Synthesis of the enantiomeric forms of alpha- and gamma-damascone starting from commercial racemic alpha-ionone

A straightforward synthesis of both enantiomers of alpha- and gamma-damascone is described. The title compounds were prepared by a divergent pathway starting from the enantiomeric forms of (6RS,7SR,9RS)-7-hydroxy-7,8-dihydro-alpha-ionol and of (6RS,7SR,9RS)-7hydroxy-7.8-dihydro-gamma-ionol. These building blocks were obtained from racemic alpha-ionone in four and five steps, respectively. The 7-hydroxy group was introduced by regio- and diastereoselective epoxidation of the conjugated double bond followed by reductive opening of the oxirane ring. The hydroxy-ketone obtained was reduced diastereo selectively to trans-alpha-diol that could be converted to the trans-gamma-diol by photochemical isomerization. Both diols were then resolved by lipase-mediated acetylation.