Salt-free esterification of α-amino acids catalysed by zeolite H-USY

A truly catalytic procedure is described for the esterification of alpha -amino acids, thereby circumventing the formation of stoichiometric quantities of salts associated with conventional procedures. The acid form of ultrastable zeolite Y (H-USY), a naphtha cracking catalyst, acted as a solid acid catalyst in the reaction of several alpha -amino acids with methanol at 100-130 degreesC (15-20 bar). For example, L-phenylalanine afforded the methyl ester in 83% yield after 20 h at 100 degreesC. Based on the (unlikely) participation of all the Al atoms of the zeolite this corresponded to a turnover number of 180. The ester product was partially racemised (52% ee). Phenylglycine, p-hydroxyphenylglycine and homophenylalanine were similarly converted to their methyl esters. The H-USY catalyst could be recycled albeit with decreased activity after each cycle owing to the adsorption of water (formed in the reaction). Its activity was completely restored, however, after calcination.