抄録
A simple and stereoselective preparation of (-)-11β-carboxysantonin (8a) was achieved by carboxylation of l-α-santonin (5b), based on the non-stereoselective two-step method described earlier. (+)-11β-Carboxy-(3α) and (+)-11α-carboxy-3α, 4β, 5α, 6βH-hexahydrosantonin (3b) were prepared in a ratio of 3 : 1 by alkoxycarbonylation of 3α, 4β, 5α, 6β, 11βH-hexahydrosantonin (HHS) (1a), followed by hydrolysis. The absolute configurations at C-11 in the three optically active 11-carboxysantonins have now been confirmed on the basis of the stereochemistry of the fourth new stereoisomer, (-)-11β-carboxysantonin (8a). The complete structure of 8a was established by its transformation to (+)-11β-carboxy-4β, 5α, 6βH-tetrahydrosantonin (7a), correlating with 3a. The proton magnetic resonance (PMR) signals of 6β-hydrogens in 11β-carboxy- or 11β-methoxycarbonyl-HHS (2a-4a) are anisotropically shifted further downfield compared to those in the 11α-oriented epimers (2b-4b). A similar result was obtained for the pairs of epimers in the 4β, 5α, 6β, 11βH-tetrahydrosantonin (THS) series (6a, 7a and 6b, 7b). The stereospecific decarboxylation of the 11β- and 11α-carboxylactone derivatives (3a, 7a, 8a, and 3b, 7b, 8b) afforded the thermodynamically more stable 11α-methyl lactone analogs (1a, 5a, 5b), and not the 11β-methyl epimers (1c, 5c, 5d). The rate of decarboxylation of the former lactone acids was faster than that of the latter, supporting the foregoing assignments, based on spectral data, for the configurations of the 11β- and 11α-carboxyl groups.
