A method for intramolecular syn delivery of an alkyl group to the proximal olefinic carbon in cyclooct-5-enols

Abstract

Various functionalized derivatives of trans-2,6-dimethylcyclooct-5-enol have been prepared and found to be recalcitrant to free radical or ionic six-ring cyclization with formation of a new carbon–carbon bond syn to the original hydroxy group at the pro-quaternary site. However, intramolecular syn delivery was achieved with complete regiocontrol by epoxidation/ring opening of the MOM ether, oxidation to the α,β-unsaturated ketone 35a, Rh^I-catalysed hydrosilylation to give 36, and exposure of this silyl enol ether to stannic chloride in the presence of ‘proton sponge’. The pivotal ring closure leads in good yield to 32, thereby solving the problem posed by untoward kinetic barriers to ring closure that are present under alternative conditions.