgelachen op het lab maar natuurlijk werd er ook keihard gewerkt. Je transformatie van een welpje in een leeuw was compleet. Ook jij hebt er een mooie publicatie aan overgehouden. Seenakshi, ook jij was een HLO studente. Jij hebt een belangrijke bijdrage geleverd aan het laatste hoofdstuk van mijn proefschrift. En dan Sara B., op voorspraak van jouw vriend (lees: Tjøstil) kwam je bij mij stage lopen . Vanaf het eerste moment klikte het tussen ons. We hebben samen, zowel op het lab als daarbuiten, veel gelachen. En ook jij hebt er een mooie publicatie aan overgehouden. Xavier, jij was officieel mijn laatste student. In het
175 Dankwoord begin keek je de kat uit de boom maar gaandeweg kwam je steeds losser. Helaas was er geen tijd om je resultaten in dit proefschrift te verwerken maar we zullen deze zeker nog gaan publiceren. Alhoewel jij niet aan mijn project werkte beschouw ik jou, Bryan, toch als een van mijn studenten. Je werd bij mij op het lab geplaatst en zodoende werd ik jouw praktische begeleider. Ook tussen ons klikte het meteen en onze muzieksmaak (sorry Sara) bevestigde dat alleen maar. Verder ben ik veel dank verschuldigd aan dr. Frans de Kanter, dr. Marek Smoluch en dr. Madeleine Helliwell. Frans, je deur stond altijd open als ik (weer) niet uit een NMR spectrum kwam. Ook voor jou is de tijd aangebroken om afscheid te nemen van de VU. Geniet van je pensioen! Marek, I want to thank you for the many HRMS measurements. Last but not least, Madeleine I am grateful for all X-ray diffraction determinations.
Daarnaast waren er vele collega’s waaronder vaste staf, post-docs, PhD’s en studenten die de afgelopen vier jaar voor een plezierige werkomgeving hebben gezorgd en die ik graag wil bedanken.
Bas G., jouw steun in mijn eerste periode als promovendus was onmisbaar. Jij hebt mij wegwijs gemaakt in het wereldje van het promoveren. Ik ben daarom heel blij dat je overvliegt uit Engeland om aanwezig te zijn tijdens mijn verdediging. Ik wil jou en Nathalie dan ook veel geluk en succes wensen in Manchester. Valentin, I am very grateful for your help and hospitality during my stay in Manchester. The work we did was not only pleasant and instructive, but eventually also very successful resulting in two joint publications. Also the (weekly) walk to Brunswick Street for ‘Fish & Chips’ was very enjoyable. I hope to see you on 26th of January. Tjøstil, zoals Bas G. mij wegwijs gemaakt heeft in het wereldje dat promoveren heet, heb ik geprobeerd dat bij jou te doen. Wij hebben vele discussies gevoerd, zowel over “acute” problemen in de zuurkast, als over de theoretische aspecten van ons onderzoek. Ik ben dan ook trots dat dit minimaal één gezamenlijke publicatie heeft opgeleverd. Aangezien ik weet hoe belangrijk traditie voor jou is draag ik hierbij mijn plek aan de koffietafel officieel aan jou over. Ik wens jou dan ook nog veel succes met je verdere onderzoek. De rol van een goede secretaresse voor het doel van wetenschappelijk onderzoek is hevig ondergewaardeerd. Hierbij wil ik dan ook Miep bedanken voor al het regelwerk dat ze voor mij heeft verricht.
Vanzelfsprekend zijn er nog een aantal collega’s en studenten die ik wil bedanken voor de afgelopen jaren: Sanne, Paul S., Leon, René, Lisette, Alay, Maurice, Gitte, Rob, Niels E., Niels T., Rachel, Halil, Helen, Chris, Jeroen, Monica, Loek, prof.dr. Koop Lammertsma, prof.dr. Fritz Bickelhaupt, Robert, Sebastiaan, Andreas, Bas de J., Federico, Federica, Wannes, Volodymyr,
176 Dankwoord
Art, Matthijs, Corien, Tom, Mark R., Elwin en Guido.
