The Synthesis and Applications of Sulfoxide Ligands and Methodology Development Towards Β-Amino Acid Incorporation in Peptides

Home , Sulfoxide, Triphosphane

The Synthesis and Applications of Sulfoxide Ligands and Methodology Development Towards β-Amino Acid Incorporation in Peptides

Priscilla Leung

A thesis submitted in conformity with the requirements for the degree of Masters of Science Department of Chemistry University of Toronto

The Synthesis and Applications of Sulfoxide Ligands and Methodology Development Towards β-Amino Acid Incorporation in Peptides

Priscilla Leung

Masters of Science

Department of Chemistry University of Toronto

2011 Abstract

The use of sulfoxide ligands for transition metal catalyzed transformations has recently been brought to the forefront in organic chemistry. The synthesis of a series of tri- and disulfoxides will be presented, and their applications investigated. Their use in rhodium catalyzed 1,4- additions of phenylboronic acid to 2-cyclohexen-1-one result in enantioselectivities up to 80%.

The incorporation of β-amino acid residues into polypeptides has resulted in new foldamers whose structures and enhanced stability provide interesting opportunities for new biological applications. A novel strategy for an iterative peptide synthesis involving β-amino acids will be proposed. Lastly, a hydroamidation-type strategy for the construction of β3-amino acids, or more specifically of β-(N-acylamino)acrylates, will be presented as preliminary work towards the goal of dipeptide synthesis.

Acknowledgments

First and foremost, I would like to thank Prof. Vy Dong for all her support and guidance throughout the past year. Thank you for your inspiration, passion for science, and for all the hard work you put in for your students.

Thank you to Peter Dornan for initiating the sulfoxide project and sharing his great ideas with me during our collaboration. Thank you to Hasan Khan and Boni Kim for always being there to talk about peptides. Thank you to Wilmer Alkhas for all his hard work, stories and jokes, and for being there to always lighten the mood. I also want to specially thank Tom Hsieh for continuously lending me a helping hand – you are a great chemist, mentor and friend. Thank you to the entire Dong group, who together make the lab a better place and one of the most positive and fun working environments I have ever been in.

Lastly, I would like to acknowledge all my family and friends who have forever believed in me and encouraged me to reach for the stars.

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Table of Contents

Acknowledgments ...... iii

Table of Contents ...... iii

List of Abbreviations ...... vii

List of Tables ...... xii

List of Schemes ...... xiii

List of Figures ...... xiv

List of Appendices ...... xv

Chapter 1 Synthesis and Applications of Novel C3- and C2-Symmetric Sulfoxide Ligands ...... 1

1.1 Introduction ...... 1

1.1.1 Recent Sulfoxide Ligands in Catalysis ...... 1

1.1.2 Sulfoxides versus Phosphines ...... 2

1.1.3 Could Sulfoxide Ligands Solve Problems in Hydroacylation? ...... 3

1.1.4 Applications of C3-Symmetry ...... 4

1.1.5 The Horeau Principle ...... 5

1.2 Research Goals ...... 6

1.3 Results and Discussion ...... 6

1.3.1 C3-Symmetric Ligand Synthesis ...... 6

1.3.1.1 Application to Asymmetric 1,4-Addition Reactions ...... 8

1.3.2 C2-Symmetric Ligands ...... 11

1.3.2.1 Application to Asymmetric 1,4-Addition Reactions ...... 12

1.3.3 Conclusions and Future Work ...... 13

1.3.4 Experimental Procedures ...... 14

1.3.4.1 General Considerations...... 14

1.3.4.2 General Procedure for Trisulfides ...... 15

1.3.4.3 General Procedure for Racemic Oxidation ...... 15

1.3.4.4 General Procedure for Asymmetric Oxidation ...... 15

1.3.4.5 General Procedure for Rhodium Catalyzed 1,4-Additions ...... 16

1.3.4.6 Characterization Data ...... 16

Chapter 2 Peptides by a Novel Condensation and Reduction Strategy ...... 19

2.1 Introduction ...... 19

2.1.1 Importance of -Peptides ...... 19

2.1.2 Traditional Methods for Peptide Synthesis and Their Applicability to - Peptide Synthesis ...... 20

2.1.3 Iterative Strategy for -Peptide Synthesis ...... 22

2.2 Research Goals ...... 23

2.3 Results and Discussion ...... 24

2.3.1 One Protecting Group ...... 24

2.3.2 Aminolysis of Starter Residue ...... 25

2.3.3 Condensation ...... 26

2.3.4 Reduction – A Catalyst-Controlled Hydrogenation ...... 27

2.3.5 Challenges of Aminolysis of a Dipeptide ...... 28

2.3.5.1 N-Methylation ...... 29

2.3.5.2 Thiol-Promoted Aminolysis ...... 30

2.3.6 Preliminary Attempts of a Second Condensation en route to a Tripeptide ...... 31

2.3.7 Preliminary Model Study for 2-Amino Acid Incorporation ...... 31

2.4 Conclusions and Future Work ...... 32

2.5 Experimental Procedures and Characterization Data ...... 32

2.5.1 First Aminolysis ...... 32

2.5.2 General Procedure for Condensation ...... 33

2.5.3 General Procedure for Asymmetric Hydrogenation ...... 34

2.5.4 General Procedure for Hydrogenations With an Achiral Catalyst ...... 35

2.5.5 General Procedure for Dipeptide Aminolysis ...... 36

2.5.6 Tripeptide ...... 37

2.5.7 β2-Test Substrates...... 37

Chapter 3 Synthesis of (Z)-β-(N-acylamino)acrylates: Progress Towards Dipeptide Synthesis via a Hydroamidation-Type Disconnection Strategy ...... 39

3.1 Introduction and Research Goals ...... 39

3.2 Results and Discussion ...... 40

3.2.1 Initial Screenings ...... 40

3.2.2 Exploring Gold Catalysis ...... 43

3.2.3 Adaptions from Other Literature Precedents ...... 45

3.2.4 Exploring Platinum Catalysis ...... 47

3.2.4.1 Early Experiments and Initial Hit ...... 47

3.2.4.2 Counterion Effects and Mechanistic Considerations ...... 50

3.2.4.3 Dual Effect of Platinum and Silver...... 52

3.2.4.4 Other Sources of Triflate ...... 53

3.2.4.5 Other Sources of Platinum ...... 54

3.2.4.6 Effect of Tridentate Ligands ...... 55

3.2.4.7 Extension to Dipeptide Synthesis ...... 57

3.2.4.8 Extension to Aryl Alkyne Esters ...... 57

3.3 Conclusions and Future Work ...... 58

3.4 Experimental Procedures ...... 58

3.4.1 General Considerations ...... 58

3.4.2 General Procedure for (Z)-β-(N-acylamino)acrylates ...... 58

Appendix A. NMR Spectra ...... 59

Appendix B. HPLC Traces…………………………………………………………....………….66

List of Abbreviations

1H proton NMR

13C carbon 13 NMR oC degrees Celsius

α alpha

β beta

γ gamma

δ chemical shift

Δ unsaturated, dehydro

μL microlitre aq. aqueous

Ac acetyl

Ala alanine

Ar aryl

BINAP 1,1’-binaphthalene-2,2’-diyl-bis-diphenylphosphine

BIPHEMP dimethylbiphenyl-2,2’-diyl-bis-diphenylphosphine br broad

Cbz benzyloxycarbonyl cod 1,5-cyclooctadiene conv. conversion

vii

Cy cyclohexyl d doublet; days dd doublet of doublets de diasteriomeric excess

DCE 1,2-dichloroethane

DCM dichloromethane

DMF N,N-dimethylformamide

DMSO dimethyl sulfoxide dppe 1,3-bis(diphenylphosphino)ethane dppp 1,3-bis(diphenylphosphino)propane dr diasteriomeric ratio dt doublet of triplets ee enantiomeric excess

EI electron impact equiv. equivalent

ESI electron spray ionization

Et ethyl

EtOAc ethyl acetate

Fmoc fluorenylmethyloxycarbonyl

GC-FID gas chromatography-flame ionization detector

viii

GC-MS gas chromatography-mass spectrometry h hours hAla homoalanine

HPLC high-pressure liquid chromatography

HRMS high resolution mass spectrometry

Hz hertz iPr isopropyl

IR infrared spectroscopy

J coupling constant

LC-MS liquid chromatography-mass spectrometry m multiplet

M molar mCPBA meta-chloroperbenzoic acid

Me methyl mg milligrams min minutes mL millilitres mmol millimoles

Ms mesylate

NHC N-heterocyclic carbene

NMR nuclear magnetic resonance

Ns nosyl ((4-nitrophenyl)sulfonyl)

NTf2 triflimidate

OTf triflate

PhCOOH benzoic acid

Phe phenylalanine

PhMe toluene ppm parts per million

PTSA p-toluenesulfonic acid py pyridine r.t. room temperature s singlet

SPPS solid-phase peptide synthesis t triplet tBu tert-butyl

TBAF tetra-n-butylammonium fluoride

TfOH triflic acid

Tf2O triflic anhydride

THF tetrahydrofuran

TLC thin layer chromatography

TMS trimethylsilane

Ts tosyl (p-toluenesulfonyl)

List of Tables

Table 1. Synthesis of C3-symmetric ligands...... 7

Table 2. Solvent screening in 1,4-additions ...... 10

Table 3. Summary of C3-symmetric ligands in 1,4-additions...... 11

Table 4. Synthesis of C2-symmetric ligands...... 12

Table 5. Summary of C2-symmetric ligands in 1,4-additions...... 13

Table 6. Initial screening with model system ...... 42

Table 7. Screening with AuCl ...... 43

Table 8. Screening with Au(I)PPh3 complexes ...... 44

Table 9. Screening with platinum...... 49

Table 10. Counterion effects on platinum catalyzed hydroamidations...... 51

Table 11. Further investigations into a platinum-silver system...... 53

Table 12. Effect of other triflate sources on hydroamidation...... 54

Table 13. Effect of other platinum sources on hydroamidation...... 55

Table 14. Effect of tridentate ligands on hydroamidation...... 57

xii

List of Schemes

Scheme 1. Example of the Horeau Principle ...... 5

Scheme 2. Solid-phase peptide synthesis ...... 21

Scheme 3. Iterative approach to -oligopeptides by Bode...... 23

Scheme 4. Disconnections: SPPS vs. CAR ...... 24

Scheme 5. Condensation and reduction approach to peptide synthesis ...... 24

Scheme 6. Aminolysis of (S)-phenylalanine starter unit...... 25

Scheme 7. Torsional strain imparted by PPP pincer ligands promotes protonolysis...... 56

xiii

List of Figures

Figure 1. C2-symmetric, disulfoxide ligands with applications in asymmetric 1,4-addition reactions...... 2

Figure 2. Effect of chelating atom...... 3

Figure 3. A C3-symmetric ligand...... 4

Figure 4. Examples of C3-symmetric ligands in catalysis...... 5

Figure 5. Retrosynthesis of trisulfoxide...... 6

Figure 6. Crystal structure of 1a...... 8

Figure 7. Crystal structure of 2d...... 13

Figure 8. - and -residues ...... 20

Figure 9. Possible protecting groups for the N-terminus...... 25

Figure 10. Crystal structure of (S,S’)-Cbz-Phe-3-hAla-OMe...... 28

Figure 11. Hydroamidation-type disconnection strategy...... 40

Figure 12. Proposed catalytic cycle involving an outer-sphere nucleophilic attack...... 52

xiv

List of Appendices

Appendix A. NMR Spectra ...... 59

Appendix B. HPLC Traces ...... 66

xv 1

Chapter 1 Synthesis and Applications of Novel C3- and C2-Symmetric Sulfoxide Ligands 1.1 Introduction

1.1.1 Recent Sulfoxide Ligands in Catalysis

The use of C2-symmetric sulfoxide ligands for rhodium catalyzed transformations has recently been brought to the forefront in organic chemistry. In 2008, Dorta and coworkers1 reported the synthesis of chiral disulfoxide ligands and the first highly enantioselective application of such ligands to late transition metal catalysis. The analogue of BINAP, named p-tol-BINASO (1,1’- binaphthalene-2,2’-diyl-bis-(p-tolylsulfoxide), could be prepared in one-pot in good yields (>70%) (eq. 1), and atropisomers could be separated with ease using column chromatography.

A rhodium complex [{(P,R,R)-p-tol-BINASO}RhCl]2 could be synthesized from precursor

[RhCl(C2H4)2]2 in greater than 90% yield. The use of this complex in Hayashi’s rhodium catalyzed 1,4-additions resulted in enantioselectivities up to 99% (eq. 2).

1 Mariz, R.; Luan, X.; Gatti, M.; Linden, A.; Dorta, R. J. Am. Chem. Soc. 2008, 130, 2172-2173.

In an analogous study in 2009, Dorta synthesized the disulfoxide analogue (p-tol-Me-BIPHESO) of BIPHEMP via a similar route, and revealed that even greater selectivity (>99% ee) could be achieved in 1,4-addition reactions.2 Other contributions from Liao3 and Li4 have also shown >99% enantioselectivities for rhodium catalyzed 1,4-additions (Figure 1).

Figure 1. C2-symmetric, disulfoxide ligands with applications in asymmetric 1,4-addition reactions.

1.1.2 Sulfoxides versus Phosphines

Although their application as ligands in asymmetric homogeneous metal catalysis has only been recently explored,5 the structure and bonding of sulfoxides in metal complexes has been thoroughly studied.6 The ability for sulfoxides to coordinate through oxygen (hard) or sulfur (soft) is dependent on the hardness or softness of the metal centre. Its mode of coordination may significantly affect reactivities and selectivies. Unlike common phosphine ligands used for asymmetric transformations, the chirality of a sulfoxide ligand can easily be set at the sulfur atom, providing a chiral environment that is closer to the metal centre if bound through sulfur.

