Synthesis of (-)- and (+)-esermethole via chemical resolution of 1,3-dimethyl-3-(2-aminoethyl)-5-methoxyoxindole

A new synthesis of (-)- and (+)-esermethole (2 and 7 respectively), penultimate intermediates to (-)- and (+)-physostigmine, is described. Phase-transfer catalyzed dimethylation of 3-cyanomethyl-5-methoxyoxindole (3) and successive hydrogenation of the cyano group gave 1,3-dimethyl-3-(2-aminoethyl)-5-methoxyoxindole (5), which was efficiently resolved with optically active tartaric acids. Subsequent conversion to a carbamate and reductive cyclization performed by a standard method afforded both the enantiomers of esermethole. The enantiomeric excesses, determined by chiral HPLC analysis, were greater than 99.5%. In particular, analytical HPLC resolution of 5, previously reported as unfeasible, was achieved.