Catalyst-controlled diastereoselectivity reversal in the formation of dihydropyrans.

An enantioselective synthesis of cis-dihydropyrans as the formal HDA reaction products was achieved through the catalyst-controlled anti-selective conjugate addition of aldehydes to β,γ-unsaturated α-keto esters. The observed unusual cis-selectivity could be attributed to the stabilization of the less favorable transition state for anti-conjugate adducts by the hydrogen bonding between the hydroxy group of the amino diol catalyst and β,γ-unsaturated α-keto esters.