Grignard reagent-promoted selective ring expansion and alkylation of formyl borneol and isoborneol: a new route to highly substituted cyclopentanes

Formyl borneol, a [2.2.1]-bicyclic carbinol, reacts with various Grignard reagents to produce corresponding alkyl [3.2.1]-bicyclic diols, which can be converted to new highly substituted cyclopentanes, and further to 3-acyl-bornylenes. These ring expansion–alkylation reactions are highly selective. Reaction of formyl isoborneol with methyl magnesium bromide gave ring expansion-only and alkylation-only products.