In both of Dorta’s systems,2 p-tol-BINASO and p-tol-Me-BIPHESO provide a larger S-Rh-S bite angle than their phosphine analogues. Dihedral angles between the planes of the atropisomeric backbones are not significantly different, however. Coordination of sulfur results in a shortening of the S=O bond, indicative of its -donation ability. Perhaps the most interesting claim is that

2 Bürgi, J. J.; Mariz, R.; Gatti, M.; Drinkel, E.; Luan, X.; Blumentritt, S.; Linden, A.; Dorta, R. Angew. Chem. Int. Ed. 2009, 48, 2768-2771. 3 Chen, J.; Chen, J.; Lang, F.; Zhang, X.; Cun, L.; Zhu, J.; Deng, J.; Liao, J. J. Am. Chem. Soc. 2010, 132, 4552. 4 Chen, Q.; Dong, X.; Chen, M.; Wang, D.; Zhou, Y.; Li, Y. Org. Lett. 2010, 12, 1928. 5 Khiar, N.; Fernandez, I. Chem. Rev. 2003, 103, 3651-3705. 6 (a) Caurgo, O.; Calligaris, M. Coord. Chem. Rev. 1996, 153, 83-154. (b) Calligaris, M. Coord. Chem. Rev. 2004, 248, 351-375.

3 sulfoxides are more electron-donating than their phosphine counterparts. Based on a study of CO stretching frequencies in rhodium carbonyl complexes, sulfoxide-containing complexes displayed a decreased CO stretch, presumably due to higher electron donation by sulfur and more backbonding into the CO * orbital. Although no other studies have been done to verify these results, they do illustrate the excellent ability of sulfoxides to act as ligands.

1.1.3 Could Sulfoxide Ligands Solve Problems in Hydroacylation?

Previous hydroacylation studies in our group have found that a chelating atom (O, N, S) on the substrate is necessary to prevent decarbonylation and favour the desired hydroacylation product (Figure 2).7 Additionally, in the synthesis of phthalides by ketone hydroacylation,8 significant counterion effects are observed when the substrate does not contain a chelating atom. Counterions with relatively good to moderate coordination abilities produced the greatest reactivities and enantioselectivities.

Figure 2. Effect of chelating atom.

(a) Chelating atom occupies a coordination site. (b) In the absence of a chelating atom, counterion effects are observed; X = O, N, S; Y = O, CH2 (ketone, olefin hydroacylation, respectively).

The use of sulfoxides in diastereoselective olefin hydroacylation has already been documented 7b by our group. Peter Dornan, a fellow group member, envisioned the design of a C3-symmetric, tripodal, chiral trisulfoxide ligand that could potentially occupy the empty coordination site

(Figure 3). This C3-symmetric ligand would bind facially to rhodium, facilitating the correct

7 (a) Shen, Z.; Khan, H. A.; Dong, V. M. J. Am. Chem. Soc. 2008, 130, 2916.; (b) Coulter, M. M.; Dornan P. K.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 6932.; (c) Khan, H. A.; Kou, K. G. M.; Dong V. M. Chem. Sci. 2010. DOI: 10.1039/C0SC00469C 8 Phan, D. H. T.; Kim, B.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 15608.

4 transition state geometry for intramolecular ketone hydroacylation without the need for a directing heteroatom in the substrate.

Figure 3. A C3-symmetric ligand.

(a) Intermediate after C-H activation; tripodal ligand enforces facial coordination. (b) Large R groups and homochirality would invoke a propeller-like geometry. (c) Proposed tridentate ligand.

1.1.4 Applications of C3-Symmetry

While C2-symmetric ligands minimize the number of diastereomeric complexes that can result from substrate-binding in a square-planar complex, it has been postulated that C3-symmetric ligands should be effective in catalytic reactions involving octahedral metal complexes.9 In the selectivity-determining step, a C3-symmetric ligand should render the remaining three coordination sites in an octahedral complex homotopic.

A few accounts of the use of C3-symmetric ligands in catalysis have been reported. For example, in 1990, Burk10 synthesized a chiral triphosphane (Figure 4) and demonstrated its application in 11 the asymmetric hydrogenation of dimethyl itaconate in up to 95% ee. Xu detailed C3- symmetric tris(β-hydroxyamide) ligands that catalyze the asymmetric alkynylation of aldehydes in up to 92% ee. Gade’s tri(oxazoline) derivatives12 have been applied to the kinetic resolution of amino acid esters. Furthermore, an achiral tripodal triphosphine ligand, triphos, is somewhat

9 Moberg, C. Angew. Chem. Int. Ed. 1998, 37, 248-268. 10 Burk, M.J.; Harlow, R.L. Angew. Chem. Int. Ed. 1990, 29, 1462. 11 Fang,T;. Du, D. M;. Lu, S. F; Xu, J. X. Org. Lett. 2005, 7, 2081. 12 Gade, L. H.; Bellemin-Laponnaz, S. Chem. Eur. J. 2008, 14, 4142.

5 analogous to our trisulfoxides, and has been used in the catalysis of hydrogenation and hydroformylation reactions.13

Figure 4. Examples of C3-symmetric ligands in catalysis.

1.1.5 The Horeau Principle

In 1973, Horeau14 described the enhancement of enantiomeric excess by preparing and then separating diasteriomeric mixtures. Starting with the enantiopure material in Scheme 1 below, carbonate formation can result from two of the same enantiomers of menthol (homo), or two different enantiomers (hetero). Separation of these two diastereomers using column chromatography is facile. By discarding the hetero-carbonate and cleaving the homo-carbonate back into the original monomers, the enantiopurity of the initial mixture increases.

Scheme 1. Example of the Horeau Principle.

13 Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F. Organometallics 1990, 9, 226-240. 14 Vigneron, J. P.; Dhaenens, M.; Horeau, A. Tetrahedron 1973, 29, 1055-1059.

The Horeau principle can be applied to the synthesis of trisulfoxide ligands, as the formation of three chiral sulfur centres can lead to an overall enhancement of enantiopurity with respect to the enantioselectivity of a single oxidation. Any undesired selectivity will result in the formation of a hetero diastereomer, which can be easily removed during the purification process. The probability that all three sulfur atoms in one molecule will be oxidized with undesired selectivity is small.

1.2 Research Goals

Based on literature precedence and the practical need to solve a problem in hydroacylation, Peter

Dornan, a fellow group member, envisioned the design and synthesis of C3-symmetric, tripodal trisulfoxide ligands. One of the goals in this collaboration was to synthesize a variety of sulfoxide ligands and to explore other potential applications in asymmetric catalysis.

1.3 Results and Discussion

1.3.1 C3-Symmetric Ligand Synthesis

Our trisulfoxdes15 can be easily synthesized in two steps from the commercially available alkyltrichloride (Figure 5).

Figure 5. Retrosynthesis of trisulfoxide.

The nucleophilic substitution of 2,2,2-tris(chloromethyl)-ethane with naphthalenethiol can be achieved in up to 91% yield (Table 1). The neopentyl group requires harsher than normal conditions (NaH, 100oC). For the 1-naphthyl derivative, slower reaction rates, presumably due to sterics, typically require reaction times three times longer relative to the 2-naphthyl derivative. By-products in the alkylation step included a small amount of disulfide (from thiol oxidation),

15 Dornan, P.K.; Leung, P.L.; Dong, V.M. Tetrahedron 2011 (Invited Article), submitted.

7 which was difficult to separate in the 1-naphthyl case, and hence the crude was carried through to the oxidation.

Table 1. Synthesis of C3-symmetric ligands.

R % Yield % Yield (Step 2) adr (homo:hetero) % ee (Step 1) (Step 2) (Step 2)

2-nap (1a) 91b 60 3.1 : 1 (1 : 2.5) 99

1-nap (1b) - 35 (over 2 steps) 3.1 : 1 (1 : 1.8) 97

aDetermined by 1H NMR. In parentheses: dr using mCPBA as an oxidant. bReaction performed by Peter Dornan.

The key feature of this ligand synthesis is the triple asymmetric oxidation; three stereocentres are formed in one step and amplification of enantiomeric excess can be achieved with the Horeau principle. Assuming that each oxidation is independent, meaning that the selectivity of one oxidation does not affect the outcome of the next, an achiral oxidant such as mCPBA should provide a statistical ratio of 1:3 (homo:hetero). This independence was observed, as the 2- naphthyl ligand (as well as ligands with substituents Ph and Cy, not listed) produced a 1:2.5 homo:hetero ratio. With the use of a catalytic vanadium oxidation system with Schiff-base ligand 3, these ratios were reversed to approximately 3:1 homo:hetero. The hetero diastereomer could be separated by column chromatography, leaving the remaining homo diastereomer in up to 99% ee.

Single crystal x-ray analysis of the 1-naphthyl trisulfoxide (Figure 6) revealed that its (S)- absolute stereochemistry was in accordance with literature precedence for the asymmetric oxidation of alkyl aryl sulfides.16

Figure 6. Crystal structure of 1a.

1.3.1.1 Application to Asymmetric 1,4-Addition Reactions

Highest yielding ligand 1a was the easiest to synthesize in terms of reaction time and purification, hence it was chosen for use in further experiments. Although the design of the trisulfoxide was intended for asymmetric hydroacylation reactions, experiments performed with cationic rhodium and 2-(1-phenylvinyl) benzaldehyde17 (eq. 3) as a substrate did not result in 18 any enantioselectivity, albeit full conversion.

16 Drago, C.; Caggiano, L.; Jackson, R. F. W. Angew. Chem. Int. Ed. 2005, 44, 7221-7223. 17 Kundu, K.; McCullagh, J. V.; Morehead, A. T., Jr. J. Am. Chem. Soc. 2005, 127, 16042. 18 Reaction performed by Peter Dornan.

Hydrogenation experiments with the new ligand also resulted in full conversion but no enantiomeric excess. Finally, its use as a ligand in asymmetric 1,4-addition reactions proved successful, with an initial hit of 60% ee (Table 2, entry 1).

1.3.1.1.1 Optimization of 1,4-Additions

The initial hit with Dorta’s conditions1 resulted in a 73% isolated yield and 60% ee (Table 2, entry 1). Reducing the temperature resulted in a reduction of yield (37%), but a 10% increase in enantiomeric excess (entry 2). Other solvents such as p-dioxane (a common solvent in 1,4- additions19), dichloromethane, tetrahydrofuran, and methanol were tested (entries 3-7), with no further increase in enantiomeric excess. The use of ethyl acetate at room temperature initially gave 65% ee, but only 3% yield (entry 8); ethyl acetate at room temperature resulted in solubility issues, explaining the lower yields. A background reaction is observed, as lower ligand loadings relative to rhodium will contribute to a reduction in enantiomeric excess (entry 9). Excessive ligand does not significantly affect enantiomeric excess (entry 10), hence a 1:1.2 rhodium:ligand ratio was decided to be optimal. The optimized conditions (entry 11) produced a 73% yield and 80% ee. Deboronation and subsequent protonation resulting in a benzene by-product is a common competitive pathway in rhodium catalyzed 1,4-additions,19 hence the use of additional equivalents of relatively cheap phenylboronic acid was justified; however further optimization to limit the amount of this reagent may be possible. The use of [Rh(C2H4)2(acac)] (entry 12 and 13), a common source of rhodium in 1,4-additions with bidentate phosphines,19 gave a 86% yield when toluene was used as a solvent, but enantioselectivity fell to 33%. Although not shown, it was also determined that reaction time did not have an influence on enantiomeric excess. Reduction or addition of more water or KOH also stalled the reaction.

19 Fagnou, K.; Lautens, M. Chem. Rev. 2003, 103, 169-196.

Table 2. Solvent screening in 1,4-additions.

Temp. PhB(OH)2 Rh/ligand Isolated Yield Entry Solvent (oC) Time (h) equiv. ratio (%) % ee

1 PhMe 40 24 1.1 1/1 73 60

2 PhMe r.t. 4 1.1 1/1 37 70

3 dioxane 40 24 1.1 1/1 17 45

4 DCM 40 24 1.1 1/1 73 60

5 DCM:PhMe (1:1) 40 24 1.1 1/1 75 60

6 THF 40 4 1.1 1/1 17 40

7 MeOH 40 4 1.1 1/1 27 57

8 EtOAc r.t. 24 4 1/1 3 65

9 EtOAc r.t. 24 4 4/1 20 42

10 EtOAc r.t. 24 4 1/3 56 80

11 EtOAc 40 24 4 1/1.2 72 80

12a EtOAc 40 24 4 1/1 0b -

13a PhMe 40 24 4 1/1 86 33

a b With [Rh(C2H4)2(acac)]. No conversion.

The use of 1b in 1,4-additions resulted in stalled reactivity and lower enantioselectivity. As seen with the reactions towards ligand synthesis, the 1-naphthyl substituent significantly increases reaction times. Table 3 shows a summary of the performance of C3-symmetric ligands in 1,4- additions.

Table 3. Summary of C3-symmetric ligands in 1,4-additions.

