ChemComm

A concise [C+NC+CC] coupling -enabled synthesis of kaitocephalin

Journal: ChemComm

Manuscript ID: CC-COM-03-2014-001692.R1

Article Type: Communication

Date Submitted by the Author: 26-Mar-2014

Complete List of Authors: Garner, Philip; Washington State University, Department of Chemistry Weerasinghe, Laksiri; Washington State University, Chemistry Van Houten, Ian; Washington State University, Chemistry Hu, Jieyu; Washington State University, Chemistry

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A concise [C+NC+CC] coupling-enabled synthesis of kaitocephalin Cite this: DOI: 10.1039/x0xx00000x

Philip Garner,* Laksiri Weerasinghe, Ian Van Houten and Jieyu Hu

Received 00th January 2012, Accepted 00th January 2012

DOI: 10.1039/x0xx00000x

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A 15-step synthesis of the iGluR antagonist kaitocephalin of the -derived aldehyde 4, (S)-glycyl sultam 5, and from aspartic acid is reported. The linchpin ring vinyl sulphone 6.9 The key C-C bond-forming step involves a [3+2] of the target molecule is efficiently assembled with in a single cycloaddition of a metalated azomethine ylide (formed in situ from 4 operation via an asymmetric [C+NC+CC] reaction. and 5) with an electronically-activated dipolarophile 6. The advantage of [C+NC+CC] coupling over other [3+2] cycloaddition Kaitocephalin is a natural product isolated from the fungus methodology stems from the ability to employ aliphatic aldehydes Eupenicillium shearii that was found to be a potent competitive such as 4 without enolization or enamine formation that would result 1 antagonist of ionotropic (iGluR) activity. This in undesired side reactions. This limitation precludes the application property is of interest because glutamate receptors are the major of existing [3+2] dipolar cycloaddition technology to the excitatory receptors in the vertebrate brain and are kaitocephalin pyrrolidine problem. While essential for activating the 2 involved in both normal and pathological neuronal functions. Drugs dipolarophile in the underlying cycloaddition process, the targeting glutamate receptors may have considerable potential for phenylsulfonyl moiety would be removed reductively at a treating such diverse disorders as , amyotrophic lateral subsequent stage in the synthesis.10 sclerosis, ischemic brain damage, major depression, drug abuse, and neuropathic pain. Glutamate receptors have also been implicated in 3 4 O O non-neurological diseases such as diabetes and cancer. Many of Cbz these disease states are associated with the selective expression of N + N OH O Boc iGluR subtypes, which are broadly classified according to their N CO Me Cl Cl 2 3 selective activation by NMDA (N-methyl-D-aspartate), AMPA (2- 2 MeO O amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionate), or kainate. 1 H N Kaitocephalin displays differential affinity for iGluR subtypes in the 2 CO2H O NH 2 order NMDA > AMPA >> kainate,5 and is thus of interest as a 3 6 OH scaffold for drug design. Recently, the crystal structure of HO2C N PhO2S H CO2H kaitocephalin bound to an iGluR was determined and provides clues Cbz 6 O S 7 kaitocephalin (1) N 2 for the development of subtype-specific antagonists. As a first step + N O H H towards this goal, we report a concise total synthesis of O H2N kaitocephalin. Our approach is distinguished from prior syntheses8a-g 4 O 5 of kaitocephalin by its effective use of the asymmetric [C+NC+CC] coupling reaction to assemble the target’s core pyrrolidine ring. Scheme 1 Retrosynthetic analysis of 1. Our key retrosynthetic analysis of kaitocephalin (1, Scheme 1) began with the introduction of the C1-C3 moiety via the union of 2 Reduction of this plan to practice began with the synthesis of the 11 12 with the -derived aldehyde 3 following an established aldol key pyrrolidine 2 (Scheme 2). The [C+NC+CC] coupling of 4, 5, 13 tactic (references 8a and 8e). The pyrrolidine precursor of 2 and 6 proceeded cleanly at ambient temperature in the presence of 14 comprising the remaining contiguous carbon skeleton of 1 would be Ag(I) ion to give the endo-cycloadduct 13 in 85% isolated yield. assembled via a Ag(I)-catalyzed asymmetric [C+NC+CC] coupling The pyrrolidine ring stereochemistry of this cycloadduct was

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 1 ChemComm Page 2 of 4 COMMUNICATION Journal Name expected based on the (endo-Si) pre-TS ensemble 12 and was oxazolidines to their N-protected α-amino acids. The resulting diacid supported by the successful conversion of 13 to the natural product 18 was treated directly with excess diazomethane to give the triester 1. Although the stereochemistry of the phenylsulphonyl group at C6 19 in 36% overall yield from 2. This three-step reaction sequence is inconsequential because this functional group is removed accomplished a total of five synthetic transformations (aldol downstream, we believe that it is endo based on our previous addition, oxazolidinone formation, bis-oxazolidine hydrolysis, diol experience with the Ag(I)-catalyzed [C+NC+CC] coupling oxidation, and bis-esterification), affording pure 19 after a single reaction.15 Post [C+NC+CC] coupling adjustment of the pyrrolidine chromatography.17 Hydrogenolysis of 19 released the amine 20, substitution pattern began with methanolysis of the acyl sultam using which was acylated with the known acid chloride 21 to give the

