Selective transformations of differently functionalized 4-ethynyl-octa-1,7-dienes and 5-ethynyl-nona-1,8-dienes via intramolecular Pauson–Khand reaction: preparation of new and useful building blocks for the synthesis of angularly fused triquinanes

The intramolecular Pauson–Khand (PK) reaction of 4-bromomethyldimethylsilyloxy-4-ethynyl-6,6-dimethyloct-1-en-7-yne (), the differently functionalized 4-ethynyl-octa-1,7-dienes (), and 5-ethynyl-5-hydroxy-2,4,4-trimethyl-nona-1,8-diene () is reported. The resulting bicyclo[3.3.0]oct-1-en-3-ones () and () have been obtained in good yield, with moderate stereoselectivity. Major isomers (, , , ) have the free or protected hydroxyl group cis to H-6a at the fused ring carbon. In dienyne compound the gem-dimethyl functional group, with the same length for both alkenyl chains, dictates the regiochemistry of the PK reaction, affording major bicyclo derivatives and (in a 4.6:1 ratio, the major isomer () having the free hydroxyl group cis to the methyl group at C-6a), and compound with the hydroxyl group trans to proton H-6a at ring junction. All new adducts are potential useful building blocks for the total synthesis of angularly fused triquinanes.