H. V. Balen, A. Broekhuis, J. Scheeren
Sep 2, 2010
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Journal
Recueil des Travaux Chimiques des Pays-Bas
Abstract
Carbonyl compounds, activated by electron-withdrawing groups, add to 1,1-dimethoxybutadiene (Ia), yielding 2,2-dimethoxy-5,6-dihydropyrans (IIa). 1,1,4-Trimethoxybutadiene (Ib) reacts in a similar way with suitably activated ketones, but gives[2 + 2]-cycloadditions with activated aldehydes. The different behaviour is ascribed to steric factors. Acid hydrolysis of the 2,2-dimethoxydihydropyrans yields α,β-unsaturated esters and corresponding lactones, or dienyl esters depending on the reaction conditions used. 1,1-Dimethoxy-3-(trimethylsilyloxy)butadiene (Ic) yields Diels-Alder adducts even with non-activated carbonyl compounds, when zinc chloride is used as a catalyst. The products can be converted into dihydro-γ-pyrones by treatment with protic solvents (e.g. methanol).