Entry Ligand Yield (%) ee (%)

1 1a 72 80 2 1b 15 50

1.3.2 C2-Symmetric Ligands

As the mechanism of rhodium catalyzed 1,4-additions typically involves bidentate phosphines,19 it was necessary to synthesize the bidentate sulfoxides for comparison (Table 4). Ligand 2a can be seen as a bridge between the C3-symmetric trisulfoxide and the other bidentate sulfoxides that resemble the bis(diphenylphosphino)alkane series. HPLC analysis after recrystallization showed diastero- and enantiopure material for all ligands. Ligand 2b has been previously synthesized, and its meso form is used for allylic C-H functionalizations.20 Unfortunately the synthesis of an enantiopure 2-naphthyl ligand with a two-carbon linker could not be achieved using column chromatography or recrystallization methods for purification. Similar derivatives of ligands 2a- 2e have been synthesized also.21

20 For example: Rice, G.T.; White, C. J. Am. Chem. Soc. 2009, 131, 11707-11711. 21 Skarzewski, J.; Ostrycharz, E.; Siedlecka, R. Tetrahedron: Asymm. 1999, 10, 3457.

Table 4. Synthesis of C2-symmetric ligands.

a a Ligand ( )n R % Yield % Yield dr (homo:meso) % ee

(Step 1) (Step 2) (Step 2) (Step 2)

b,c 2a CH2C(Me)2CH2 2-nap 79 38 >99 : 1 (1 : 1.8) >99

c 2b (CH2)2 Ph 85 9 99 : 1 (1 : 4.0) >99

c 2c (CH2)3 2-nap 72 12 49 : 1 (1 : 1.2) >99

c 2d (CH2)4 2-nap 70 49 20 : 1 (1 : 1.0) >99

2e (CH2)5 2-nap - 22 (over 24 : 1 (1 : 1.0) >99 2 steps) aDetermined by HPLC after recrystallization. In parentheses: dr using mCPBA as an oxidant. bReaction conditions same as 1a, 1b. cReaction performed by Peter Dornan.

1.3.2.1 Application to Asymmetric 1,4-Addition Reactions

C2-symmetric ligands were subsequently applied to 1,4-addition reactions. Similar to the trisulfoxide ligands, 2a resulted in 80% ee; however isolated yield improved to 89% (Table 5, entry 1). Although no kinetic studies were performed, the reaction with 2a reached full conversion in less time (18 h) than with 1a. The use of phenyl substituted 2b gave no selectivity (entry 2). Furthermore, an interesting trend of inversed enantio-induction was observed with the use of ligands 2c-2e, where enantioselectivity increases in the opposite direction with increasing carbon-linker length. Additionally, it is postulated that the increased bite angle in 2e may not allow for bidentate coordination of the ligand to rhodium; future study of a mono-sulfoxide analogue would be interesting.

Table 5. Summary of C2-symmetric ligands in 1,4-additions.

Entry Ligand Yield (%) ee (%)

1 2a 89 80 2 2b 82 0 3 2c 66 -20 4 2d 35 -31

5 2e 15 -43

To verify that the disulfoxides that demonstrated opposite enantio-induction had the predicted (S)-absolute configuration, crystals were grown for x-ray analysis (Figure 7).

Figure 7. Crystal structure of 2d.

Similar rigidity in the backbone of 2a to the trisulfoxide 1a can be used to rationalize its relative success in enantio-induction. Nonetheless, it can be concluded that the third arm of the trisulfoxide remains a spectator in the 1,4-addition reaction.

1.3.3 Conclusions and Future Work

In collaboration with Peter Dornan, a series of trisulfoxide ligands were designed and synthesized. Although originally designed to solve problems in hydroacylation, no enantio- induction was observed; however, their application in rhodium catalyzed 1,4-addition reactions produced up to 80% ee. The testing of a series of disulfoxides in 1,4-additions concluded that the

14 third arm of the trisulfoxide was unnecessary in this reaction. An interesting trend of inversed enantio-induction can be observed with disulfoxides containing flexible backbones, and increased rigidity can be associated with greater enantioselectivity. Further testing of the applications of these sulfoxide ligands, as well as investigations into coordination chemistry are areas of future focus.

1.3.4 Experimental Procedures

1.3.4.1 General Considerations

Commercial reagents were purchased from Sigma Aldrich, Strem or Alfa Aesar and used without further purification. All reactions were carried out under an argon atmosphere unless otherwise indicated. Reactions were monitored using thin-layer chromatography (TLC) on EMD Silica Gel

60 F254 plates and a Waters 2795 LC-MS. Visualization of the developed plates was performed under UV light (254 nm), or with KMnO4, or 2,4-dinitrophenylhydrazine stain. Organic solutions were concentrated under reduced pressure on a Büchi rotary evaporator. 1H and 13C NMR spectra were recorded on a Varian Mercury 300, Varian Mercury 400, VRX-S (Unity) 400, or Bruker AV-III 400 spectrometer. NMR spectra were internally referenced to the residual solvent signal or TMS. Data for 1H NMR are reported as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad), coupling constant (Hz), integration. Data for 13C NMR are reported in terms of chemical shift (δ ppm).

High resolution mass spectra (HRMS) were obtained on a micromass 70S-250 spectrometer (EI) or an ABI/Sciex QStar Mass Spectrometer (ESI). Infrared (IR) spectra were obtained on a Perkin-Elmer Spectrum 1000 FT-IR Systems and are reported in wavenumber (cm-1). GC-MS analysis was performed on an Agilent Technologies 7890A (GC) and 5975C inert XL EI/CI MSD system with an HP-5 column (0.320 mm, 0.25 μm film). GC-FID analysis was performed on an Agilent Technologies (7890A) system. Melting point ranges were determined on a Fisher- Johns Melting Point Apparatus. Enantiomeric excesses were ascertained on an Agilent 1100 Series HPLC. Diastereomeric ratios were determined by integration of crude 1H NMR spectra or HPLC analysis. Optical rotations were measured on a Rudolph Research Analytical Autopol IV Automatic Polarimeter. Column chromatography was performed with Silicycle Silia-P Flash Silica Gel, using glass columns. All salts were purchased from Aldrich and used without purification. Solvents were purchased from Caledon. Tetrahydrofuran, diethyl ether, N,N-

15 dimethylformamide, toluene, and dichloromethane were used directly from a Pure Solv. solvent purification system (Innovative Technology, Inc.).

1.3.4.2 General Procedure for Trisulfides

Sodium hydride (6 equiv., 60% dispersion in oil) and sodium iodide (1 equiv.) were dissolved in N,N-dimethylformamide (0.1 M) in a flame-dried round bottom flask. The appropriate thiol (6 equiv.) was added dropwise, followed by 2,2,2-tris(chloromethyl)-ethane (1 equiv.). The mixture was stirred at 100°C for 24 h. The crude mixture was diluted with ethyl acetate, washed twice with 1:1 1 M NaOH/brine and the combined organic extracts were dried with Na2SO4. The solvent was removed in vacuo.

1.3.4.3 General Procedure for Racemic Oxidation

Sulfide (1 equiv.) was dissolved in DCM (1 M) and cooled to 0°C. mCPBA (3.05 equiv. for trisulfides, 2.05 equiv. for disulfides; 74% by weight) was then added, and the reaction was stirred at 0°C for 30 minutes. The crude mixture was poured into brine and aqueous 1 M NaOH was added to basify. This mixture was extracted twice with DCM and the organic extracts were dried with Na2SO4. The solvent was removed in vacuo. The homo diastereomer was isolated by preparative TLC. For ligands 2b-2e, the meso diastereomer could not be removed by chromatography, but could be resolved during HPLC analysis.

1.3.4.4 General Procedure for Asymmetric Oxidation

Ligand 3 (8 mol% for trisulfides, 5 mol% for disulfides; synthesized according to Jackson22), was dissolved in one third of total volume DCM to give a yellow solution. VO(acac)2 (4 mol% for trisulfides, 2.5 mol% for disulfides) was dissolved in another equal portion of DCM, and this was added to the first solution. The mixture was initially greenish-blue, and after stirring at room temperature for 30 minutes, the solution turned green-brown. The trisulfide (1 equiv., 0.17 M final concentration) was added in the final portion of DCM, and this was stirred at room temperature for 30 minutes. The reaction was then cooled to 0°C and 30% H2O2 was added dropwise (3.2 equiv. for trisulfides, 2.4 equiv. for disulfides). The reaction was stirred at 0°C

22 Pelotier, B.; Anson, M. S.; Campbell, I. B.; Macdonald, S. J. F.; Priem, G.; Jackson, R. F. W. Synlett 2002, 1055.

16 until the first appearance of over-oxidation as judged by LC-MS analysis. The reaction was quenched by addition of sodium thiosulfate, and extracted twice with DCM. The organic layer was dried with Na2SO4, and the solvent was removed in vacuo.

1.3.4.5 General Procedure for Rhodium Catalyzed 1,4-Additions

In a glovebox, the sulfoxide ligand (0.0136 mmol) was dissolved in ethyl acetate and added to

[Rh(C2H4)2Cl]2 (2.2 mg, 0.006 mmol) in ethyl acetate (3 mL total). The resulting mixture was allowed to stir at room temperature for 20 minutes, upon which degassed 2-cyclohexen-1-one (36 mg, 0.375 mmol) and phenylboronic acid (183 mg, 1.5 mmol, 4 equiv.) were added. The reaction flask was sealed with a rubber stopper and brought out of the glovebox, upon which 2.5

M KOH(aq) (0.075 mL, degassed) was added via syringe. The reaction was performed under argon at 40°C and monitored by LC-MS. Upon completion, the mixture was filtered through celite, concentrated, and purified by preparative TLC (eluent 30% ethyl acetate in hexanes).

HPLC analysis: OD-H, 1:99 isopropanol:hexanes, 0.5 mL/min, 254 nm, tR1 = 34.5 min (R), tR2 = 38.9 min (S). Other characterization data is in accordance with literature.2

1.3.4.6 Characterization Data

2,2,2-tris((2-naphthylsulfinyl)methyl)-ethane (1a). Synthesized according to the general procedure for asymmetric oxidation (reaction time: 3 h, 1.41 mmol trisulfide). The product was isolated by column chromatography (65% ethyl acetate in hexanes) to give a white solid (500 ° 1 mg, 60%); m.p. 87-90 C. H NMR (400 MHz, CDCl3) δ 2.11 (s, 3H), 3.21 (d, J = 14.0 Hz, 3H), 3.71 (d, J = 13.9 Hz, 3H), 7.55–7.63 (m, 6H), 7.74 (dd, J = 8.6, 1.8 Hz, 3H), 7.89-7.95 (m, 6H), 13 8.00 (d, J = 8.6 Hz, 3H), 8.26 (s, 3H); C NMR (100 MHz, CDCl3) δ 25.3, 39.6, 67.5, 120.0, 124.6, 127.3, 127.9, 128.0, 128.6, 129.8, 132.9, 134.5, 140.9; IR (neat): 2957, 1714, 1504, 1345 -1 + 1066, 1032, 812, 744 cm ; HRMS (ESI+) Calcd. for [C35H30S3O3H] 595.1429, found 595.1434. 25 [α] D = –383 (c 1.1, CHCl3). HPLC analysis: 99% ee (AD-H, 4:6 isopropanol:hexanes, 1 mL/min, 254 nm, tR1 = 49.8 min (minor, RRR), tR2 = 54.1 min (major, SSS).

2,2,2-tris((1-naphthylsulfinyl)methyl)-ethane (1b). Synthesized according to general procedure for the synthesis of trisulfides (reaction time: 2.5 d, 0.78 mmol 2,2,2- tris(chloromethyl)-ethane). Purification was difficult, therefore the crude material was subjected to asymmetric oxidation directly according to the general procedure for asymmetric oxidation (reaction time: 6 h, 287 mg crude, 0.535 mmol (assuming pure trisulfide)). The product was isolated by column chromatography (60% ethyl acetate in hexanes) to give a white solid (110 mg, 35% over two steps). A single crystal suitable for X-ray diffraction was grown by slow evaporation from a solution of 60% ethyl acetate in hexanes. m.p. 190oC (decomposed). 1H

NMR (400 MHz, CDCl3) δ 2.37 (s, 3H), 3.36 (d, J = 14.0 Hz, 3H), 3.75 (d, J = 14.0 Hz, 3H), 7.66-7.56 (m, 9H), 7.95 (d, J = 8.3 Hz, 3H), 7.97 (d, J = 8.2 Hz, 3H), 8.09 (d, J = 8.4 Hz, 3H), 13 8.14 (d, J = 7.3 Hz, 3H); C NMR (100 MHz, CDCl3) δ 24.6, 40.4, 65.2, 121.9, 123.7, 126.0, 127.1, 127.9, 128.8, 129.3, 131.8, 133.8, 140.1; IR (neat): 3049, 2955, 1503, 1379, 1045, 808, -1 + 25 772, 743 cm ; HRMS (ESI+) Calcd. for [C35H30S3O3H] 595.1429, found 595.1406. [] D =

–783 (c 0.24, CHCl3). HPLC analysis: 97% ee (AD-H, 4:6 isopropanol:hexanes, 1 mL/min, 254 nm, tR1 = 17.5 min (minor, RRR), tR2 = 41.9 min (major, SSS).

1,5-bis(naphthalen-2-ylsulfinyl)pentane (2e). To 1,5-pentanediol (2.0 g, 0.019 mol) was added 4 ml of pyridine and the solution was stirred and cooled to 0°C. p-Toluenesulfonyl chloride (7.2 g, 0.038 mol, 2 equiv.) in 12 ml pyridine was added by addition funnel over 30 minutes. The reaction mixture was stirred at 0oC for 4 h. Addition of 20 ml of cold water precipitated a white solid. The desired product was separated from the mono-tosylate (3:1 ratio) by recrystallization from hot methanol, affording 2.54 g (32%) of pentane-1,5-diyl bis(p-toluenesulfonate), as clear

18 long crystals whose characterization data is in agreement with literature.23 Pentane-1,5-diyl bis(p-toluenesulfonate) (2.54 g, 0.0061 mmol), 2-thionaphthol (2.44 g, 0.015 mmol, 2.5 eq.), and potassium carbonate (2.5 g, 0.018 mmol, 3 equiv.) were dissolved in 50 ml of acetone and stirred at 45oC for 24 h. The mixture was then filtered, concentrated, taken up in dichloromethane, and washed with 1M NaOH and brine. The aqueous layer was washed twice with dichloromethane. 1 Combined organic layers were dried over Na2SO4, and evaporated to afford a beige solid. H NMR analysis indicated a 3:1 ratio of the desired disulfide to the disulfide resulting from oxidation of 2-thionaphthol, which was carried through to the oxidation step without further purification.

