Specificity of the reaction of (-)-1-{(1S,2R,4R)-1-ethenyl-2-hydroxy-7,7-dimethylbicyclo[2.2.1]hept-2-yl}ethanone with ethenylmagnesium bromide

Ethenylmagnesium bromide (1.5 equiv) forms a chelate with (-)-1-{(S,2R,4R)-1-ethenyl-2-hydroxy-7,7-dimethylbicyclo[2.2.1]hept-2-yl}ethanone in THF and promotes its fast primary a-ketol rearrangement into 1-ethenyl-2-hydroxy-2,8,8-trimethylbicyclo[3.2.1]octan-3-one. The latter reacts with excess magnesium reagent (0.5 equiv) according to common 1,2-addition pattern at the carbonyl group and is simultaneously involved in the second alpha-ketol rearrangement which leads to 1-ethenyl-3-hydroxy-3,8,8-trimethylbicyclo-[3.2. 1]octan-2-one as thermodynamically more stable regioisomer.