The importance of A-strain in the stereochemical control of ring opening of tetrahydropyrans

Readily available cyclic β-alkoxy-α,β-unsaturated trifluoromethylketones react with excess phenyl magnesium bromide by initial 1,4-addition, followed by ring opening to give a variety of products. Both the major products, unsaturated diols, and the minor products, as shown by X-ray diffraction studies, are generated with high stereoselectivity, which is attributed to A1,3-strain.