Enantioselective Friedel-Crafts Alkylation of Furans and Indoles with Simple α,β-Unsaturated Ketones Catalyzed by Oxazaborolidinone

Abstract

allo-Threonine-derived oxazaborolidinone (OXB) catalyzes the Friedel―Crafts alkylation of furans and indoles with simple acyclic α,β-unsaturated ketones to give the products in high yields and with high enantioselectivities. With 5―10 mol% of the OXB catalyst, enantioselectivities of up to 94% ee could be achieved for a variety of substrates. The use of N,N-dimethylaniline (2.5―10 mol%) as an additive is found to be essential for enantioselectivity. The effects of the additive are discussed in terms of the retardation of a proton-catalyzed racemic pathway, which deteriorates the enantioselectivity of the Friedel―Crafts reactions.