Synthetic utility of active esters and carbonatescontaining 2-pyridyl moiety

Abstract

2-Pyridyl esters or S-(2-pyridyl) thioates were prepared by treatment of the mixed anhydride, generated from an equimolar mixture of carboxylic acids, triethylamine, and 2-pyridyl chloroformate, with a catalytic amount of 4-dimethylaminopyridine. Reaction of lithium dialkylcuprates with 2-pyridyl esters afforded the corresponding ketones in high yields under mild conditions. Preparation of hindered esters could be accomplished by the reaction of S-(2-pyridyl) thioates or 2-pyridyl esters with alcohols in the presence of cupric bromide. Di-2-pyridyl carbonate (2-DPC) was conveniently prepared in high yields by the reaction of phosgene with 2 equiv of 2-hydroxypyridine in the presence of triethylamine. Carboxylic acids, upon treatment with equimolar amounts of alcohol and 2-DPC in the presence of 0.1 equiv of DMAP, yielded the corresponding esters in high yields. Also, aliphatic carboxylic esters were prepared in high yields by the reaction of acid with equimolar amounts of chloroformates and triethylamine in the presence of a catalytic amount of DMAP. New amino protective reagents, t-butyl 2-pyridyl carbonate (BPC) and benzyl 2-pyridyl carbonate (BZPC), were prepared by the reaction of 2-pyridyl chloroformate with t-butyl alcohol in the presence of pyridine or benzyl chloroformate with 2-hydroxypyridine in the presence of triethylamine. BPC and BZPC were found to be very effective in the t-butoxycarbonylation and benzyloxycarbonylation of amines and amino acids.