Mg(OMe)2 to give the pyrrolidine ester 14 which was N- amide 22 in good overall yield. Synthesis of 22 constituted a formal carbamoylated with methyl chloroformate to provide the methyl synthesis of the target molecule 1 (reference 8e). Removal of the carbamate 15. Chemoselective reductive cleavage of the Boc, benzyl ether, and three methyl ester protecting groups was phenylsulfonyl group was accomplished with sodium amalgam to effected by treating 22 with the Fujita reagent18 to afford 23. This provide 2 in 62% overall yield from 13 concluding the 3-step was followed by hydroxide-mediated hydrolysis of the sequence. Alternatively, the cycloadduct 13 could be converted to 2 oxazolidinone to give kaitocephalin (1). Our material was identical in just two steps in comparable overall yield by exposing 13 to to an authentic sample of 1. excess metal in MeOH to produce the methyl ester (desulphonation and methanolysis) followed by N-carbamoylation to This short (15 steps from aspartic acid) total synthesis of afford 2. kaitocephalin demonstrates the value of the asymmetric [C+NC+CC] coupling reaction for the construction of complex pyrrolidine O S structures and paves the way for the generation of novel H2NCH2COX (5) S H Ph kaitocephalin analogues for biological studies. CH2=CHSO2Ph (6) O Cbz 10 mol% AgOAc S N N O THF, rt Ag Our research was supported by a grant from the National Science O CHO O S N L H Foundation (CHE-1149327). We thank Vertex Pharmaceuticals for a 85% O 4 H supply of 5•HCl and the Ohfune lab at Osaka City University for 12 providing us with a sample of kaitocephalin for comparison. CbzN O

[3+2]

Cbz SO2Ph Mg(OMe)2 Cbz SO2Ph N MeOH N O2S O O N H N CO2Me 83% N H H H O 14 13 1. Mg/MeOH ClCO2Me 0° to rt aq NaHCO3,THF 99% 2. ClCO2Me aq NaHCO3,THF Cbz 61% (2 steps) Cbz SO2Ph Na(Hg) N N Na2HPO4 O O MeOH N CO2Me N CO2Me 75% CO2Me CO2Me 2 15

Scheme 2 Synthesis of Pyrrolidine 2.

With the pyrrolidine 2 in hand, we were poised to complete the synthesis of kaitocephalin (Scheme 3). We found that the lithium enolate of 2 reacted with the serinal derivative 3 to give a 2:1 mixture of oxazolidinone 16 and undesired (3R) aldol diastereomer 17 in 83% combined yield. Presumably, 16 arose from the preferential cyclization of the major (3S) aldolate. This result was in line with Ma’s report (reference 8e). Because of the difficulty in separating 16 from 17, the mixture was used for the next reaction. Following Dondoni’s protocol,16 exposure of this mixture to Jones reagent resulted in clean conversion of both N-protected

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Cbz LiHMDS, THF Cbz Cbz N N CO2Me N CO2Me -78 to -30 °C O O O O S + O R CO Me then N 2 O N N N N CO Me O OH 2 OHC Boc MeO O Boc N O 2 3 Boc 16 (2:1) mixture 17 -78 °C to rt Jones reagent

H2, Pd-C CO2Me CO Me CO Me NH2 CbzHN 2 CH2N2, Et2O CbzHN 2 NHBoc EtOAc NHBoc NHBoc

MeO2C N MeO2C N HO2C N CO Me O 2 O CO2Me O CO2H O O O 18 20 19 (36% from 2) OBn Cl Cl 21 OBn OH OH COCl Cl Cl Cl Cl Cl Cl Et3N, DCM

H N KOH, EtOH 2 CO2H O NH CO2Me AlCl3, Me2S O NH CO2H 40 °C O NH NHBoc NH 2 OH MeO2C N HO2C N HO2C N H CO2H O CO2Me O CO2H O O 1 (21% from 22) 22 (49% from 19) 23 M.; Shinda, T.; Ohfune, Y. Org. Lett. 2009, 11, 4664. (e) Yu, S.; Zhu, S.; Pan, X.; Yang, J.; Ma, D. Tetrahedron 2011, 67, 1673. (f) Scheme 3 Kaitocephalin end-game. Takahashi, K.; Yamaguchi, D.; Ishihara, J.; Hatakeyama, S. Org. Lett. 2012, 14, 1644. (g) Lee, W.; Youn, J. H.; Kang, S. H. Chem. Notes and references Commun. 2013, 49, 5231. 9 Garner, P.; Kaniskan, H. Ü.; Hu, J.; Youngs, W. J.; Panzner, M. Org. Lett. 2006, 8, 3647. Department of Chemistry, Washington State University, Pullman, WA 10 A related approach to 1 had been suggested but never 99164-4630, USA. E-mail: [email protected] implemented, apparently due to the poor yield and † Electronic Supplementary Information (ESI) available: Experimental stereoselectivity associated with the underlying azomethine ylide cycloaddition (Kudryavtsev, K. V.; Nukolova, N. V.; Kokoreva,

