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19.4 g of the thus-obtained dibenzyl L-aspartate paratoluenesulfonate was suspended in 150 ml of chloroform, to which was added 16 g (0.04 mols) of aqueous 10% sodium hydroxide solution, and stirred at room temperature for 1 hour. Next, the chloroform layer was separated, to which was added 14.7 g (0.04 mols) of O,O′-diparatoluoyl-L-tartaric anhydride, and again stirred at room temperature for 2 hours. The chloroform layer was analyzed through HPLC. Its chart gave two peaks based on O,O′-diparatoluoyl-L-tartaric acid mono-L-(1,2-dibenzyloxycarbonyl)ethylamide (L-L form) of formula (16) and on O,O′-diparatoluoyl-L-tartaric acid mono-D-(1,2-dibenzyloxycarbonyl)ethylamide (L-D form) of formula (17), and the optical purity of the dibenzyl L-aspartate paratoluenesulfonate used in the reaction was found to be 59.0% ee (FIG. 1—column, CAPCELL PAC SG120; eluent, aqueous phosphoric acid solution (pH 2.2)/methanol=38/62 v/v).

1. customNext, the chloroform layer was separated, to which
2. stirringagain stirred at room temperature for 2 hours
3. customIts chart gave two peaks