Synthesis of Medium Ring Ethers. Part 4. Stereoselective Claisen‐ Mediated Ring Expansion as a Route to Homochiral Disubstituted Medium Ring Lactones.

Thermal elimination/Claisen rearrangement of selenoxides derived from the enantiomerically pure phenylselenomethyl-substituted cyclic acetals 4 and 8 proceeds stereospecifically to provide the homochiral 8- and 7-membered lactones 5 and 9 respectively.