A novel catalytic and highly enantioselective approach for the synthesis of optically active carbohydrate derivatives

A catalytic enantioselective inverse-electron demand hetero-Diels-Alder reaction of alpha,beta-unsaturated carbonyl compounds with electron-rich alkenes catalyzed by chiral bisoxazolines in combination with Cu(OTf)2 as the Lewis acid is presented. The reaction of gamma-substituted beta,gamma-unsaturated alpha-keto esters with vinyl ethers and various types of cis-disubstituted alkenes proceeds in good yield, high diastereoselectivity, and excellent enantioselectivity. The potential of the reaction is demonstrated by the synthesis of optically active carbohydrates such as spiro-carbohydrates, an ethyl beta-D-mannoside tetraacetate, and acetal-protected C-2-branched carbohydrates. On the basis of X-ray crystallographic data and the absolute configuration of the products, it is proposed that the alkene approaches the si-face of the reacting alpha,beta-unsaturated carbonyl functionality when coordinated to the catalyst.