LICKOR-Promoted 1,2-elimination in 1,1-dimethoxy-2-phenylethane and 1,1-dimethoxy-2-phenylpropane: synthesis of substituted enol ethers and alkynes

Abstract

Dimethyl acetals of phenylacetaldehyde 1a and 2-phenylpropionaldehyde 1b, upon treatment with 1.25 equiv. of LICKOR reagent, undergo 1,2-elimination (THF at –95 °C) promoted by metallation at the benzylic site, and afford the corresponding enol ethers in the E-form. When the substrate is treated with an excess of the base (2.5 equiv.), further hydrogen–metal exchange takes place at the α-vinyl site of the elimination product, and carbonyl electrophiles can be added to the carbanionic intermediate yielding allyl alcohols. Experimental procedures are given for the conversion of the α-substituted derivatives into carbonyl compounds, according to an inverse polarity approach. Moreover, allyl alcohols synthesised starting from acetal 1a, can be transformed into prop-2-ynyl alcohols by treatment with LICKOR base.