A synthetic approach to (+/-)-forskolin .1. Preparation of key hydrobenzofuran intermediates

In a synthetic approach to (+/-)-forskolin 1, a stereoselective preparation of the unsaturated lactone 2 was: envisaged. Propargylic derivatives 18a-c were prepared from available alpha-ionone 5 and treated with Bu3SnH/AIBN to give the bicyclic vinylstannanes 19a-c in high yield. From these compounds we then performed transmetallation reactions to obtain the 21a-c and 22b homologous derivatives. The two enyne compounds 29 and 30 were then prepared by our previous approach to lactone 2 involving a radical C7-C8 bond formation. In a second radical approach promoted by SmI2, the diol 10, was used to synthesize a potential precursor of the dialdehyde 9.