Carboxylic acids possessing a strong electron-withdrawing group in the α-position undergo facile dehydration upon reaction with carbodiimides to form the corresponding substituted ketenes that can react in situ with alcohols providing esters in a high yield. The ketene formed by the treatment of ethyl 2-methylmalonate with DCC was trapped in situ by a [4+ 2] cycloaddition with a second DCC molecule. The chemoselectivity of the acylation through ...