Comparative Cyclotrimerization of Enantiopure vic-Bromo(trimethylstannyl)bicycloolefins Derived from (+)-Camphor (+)-Fenchocamphorone and (–)-Epicamphor. Effect of the Bridgehead Methyl Group in the syn to anti Products Ratio

The influence of the bridgehead methyl group on syn/anti diastereoselectivity in the cyclotrimerisation of polycyclic alkenes was investigated. To this end, three different enantiopure vic-bromo(trimethylstannyl)-substituted bicyclic olefins were prepared and used as probes. The effects of different copper(II) or -(I) salts were studied, and most dimeric intermediates were independently synthesised and characterised. Some indications of the mechanism of cyclotrimerisation were furnished by experiments aimed at determining the relative rates of dimerisation. The results gave useful indications of how to achieve highly effective syn-cyclotrimerisation reactions.