The title compound was prepared according to the general procedure for asymmetric oxidation from the crude mixture of 1,3-bis(naphthalen-2-ylthio)pentane, (1.0 g crude material, reaction time: 3.75 h). The off-white product was isolated by column chromatography eluting with ethyl acetate. Two recrystallizations from boiling acetone yielded a white solid. (181 mg, 22%, calculated based on the NMR-determined ratio of desired disulfide added in the oxidation 1 reaction); m.p. 104-105°C. H NMR (400 MHz, CDCl3) δ 1.52-1.67 (m, 4H), 1.77-1.86 (m, 2H), 2.76-2.90 (m, 4H), 7.52 (dd, J = 8.6, 1.7 Hz, 2H), 7.55-7.60 (m, 4H), 7.87-7.92 (m, 4H), 7.94 (d, 13 J = 8.6 Hz, 2H), 8.14 (s, 2H); C NMR (100 MHz, CDCl3) δ 21.6, 27.7, 56.2, 119.7, 124.6, 127.3, 127.8, 128.0, 128.5, 129.5, 132.8, 134.4, 140.7. IR (neat): 3050, 2935, 1070, 1029, 867,

-1 + 25 813, 755, 742 cm ; HRMS (ESI) Calcd. for [C25H24O2S2+H] 421.1290, found 421.1286; [α] D=

–193 (c = 0.27, CHCl3). HPLC analysis: >99% ee (AD-H, 4:6 isopropanol:hexanes, 1 mL/min,

210 nm, tR1 = 17.4 min (minor, RR), tR2 = 20.4 min (major, SS), tR3 = 21.6 min (meso).

23 Walczak, R. M.; Cowart, J. S.; Reynolds, J. R. J. Mat. Chem. 2007, 17, 254.

Chapter 2 Peptides by a Novel Condensation and Reduction Strategy 2.1 Introduction

2.1.1 Importance of -Peptides

The synthesis and study of peptides containing -amino acids has been driven by their significance and contributions in biological applications. Structure and function is well studied in the field of biochemistry; substitution of a natural amino acid residue for an unnatural one may result in very different properties. The design of new foldamers is a topic of much interest. Foldamers are unnatural oligomers whose conformational properties mimic natural oligomers, such as nucleic acids and proteins.24 Studying protein-protein interactions and their disruptions in cellular processes is significant, as unwanted molecules that bind to protein surface sites can contribute to disease. It is often difficult to find high-affinity and metabolically stable ligands capable of binding to protein surfaces. Research on -peptide inhibitors of protein-protein interactions has been extensive; however, this class of molecules is ultimately susceptible to protease catalyzed degradation.25 Foldamer backbones can be strengthened and less susceptible to degradation when -residues are substituted for beta ones, or when -peptides are used exclusively. The possibility for substitution at the - and -positions (Figure 8), giving, respectively, 2 and 3 (or 2,3) isomers, also increases the potential for biological screening.

24 (a) Horne, W.S.; Boersma, M.D.; Windsor, M.A.; Gellman, S.H. Angew. Chem Int. Ed. 2008, 47, 2853.; (b) Bruckner, A.M.; Chakraborty, P.; Gellman, S.H.; Diederichsen, U.; Angew. Chem. Int. Ed. 2003, 42, 4395-4399.; (c) Horne, W.S.; Gellman, S.H. Acc. Chem. Res. 2008, 41, 1399-1408. 25 (a) Sadowsky, J.D.; Murray, J.K.; Tomita, Y., Gellman, S.H. ChemBioChem 2007, 8, 903-916.; (b) Sadowsky, J.D.; Schmitt, M.A.; Lee,H.-S.; Umezawa, N.; Wang,S.; Tomita,Y.; Gellman, S.H. J. Am.Chem.Soc. 2005, 127, 11966-11968

Figure 8. - and -residues.

Studies by Seebach26 and Gellman27 have shown that 3-oligopeptide motifs can be used to design molecules with predictable secondary and tertiary structures. The incorporation of 3- amino acids has led to other biological properties such as antimicrobial activity28 and the inhibition of -secretase29. The inclusion of -building blocks provides the properties and activities one may achieve by using -residues, with the advantage of increased metabolic stability and the opportunity to create more diverse sequences that lead to a larger variety of conformations that can resemble protein structures.24

2.1.2 Traditional Methods for Peptide Synthesis and Their Applicability to -Peptide Synthesis

Solid-phase peptide synthesis (SPPS) has been the dominant method for synthesizing peptides since its introduction in 1963 by Bruce Merrifield.30 SPPS (Scheme 2) relies on a continuous cycle of activation and deprotection – the C-terminus of the incoming N-terminus protected amino acid residue undergoes carbodiimide and/or triazole activation, followed by deprotection of the N-terminus.31 Unlike ribosomal protein synthesis, SPPS is accomplished in the C- to N-

26 Seebach, D.; Overhand, M.; Kühnle, F.N.M.; Martinoni, B. Helv.Chim.Acta 1996, 79, 913-941. 27 Appella, D.H.; Christianson, L.A.; Karle, I.L.; Powell, D.R.; Gellman, S.H. J. Am. Chem. Soc. 1996, 118, 13071-13072. 28 Imamura,Y.; Watanabe, N.; Umezawa, N.; Iwatsubo, T.; Kato, N.; Tomita, T.; Higuchi, T. J. Am.Chem.Soc. 2009, 131, 7353- 7359. 29 Porter, E.A.; Wang, X.; Lee, H.S.; Weisblum, B.; Gellman, S.H. Nature 2000, 404, 565. 30 Merrifield, R.B. J. Am. Chem. Soc. 1963, 85, 2149-2154. 31 Bray, B. L. Nature Reviews 2003, 2, 587-593.

21 terminus direction. The C-terminus residue is attached to a resin polymer, which is removed at the end of the synthesis, followed by the deprotection of remaining protecting groups.

Scheme 2. Solid-phase peptide synthesis.

The application of traditional activation/deprotection peptide coupling approaches to the assembly of -oligopeptides is often hampered by slow reactivity, and several equivalents of precious -amino acids are often needed.32 Most 3- building blocks (which can be obtained from their respective natural -amino acids using Arndt-Eistert homologation33) are commercially available for solid-phase; however, 2- residues are specially constructed via solution phase synthesis.34 Epimerization of 2-residues is facile; di- or tripeptides containing such residues may be synthesized first in the solution phase before being subjected to solid phase for peptide extension.35 The extension of exclusively 3-peptides beyond five or six units is

32 Carrillo, N., Davalos, E.A., Russak, J. A., Bode, J.W. J. Am. Chem. Soc. 2006, 128, 1452-1453. 33 Arndt, F.; Eistert, N. Ber. Dtsch. Chem. Ges. 1935, 68, 200-208. 34 The enantioselective organocatalytic Mannich reaction by Gellman is a useful approach: Chi, Y.; Gellman, S.H. J. Am. Chem. Soc. 2006, 128, 6804-6805. Also Seebach’s DIOZ auxiliary: Seebach, D.; Schaeffer, L.; Gessier, F.; Bindschadler, P.; Jager, C.; Josein, D.; Kopp, S.; Lelais, G.; Mahajan, Y.R.; Micuch, P.; Sebesta, R.; Schweizer, B.W. Helv. Chim. Acta 2003, 86, 1852- 1861. 35 Arvidsson, P.I.; Frackenpohl, J.; Seebach, D. Helv. Chim. Acta 2003, 86, 1522-1553.

22 challenging with SPPS, resulting in low purity of the desired peptide chain.35,36 Aggregation, folding, and helix formation are common problems. Microwave conditions have been shown to improve purity slightly for longer chains that form 14-helices.36

For the synthesis of longer -peptides, native chemical ligation37 has been explored by Seebach.38 Unfortunately, there are not yet methods to perform -peptide ligations without a thiol-containing side chain, and enantiopure 3-cysteine is one of the few 3-amino acids that is not commercially available.39

2.1.3 Iterative Strategy for -Peptide Synthesis

In 2006, the Bode group developed an iterative, aqueous solution phase synthesis for - oligopeptides (Scheme 3).30 This novel concept presents an extension to his earlier work on - peptides40 – it is a simple and environmentally begin approach to creating peptide bonds without the use of standard coupling reagents. Coupling aldehydes containing the desired monomer side chain with sugar-derived hydroxylamines results in the in situ formation of a nitrone. By applying Vasella’s known diastereoselective nitrone cycloaddition41 to 2-methoxyacrylates, isoxazolidine acetals can be formed in >99% enantiomeric purity. These isoxazolidine acetals undergo decarboxylative couplings with -ketoacids in 1:1 tBuOH/H2O in up to 93% yield in 1 hour. Hydrolysis of the newly formed -ketoester at the C-terminus allows for continuous iteration. Tripeptides may be formed with up to 90% yield during the third iteration. Aliphatic and protected lysine and glutamic acid side chains can be tolerated in this green method.

36 Murray, J.K.; Gellman, S.H.; Org. Lett. 2005, 7, 1517-1520. 37 Dawson, P.E.; Muir, T.W.; Clark-Lewis, I.; Kent, S.B. Science. 1994, 266, 776-669. 38 Kimmerlin,T.; Seebach,D.; Hilvert,D. Helv.Chim.Acta 2002, 85, 1812-1826. 39 Seebach, D.; Kimmerlin, T.; Sebesta, R.; Campo, M.A.; Beck, A.K. Tetrahedron, 2004, 60, 7455-7506. 40 Bode, J.W.; Fox, R.M.; Baucom, K.D. Angew. Chem. Int. Ed. 2006, 45, 1248 –1252. 41 Vasella, A. Helv. Chim. Acta 1977, 60, 1273-1295.

Scheme 3. Iterative approach to -oligopeptides by Bode.

2.2 Research Goals

We have envisioned a novel strategy for the construction of peptides that will be complementary to the strategies discussed previously. One of the goals of our group is to develop more efficient and environmentally friendly methodologies. In line with this, we have proposed a condensation and reduction (termed “CAR”) approach to peptide synthesis (Scheme 4), where water is an environmentally benign by-product, and atom economical hydrogenations determine the stereochemical outcome of the peptide residues. Only one protecting group at the N-terminus is required, and activating agents seen in traditional peptide synthesis are not employed. Contrary to SPPS, this method synthesizes peptides in the opposite direction, N to C-terminus, like nature does, and relies on a unique disconnection that does not involve the amide bond. This method has been applied to the synthesis of an -tripeptide,42 and its application towards the synthesis of peptides containing -peptides will be discussed.

42 Unpublished work by fellow group member, Byoungmoo Kim.

Scheme 4. Disconnections: SPPS vs. CAR.

Scheme 5. Condensation and reduction approach to peptide synthesis.

2.3 Results and Discussion

2.3.1 One Protecting Group

In our proposed strategy, one protecting group on the N-terminus of the starting amino acid is required, which is somewhat analogous to the resin used in SPPS. A suitable protecting group (Figure 9) must be orthogonal to acid catalyzed condensation, homogenous hydrogenation, and basic aminolysis conditions. The popular SPPS Fmoc group was deemed unsuitable in 7 N ammonia in methanol aminolysis conditions – the unprotected amino acid was detected by LC- MS analysis. This is not surprising as a weak base, piperidine, is commonly used in SPPS deprotections. A sulfonamide-based nosyl group (Ns) was experimented with, as its deprotection requires milder conditions (PhSH, K2CO3 in DMF) relative to the tosyl group. Unfortunately, addition of the nosyl protecting group to (S)-phenylalanine methyl ester already created solubility issues. The Cbz protecting group was finally chosen, as commercially available protected residues were easily accessible, and deprotection requires (preferably) hydrogenolysis via a heterogeneous mechanism, or use of a strong acid such as HBr.

Figure 9. Possible protecting groups for the N-terminus.

2.3.2 Aminolysis of Starter Residue

We chose a Cbz-protected (S)-phenylalanine for the first amino acid, taking advantage of commercially available N-protected natural amino acids for the ease of the initial testing of our method. The amino acid was first converted to the methyl ester by forming a reactive acid chloride in situ (Scheme 6). Aminolysis was achieved with 20 equivalents of ammonia (7 N solution in methanol), and the driving force is presumed to be the precipitation of the polar amide during the course of the reaction. Concentration and filtration of the resulting suspension of product in methanol solution afforded Cbz-Phe-NH2 in 50% yield. On a 13.5 g scale, the amide was isolated in 88% yield. It is possible to start with the desired -unit by forming an unprotected enamine from ammonium acetate and a -ketoester, subsequently performing an asymmetric hydrogenation with a rhodium-Josiphos system,43 protecting using Cbz-Cl, and finally subjecting the ester to aminolysis conditions.

Scheme 6. Aminolysis of (S)-phenylalanine starter unit.

43 Hsiao, Y.; Rivera, N.R.; Rosner, T.; Krska, S.W.; Njolito, E.; Wang, F.; Sun, Y.; Armstrong III, J.D.; Grabowski, E.J.J.; Tillyer, R.D.; Spindler, F.; Malan, C. J. Am. Chem. Soc. 2004, 126, 9918-9919.

