First example of reversal of normal stereoselectivity in the intramolecular Pauson-Khand reaction

Stereoselectivity favoring exo substituent orientation in the intramolecular Pauson-Khand reaction of 3-methyl-4-hydroxy-1,6-heptenynes to form bicyclo[3.3.0]octenones is affected by the relative stereochemistry at C3 and C4. Cycloaddition of one stereoisomer of 3,5-dimethyl-4-propargyl-1,6-heptadiene gives predominantly the bicyclo[3.3.0]octenone isomer containing both substituents in more hindered endo orientations. An explanation based upon the effect of conformational preferences prior to metallacycle formation is proposed.