OXIDATION OF NATURAL TARGETS BY DIOXIRANES .4. HIGH STEREOSELECTIVE AND REGIOSELECTIVE CONVERSION OF VITAMIN-D-2 TO ITS (ALL-R) TETRAEPOXIDE AND C-25 HYDROXY DERIVATIVE

Upon reaction with methyl(trifluoromethyl)dioxirane (Ib) at -40 degree s C, vitamin D-2 (2a) or its 3 beta-acetyl derivative (2b) give in hig h yield (78-80%) the corresponding tetraepoxide (3a,b) as a single dia stereoisomer having the 6(beta);7,8(beta);10,19(alpha);22,23(pseudo-al pha) configuration. Transformation of tetraepoxide 3a into its 3 beta- (p-bromobenzoyl) derivative 3c allowed X-ray diffraction analysis; thi s permitted us to ascertain that the stereomeric tetraepoxide product has the R configuration at all of the seven newly generated stereocent ers, i.e. 5R,6R;7R,8R;10R(19);22R,23R. The oxidation of 3 beta-acetyl vitamin D-2 (2b) with the less powerful dimethyldioxirane (la) led to the corresponding 5,6(beta);7,8(beta);10,19(alpha)-triepoxide 4 as the major product (62%), accompanied by tetraepoxide 3b (26%). Parallel t o vitamin D-3 triepoxide, remarkable site selectivity was monitored in oxyfunctionalization of 3 beta-acetyl vitamin D-2 tetraepoxide (3b) a t the side-chain tertiary C-25. Reaction of tetraepoxide 3b with the p owerful dioxirane 1b at 0 degrees C left the epoxide groups and remain ing C-H functionalities intact, affording the 25-hydroxy derivative 5 in good isolated yield (61%).