An improved route to an intermediate in podophyllotoxin synthesis

Abstract

Cyclopropanation of the chalcone (3) with dimethylsulphonium ethoxycarbonylmethylide affords a 1 : 1 mixture of the cyclopropanyl keto-ester epimers (4) and (5); both stannic chloride and boron trifluoride etherate in nitromethane catalyse the stereoselective cyclisation of this mixture to the tetralone (2), the known podophyllotoxin precursor, in 51% overall yield.