Studies towards the synthesis of (±)-reserpine: Photocyclization mediated a novel and efficient synthesis of 11,18-dimethoxy-(20α)-18,19-didehydro-yohimban-17-one

A short, highly efficient synthesis of advanced intermediates to reserpine 1 has been developed starting from enamide 8. The enamide underwent photocyclization reaction using high pressure mercury lamp to afford the lactam 9 in excellent yield. Then lactam was reduced to the required amine 10, which upon acidic hydrolysis gave the nonconjugate ketone product 11, followed by reaction with sodium hydroxide resulted the desired conjugate ketone 12. Epoxidation, and then ring opening of the epoxide 13 with methanol yielded the desired product 14, which is key intermediate to the total synthesis of ()-reserpine.