Short synthesis of (±)-hirsutine: Direct addition of dimethyl malonate anion to a 1,4-conjugate iminium salt of appropriate 3-ethylindolo[2,3-a]quinolizidine

Direct addition of dimethyl malonate anion to a 1,4-conjugate iminium salt of 3-ethylindolo[2,3- a ]quinolizidine 4a (regenerated from the corresponding α-aminonitrile 4b ) afforded enamine 5. Catalytic hydrogenation of compound 5 led stereoselectively to compound 6 ( pseudo ), which is the highly desired intermediate for the preparation of several Corynanthe alkaloids, including (±)-hirsutine 7.