A solution of l-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (12.5 g) in dichloromethane (200 ml) was added to a solution of (±)-trans-1,2,4,5-tetrahydro-4-methyl-7,8-methylenedioxy-5-oxo-3-benzothiepin-2-carboxylic acid (15.34 g) and methyl (S)-(+)-mandelate (18.19 g) in N,N-dimethylformamide (DMF) (200 ml) at 0° C., followed by the addition of 4-dimethylaminopyridine (DMAP) (3.34 g). This mixture was stirred at 0° C. for 1 hour and at room temperature for 15 hours, after which it was poured into water and extracted with ethyl acetate. The ethyl acetate layer was washed with water and dried (MgSO4), after which the solvent was distilled off; the residual crystals were collected by filtration and washed with ether-hexane, followed by recrystallization twice from ethyl acetate-hexane to yield (S)-α-methoxycarbonylbenzyl ester of (2S,4R)-(+)-1,2,4,5-tetrahydro-4-methyl-7,8-methylenedioxy-5-oxo-3-benzothiepin-2-carboxylic acid (4.57 g, 19%). Colorless needles. Melting point 141-142° C. Optical rotation [a]D (23° C.) +239.7° (c=0.50, CHCl3).
