Asymmetric total synthesis of (-)-isolaurallene.

Medium-ring ethers of various structural types have been isolated from marine organisms.1 While medium-ring ethers such as the ladder ether toxins have important implications with regard to their biological impact, it is the exquisite structures of naturally occurring medium-ring ethers that has provoked the imagination of synthetic chemists. Because eight-membered cyclic ethers are more prevalent, many unique and interesting approaches to their construction have been designed.2 Nine-membered ethers are perhaps less common, but are present in the ladder toxins brevetoxin A,3 ciguatoxin,4 gambieric acid A,5 the eunicellins,6 and the simpler metabolites obtusenyne,7 neolaurallene,8 and isolaurallene.9 Synthetic approaches to nine-membered ethers have been limited not only because of their infrequent occurrence, but also because of the challenges associated with their stereoselective assembly.10 Isolaurallene, which contains a nine-membered cyclic ether, as well as a bromoallene-substituted tetrahydrofuran, was isolated by Kurata from laurencia nipponica yamada collected in Izumihama near Hiroo on the Pacific Coast of Hokkaido. The structure of isolaurallene was proposed based on spectroscopic information and later confirmed by a single-crystal X-ray study.9 The closely related metabolite, neolaurallene, was subsequently isolated and its structure was also elucidated by X-ray crystallography.8 As part of our continuing program directed toward the development of flexible strategies for the asymmetric construction of medium-ring ether metabolites,2 we designed an approach to the synthesis of isolaurallene that focused on construction of the core nine-membered ether through a ring-closing metathesis without the assistance of a cyclic conformational constraint (Scheme 1).11 A similar asymmetric alkylation-olefin metathesis approach was employed in our recent laurencin synthesis.12 It was anticipated that diene 3 would undergo rapid closure to the ∆5oxonene because of the gearing effect created by two synergistic gauche effects at C6-C7 and C12-C13.13 The assembly of diene 3 would be completed by an asymmetric glycolate alkylation14 of oxazolidinone 4 followed by additional functionalization. The implementation of this plan culminating in the first total synthesis of isolaurallene is the subject of this report. The oxazolidinone 4 was prepared from the glycolic acid derivative as described for the antipode.12 Exposure of 4 to NaN(SiMe3)2 and iodide 515 produced the alkylation product 6 in high yield with excellent (97:3) diastereoselectivity (Scheme 2).14 The auxiliary was reductively removed with sodium borohydride to provide the alcohol 7 in 88% yield.16 Oxidation of the alcohol 7 under Swern conditions17 gave the aldehyde 8, which was immediately exposed to Brown’s asymmetric allylation18 to generate a 96:4 mixture of the two diastereomeric secondary alcohols in 93% yield. The major isomer was subsequently converted to the corresponding acetate 9. Removal of the benzyl ethers of 9 under oxidative conditions19 also resulted in oxidation of the allylic alcohol to the aldehyde. The aldehyde was immediately reduced with sodium borohydride to provide diol 10, which was readily converted to the bis silyl ether 11 upon treatment with Et3SiOTf (67% for 3 steps). The allylic alcohol 12 was prepared by selective removal of the primary silyl ether with catalytic PPTS in ethanol and dichloromethane. Epoxidation of the allylic alcohol 12 under