Syntheses with organoboranes. Part 14: Enolization–aldolization of conjugated cyclohexenones via dienolborinates

Enolization of cyclohex-2-enone (1), 3-methyl- (2), 3,5-dimethyl- (3), 3,5,5-trimethyl- (4), and 3,4,5,5-tetramethylcyclohex-2-enone (5) with chlorodicyclohexylborane proceeds by deprotonation at the 6-position. Aldolization of the dienolborinates with benzaldehyde, and acetaldehyde, provides the corresponding anti aldols with 87–95% selectivity. Ketones 4 and 5 undergo competitive deprotonation at the 3-methyl group and aldolization at the 2-position. In contrast, lithium dienolates derived from 4 and 5 gave syn aldols with 95% selectivity.