(<i>E</i>)-2-Oxo-1-sulfonyl-3-alkenes as Reactive Hetero 1,3-Dienes. Absolutely <i>endo</i>-Selective Hetero Diels-Alder Reactions with Vinyl Ethers in the Presence of a Lewis Acid Catalyst

Abstract

Abstract (E)-2-Oxo-1-sulfonyl-3-alkenes as new hetero 1,3-dienes undergo smooth hetero Diels-Alder reactions with vinyl ethers in the presence of Eu(fod)3 or TiCl2(i-PrO)2. The reactions are absolutely endo-selective producing 2,4-cis-3,4-dihydro-2H-pyrans in excellent yields, sufficient catalytic cycles being attained. The sulfonyl-stabilized carbanions derived from the cycloadducts are alkylated followed by reductive desulfonylation.