Exclusively endo-selective Lewis acid-catalyzed hetero Diels-Alder reactions of (E)-1-phenylsulfonyl-3-alken-2-ones with vinyl ethers

(E)-1-Phenylsulfonyl-3-alken-2-ones as new hetero 1,3-dienes undergo smooth hetero Diels-Alder reactions with vinyl ethers in the presence of a catalytic amount of Lewis acid such as ZnI2, Eu(fod)(3), and TiCl2(i-PrO)(2). The reactions are absolutely endo-selective producing 2,4-cis-3,4-dihydro-2H-pyrans in excellent yields, the configuration at 3-position depending upon the stereochemistry of the starting vinyl ethers. Reductive ring opening reactions of the 3,4-dihydro-2H-pyran cycloadducts with Et(3)SiH/TiCl4 lead to 6-alkoxy-1-phenylsulfonyl-2-hexanones, and the sulfonyl-stabilized carbanions derived from the 3,4-dihydro-2H-pyran cycloadducts are alkylated followed by reductive desulfonylation to give 2,4,6-trisubstituted 3,4-dihydro-2H-pyran derivatives.