19 タキサン型ジテルペノイドの特異な反応(口頭発表の部)

抄録

Since the discovery of anticancer activity of taxol, much attention has been paid for the isolation of new taxane diterpenoids from various yew trees and the structure-activity relationships and the total syntheses of taxoids. In our continuing chemical derivatization from taxinine, one of major taxoids obtained from the Japanese yew Taxus cuspidata Sieb. et Zucc., we found unusual reactions of taxinine derivatives. Here we describe the formation of unexpected products and propose plausible mechanism of these reactions. 1. Stereoselective Epoxidation of 4(20)-Exomethylene in Taxinine Derivatives Epoxidation of taxinine (1), taxinine A (2), and taxinine derivative 7 with m-chloroperbenzoic acid afforded the α-4(20)-epoxides selectively (α:β=99:1), while epoxidation of taxinine derivatives 7 and 8 with dimethyldioxirane gave the β-4(20)-epoxides predominantly (α:β=1:4〜5). 2. Unusual Boron Trifluoride-catalyzed Reactions of Taxinine Derivatives with α- and β-4(20)-Epoxides Boron trifluoride-catalyzed reaction of β-4(20)-epoxy-5-O-triethylsilyltaxinine A (8b) gave the 3,5-diene (9, 42%), the 3,8-cyclopropane (10, 16%), the cyclobutane (11, 4%), and the dioxane (12, 3%) derivatives, while the similar reaction of the corresponding α-4(20)-epoxide (8a) afforded the ring contracted derivative (13, 41%), its hemiacetal dimer (14, 16%), and the orthoester (15, 9%) derivatives. The structures of 9〜15 were elucidated by spectral data including MS and 2D NMR data. Plausible mechanisms of these reactions containing 1,2-hydride shift (9 and 10 from 8b) or pinacol-type rearrangement (13 and 14 from 8a) are proposed. 3. Occurrence of a New Dimeric Compound of 5-Oxotaxinine A through Diels-Alder Cycloaddition Oxidation of taxinine A (2) with tetrapropylanunonium perruthenate afforded 5-oxotaxinine A (16) which subsequently gave a new dimeric compound (17) through regio- and stereo-specific Diels-Alder cycloaddition. The relative stereostructure of 17 was established by spectral data and X-ray analysis.