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Download Author Version (PDF) RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 3 Journal Name RSC Advances Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc. org/xxxxxx ARTICLE TYPE Synthesis of ( –)-Epibatidine Wen-Hua Chiou* and Yu-Min Chiang Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 9b 5 An asymmetric synthesis to dendrobatid alkaloid (–)- As a part of the project devoted to asymmetric syntheses of epibatidine has been described, featuring chiral resolution of 45 alkaloids and derivatives for pharmaceutical purpose, here we both optical pure 7-azabicyclo[2.2.1]heptanecarboxylic acid, describe a different approach to synthesize (–)-epibatidine (1). and subsequent transformations to (–)-epibatidine. The This strategy takes advantage of readily available carboxylic methodology provides a flexible access to various substituted acid 3, and features a practical preparation of enantiopure acid 3 and construction of the 2-chloropyridine moiety from the 10 chiral epibatidine analogues. 50 carboxylic acid end. In 1992, Daly and coworkers have reported a dendrobatid Coupling H Cl alkaloid, epibatidine (1), isolated in trace amount from the N 1,2 Transannular skin of the Ecuadoran poison frog Epipedobates tricolor. N Manuscript Epibatidine (1) has been found to be 200-400 times more SN2 Displacement This work 15 potent than morphine as an analgesic, and appeared to act via Cycloaddition a non-opioid mechanism since its effects have not blocked by the opiate receptor antagonist naloxone. In addition, Figure 2. Approaches towards epibatidine. epibatidine (1) is an extremely potent agonist of the nicotinic Our approach commences with the preparation of racemic 7- 3 acetylcholine receptor. Due to its pharmacological activity, azabicyclo[2.2.1]heptanecarboxylic acid, using Fevig’ 20 epibatidine (1) has attracted much attention from synthetic conditions,9c a modification based on Bai’s procedure. 9a, 9b We chemists, and resulting in abundant approaches. However, its 55 envision both optically active 7-azabicyclo[2.2.1]heptane high toxicity also prevents therapeutic applications, and have carboxylic acids 3 are available through covalent bond prompted as search for safer analogues such as epiboxidine modification with a chiral compound to two separable Accepted 4 (2). diastereomers, followed by removal of the chiral auxiliary. Cl N H H O Subsequent functionality transformations of the carboxylic N N N 60 acid end effect the formation of the 2-chloropyridine moiety in epibatidine. Such an approach does not only synthesize the 2-chloropyridine moiety, but also allow diversity by epibatidine (1) epiboxidine (2) construction of various bioisosteric rings or other modi- 25 fication. For example, epiboxidine (2) can be achieved by an Figure 1. (–)-epibatidine (1) and epiboxidine (2). 8g 65 acetoxime addition-cyclization protocol. With racemic acid Various strategies have been developed for efficient syntheses 3 in hand, transformation of (±)-3 to separable diastereomers of the molecule, which have been reviewed by Trudell 5 and has been carried out by treating of acid 3 with SOCl 2 to the Advances Olivo.6 One of these approaches to synthesize the unique 7- resulting acid chloride, followed by reaction with various chiral auxiliaries. 30 azabicyclo[2.2.1]hepatane framework in epibatidine is the cycloaddition reaction. N-protected pyrroles could undergo 70 We have utlized L-menthol and L-borenol as the chiral Diels-Alder reactions with substituted acetylene or ethylene auxiliary, but both have failed to give separable diastereo- meric adducts. Fortunately, after racemic acid 3 has been RSC derivatives. Transannular SN2 displacement also provides a practical route to synthesize epibatidine. 7 A well-established coupled with (4 S)-benzyl-2-oxazolidinone, readily available from L-phenylalanine, two diastereomeric adducts were easily 35 arrangement in a 1,4-disubstituted cyclohexyl ring system 75 separated by column chromatography, to yield the less polar could trigger an SN2 reaction to give the 7-azabicyclo- [2.2.1]heptane structure. Direct coupling of the 7-azabi- product 4a in 44% yield and the more polar product 4b in cyclo[2.2.1]heptan-2-one or its derivatives with the aromatic 41% yield, respectively. (Scheme 1) In addition, ring is an effective strategy for syntheses of epibatidine and recrystallization of oxazolidinone 4a within ethyl acetate and hexane provided a crystal for an X-ray analysis, which 40 analogues. Due to the diversity of the approach, many elegant 80 confirmed the absolute configuration as (1 R, 2 S, 4 S)-7- approaches to synthesize 7-azabicyclo[2.2.1]heptane 10 derivatives have been reported, 8 including Aza-Prins-Pinacol azabicyclo[2.2.1]heptane moiety (Figure 2). The results also rearrangement 8e and Favorskii rearrangement of tropinone. 