1,5-Asymmetric induction in the boron-mediated aldol reaction of beta-oxygenated methyl ketones

摘要

High levels of substrate-based 1,5-stereoinduction are obtained in the boron-mediated aldol reactions of b-oxygenated methyl ketones with achiral and chiral aldehydes. Remote induction from the boron enolates gives the 1,5-anti adducts, with the enolate p-facial selectivity critically dependent upon the nature of the b-alkoxy protecting group. This 1,5-anti aldol methodology has been strategically employed in the total synthesis of several natural products. At present, the origin of the high level of 1,5-anti induction obtained with the boron enolates is unclear, although a model based on a hydrogen bonding between the alkoxy oxygen and the formyl hydrogen has been recently proposed.