2.3.3 Condensation

Preparation of -ketoenamides via the direct acid catalyzed condensation of 1,3-dicarbonyl compounds with acetamide has been shown by Wu and Zhang.44 There has also been precedence 45 with -ketoesters. Condensation of Cbz-Phe-NH2 with methyl acetoacetate to afford the dehydro-dipeptide (Z)-Cbz-Phe-3-hAla-OMe was achieved in 65% yield (eq. 4) using PTSA as a catalyst and toluene as a solvent under Dean Stark conditions. On larger scales, the yield decreases to 35%. The (Z)-isomer is formed exclusively using this method, and its characterization can be made by 1H NMR, where the NH proton appears much more downfielded (~11.5 ppm relative to ~9 ppm) due to intramolecular hydrogen bonding.46

Studies on this system have shown that byproducts result when the product is refluxed under the same conditions with and without additional methyl acetoacetate equivalents. Polymerization of the desired product is likely, as the byproduct(s) lie on the baseline of a TLC and possess hard and sticky physical characteristics. At dilute concentrations (<0.05 M), solubility of the refluxed reaction mixture improves and byproducts seem to be slightly minimized. Achieving a moderate yield results from stopping the reaction at an ideal time, where conversion of the starting material is maximal and byproduct formation is minimal. Up to 33% of the initial starting material can be recovered cleanly by concentrating the reaction mixture in vacuo and recovering the solid by filtration.

44 Geng, H. Zhang, W. Chen, J. Hou, G, Zhou, L. Zou, Y. Wu, W. Zhang, X. Angew. Chem. Int. Ed. 2009, 48, 6052-6054. 45 Ovenden, S.P.B., Capon, R.J., Lacey, E., Gill, J. H., Friedel, T., Wadsworth, D. J. Org. Chem. 1999, 64, 1140-1144. 46 Lee, J.M., Ahn, D.S., Jung, D.Y., Lee, J., Do, Y., Kim, S.K., Chang, S. J. Am. Chem. Soc. 2006, 128, 12954-12962.

Milder conditions and different sources of dehydrating agents were used in an attempt to improve yields. Procedures adapted from known literature used acetic anhydride,47 molecular 48 sieves, and Ti(OEt)4 (also an activating agent) as dehydrating sources. Substitution of the acid for pivalic acid49 offered no reactivity. The use of 4-methyoxyphenol as a reagent to alleviate polymerization also had no effect. 50

2.3.4 Reduction – A Catalyst-Controlled Hydrogenation

β-(N-acylamino)acrylates are considered benchmark substrates in the testing of new catalysts for enantioselective hydrogenations. Extensive studies51 provided excellent precedence for the hydrogenation of our dehydro-peptides. TangPhos,52 DuPhos,53 and DuanPhos54 ligands are capable of achieving greater than 90% enantioselectivities. Such ligands were applied to the hydrogenation of Cbz-(S)-Phe-3-hAla-OMe using cationic rhodium. Et-DuPhos and TangPhos did not result in full conversion after 12 h (40oC, 200 psi), while full conversion with iPr-DuPhos and Me-DuPhos afforded a dr (S,S’):(S,R’) of 6.3 : 1 and 2.5: 1, respectively, by NMR analysis. . o (S,S’,R,R’)-DuanPhos (1 mol%) with Rh(cod)BF4 H2O (25 C, 15 psi, 12 h) was determined to be the optimal catalyst, affording quantitative yield and 99:1 dr (HPLC) before recrystallization (eq.

5). Interestingly, the analogous Rh(cod)BF4 required a 10 mol% loading for full conversion over 12 h.

47 Vieira, Y.W.; Nakamura, J.; Finelli, F.G.; Brocksom, U.; Brockson, T.J.; J. Braz. Chem. Soc. 2007, 18, 448-451. 48 Liu, G.; Cogan, D. A.; Owens, T.D.; Tang, T.P.; Ellman, J.A. J. Org. Chem. 1999, 64, 1278-1284. 49 Tudge, M.; Savarin, C.G.; DiFelice K.; Maligres, P.; Humphrey, G.; Reamer, B.; Tellers, D.M.; Hughes, D. Org. Process Res. Dev. 2010, 14, 787–798. 50 Crestey, F.; Collot, V.; Stiebing, S.; Rault, S. Synthesis 2006, 3506-3514. 51 de Vries, J.G. Elsevier, C.J. Handbook of Homogeneous Hydrogenation. Wiley, 2006. 52 Tang, W., Zhang, X. Org. Lett. 2002, 4, 4159-4161. 53 Heller, D., Holz, J., Derexler, H.J., Lang, J., Drauz, K., Krimmer, H.P., Borner, A. J. Org. Chem. 2001, 66, 6816-6817. 54 Liu, D., Zhang, X. Eur. J. Org. Chem. 2005. 646-649.

Single crystal x-ray analysis determined that (S,S’,R,R’)-DuanPhos gave the (S)-enantiomer of 3-hAla (Figure 10), which is in line with literature precedence.54 (R,R’,S,S’)-DuanPhos produced the (S,R’)-diasteriomer in similar yield and enantiopurity. An attempted diastereoselective hydrogenation with achiral catalysts, dppp and dppe, produced a 1:1 diasteriomeric ratio (NMR analysis). Evidence implies that the asymmetric hydrogenations are completely catalyst-controlled, and that the initial stereochemistry of the starting material has no influence on hydrogenation selectivities.55 Further optimization of reaction time may be considered, as it is suspected that the reaction does not need 12 h for full completion.

Figure 10. Crystal structure of (S,S’)-Cbz-Phe-3-hAla-OMe.

2.3.5 Challenges of Aminolysis of a Dipeptide

With the dipeptide methyl ester, Cbz-Phe-3-hAla-OMe, in hand, the second aminolysis was attempted. Using previously established conditions, 20 equivalents of ammonia with the source

55 In the asymmetric hydrogenation of three enamides of a cyclic peptide in one step, a DuanPhos and Rh(cod)BF4 system also displays complete catalyst control. Hasan Khan, unpublished work.

29 of ammonia being 7 N in methanol, were unsuccessful, resulting in a recovery of clean starting material (eq. 6).

To increase reactivity, various solvents,56 ammonia sources,57 and Lewis acids58 were considered, though starting material was recovered on each attempt. Harsh microwave conditions produced reactivity after 15 minutes; however, 1H NMR showed an unidentifiable mixture of byproducts along with starting material. Interestingly, the aminolysis of an ethyl ester 59 dipeptide in 7 N NH3 in MeOH first goes through a transesterification mechanism to form the methyl ester, followed by aminolysis. Work by Högberg supports such a mechanism.60 Subjecting the analogous ethyl ester, Cbz-Phe-3-hAla-OEt, to originally established aminolysis conditions indeed results in the formation of the methyl ester. This result may suggest that for our dipeptide system, the equilibrium heavily favours the methyl ester over the amide, and that any attempts to enhance reaction rates (e.q. Lewis acids) may be ineffective. It is suspected that the transformation is driven by the precipitation of the amide product, as this has been observed in all cases of successful aminolysis. Adding additional equivalents of ammonia (up to saturation point) and increasing the reaction concentration did not result in reactivity.

2.3.5.1 N-Methylation

It was speculated that Cbz-Phe-3-hAla-OMe may exist in solution in a conformation that is undesired for aminolysis. Single crystal x-ray analysis did not indicate any unusual conformations in the crystal lattice; however, solid state and solution phase conformations may differ significantly. Although the use of a polar solvent such as methanol should in theory destroy self-interactions by hydrogen-bonding, comparisons to N-methylated substrates would be

56 Notably, iPrOH, EtOH, THF, DCM, and dioxane. 57 Commercially available ammonia solutions, gaseous, and liquid ammonia. 58 For example: Sc(OTf)3, In(OTf)3, Al(OTf)3, Yb(OTf)3, Sb(OTf)3 Cu(OTf)2, Zn(OTf)2, etc. 59 Unpublished work by fellow group member, Byoungmoo Kim. 60 Högberg, T.; Ström, P.; Ebner, M.; Rämsby, S. J. Org. Chem. 1987, 52, 2033-2036.

30 interesting. In attempted methylation experiments, the use of a strong base, NaH, led to decomposition as well as epimerization, and weakly basic K2CO3 did not lead to reactivity (eq. 7), which is not highly surprising. Addition methylation at the α-carbon may be a factor. A screening of other bases for methylation would be an area of future focus. N-alkylated substrates are an interesting alternative in our synthetic strategies, and may help alleviate challenges resulting from highly polar peptides.

2.3.5.2 Thiol-Promoted Aminolysis

Native chemical ligation37 is used to construct large polypeptides from two smaller peptide chains. The C-terminus of the first fragment (of the soon to be created polypeptide) is composed of a thioester. The N-terminus of the second fragment contains an unprotected cysteine residue. The thiol moiety of cysteine undergoes transthioesterification with the thiol ester, upon which a spontaneous and irreversible S to N rearrangement occurs, resulting in the attack of the free amino group on the carbonyl, and producing a native amide bond. This concept initiated the idea of adding a thiol, in hopes of forming a thioester61 in situ that would be more reactive towards aminolysis (eq. 8).

One equivalent of thiophenol promoted a 26% conversion (analysis by 1H NMR) to the desired product overnight. Another equivalent added the following day resulted in a total 46% conversion after 18 hours. These experiments were not performed in an oxygen-free atmosphere; disulfide from the oxidation of thiophenol was visible in the 1H NMR. Furthermore, after

61 It is speculated that basic conditions also aid in the formation of a thioester.

31 analysis of the 46% conversion (which requires long-term exposure to air), no additional thiol was added and the reaction halted. Ideally, under an ammonia atmosphere, the transformation may be achieved with a catalytic amount of thiol. However, repeating the reaction under an ammonia atmosphere only resulted in a 13% conversion after 1 day. Further investigations may need to be conducted to determine if this is a viable route for amide formation.

2.3.6 Preliminary Attempts of a Second Condensation en route to a Tripeptide

3 Cbz-Phe- -hAla-NH2 from the thiophenol promoted aminolysis was not isolated; however, the second condensation was achieved using the amide synthesized from the Cbz-Phe-3-hAla-OH and ammonia (eq. 9). Hydrolysis of the methyl ester, Cbz-Phe-3-hAla-OMe, gave the carboxylic acid in quantitative yield and it was used without further purification. Aminolysis of the in situ formed acid chloride resulted in a 28% isolated yield; it is speculated that isolation of the polar compound was the cause of the lower yield. For characterization purposes (due to the small amount of precious material isolated) and to fulfill the goal of extending the peptide chain, a small-scale condensation was performed, resulting in the formation of the desired tripeptide in 35% isolated yield (eq. 10), which was characterized by 1H NMR and HRMS.

2.3.7 Preliminary Model Study for 2-Amino Acid Incorporation

As a preliminary test to indicate whether the established condensation conditions could work for the incorporation of 2-residues, a constitutional isomer of methyl acetoacetate, methyl 2-

32 methyl-3-oxopropanoate, was prepared.62 Condensation with acetamide (eq. 11), the model compound for the peptide chain, resulted in a 37% isolated yield of the (Z)-isomer. It can be concluded that an aldehyde precursor can be used for condensation; however, the lower yield with acetamide (relative to its reaction with methyl acetoacetate45 for the analogous 3-system) may suggest that a lower yield may result with the desired system.

2.4 Conclusions and Future Work

A dipeptide, Cbz-Phe-3-hAla-OMe, containing an α- and β-amino acid residue has been fully characterized and synthesized using a condensation and reduction strategy. A potentially new methodology for the conversion of the dipeptide methyl ester to its amide has been proposed, and preliminary work towards a second iteration towards a tripeptide and also towards β2-amino acid incorporation has been achieved. Expansion of substrate scope and further studies on aminolysis will be areas of focus for the future.

2.5 Experimental Procedures and Characterization Data

2.5.1 First Aminolysis

(S)-benzyl (1-amino-1-oxo-3-phenylpropan-2-yl)carbamate (Cbz-Phe-NH2).

62 Nakatsuji, H.; Nishikado, H.; Ueno, K.; Tanabe, Y. Org. Lett. 2009, 11, 4258-4261.

Cbz-(S)-phenylalanine (900 mg, 3 mmol) was dissolved in 15 ml of anhydrous methanol. To this stirring solution was added SOCl2 (6.6 mmol, 0.48 mL) dropwise. The reaction was monitored by LC-MS (typical reaction time for this scale: 3-5 h). The reaction mixture was concentrated to a yellow oil in vacuo, and the crude methyl ester was carried over to the aminolysis step without purification. The methyl ester was dissolved in 7 N NH3 in MeOH (60 mmol, 20 equiv. NH3; 8.6 mL) and stirred at room temperature overnight. The reaction was monitored by LC-MS. Upon completion, the amide product appeared as a suspension in methanol. The mixture was concentrated and filtered to afford the title compound whose characterization was in accordance with literature63 (449 mg, 50%).

A large scale esterification and subsequent aminolysis starting from Cbz-(S)-phenylalanine (13.5 g, 45 mmol) was achieved. At least 20 equiv. of NH3 in MeOH was necessary for the reaction to go to completion. Additional equivalents of NH3 in MeOH were added periodically (up to 20 equiv.), and the reaction was complete after 2.5 d. Upon concentration and filtration, the title compound was isolated (11.8 g, 88%).

Aminolysis of the enantiopure starting material did not result in racemization of the chiral centre under these conditions. HPLC analysis: OD-H, 10:90 isopropanol:hexanes, 0.8 mL/min, 210 nm, tR1 = 23.0 min (R), tR2 = 28.4 min (S).

