New Chiral Synthesis of Methyl and Allyl Disubstituted Butyrolactone: A Formal Synthesis of (-)-Ngaione

A new chiral synthesis of butyrolactone [(-)-7] bearing methyl and allyl substituents at the γ-position has been established via diastereoselective continuous nucleophilic addition to the chiral tricyclic lactone [(-)-1] as the key step.In practice, continuous nucleophilic addition to the tricyclic lactone [(-)-1] by using the combination of methyllithium and allylmagnesium bromide proceeded to yield the corresponding diol (8) diastereoselectively.After oxidation of 8, enantiopure methyl and allyl disubstituted butenolide (10) was obtained via retro-Diels-Alder reaction.Selective 1,4-reduction of 10 afforded the corresponding butyrolactone [(-)-7], which is the key intermediate for (-)-ngaione synthesis.Chiral γ-substituted butenolides (4) are known to be useful synthons for the enantiocontrolled