Structures and Reactivities of O-Methylated Breslow Intermediates

摘要

Since the report by Ukai et al. in 1943, thiamine and related thiazolium ions A have been known to be catalysts for the umpolung of aldehydes. The mechanism generally accepted for these reactions was proposed by Breslow in 1958, when he described that the thiazolium ring first undergoes depro-tonation at the most acidic position to give an ylide or carbene B (Scheme 1). The subsequent nucleophilic addition of B to an aldehyde generates the zwitterion C, which undergoes a proton shift to give the Breslow intermediate D, a nucleophilic acyl anion equivalent. The reaction of D with a second molecule of aldehyde followed by a proton shift and release of B generates the benzoin.