71(P2-1) 軟サンゴ、オオウミキノコからの新センブラン型ジテルペン類の構造と分子内閉環反応(ポスター発表の部)

抄録

Six new cembranoids sarcophytol P (3), R (4), S (5), K (8), F (11), and T (13) were isolated from the soft coral Sarcophyton glaucum. Sarcophytol P (3) was shown to be the 20-hydroxy derivative of the major component sarcophytol A (1), and afforded the cyclization product 6 in CHCl_3 at room temperature, in a same way as in 1. Sarcophytols R (4) and S (5) were correlated to 1, by conversion of its 7R,8R and 7S,8S epoxide derivatives. Sarcophytol K(8) was a 13, 14-dihydroxycembranoid having a 1E,3Z-diene moiety. The absolute configuration of 8 and its 1Z,3E- and 1Z,3Z-isomers sarcophytol B (2) and J (10) were determined by circular dichroism study of their bis-p-dimethylaminobenzoate derivatives. Sarcophytols F (11) and T (13) were 1E- and 1E,3Z-isomers of 1. Compound 11 showed characteristic broadening of the ^1H-NMR chemical shifts, due to the restricted conformational interconversion. Using the three cembranoids sarcophytols F (11, 1E,3E), N (15, 1Z,3Z) and T (13,1E,3Z), spontaneous autoxidation-cyclization, in CHCl_3,was examined, in order to compare the stereochemical course of the reaction with that of 1 (1Z,3E), which affords trans-fused bicyclo[9.3.0]tetradecene systems. The 1E,3Z-isomer 13 gave the same cyclization product 18, as in the reaction of 1, even though it is isomeric at C-1,3. The 1Z,3Z isomer 15 gave 19, in consequence of the reversed geometry at C-3 of 15. The 1E,3E-isomer (11) gave the bicyclic product 22, having an antipodal fusion as compared with 19. This was confirmed by PCC oxidation of 19 and 22, giving enantiomeric ketones 23 and 24 respectively. The 1(14)-epoxide 26 was shown to be the immediate precursor of 22, and acounted for the inversion of the geometry at C-1 of the cyclization product. Compound 26 is isomeric with the epoxide 17 derived from 1. The epoxide 17 is the postulated precursor in the conversion of 13 to 18. It is noteworthy that the mode of antipodal fusion of the cyclopentane rings, in 18 and 22, was controlled by the chirality of the epoxy rings. The C-14 hydroxyl group participates in the transannular cyclization, but was found not to be the requisite functional group for the reaction. Similar treatment of cembrene C (30), the parent hydrocarbon of 1, also reacted in CHCl_3 giving the bicyclic product 32.