2.5.2 General Procedure for Condensation

(S,Z)-methyl 3-(2-(((benzyloxy)carbonyl)amino)-3-phenylpropanamido)but-2-enoate (Cbz- Phe-3-hAla-OMe ).

To a suspension of Cbz-Phe-NH2 (308 mg, 1.03 mmol) in 20 mL toluene was added methyl acetoacetate (240 mg, 2.06 mmol). p-Toluenesulfonic acid-monohydrate (10 mol%, 38 mg, 0.2

63 Giacomelli, G. Tetrahedron: Asymmetry 1998, 9, 1419–1426.

34 mmol) was added. The suspension, which becomes a slightly cloudy white solution upon heating, was refluxed at 135oC with a 5 mL Dean Stark apparatus filled with toluene for 17 h.

Upon cooling the reaction mixture, it was quenched with sat. NaHCO3. The toluene layer was extracted and the aqueous layer was washed with toluene. The organic layers were combined, washed with brine, and dried over Mg2SO4. Upon filtering and concentration of the filtrate in vacuo, a thick yellow-orange oil was obtained. Purification by glass column (eluent: 20% ethyl acetate in hexanes) afforded a crystalline white solid (266 mg, 65%).

1 H NMR (400 MHz, CDCl3) δ 2.36 (s, 3H), 2.23 (dd, 1H, J = 5.7 Hz, J = 16.0 Hz), 3.11 (dd, 1H, J = 7.2 Hz, J = 14.0 Hz), 3.19 (dd, 1H, J = 5.6 Hz, J = 14.0 Hz), 3.64 (s, 3H), 4.56 (dd, 1H, J = 6.6 Hz, J = 13.0 Hz), 4.94 (s, 1H), 5.11 (q, 2H, J = 12.1 Hz), 5.31 (s, br, 1H), 7.15-7.33 (m, 10H) 13 11.5 (s, 1H); C NMR (100 MHz, CDCl3) δ 21.8, 38.1, 51.1, 57.2, 67.2, 97.7, 127.2, 128.1, 128.2, 128.5, 128.8, 129.3, 135.8, 136.2, 154.0, 155.9, 169.1, 170.2; IR (neat): 3370, 1721, 1703, -1 + 1676, 1619, 1530, 1493, 1252, 1239, 1055 cm ; HRMS (ESI+) Calcd. for [C22H25N2O5] o 25 397.1758, found 397.1757. Mp. 89-90 C. [] D = -24.0 (c 1.15, CHCl3).

2.5.3 General Procedure for Asymmetric Hydrogenation

Methyl 3-(2-(((benzyloxy)carbonyl)amino)-3-phenylpropanamido)butanoate ((S,S’)- or (S,R’)-Cbz-Phe-3-hAla-OMe).

In a glovebox, 2 mL of degassed methanol was added to a 1 dram vial equipped with a stir bar and (S,S’,R,R’)-DuanPhos (1.4 mg, 0.0036 mmol). This was added to another 1 dram vial . containing Rh(cod)BF4 H2O (1.3 mg, 0.003 mmol) dissolved in 2 mL methanol. The metal- ligand solution was stirred for 20 minutes. Cbz-Phe-3-hAla-OMe (119 mg, 0.3 mmol) was brought into the glove box and transferred into an Endeavor® reaction vessel using the orange metal-ligand solution. This was stoppered and quickly placed into the Endeavor for hydrogenation (15 psi, 500 rpm, 25oC, 12 h). After 12 h, the reaction was deemed complete by LC-MS analysis. The reaction mixture was evaporated and taken up in 50% ethyl acetate in hexanes, adding DCM until soluble. This was filtered through a silica plug (1:1 dichloromethane:

50% ethyl acetate in hexanes) carefully to ensure that eluate did not contain the yellow metal complex. (An adequate amount of DCM was added such that the product did not crash out, and so that the eluate did not contain the yellow metal complex.) The eluate was concentrated in vacuo to an oil. In some cases, crystallization occurred spontaneously to give the major diasteriomer. Crystallization was promoted by dissolution in 1:1 dichloromethane:ethyl acetate at room temperature. Upon crashing out of the pure diasteriomer, the crystals were filtered and washed with diethyl ether. The major (S,S’)-diasteriomer was isolated as a white crystalline powder (76 mg, 64%). (R,R’,S,S’)-DuanPhos produced the (S,R’)-diasteriomer in similar yields. Crystals for x-ray analysis were grown from slow evaporation from dichloromethane/ethyl acetate.

1 H NMR of (S,S’)-diasteriomer (400 MHz, CDCl3) δ 1.11 (d, 3H, 6.8 Hz), 2.23 (dd, 1H, J = 5.7 Hz, J = 16.0 Hz), 2.37 (dd, 1H, J = 4.5 Hz, J = 16.0 Hz), 2.97 (dd, 1H, J = 7.8 Hz, J = 13.5 Hz), 3.12 (dd, 1H, J = 5.8 Hz, J = 13.5 Hz), 3.62 (s, 3H), 4.20-4.35 (m, 2H), 4.33 (m, 1H), 5.09 (s, 2H), 5.36 (d, br, 1H, J = 6.6 Hz), 6.22 (d, br, 1H, J = 8.5 Hz), 7.18-7.34 (m, 10H); 1H NMR of

(S,R’)-diasteriomer (400 MHz, CDCl3) δ 0.99 (d, 3H, 6.8 Hz), 2.23 (dd, 1H, J=5.7 Hz, J = 16.0 Hz), 2.37 (dd, 1H, J = 4.5 Hz, J = 16.0 Hz), 2.97 (dd, 1H, J = 7.8 Hz, J = 13.5 Hz), 3.12 (dd, 1H, J = 5.8 Hz, J = 13.5 Hz), 3.62 (s, 3H), 4.33 (m, 1H), 4.20-4.35 (m, 2H), 5.09 (s, 2H), 5.36 (d, br, 1H, J = 6.6 Hz), 6.07 (d, br, 1H, J = 8.2 Hz), 7.18-7.34 (m, 10H); 13C NMR of both diastereomers (100 MHz, CDCl3) δ 19.7, 38.9, 39.3, 41.8, 51.6, 56.5, 67.1, 127.0, 128.1, 128.2, 128.6, 128.7, 129.2, 136.2, 136.5, 155.8, 169.7, 171.4; IR (neat): 3295, 3261, 1732, 1692, 1652, -1 + 1546, 1261, 1236, 1024 cm ; HRMS (ESI+) Calcd. for [C22H27N2O5] 399.1914, found 399.1914. HPLC analysis: >99:1 dr with recrystallization; 99:1 dr, (S,S’) major, with ((S,S’,R,R’)-DuanPhos), 97.5 : 2.5 dr, (S,R’) major with ((R,R’,S,S’)-DuanPhos) without recrystallization. OD-H, 5:95 isopropanol:hexanes, 0.8 mL/min, 254 nm, tR1 = 28.2 min (S,S’), o 25 tR2 = 32.3 min (S,R’). Mp. 130-131 C (97.5 : 2.5 dr, (S,R’) major), [] D = 20.8 (c 0.66, CHCl3), (97.5 : 2.5 dr, (S,R’) major).

2.5.4 General Procedure for Hydrogenations With an Achiral Catalyst

In a glovebox, 0.5 mL of degassed methanol was added to a 1 dram vial equipped with a stir bar and dppp (3.1 mg, 0.006 mmol). This was added to another 1 dram vial containing . Rh(cod)BF4 H2O (2.7 mg, 0.006 mmol) dissolved in 0.5 mL methanol. The metal-ligand solution

36 was stirred for 20 minutes. The dehydro-dipeptide (25 mg, 0.06 mmol) was brought into the glove box and transferred into an Endeavor® reaction vessel using the metal-ligand solution. This was stoppered and quickly placed into the Endeavor® for hydrogenation (200 psi, 500 rpm, 45oC, 6 h). After 6 h, the reaction was deemed complete by LC-MS analysis. The reaction mixture was concentrated in vacuo and taken up in DCM. Purification by preparative TLC (eluent: 50% ethyl acetate in hexanes) resulted in a white powder (18 mg, 75% yield, 1:1 dr by NMR). Separation of the diastereomers by HPLC (OD-H, 5:95 isopropanol:hexanes, 0.8 mL/min, 210 nm, tR1 = 28.2 min (S,S’), tR2 = 32.3 min (S,R’)) confirmed the 1:1 diasteriomeric ratio.

2.5.5 General Procedure for Dipeptide Aminolysis

Benzyl ((S)-1-(((S)-4-amino-4-oxobutan-2-yl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate 3 ((S,S’)-Cbz-Phe- -hAla-NH2).

From (S,S’)-Cbz-Phe-3-hAla-OMe: (N.B. This reaction was not performed under an oxygen- free environment.) (S,S’)-Cbz-Phe-3-hAla-OMe (0.12 mmol, 48 mg) was dissolved in 0.35 mL of 7 N NH3 in MeOH in a 5 mL round-bottom flask and capped with a septum. Thiophenol (0.12 mmol, 12 μL) was added by syringe. Another equivalent of thiophenol was added after 1H NMR analysis after 24 h. The reaction was stirred at room temperature for a total of 2 days, resulting in 46% conversion to the amide.

From (S,S’)-Cbz-Phe-3-hAla-OH: (S,S’)-Cbz-Phe-3-hAla-OH (0.104 mmol, 40 mg) was o dissolved in 25 mL DCM. Upon cooling to 0 C, SOCl2 (2.2 equiv., 0.22 mmol, 0.02 mL) was added dropwise. The reaction mixture was stirred as it warmed to room temperature for 30 minutes. Cooling again to 0oC, it was purged with ammonia gas, upon which solid immediately crashed out and the reaction mixture appeared orange coloured. The reaction was monitored by LC-MS, quenching with MeOH. After 2 hours, the then beige mixture was concentrated. Purification by preparative TLC (5% MeOH in DCM) afforded 3 mg (28%) of an off-white solid.

1H NMR (400 MHz, DMSO) δ 1.12 (d, 3H, J = 6.6 Hz), 1.97-2.11 (m, 2H), 2.26 (dd, 1H, J = 5.5 Hz, J = 14.2 Hz), 2.79 (dd, 1H, J = 10.4 Hz, J = 13.6 Hz), 2.99 (dd, 1H, J = 4.4 Hz, J = 13.7 Hz), 4.01-4.13 (m, 1H), 4.22 (dt, 1H, J = 4.5 Hz, J = 10.0 Hz), 4.96 (s, 2H), 6.85 (s, br, 1H), 7.52 (d, 1H, J = 8.6 Hz), 7.18-7.37 (m, 10H), 8.00 (d, 1H, J = 8.0 Hz); 13C NMR (100 MHz, DMSO) δ 20.0, 37.7, 41.6, 42.2, 56.2, 65.1, 126.2, 127.4, 127.6, 128.0, 128.3, 129.2, 137.1, 138.1, 155.8, + 170.3, 172.1; HRMS (ESI+): Calcd. for [C21H26N3O4] 384.1917, found 384.1916.

2.5.6 Tripeptide

(5S,8S,Z)-methyl 5-benzyl-8,12-dimethyl-3,6,10-trioxo-1-phenyl-2-oxa-4,7,11- triazatetradec-12-en-14-oate ((S,S’)-Cbz-Phe-3-hAla-3-hAla-OMe). Synthesized according to the General Procedure for Condensation. Isolated 1 mg (35% yield) after 1 purification with preparative TLC (20% ethyl acetate in hexanes). H NMR (400 MHz, CDCl3) δ 1.16 (d, 3H, J = 6.6 Hz), 2.23 (m, 1H), 2.33 (s, 3H), 2.37 (m, 1H), 2.97 (dd, 1H, J = 7.9 Hz, J = 13.5 Hz), 3.10-3.16 (m, 1H,), 3.70 (s, 3H), 4.22-4.35 (m, 2H), 4.93 (s, 1H), 5.33-5.39 (m, 1H), 5.31 (s, br, 1H), 6.46 (d, 1H, J = 8.0 Hz), 7.17-7.37 (m, 10H) 11.0 (s, br, 1H); HRMS (ESI+): + Calcd. for [C26H32N3O6] 482.2285, found 482.2302.

2.5.7 β2-Test Substrates

Methyl 2-methyl-3-oxopropanoate. Adapted from known literature procedure.62 A 250 mL round-bottom flask was charged with methyl propionate (40 mmol, 3.8 mL) and methyl formate o (120 mmol, 7.4 ml) in 50 ml DCM. Upon cooling to 0 C, TiCl4 (80 mmol, 8.8 mL) was added dropwise, turning the reaction mixture yellow. Dropwise addition of Et3N (96 mmol, 13.3 mL) turned the reaction deep red. The reaction was left to stir to room temperature for 4 hours, upon

38 which it was quenched with distilled water. The aqueous layer was washed twice with ethyl acetate. Combined organic layers were washed with brine, dried over MgSO4, filtered and concentrated in vacuo to give an orange oil. Purification by distillation resulted in 593 mg (13%) of a colourless oil. Characterization data matched that in the literature. 62

(Z)-Methyl 3-acetamido-2-methylacrylate. Synthesized from methyl 2-methyl-3- oxopropanoate and acetamide according to the General Procedure for Condensation. Isolated as a light yellow oil (58 mg, 37%) after column chromatography (10% ethyl acetate in hexanes). 1H

NMR (400 MHz, CDCl3) δ 1.84 (s, 3H), 2.13 (s, 3H), 3.78 (s, 3H), 7.38 (d, 1H, J = 11.1 Hz), 13 10.4 (s, br, 1H); C NMR (100 MHz, CDCl3) δ 16.0, 23.7, 51.6, 104.3, 134.9, 168.1, 170.1. IR (neat): 3324, 2954, 1685, 1629, 1435, 1339, 1177, 1145, 777, 593 cm-1. HRMS (ESI+): Calcd. + for [C7H13NO3] 158.0811, found 158.0811.

