ASYMMETRIC-SYNTHESIS AND ABSOLUTE-CONFIGURATION OF SUBSTITUTED DIHYDROPYRANS, BY INTRAMOLECULAR HETEROCYCLOADDITION OF CHIRAL VINYL ETHERS

The alkyl vinyl ethers 2b-8b (deriving from the alcohols 2a-8a) smoothly reacted with methyl E-benzylidenepyruvate 10 in the presence of catalytic amounts of Eu(fod)(3) or Yb(fod)(3) in refluxing hexane, thus leading to the dihydropyran endo adducts 13-19 in high yields (80-95%). The endo-cycloadducts 13-17 were obtained with diastereofacial selectivities (ds) ranging from 76/24 to 87.5/12.5. Asymmetric induction was found to culminate with the vinyl ether (-)-7b deriving from n-butyl (R)-(-)-mandelate : the best ds (92.5/7.5) was obtained for the corresponding adduct 18 after 6 days at room temperature. The absolute configurations of the major isomers of the dihydropyrans 16-18, 20 and 21 were established by oxydative degradation to optically active alpha-phenylsuccinic acid derivatives.