Enantiomerically Pure Dibenzyl Esters of L‑Aspartic and L‑Glutamic Acid

S)-Dibenzyl aspartate p-toluenesulfonate [(S)-1 center dot TsOH] and (S)-dibenzyl glutamate p-toluenesulfonate [(S)-2 center dot TsOH] were efficiently prepared from the respective L-amino acids and benzyl alcohol with very high yields by using cyclohexane as a water azeotroping solvent instead of benzene, carbon tetrachloride, toluene, or benzyl alcohol itself; as reported in literature methods. Preventively, chiral HPLC methods were developed to determine the enantiomeric excess of the two diesters and DSC analyses were performed on the respective p-toluenesulfonates. With the aid of such investigation tools, we demonstrated that (S)-1 center dot TsOH and (S)-2 center dot TsOH were formed enatiomerically pure in cyclohexane, whereas more or less pronounced racemization occurred both in toluene and in benzyl alcohol. The two one-pot procedures, which did not require crystallization of the product or any other purification step, were accomplished on multigram scale.