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

A truly catalytic procedure is described for the esterification of alpha -a mino acids, thereby circumventing the formation of stoichiometric quantitie s of salts associated with conventional procedures. The acid form of ultras table 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 10 0-130 degreesC (15-20 bar). For example, L-phenylalanine afforded the methy l ester in 83% yield after 20 h at 100 degreesC. Based on the (unlikely) pa rticipation of all the Al atoms of the zeolite this corresponded to a turno ver number of 180. The ester product was partially racemised (52% ee). Phen ylglycine, p-hydroxyphenylglycine and homophenylalanine were similarly conv erted to their methyl esters. The H-USY catalyst could be recycled albeit w ith decreased activity after each cycle owing to the adsorption of water (f ormed in the reaction). Its activity was completely restored, however, afte r calcination.