Citronellal cyclisation in superacids

Abstract

The cyclisation of citronellal in superacids parallels closely the reaction in ‘normal’ acids, yielding iso-pulegol and neoisopulegol. Each alcohol then undergoes protonation at the double bond; neoiso-pulegol loses water to give 2-(4-methylcyclohex-1-en-1-yl)prop-2-yl cation, but in isopulegol this process is stereochemically unfavourable, and competes with a 1,5-hydride shift to yield the 1-methyl-4-isopropyl cyclohexenyl cation. Hydroxycitronellal yields only the 2-(4-methylcyclohex-1-en-1-yl)prop-2-yl cation by an unknown route.