Stereocontrol in the nucleophilic epoxidation of α-(1-hydroxyalkyl)-α,β-unsaturated sulfones

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Abstract
Epoxidation of β-unsubstituted-α-(1-hydroxyalkyl)-α,β-unsaturated sulfones 1 with lithium tert-butyl peroxide proceeds with high diastereoselectivity to give the syn epoxides 10. Epoxidation of the triisopropylsilyl ethers 7, however, leads to the anti epoxides 13 with moderate to good selectivity. In contrast to this, epoxidation of (E)-α-(1-hydroxyalkyl)-β-phenyl-α,β-unsaturated sulfones 2 proceeds with high diastereoselectivity to give the anti epoxides 15. Epoxidation of the corresponding triisopropylsilyl ethers 9 leads to a reversal in diastereofacial selectivity, giving the syn epoxides 16 with moderate selectivity. The sense of diastereoselectivity has been determined by X-ray crystal-structure analyses of compounds 10c, 13a, 13b and 16a and chemical correlation. Use of potassium tert-butyl peroxide as the epoxidising agent for all these epoxidation reactions results in very similar levels of diastereoselectivity to those observed using lithium tert-butyl peroxide. A rationalisation for these results, based on the influences of 1,2- and 1,3-allylic strain, is proposed.