Thioacetals metal catalysis

Corey s method20 relies on metal exchange with the bromocyclopropane 69 prepared by carbene addition. The extra stabilisation of cyclopropyl anions (chapter 8) makes both this lithium derivative and the ylid 63 more easily handled. Addition to aldehydes or ketones gives mixtures of adducts 70 [it turns out that none of the stereochemistry of 69 or 70 matters] which fragment under Lewis acid catalysis to give the thioacetal 71. Careful hydrolysis releases the 3,4-enal -72, the product of a homoaldol reaction with an aldehyde homoenolate and RCHO and a difficult compound to make as the double bond moves into conjugation very easily. 

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