The dienolate aldol reaction of (E)-N-crotonoyl C(4)-isopropyl SuperQuat: asymmetric synthesis of alpha-vinyl-beta-hydroxycarboxylic acid derivatives and conversion to alpha-ethylidene-beta-hydroxyesters (beta-substituted Baylis-Hillman products)

摘要

The synthesis of alpha-vinyl-beta-hydroxyesters and alpha-ethlylidene-beta-hydroxyesters (beta-substituted Baylis-Hillman products) Via the dienolate aldol reaction of (E)-N-crotonoyl C(4)-isopropyl SuperQuat is described. High levels of syn-diastereoselectivity (up to >98% de) are observed for the dienolate aldol reaction with boron enolates, generated either directly with Bu2BOTf or by transmetalation of the potassium enolate with B-bromocatecholborane. Cleavage of the resultant syn-aldol products from the auxiliary gives alpha-vinyl-beta-hydroxyesters in >98% de and >98% ee. Subsequent isomerisation of the double bond into conjugation provides alpha-ethylidene-beta-hydroxyesters (beta-substituted Baylis-Hillman products) in high diastereo- and enantiopurity (>= 91:9 |(E):(Z)| and >98% ee).