Preparation of linearly fused 6-6-5 and 6-5-5 carbotricyclic ring systems related to the natural polyquinanes

A short pathway to the formation of the title framework is elaborated via (i) the tandem addition of arylsulfenium chloride and 1-siloxycycloalkenes across the double bond of dicobalthexacarbonyl complex of vinylacetylene, (ii) the diastereoselective Grignard reaction at the carbonyl group of the formed adducts; and (iii) the Pauson–Khand cyclization of 1,6- or 1,7-enyne precursors thus prepared.