Mijn paranimfen, Jalal en Illiass, ik ben zeer vereerd dat jullie mij tijdens de promotie terzijde willen staan. Alvast bedankt voor alles!
En dan mijn zusjes, Ik wil jullie bedanken voor alle steun die ik de afgelopen jaren van jullie heb mogen ontvangen. Het is wellicht niet altijd duidelijk geweest waar ik nou eigenlijk mee bezig was, maar de belangstelling is altijd gebleven.
Daar aangekomen waar dankbaarheid niet meer in woorden uit te drukken is, toch maar een enkele aanduiding. Mijn ouders hebben me altijd aangemoedigd en gesteund in mijn schoolloopbaan en studie, en creëerden de randvoorwaarden om mijn mogelijkheden te ontplooien.
أس
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List of Publications/Patents
Publications/ Patents
“Stereoselective Synthesis of N-Aryl Proline Amides by Biotransformation/Ugi-Smiles Sequence” A. Znabet, S. Blanken, E. Janssen, F. J. J. de Kanter, M. Helliwell, N. J. Turner, R. V. A. Orru, E. Ruijter, Org. Biomol. Chem. 2011, accepted.
“Palladium-Catalyzed Synthesis of 4-Aminophthalazin-1(2H)-ones by Isocyanide Insertion” Vlaar T, Ruijter E, Znabet A, Janssen E, De Kanter FJJ, Orru RVA, Org.Lett. 2011, accepted.
“Process for the Preparation of Alpha-Acyloxy Beta-Formamido Amides” E. Ruijter, R. Orru, A. Znabet, M. Polak, N. Turner, WO2011/103933A1.
“A Process for the Preparation of Substituted Prolyl Peptides and Similar Peptidomimetics” E. Ruijter, R. Orru, A. Znabet, M. Polak, N. Turner, WO2011/103932A1.
“A Highly Efficient Synthesis of Telaprevir by Strategic use of Biocatalysis and Multicomponent Reactions” A. Znabet, M. M. Polak, E. Janssen, F. J. J. de Kanter, N. J. Turner, R. V. A. Orru, E. Ruijter, Chem. Commun. 2010, 46, 7918-7920.
“Asymmetric Synthesis of Synthetic Alkaloids by a Tandem Biocatalysis/Ugi/Pictet-Spengler-type Cyclization Sequence” A. Znabet, J. Zonneveld, E. Janssen, F. J. J. De Kanter, M. Helliwell, N. J. Turner, E. Ruijter, R. V. A. Orru, Chem. Commun. 2010, 46, 7706-7708.
“Highly Stereoselective Synthesis of Substituted Prolyl Peptides Using a Combination of Biocatalytic Desymmetrization and Multicomponent Reactions” A. Znabet, E. Ruijter, F. J. J. de Kanter, V. Koehler, M. Helliwell, N. J. Turner, R. V. A. Orru, Angew. Chem. 2010, 122, 5417-5420; Angew. Chem. Int. Ed. 2010, 49, 5289-5292.
“Enantioselective Biocatalytic Oxidative Desymmetrization of Substituted Pyrrolidines” V. Koehler, K. R. Bailey, A. Znabet, J. Raftery, M. Helliwell, N. J. Turner, Angew. Chem. 2010, 122, 2228-2230; Angew. Chem. Int. Ed. 2010, 49, 2182-2184.
“Scope and Limitations of an Efficient Four-Component Reaction for Dihydropyridin-2-ones” R. Scheffelaar, M. Paravidino, A. Znabet, R. F. Schmitz, F. J. J. de Kanter, M. Lutz, A. L. Spek, C. F. Guerra, F. M. Bickelhaupt, M. B. Groen, E. Ruijter, R. V. A. Orru, J. Org. Chem. 2010, 75, 1723- 1732.
“Diastereoselective Multicomponent Synthesis of Dihydropyridones with an Isocyanide Functionality” M. Paravidino, R. S. Bon, R. Scheffelaar, D. J. Vugts, A. Znabet, R. F. Schmitz, F. J. J. de Kanter, M. Lutz, A. L. Spek, M. B. Groen, R. V. A. Orru, Org. Lett. 2006, 8, 5369-5372.
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