Chapter 3 Synthesis of (Z)-β-(N-acylamino)acrylates: Progress Towards Dipeptide Synthesis via a Hydroamidation-Type Disconnection Strategy 3.1 Introduction and Research Goals

This final chapter will detail a project that arose from the work outlined in Chapter 2. Many of the references that pertain to the background of this project can also be found in Section 2.1. The concept will be presented as follows, and literature precedence that fueled the ideas in methodology development will be presented chronologically in Section 3.2.

In Chapter 2, the use of a condensation and reduction approach for the iterative synthesis of β- peptides has been outlined. As an extension and optimization of this method, we have proposed a new disconnection (Figure 11) involving an intermolecular conjugate addition or hydroamidation strategy for installing the incoming C-terminus amino acid residue. To date, the installation of an incoming residue to form a dipeptide has yet to be achieved; however, this new methodology has allowed for the synthesis of (Z)-β-(N-acylamino)acrylates with complete (Z)-selectivity using a catalytic amount of cationic platinum. (Z)-β-(N-acylamino)acrylates are benchmark substrates used in hydrogenations, and are often synthesized in a 2-step approach,64 where the product is a mixture of (E)- and (Z)-isomers. In our condensation reaction (Section 2.3.3), harsh reaction conditions often lead to polymerization or product instability.65 Herein, the methodology development towards dipeptide synthesis and the synthesis of β-(N-acylamino)acrylates will be described.

64 Zhu, G.; Chen, Z.; Zhang, X. J. Org. Chem. 1999, 64, 6907-6910. 65 Refer to Chapter 2.

Figure 11. Hydroamidation-type disconnection strategy.

3.2 Results and Discussion

3.2.1 Initial Screenings

As a model system for amide addition, acetamide or benzamide and ethyl 2-butynoate were used in hopes of synthesizing an N-acyl, unsaturated derivative of β3-hAla. There has been literature precedence involving the addition of secondary amides or carbamates to nitroalkenes (eq. 12 and 13).66,67

66 Lucet, D.; Toupet, L.; Le Gall, T.; Mioskowshi, C. J. Org. Chem. 1997, 62, 2682-2683. 67 Kamimura, A.; Kadowaki, A.; Nagata, Y.; Uno, H. Tetrahedron Lett. 2006, 47, 2471-2473.

Initial experiments (Table 6) were adaptations of these known conditions. The use of potassium tert-butoxide and 18-crown-6 65,66 in the presence of various Lewis acids resulted in recovered starting material, often with decomposition of the alkyne ester (entries 1-5). The use of Lewis acids alone (entries 6-12) were unsuccessful, with the exception of AuCl or AuCl3, where trace product could be detected by 1H NMR and LC-MS (vide infra, Section 3.2.2). A strong base for amide deprotonation (entries 13-18) may result in a 1,2-addition type mechanism where the ethoxide from the alkyne ester is expelled and subsequently does a conjugate addition on to the remaining ethyl 2-butynoate starting material to give ethyl 3-methoxybut-2-enoate as the only isolatable product. The role of hydrolysis in this process has been eliminated, as reactions were repeated under air and moisture-free conditions using dried reagents. The initial hit with gold Lewis acids sparked a further investigation into this area.

Table 6. Initial screening with model system.

Entrya Additives or LA (50 mol%) Solvent Temperature (oC)

1 tBuOK, 18-crown-6 THF -780

2 tBuOK. Bi(NO3)3, THF -780

3 tBuOK. AuCl3, 18-crown-6 THF 50

4 tBuOK. In(OTf)3, 18-crown-6 THF 50

5 tBuOK. Sc(OTf)3, 18-crown-6 THF 50

6 In(OTf)3 DCM 40

7 Sc(OTf)3 DCM 40

8 Yb(OTf)3 DCM 40

9 Zn(OTf)2 DCM 40

10 AgOTf DCM 40

11 Cu(OTf)2 DCM 40

c 12 AuCl or AuCl3 DCM 40

13b NaH THF/DCM r.t.

14b NaH THF r.t.

. 15 NaH, Bi(NO3)3 5H2O THF r.t.

16 NaH, AuCl THF 50

17 NaH, AuCl3 THF 50

18 NaH, Yb(OTf)3 THF 50 aStarting material recovered along with some alkyne decomposition, unless noted otherwise. bFormation of 2 detected. cTrace product detected by 1H NMR and GC-MS.

3.2.2 Exploring Gold Catalysis

The ability of gold(I) to activate alkynes has been well studied,68 so we decided to proceed with screening using such complexes (Table 7). A model system was not used in this case. Unfortunately, the use of sub-stoichiometric loadings of gold were needed to observe trace product by 1H NMR (entry 1 and 2); this may have been partially due to the poor solubility of AuCl.

Table 7. Screening with AuCl.

Entrya x mol % Ligands and Additives Solvent

1 25 - DCE 2b 50 - DCE 3 25 (IPr)AuCl only DCE 4b 50 - DCM 5b 50 - o-dichlorobenzene 6 50 - THF 7b 50 - EtOAc aStarting material recovered, along with some alkyne decomposition, unless noted otherwise. bTrace product detected by 1H NMR and LC-MS.

68 For extensive reviews: (a) Gorin, D.J.; Sherry, B.D.; Toste, F.D. Chem. Rev. 2008, 108, 3351-3378.; (b) Gorin, D.J.; Toste, F.D. Nature, 2007, 446, 395-403.

Additionally, a commercially available NHC complex, (IPr)AuCl (entry 3), did not exhibit any reactivity. For cases where reactivity was observed, crude 1H NMR showed traces of desired product. Product resulting from the reaction of a nucleophilic carbamate was also not observed.

The use of commercially available AuPPh3Cl with various counterions did not produce any desired product (Table 8). It was hypothesized that ligands on the Lewis acid hinder reactivity by placing additional electron density on the metal, decreasing its electropositive nature and its π- philicity.

Table 8. Screening with Au(I)PPh3 complexes.

Entrya AgX

1 none

2 AgNO3

3 AgBF4

4 AgSbF6

5 AgOTf

6 AgNTf2 aStarting material recovered, along with some alkyne decomposition.

In retrospect, it is noted that toluene was not tried as a solvent (vide infra, Section 3.2.4); however, the high loading of gold was the largest concern in terms of the feasibility of this transformation. Further exploration of gold catalysis for the model system with acetamide and ethyl 2-butynoate may also want to be considered in the future.

3.2.3 Adaptions from Other Literature Precedents

Before stumbling upon a successful platinum catalyzed system, it is worth mentioning other conditions adapted from known hydroamination literature that were attempted. Yamamoto and coworkers have demonstrated the success of a palladium/benzoic acid system for both intra- and intermolecular hydroaminations.69 In 2006, he reported an intramolecular hydroamidation of N– tosylamides (eq. 14); however, the tosyl group is crucial for the suppression of undesired β- hydride elimination.

It was hopeful that the electron deficient, activated alkyne in our model system may lower the energy barrier for reaction that may have been accomplished by tethering the reactants in the intramolecular version. However, recovery of the acetamide, alkyne decomposition, and no desired product were observed. This is in line with Yamamoto’s 2004 results,70 which demonstrated that N-acyl derivatives impair reactivity (eq. 15).

Rhenium is known catalyze the hydroamidation of terminal alkynes.71 In Takai’s system, (eq. 16) conditions are fairly harsh and yields are moderate. Under the same conditions, our model system did not show any reactivity – clean starting materials were recovered.

69 Patil, N. T.; Huo, Z.; Bajracharya, G. B.; Yamamoto, Y. J. Org. Chem. 2006, 71, 3612-3614 and references therein. 70 Patil, N. T.; Wu, H.; Kadota, I.; Yamamoto, Y. J. Org. Chem. 2004, 69, 8745-8750. 71 (a) S., S. Y; Kuninobu, Y.; Takai, K. Org. Lett., 2007, 9, 5609-5611. (b) Ouh, L.L.; Müller, T.E.; Yan, Y.K.; J. Organomet.Chem. 2005, 690, 3774-3782.

The ruthenium catalyzed addition of N-aryl secondary amides to alkynes was reported by Watanabe72 in 1995 (eq. 17). A trace amount of product was observed by GC-MS for our model system. Other examples of ruthenium activation include those by Gooβen72b and Tobisu.72c

In the synthesis towards linear tetrapyrroles and corrins, TBAF promotes intramolecular hydroamidation with primary amides (eq. 18); however, reactivity relies heavily on Thorpe- Ingold effects (substituents A-D).73 Without an electron-withdrawing substituent activating the pyrrole ring, yields decrease to 5-20%. Acetamide recovery and alkyne decomposition was observed with similar conditions on our model system.

72 (a) Kondo, T,; Tanaka, A.; Kotachi, S.; Wantanabe, Y. J. Chem. Soc. Chem. Commun., 1995, 413-414.; (b) Gooβen, L.J.; Rauhaus, J.E.; Deng, G. Angew. Chem. Int. Ed., 2005, 44, 4042-4045.; (c) Ru alkyne activation: Tobisu, M.; Nakai, H. Chantani, N. J. Org. Chem. 2009, 74, 5471–5475. 73 Jocobi, P.A.; Brielmann, H.L.; Hauck, S.I. J. Org. Chem. 1996, 61, 5013-5023.

Furthermore, a CsF promoted conjugate addition74 developed for acrylates and a halogenation- amination approach introduced by Larock75 did not lead to desired product for our model system. Oxidative amidations were also attempted using a palladium system.76

3.2.4 Exploring Platinum Catalysis

3.2.4.1 Early Experiments and Initial Hit

Although not as highly precedented relative to gold activation, the activation of alkynes via platinum catalysis has been documented.77 Initial screening (Table 9, entry 1) with acetamide gave promising results, producing the desired product in up to 14% isolated yield with toluene as a solvent. Reactions in p-xylene behave similarly. Intramolecular hydroaminations with tBuDavePhos have recently been reported by Widenhoefer (entry 2).78 Hartwig has demonstrated triflic acid promoted intramolecular hydroamination of secondary amines and amides (entry 3 and 4).79 Platinum alkyne activation under an atmosphere of CO is precedented as more facile due to the increased electron deficiency on platinum (presumably due to backbonding) and the (hemi) lability of the CO ligand (entry 5).80 Both 1,5-cyclooctadiene (entry 6 and 7) and 1,5-hexadiene (entry 8) presumably break up aggregates of polymeric platinum(II) chloride, making it more reactive.81 None of these concepts could be applied to increase the reactivity of this system – the additional ligands had a detrimental effect on reactivity. The addition of a co-Lewis acid (entries 9-11) for carbonyl activation on the alkyne ester was not successful. Additionally (not presented in Table), more polar solvents such as DMF and DMSO

74 Anh, K, H.; Lee, S. J. Tetrahedron Lett., 1994, 35, 1875-1878. 75 Yao, T.; Larock, R.C. J. Org. Chem. 2005, 70, 1432-1437. 76 (a) Lee, J.M.; Anh, D,; Jung, D.J.; Do, Y.; Kim, S.K.; Chang, S. J. Am. Chem. Soc, 2006, 128, 12954-12962.; (b) Liu, X.; Hii, K.K.; Eur. J. Org. Chem. 2010, 5181-5189.; (c) Hosokawa, T.; Takano, M.; Kuroki, Y.; Murahasi, S. Tetrahedron Lett. 1992, 33, 6643-6646. 77 (a) Pertaining to hydroamination: Müller, T.E.; Hultzsch, K.C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795- 3892. Alkene activation: (b) Chianese, A.R.; Lee, S. J.; Gagné, M. R. Angew. Chem. Int. Ed., 2007, 46, 4042-4059. 78 (a) Bender, C.F.; Widenhoefer, R.A. J. Am. Chem. Soc, 2005, 127, 1070-1071.(b) Bender, C.F.; Hudson, W.B.; Widenhoefer, R.A. Organometallics. 2008, 27, 2356-2358. 79 Schlummer, B.; Hartwig, J.F. Org. Lett. 2002, 4, 1471-1474. 80 Fürstner, A.; Aïssa, C. J. Am. Chem. Soc. 2006, 128, 6306. 81 Kirsch, S.F. Synthesis 2008, 20, 3183-3204.

48 could potentially allow for lower reaction temperatures; however, these solvents did not give desired product. Trace product was also observed by GC-MS with DCM, THF, benzene, and p- dioxane. Additional complexes such as Pt(py)Cl2 and Pt(PPh3)Cl2 were also tried, with relatively no reactivity observed. This evidence further supports the ligand hypothesis discussed earlier (Section 3.2.2). Platinum is known for its strong metal-ligand bonds and slow ligand substitution kinetics, hence additional ligands leave less room for metal-substrate interaction.82 Furthermore, use of PtBr2 and PtI2 did not improve the system significantly.

82 Chianese, A.R.; Lee, S. J.; Gagné, M. R. Angew. Chem. Int. Ed., 2007, 46, 4042-4059.

Table 9. Screening with platinum.

Entrya Pt Source / Ligands or Additivesd Isolated Yield

1 PtCl4 or PtCl2 14%

b 2 PtCl2 / DavePhos -

3 TfOH only -

c 4 PtCl2 / TfOH (1 equiv.) -

5 10% PtCl2 / CO (1 atm) -

6 10% PtCl2 / COD (40%) -

7 10% Pt(COD)Cl2 -

8 10% PtCl2 / 1,5-hexadiene (40%) -

9 PtCl2 / In(OTf)3 -

10 PtCl2 / Al(OTf)3 -

b 11 PtCl2 / Cu(OTf)2 - aStarting material recovered, along with some alkyne decomposition, unless noted otherwise. bTrace product detected by 1H NMR and GC-MS. cFull conversion; Mixture of unidentifiable products. dPt:ligand or additive 1:1, unless otherwise noted.

