Synthesis and stereochemistry of an optically active selenonium ylide. X-Ray molecular structure of (+) Se -{4′-[(–)-menthyloxycarbonyl]phenyl}(methyl)-selenonium 4,4-dimethyl-2,6-dioxocyclohexylide

Fractional recrystallization of diastereoisomeric {4′-[(–)-menthyloxycarbonyl]phenyl}(methyl)-selenonium 4,4-dimethyl-2,6-dioxocyclohexylide from hexane–diethyl ether gave the optically pure (+)-selenonium ylide as stable crystals. The absolute configuration around the selenium atom was determined to be R by X-ray crystallographic analysis. The epimerization of the optically active selenonium ylide by pyramidal inversion was studied.