Reaction of Alkyl and Aryl Grignard Reagents with Trifluoroacetyldihydropyrans and Other Cyclic β-Alkoxy-α,β-unsaturated Trifluoromethylketones

Alkyl and aryl Grignard reagents react with cyclic β-alkoxy-α,β-unsaturated trifluoromethylketones by 1,4-addition to give as the major products, for example, cis-2,3-disubstituted tetrahydropyrans. In most examples ring opening leads to stereoselective formation of rearranged hemiketals as minor products. Under other conditions with 2,2,2-trifluoro-1-(2-ethoxy-3,4-dihydro-2H-5-pyranyl)-1-ethanone stereoselective ring opening leads to acyclic aldehydes, as minor products and, by the major pathway, to a series of diols by addition of further equivalents of the Grignard reagent. In the absence of further Grignard reagent at higher temperatures internal hydride transfer can afford acyclic esters.