Diastereomeric acetoxy sulfones

Since the cis- and trans-vinyl radicals can equilibrate at this stage and the trcms-radical is the more stable of the two, both diastereomeric acetoxy sulfones would still lead selectively to the same product. 

The reduction of the diastereomeric acetoxy-sulfone with sodium amalgam gives selectively the ( )-alkene. 

Reduction of the mixture of erythro- and t/rreo-sulfones with Na(Hg) leads stere-oselectively to the trans-alkene, indicating that the alkene-forming steps for both diastereomeric sulfones must involve a common intermediate. The (E)-selectivity of the elimination step may involve two successive electron transfers from the sodium to the sulfone, resulting in the loss of benzenesulfinate. The anion that is formed must have a lifetime sufficient to equilibrate and assume the low-energy conformation, which places (1) the p-orbital of the carbanion antiperiplanar to the acetoxy leaving group and (2) the bulky substituents as far apart as possible. Therefore, the... 

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