3.2.4.2 Counterion Effects and Mechanistic Considerations

It was speculated that a more electropositive platinum would be needed for increased alkyne activation. The use of cationic platinum with hydroaminations has been shown by Tilley (eq. 19), 83 however literature in this area remains sparse.

Cationic platinum without the coordination of ligands was speculated to be fairly sensitive and was formed in situ, and all reactions were performed in the glove box. Using silver salts with counterions of various coordination abilities (from very coordinating, NO3, to very non- coordinating, BArF), a general trend can be observed (Table 10). Moderately non-coordinating counterions such as triflate (entry 5) and trifimidate (entry 6) produced the highest yields, 55% and 60%, respectively, while more sluggish reactivity was observed for very coordinating 84 (entries 1-4) and non-coordinating counterions (entries 8-10). The use of PtCl4 and AgNTf2 gave the best yield of 65%. The use of platinum(IV) may further enhance softness of the metal and increase alkyne activation. Note that with the optimized conditions (entries 5-7), the reaction is complete in 5 h; however the product remains stable under the reaction conditions, with no decrease in yield after 24 h.

83 Karchtedi, D.; Bell, A.T.; Tilley, T.D. J. Am. Chem. Soc. 2005, 127, 12640-12646. 84 Antoniotti, S.; Dalla, V; Elisabet Dunach, E. Angew. Chem. Int. Ed. 2010, 49, 7860-7888.

Table 10. Counterion effects on platinum catalyzed hydroamidations.

a Entry Pt Source (10 mol% Pt) / Additive (mol%) Yield (%)

1 PtCl2 or PtCl4 / none (14)

2 PtCl2 / AgNO3 (20) 0

3 PtCl2 / AgMs (20) 29

4 PtCl2 / AgTs (20) (25)

5 PtCl2 / AgOTf (20) (55)

6 PtCl2 / AgNTf2 (20) 60 (60)

7 PtCl4 / AgNTf2 (20) 65

8 PtCl2 / AgPF6 (20) (49)

9 PtCl2 / AgSbF6 (20) (44)

10 PtCl2 / NaBArF (20) (32)

aGC-FID yield, isolated yields in parentheses.

The counterion trend may provide evidence for a platinum catalyzed hydroamidation81 – although a more highly electropositive metal promotes nucleophilic attack, protonolysis becomes decreasingly favourable. The ideal counterion favours alkyne activation, but does not heavily disfavour protonolysis. Figure 12 shows a proposed catalytic cycle with an outer-sphere mechanism that is generally preferred in the platinum activation of alkenes.

Figure 12. Proposed catalytic cycle involving alkynes and an outer-sphere nucleophilic attack. Exact oxidation states are not specified.

3.2.4.3 Dual Effect of Platinum and Silver

The ratio of platinum to silver salt appears to be crucial (Table 11, entries 1 and 2). The decrease in yield (55% (in Table 10, entry 5) to 28%) observed with the use of 1:1 platinum to silver may suggest that 2 equivalents of silver salt are needed to create the more active cationic platinum 85 species, Pt(OTf)2. The use of AgOTf alone is able to promote this transformation (entries 3 and 4); however the use of 24 mol% versus 1 full equivalent does not increase yield proportionally, and the reaction stalls, as full conversion cannot be achieved with 1 equivalent of AgOTf after 5 days. A control reaction was performed to eliminate the possible role of alkyne activation by AgCl (entry 5). A strong protic acid, such as TfOH, could also form in situ from the reagents (or even from trace water) used in this transformation; however, a triflic acid promoted reaction had been ruled out previously (Table 9, entry 3 and 4).

It can be concluded that the use of platinum and silver have a dual effect, presumably forming the active catalyst in situ. As further support (referring back to Table 10, entry 10), in a separate experiment, the use of NaBArF alone produces no reactivity. Sodium is not known to activate alkynes, therefore some extent of counterion exchange must be occurring in situ.

85 Silver can activate alkynes: (a) Tsuchimoto, T.; Aoki, K.; Wagatsuma, T.; Suzuki, T. Eur. J. Org. Chem. 2008, 4035–4040.; (b) Ross, R.S.; Dovey, M.C.; Gravestock, D. Terahedron Lett. 2004, 45, 6787-6789.

Table 11. Further investigations into a platinum-silver system.

Entry Pt Source (10 mol% Pt) / Additive (mol%) Isolated Yield (%)

1 PtCl2 / AgOTf (10) 28

2 PtCl2 / AgOTf (100) 28

3 AgOTf (24) 26

4a AgOTf (100) only 41b

5 PtCl2 / AgCl (20) Trace by GC-MS

aWith benzamide. bDid not reach full conversion after 5 days.

3.2.4.4 Other Sources of Triflate

The use of several different triflate sources was also explored (Table 12, entries 1-3). Any salt which has the propensity to exchange into a stable chloride species (silver, zinc, copper (Table 9, entry 11), sodium) and allow for the formation of cationic platinum may have the ability to promote this transformation to some extent. Interestingly, the use of PtCl2 and Zn(OTf)2 resulted in a 35% yield; however Zn(OTf)2 alone cannot promote this transformation.

This transformation can also be catalyzed using 10 mol% of triflic anhydride (entry 4). It was initially speculated to operate as a dehydrating agent; however, the use of acetic anhydride produces only trace product (entry 7). Again, a triflic acid promoted reaction had been ruled out previously (Table 9, entry 3 and 4). The combination of triflic anhydride and silver triflimidate led to a 45% yield (entry 8).

Table 12. Effect of other triflate sources on hydroamidation.

Entry Pt Source (10 mol% Pt) / Additive (mol%) Yield (%)b

1 PtCl2 / NaOTf (20) Trace by GC-MS

a 2 PtCl2 / ZnOTf (20) (35)

3a none / ZnOTf (100) 0 after 5 d

4 PtCl2 / Tf2O (10) 49

5 PtCl2 / Tf2O (50) 9

6 PtCl / Tf O (100) 0 2 2

7 PtCl2 / Ac2O (10) < 3

8 PtCl4 / Tf2O (10) / AgNTf2 (20) 45

aWith benzamide. bGIC-FID yield, isolated yields in parentheses.

3.2.4.5 Other Sources of Platinum

As these results were somewhat analogous to Tilley’s work83 (eq. 19), Zeise’s dimer was also 80 tested, but resulted in no desired product (Table 13, entry 1 and 2). Pt(cod)Cl2 and AgNTf2 gave only a 55% yield (entry 3).

Table 13. Effect of other platinum sources on hydroamidation.

Entry Pt Source (10 mol% Pt) / Additive (mol%) GC-FID Yield (%)

1 Zeise’s dimera Trace

a 2 Zeise’s dimer / AgOTf (20) 0

3 Pt(COD)Cl / AgNTf (20) 55 2 2

a Zeise’s dimer = [PtCl2(C2H4)]2

3.2.4.6 Effect of Tridentate Ligands

Gagné and coworkers86 have shown that pincer triphos-type ligands can create torsional strain on square planar platinum(II) complexes, which can be relieved upon protonolysis (Scheme 7). . Using cationic Pt–CH3 complexes with Ph2NH2 BF4 as an ammonium acid source and C6F5CN as a dication trap, PPP tridentate ligands promote protonolysis compared to inert mono- or bidentate phosphine ligands. It is hypothesized that PPP pincer ligands impart torsional strain on the square-planar platinum(II) complex, which can be relieved upon protonation to give a five- coordinate platinum(IV) complex that gives methane upon reductive elimination.81 Greater than 50 000 times rate enhancements can be seen with tridentate phosphine pincer ligands over non- pincer analogues that lack torsional strain.

86 Feducia, J.A.; Campbell, A.N.; Anthis, J.W.; Gagné, M. R. Organometallics. 2006, 25, 3114-3117.

Scheme 7. Torsional strain imparted by PPP pincer ligands promotes protonolysis.

The tridentate PPP ligands used by Gagné prefer to bind in a meridional geometry. Due to their commercial unavailability and lack of time, triphos (Figure 4), a facial binding ligand,13 was tested in our transformation. Using 5 mol% of triphos with optimized conditions, an 80% GC- FID yield was obtained (Table 14, entry 1). In accordance with the ligand effects mentioned earlier (Section 3.2.2), the percentage of triphos is crucial for reactivity (entry 2 and 3). A mere 5% yield was obtained when the percentage of triphos was increased to 10 mol%, and no conversion was observed upon the use of 15 mol%. The addition of our trisulfoxide ligand (entry 4) did not show any improvement over optimized conditions; it can be concluded that it does not interact with platinum.87 The use of meridional binding tridentate phosphine pincer ligands would be the next goal in further optimization.

87 However, it may be useful in other enantioselective hydroamination transformations – for example, the asymmetric synthesis of tertiary allylic amines via cyclopropene ring-opening. Unpublished work by fellow group member, Thi Phan.

Table 14. Effect of tridentate ligands on hydroamidation.

Entry Pt Source (10 mol% Pt) / Additive (mol%) GC-FID Yield (%)

1 PtCl4 / AgNTf2 (20) / triphos (5) 80

2 PtCl4 / AgNTf2 (20) / triphos (10) 5

3 PtCl4 / AgNTf2 (20) / triphos (15) 0

4 PtCl4 / AgNTf2 (20) / trisulfoxide (5) 64

3.2.4.7 Extension to Dipeptide Synthesis

The PtCl4 and AgNTf2 conditions were attempted in the synthesis of a dipeptide, using Cbz-Phe-

NH2 as a nucleophile. Unfortunately, no conversion was observed after 24 hours. The use of the triflic anhydride system also did not lead to desired product. The newly optimized system with triphos has not yet been attempted.

3.2.4.8 Extension to Aryl Alkyne Esters

An extension of scope to aryl substituents on the prochiral carbon was attempted. A phenyl substituent gave a 15% NMR yield, but did not reach full conversion after 23 h (eq. 20). Further optimization in this area would most likely lead to much improved yields.

3.3 Conclusions and Future Work

To the best of our knowledge, a new methodology has been developed for the first platinum catalyzed hydroamidation of alkynes. Although originally targeted for the synthesis of dipeptides, it is a novel method for the selective synthesis of (Z)-β-(N-acylamino)acrylates. PtCl4 and AgNTf2 are believed to form the active catalytic species in situ. The use of a tridentate phosphine ligand provides 80% yield and promising results for further optimization. Extension of substrate scope and its expansion to dipeptide synthesis are two areas of future work. Intramolecular variants and alkene activation with the use of chiral ligands may also be possible.

3.4 Experimental Procedures

3.4.1 General Considerations

Acetamide (Aldrich) was crushed with a mortar and pestle and dried under vacuum at 40oC for 48 hours. Toluene from a Pure Solv. solvent purification system (Innovative Technology, Inc.) was degassed with three freeze-pump thaw cycles and stored under 4Å molecular sieves. All other reagents (Aldrich, Strem) were used without further purification.

3.4.2 General Procedure for (Z)-β-(N-acylamino)acrylates

To a flame-dried amber 1 dram vial with a stir bar was added the amide (0.2 mmol) and internal standard (adamantane, 0.2 mmol, 27.3 mg). This was brought into the glove box upon which

PtCl4 (0.02 mmol, 6.7 mg) was added. A solution of AgNTf2 (0.04 mmol, 15.5 mg) in 1 mL degassed toluene was then added. Any ligands or additives were added at this point. Ethyl 2- butynoate (2 equiv., 0.04 mmol, 46 μL) was added with a micro-pipette, upon which the reaction mixture was heated to 100oC under a nitrogen atmosphere. Upon reaction completion, the desired product was isolated using preparative TLC (5-10% ethyl acetate in hexanes). Characterization data was in agreement with literature sources.46

Appendix A. NMR Spectra

1 H NMR in CDCl3

2.92 2.97 5.95 5.82 3.17 3.00 3.02 2.85

10.0 5.0 ppm (f1)

139.823 133.560 131.584 129.073 128.552 127.680 126.869 125.715 123.459 121.640 64.973 40.133 24.372

13 C NMR in CDCl3

150 100 50 ppm (f1)

1 H NMR in CDCl3

2.70 3.13 6.19 3.00 6.24 2.89 2.82 2.75

10.0 5.0 ppm (t1)

140.925 134.522 132.930 129.768 128.559 128.048 127.880 127.323 124.615 120.032 67.465 39.642 25.269

13 C NMR in CDCl3

150 100 50 ppm (f1)

1 H NMR in CDCl3

2.00 6.30 4.15 1.99 4.42 2.19 4.48

10.0 5.0 ppm (t1)

140.709 134.390 132.833 129.451 128.451 128.043 127.750 127.327 124.639 119.734 56.211 27.660 21.606

13 C NMR in CDCl3

150 100 50 ppm (t1)

1 H NMR in CDCl3

10.0 5.0 0.0 ppm (t1)

13 C NMR in CDCl3

150 100 50 0 ppm (t1)

1 H NMR in CDCl3

7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 ppm (t1)

13 C NMR in CDCl3

150 100 50 0 ppm (t1)

1H NMR in DMSO

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 ppm (t1)

13C NMR in DMSO

150 100 50 0 ppm (t1)

13C NMR in DMSO

150 100 50 0 ppm (t1)

Appendix B. HPLC Traces

racemic

meso

Racemic Cbz-Phe-NH2

(S)-Cbz-Phe-NH2

With achiral catalyst:

Enantioselective hydrogenation, before recrystallization:

Enantioselective hydrogenation, after recrystallization: