FREE-RADICAL FRAGMENTATION OF [2- FORMAL SYNTHESIS OF PENTALENENE(2] PHOTOADDUCT DERIVATIVES )

A series of [2 + 2] photoadducts 3a-d are converted to cyclobutylcarbi nyl iodides 6a-d. Free radical fragmentation of the iodides yields bic yclo[x.y.0] carbon skeletons 7a-d which are present in a variety of na tural products. Application of this methodology to terpenoid synthesis is illustrated by the preparation of unsaturated ketone 19 in only si x steps (33% overall yield). As 19 has been converted to pentalenene ( 1) this constitutes a formal synthesis of the angular triquinane sesqu iterpenoid.