ASYMMETRIC SYNTHESES OF SUBSTITUTED DIHYDROPYRANS BY DIELS-ALDER HETEROCYCLOADDITIONS INVOLVING CHIRAL VINYL ETHERS AS DIENOPHILES

The alkyl vinyl ethers 2b-8b (deriving from the alcohols 2a-8a) smoothly reacted with methyl E-benzylidenepyruvate 1 in the presence of catalytic amounts of Eu(fod)3, thus leading to the endo cycloadducts 9-15 in high yields. The cycloadducts 12-15 were obtained with moderate to good diastereomeric excesses (43-72%). Thus the vinyl ether Bb deriving from methyl (R)-mandelate gave the endo adduct IS in 83% yield and with 72% diastereomeric excess.