Sharpless conditions20 led to exclusive formation of the desired epoxyalcohol which was immediately exposed to the Grubbs catalyst [(Cy3P)2Cl2Rud (1) Faulkner, D. J. Nat. Prod. Rep. 1999, 16, 155-198. Faulkner, D. J. Nat. Prod. Rep. 1998, 15, 113-158. Faulkner, D. J. Nat. Prod. Rep. 1997, 14, 259-302. Faulkner, D. J. Nat. Prod. Rep. 1996, 13, 75-125 and earlier reviews in the same series. Yasumoto, T.; Murata, M. Chem. ReV. 1993, 93, 1897-1909. (2) For recent approaches to the synthesis of eight-membered-ring ethers see: Crimmins, M. T.; Tabet, E. A. J. Am. Chem. Soc. 2000, 122, 54735476 and references therein. (3) Pawlak, J.; Tempesta, M. S.; Golik, J.; Zagorski, M. G.; Lee. M. S.; Nakanishi, K.; Iwashita, T.; Gross, M. L.; Tomer, K. B. J. Am. Chem. Soc. 1987, 109, 1144-1150. (4) Murata, M.; Legrand, A. M.; Ishibashi, Y.; Yasumoto, T. J. Am. Chem. Soc. 1989, 111, 8929-8931. Murata, M.; Legrand, A. M.; Ishibashi, Y.; Fukui, M.; Yasumoto, T. J. Am. Chem. Soc. 1990, 112, 4380-4386. (5) Nagai, H.; Torigoe, K.; Satake, M.; Murata, M.; Yasumoto, T.; Hirota, H. J. Am. Chem. Soc. 1992, 114, 1102-1103. Nagai, H.; Murata, M.; Torigoe, K.; Satake, M.; Yasumoto, T. J. Org. Chem. 1992, 57, 5448-5453. (6) Uchio, Y.; Kodama, M.; Usui, S.; Fukazawa, Y. Tetrhedron Lett. 1992, 33, 1317-1320. (7) King, T. J.; Imre, S.; Oztunc, A.; Thomson, R. H.; Tetrahedron Lett. 1979, 1453-1454. Howard, B. M.; Schulte, G. R.; Fenical, W.; Solheim, B.; Clardy, J. Tetrahedron 1980, 36, 1747-1751. Norte, M.; Gonzalez, A. G.; Cataldo, F.; Rodriguez, M. L.; Brito, I. Tetrahedron 1991, 47, 9411-9418. (8) Notaro, G.; Piccialli, V.; Sica, D.; Mayol, L.; Giordano, F. J. Nat. Prod. 1992, 55, 626-632. (9) Kurata, K.; Furusaki, A.; Suehiro, K.; Katayama, C.; Suzuki, T. Chem. Lett. 1982, 1031-1034. (10) For approaches to nine-membered-ring ethers see: Fujiwara, K.; Awakura, D.; Tsunashima, M.; Nakamura, A.; Honma, T.; Murai, A. J. Org. Chem. 1999; 64, 2616-2617 and references therein. (11) For a discussion of conformational effects on rates of ring closing metathesis of medium-ring ethers see: Crimmins, M. T.; Emmitte, K. A. Synthesis 2000, 899-903. Crimmins, M. T.; Choy, A. L. J. Am. Chem. Soc. 1999, 121, 5653-5660. Crimmins, M. T.; Choy, A. L. J. Org. Chem. 1997, 62, 7548-7549. For other applications of ring-closing metathesis in the synthesis of medium-ring ethers see: Maier, M. C. Angew. Chem., Int. Ed. 2000, 39, 2073-2077. Fürstner, A. Angew. Chem., Int. Ed. 2000, 39, 30123043. Clark, J. S.; Hamelin, O. Angew. Chem., Int. Ed. 2000, 39, 372-374 and references therein. (12) Crimmins, M. T.; Emmitte, K. A. Org. Lett. 1999, 1, 2029-2032. (13) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; John Wiley and Sons: New York, 1994; pp 609-610. (14) Crimmins, M. T.; Emmitte, K. A.; Katz, J. D. Org. Lett. 2000, 2, 2165-2167. (15) Prepared from 2-butyne-1,4-diol as follows: (1) BnBr, NaH, THF; (2) LiAlH4, Et2O; (3) MsCl, Et3N, CH2Cl2; (4) NaI, THF. Sugiyama, H.; Yokokawa, F.; Shioiri, T.; Katagiri, N.; Oda, O.; Ogawa, H. Tetrahedron Lett. 1999, 40, 2569-2572. (16) Prashad, M.; Har, D.; Kim, H.-Y.; Repic, O. Tetrahedron Lett. 1998, 39, 7067-7070. (17) Swern, D.; Mancuso, A. J.; Huang, S.-L. J. Org. Chem. 1978, 43, 2480-2482. (18) Brown, H. C.; Randad, R. S.; Bhat, K. S.; Zaidlewicz, M.; Racherla, U. S. J. Am. Chem. Soc. 1990, 112, 2389-2392. (19) Schreiber, S. L.; Ikemoto, N. J. Am. Chem. Soc. 1992, 114, 25242536. (20) Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765-5780. Scheme 1 1533 J. Am. Chem. Soc. 2001, 123, 1533-1534