Ready access to bicyclo[5.3.0]decan-1-ones and to bicyclo[6.3.0]undecan-1-ones by intramolecular Pauson-Khand reactions using a temporary sulfur bridge

An intramolecular Pauson-Khand-based route to octahydroazulenones and decahydrocyclopentacyclooctenones is described. The amine oxide-induced cycloaddition of the dicobalt hexacarbonyl complexes of the sulfur-bridged enynes 1 and 2 leads to the exclusive formation of the tricyclic cyclopentenones 3 and 4, respectively. This stereochemical outcome, which implies that the olefin moieties in both 1 and 2 react by the less substituted alpha-face, is rationalized on the basis of the steric requirements of the putative cobaltacyclic intermediates of the Pauson-Khand reaction. The totally stereoselective conjugate addition of methyl or phenyl groups to enones 3 and 4, effected by the corresponding lithium diorganocuprates, and the subsequent reductive desulfurization with Raney nickel in refluxing ethanol, lead to the cis-bicyclo[5.3.0]decan-2-ones 5a,b and to the cis-bicyclo[6.3.0]undecan-2-one 6, respectively, in good overall yields. The stereochemistry of intermediates 3, 4, 7b, 8, and 9 has been firmly established with the aid of both NOESY experiments and AM1 MO semiempirical MO calculations.