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 (1), the differently functionalized 4-ethynyl-octa-1,7-dienes (2-6), and 5-ethynyl-5-hydroxy-2,4,4-trimethyl-nona-1,8-diene (7) is reported. The resulting bicyclo[3.3.0]oct-1-en-3-ones (22-29) and (33-35) have been obtained in good yield, with moderate stereoselectivity. Major isomers (22, 24, 26, 28) have the free or protected hydroxyl group cis to H-6a at the fused ring carbon. In dienyne compound 7 the gem-dimethyl functional group, with the same length for both alkenyl chains, dictates the regiochemistry of the PK reaction, affording major bicyclo derivatives 33 and 34 (in a 4.6:1 ratio, the major isomer (33) having the free hydroxyl group cis to the methyl group at C-6a), and compound 35 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. (C) 2002 Elsevier Science Ltd. All rights reserved.