posted on 1996-11-29, 00:00authored byThomas A. Engler, Mohammed Hashmat Ali, Fusao Takusagawa
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.