Studies on the Synthesis of Acanthodoral and Nanaimoal: Evaluation of Cationic Cyclization Routes

摘要

Intramolecular Lewis acid-promoted reactions of α,β-unsaturated ketone 6 and aldehydes 7 and 8 were examined as potential routes to acanthodoral (1), a structurally interesting natural product. Ketone 6 afforded ene product 22 exclusively, and both 7 and 8 gave mixtures of bicyclic aldehydes 3 and 26 and tricyclic aldehyde 25. The latter most likely results from 7 by intramolecular cyclization of the alkene onto the Lewis acid-activated carbonyl moiety affording carbocation 31 followed by a 1,2-hydride shift and ring closure. Starting from 8, tricyclic aldehyde 25 apparently forms by cyclization to cation 35 and ring closure to cyclobutane 36, followed by ring opening to 31, the same cation as formed in reactions of 7. Nanaimoal (3) results from loss of H~+ from 31, and bicyclic aldehyde 26 may be formed in a similar manner or by a concerted ene reaction. The configuration of 25 establishes that the stereochemistry of the initial cyclization to 31 precludes the possible use of this strategy for the synthesis of acanthodoral. However, acid-promoted cyclization of allylic alcohol 23 efficiently gives diene 29 which undergoes selective hydroboration/ oxidation to afford nanaimoal.