procedures and characterization data for all new compounds are provided. O. V.; Smolin, E. S. Russ. J. Org. Chem. 2006, 42, 412). See DOI: 10.1039/x0xx00000x 11 Aldehyde 4 was synthesized from aspartic acid 7 as shown below. ‡ This paper is dedicated to Professor Yasufumi Ohfune on the NH NH •HCl occasion of his retirement from Osaka City University. 2 EtOH,HCl 2 CO2H CO2Et HO2C EtO2C 1 Shin-ya, K.; Kim, J.-S.; Furihata, K.; Hayakawa, Y.; Seto, H. 7 8 Tetrahedron Lett. 1997, 38, 7079. CbzCl, NaHCO3 2 Traynelis, S. F.; Wollmuth, L. P.; McBain, C. J.; Menniti, F. S.; 74% overall Vance, K. M.; Ogden, K. K.; Hansen, K. B.; Yuan, H.; Myers, S.

J.; Dingledine, R. Pharmacol. Rev. 2010, 62, 405. NHCbz NaBH4, MeOH NHCbz 3 Cryer, P. E. Endocrinology 2012, 153, 1039. HO CO2Et 4 Rzeski, W.; Ikonomidou, C.; Turski, L. Biochem. Pharmacol. OH 80% EtO2C 2002, 64, 1195. 10 9 5 Limon, A.; Reyes-Ruiz, J. M.; Vaswani, R. G.; Chamberlin, A. R.; Miledi, R. ACS Chem. Neurosci. 2009, 19, 132. Me2C(OMe)2 6 Vaswani, R. G.; Limon, A.; Reyes-Ruiz, J. M.; Miledi, R.; TsOH, Chamberlin, A. R. Bioorg. Med. Chem. Lett. 2009, 19, 132. 36% 7 Ahmed, A. H.; Hamada, M.; Shinada, T.; Ohfune, Y.; Dess-Martin Weerasinghe, L.; Garner, P. P.; Oswald, R. E. J. Biol. Chem. Cbz periodinane Cbz 2012, 287, 41007. N CH2Cl2 N 8 (a) Watanabe, H.; Okue, M.; Kobayashi, H.; Kitahara, T. O O CHO Tetrahedron. Lett. 2002, 43, 861. (b) Kawasaki, M.; Shinada, T.; OH 92% Hamada, M.; Ohfune, Y. Org. Lett. 2005, 7, 4165. (c) Vaswani, R. 11 4 G.; Chamberlin, A. R. J. Org. Chem. 2008, 73, 1661. (d) Hamada,

This journal is © The Royal Society of Chemistry 2012 J. Name., 2012, 00, 1-3 | 3 ChemComm Page 4 of 4 COMMUNICATION Journal Name

12 Glycyl sultam 5 can be prepared by a route involving azide reduction (Kaniskan, H. Ü.; A Formal Total Synthesis of Bioxalomycin Beta 2, Ph.D dissertation, Case Western Reserve University, 2007) or the Delépine reaction (Isleyen, A.; Gonsky, C.; Ronald, R. C.; Garner, P. Synthesis 2009, 1261). 13 Paquette, L. A.; Carr, R. V. C. Organic Synthesis, 1990, 7, 453. 14 This multicomponent reaction could be performed on a 10 g scale (aldehyde 4 is the limiting reagent) but with a slight reduction in yield of 13 to 70%. 15 Indirect evidence for the endo-stereochemistry in 13 comes from the fact that the Cu(I)-catalyzed [C+NC+CC] coupling of 4, 5 and 6, conditions that favour the exo-product (Garner, P.; Hu, J.; Parker, C. G.; Youngs, W. J.; Medvetz, D. Tetrahedron Lett. 2007, 48, 3867), produced a cycloadduct that was isomeric to 13. (See Supplementary Information.) 16 Dondoni, A.; Marra, A.; Massi, A. Tetrahedron 1998, 54, 17 Compound 25 was also isolated in 11% yield, presumably arising from a retro-aldol reaction at some stage during the reaction sequence. CbzHN

MeO2C N CO2Me

CO2Me 25 18 Node, M.; Nishide, K.; Sai, M.; Fuji, K.; Fujita, E. J. Org. Chem. 1981, 46, 1991.

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