Stereoselective epoxidation of 4(20)-exomethylene in taxinine derivatives and assignment of the epoxide orientation by NMR

Epoxidation of taxinine (1), taxinine A (2), and taxinine derivative 7 with m-chloroperbenzoic acid afforded the alpha-4(20)-epoxides selectively (alpha : beta = 99 : 1), while epoxidation of taxinine derivatives 7 and 8 with dimethyldioxirane gave the beta-4(20)-epoxides predominantly (alpha : beta= 1 : 4 similar to 5). The epoxide orientation was found to be assignable by magnitude of chemical shift differences between the geminal epoxide protons in the H-1 NMR spectra. Relative stereochemistry of 1 beta-hydroxybaccatin I (9) possessing beta-4(20)-epoxide moiety was established by X-ray analysis to provide valid assignment of the epoxide orientation. (C) 1998 Elsevier Science Ltd. All rights reserved.