C1?C2 Cleavage of C1-Functionalized 8-Oxabicyclo-[3.2.1]-oct-6-en-3-one. Stereoselective Preparation of 4-Substituted Butenolides

Diastereoisomeric epimers at C1′ of 4-(1-methyl-2-oxo-butyl)-2-butenolide, and the corresponding saturated γ-lactones, were synthesized by hydrolysis of 1-methoxy-8-oxabicyclo[3.2.1] oct-6-en-3-one, under acidic conditions. These butenolides are interesting synthetic building blocks, precursors of biologically active natural products like insect pheromones. Their formation could be explained by a cleavage at the C1–C2 bond of the oxabicyclic precursor. On the basis of the experimental data we have proposed a mechanism of hydrolytic cleavage which is formally an intramolecular reverse Dieckmann process.