In order to delineate the mechanism of biotin-dependent carboxylation reactions, the following model experiments have been carried out. In reactions of carbonate derivatives such as methyl chloroformate with 2-ethoxy-2-imidazoline (1), a putative enolic biotin model, or 2-imidazolidinone (2), the former showed a much higher reactivity to provide the methoxycarbonylated 1 (3). Reactions of 3 with benzenethiolate, cyclohexylamine and ...