N,O-Benzyl Protection of Structurally Varied Amines and Phenols Using Wells-Dawson Heteropolyacid Catalyst

摘要

In the last two decades, the investigation of green chemistry processes1 in the protec-tion/deprotection of functional groups has received much attention. Acylation of amine and hydroxyl groups is among the most used protective strategies in organic synthesis. In the same vein, benzoylation8 is used for the N-protection of amino acids via the widely-used Schotten-Bauman procedure.9 The benzoyl (Bz) group is one of the most attractive protecting groups for amines, alcohols and phenols: this group is stable under hydrogenolysis conditions;; it can be easily introduced using such bases as pyridine, TEA, or DMAP; and it can be removed by hydrolysis.11 There are a number of reagents available for the benzoylation of amine and hydroxyl groups, including 2V-ben-zoyltetrazole,12 benzoyl cyanide,13 benzoic anhydride,14 2-benzoylthio-l-methylpyridi-nium chloride15 and benzoyl chloride. This last is widely used because of its ready availability and low cost. Benzolyation is a well-known reaction in classical organic chemistry. Among some of the newer methods, a number of procedures have been developed for benzoylation using acidic or basic conditions. Benzoylation of primary amines has even been described without the use of any solvent or alkali.20 A catalyst-free and solvent-free procedure using ultrasound irradiation was also reported.21 No matter whether the methods are classical or more up-to-date, many of them have such limitations as highly basic conditions, long reaction times, low yields of products, environmental pollution caused by toxic reagents and solvents, or the use of expensive catalysts.