9a, disclosed the absolute configuration of the other diastereomer, This journal is © The Royal Society of Chemistry [year] [journal] , [year], [vol] , 00–00 | 1 RSC Advances Page 2 of 3 amide 4b, as a (1 S, 2 R, 4 R)-7-azabicyclo[2.2.1]heptane integrity arrived intact during the processes mentioned above moiety. Subsequent basic hydrolysis conditions using H2O2 in (> 99% ee, Scheme 2). THF for the less polar oxazolidinone derivative 4a proceeded PPh =CHOMe Cbz Dess-Martin Cbz 3 BH3-Me2S N N successfully to yield optically pure (‒)-acid 3 in 98% yield, THF, rt THF, rt CH2Cl2, rt CHO 5 while the more polar one 4b gave optically pure (+)-acid 3 in (-)-3 OH 96% 93% 88% 93% yield (Scheme 1). Chiral HPLC analyses display the (-)-5 (+)-6 optical purity of (‒)-acid 3 is more than 99% ee, (tR:18.4 min Cbz OMe Cbz Cbz N N CHO CrO3,H2SO4, N CO2H for (+)-3, 26.3 min for (:)-3, See supporting information). HCl, acetone, 0 oC, THF rt 95%, Cbz (-)-8 N 7 92% (-)-9 > 99% ee Cbz N O O Cbz CO2H N H2O2, LiOH Scheme 2. Synthesis of chiral aldehyde (-)-8. (¡Ó)-3, ref 9c. N O CO2H THF/H2O, rt 1. SOCl , DMF, 40 Homoaldehyde (‒)-8 was treated with pyrrolidine in the 2 4a, 44%Bn (-)-3, 98% CH Cl presence of K 2CO 3 to the corresponding enamine, followed by 2 2 + O Cbz N reaction with ethyl acrylate, and then hydrolysis in an acidic 2. THF N Cbz O O media to furnish glutarate semialdehyde 10 in 55% yield. The CH2Cl2 H O , LiOH LiN O 2 2 yield was improved to 81% yield by Hagiwara’s protocol, 12 O N O -40~-50 C THF/H2O, rt CO2H 45 using TMSNEt 2 and ethyl acrylate in refluxing acetonitrile. Bn 4b, 41% (+)-3, 93% Bn Glutarate semialdehyde 10 underwent a double condensation 10 process with NH 4OAc in refluxing benzene to yield dihydropyridone (‒)-11 in 93% yield with 99% ee. Oxidation Scheme 1. Separation of racemic acid 3 using (4 S)-benzyl-2- with 9 equivalents of MnO in refluxing benzene produced 5- 2 Manuscript oxazolidinone.` 50 substituted 2-pyridone ( +)-12 in 90% yield. Slow addition of MnO 2 during a long period was crucial for the yield in that addition at once brought about a yield decrease in this reaction. Treatment of 2-pyridone ( +)-12 with POCl 3 and in DMF did not only convert the 2-pyridone group to the 2- 55 chloropyridine group, but also replaced the Cbz group by a formyl group as formamide 13 in 73% isolated yield. Since two sets of peaks with almost equal intensity have been observed in 13 C-NMR, the product appeared to be a mixture of E/Z isomers with equal amount due to the restrict rotation of Accepted 60 the formamide bonding. Exposure of formamide 13 in 5% HCl in MeOH afforded final product epibatidine (–)-1 in 92% Figure 3. ORTEP drawing of oxazolidinone 4a yield with > 99% ee. The specific rotation value of (–)-1, 25 [α]D –6.8 ( c: 1.04 CHCl 3) was consistent with the reported 25 13 The next efforts were involved with the construction of the 5- value, [ α]D –6.5 ( c: 1.00, CHCl 3). Treatment of formamide 15 substituted-2-chloropyridine moiety (Scheme 2 and 3). With 65 13 with alane complex, prepared by mixing LiAlH 4 and 14 optically pure acid (‒)-3 in hand, we converted acid 3 to its Et 3N•HCl, produced N-methyl epibatidine analogue (+)-14 homoaldehyde derivative by one-carbon homologation via an in 76% yield with 99% ee (Scheme 3). enol ether intermediate. 11 Thus, reduction of acid (‒)-3 with BH 3•Me 2S in THF produced alcohol (‒)-5 in 96% yield with Cbz Cbz CH2=CHCO2Et CO2Et N N Advances 20 99% ee. Oxidation of alcohol (‒)-5 with Dess-Martin TMSNEt2 MeCN, reflux CHO periodinane in CH 2Cl 2 afforded aldehyde ( +)-6 in 93% yield. CHO 81% 10 Direct treatment of oxazolidinone 4a with BH 3•Me 2S in THF, (-)-8 Cbz O followed by Dess-Martin oxidation also afforded aldehyde Cbz O MnO2, C6H6 N (+)-6 in 86% yield over two steps.
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