title: Diastereoselective synthesis of tetrahydrofurans by Lewis acid catalyzed intermolecular carbenoid-carbonyl reaction-cycloaddition sequences: unusual diastereoselectivity of Lewis acid catalyzed cycloadditions.
abstract: The effects of including metal salts for three-component reactions involving α-alkyl-α-diazo esters, aromatic aldehydes, and 3-(2-alkenoyl)-2-oxazolidinones are described, in terms of yields and diastereoselectivities. Metal tetrafluoroborates (10-30 mol %) such as Co(BF4)2·6H2O, Ni(BF4)2·6H2O, and AgBF4 were effective in delivering high yields and diastereoselectivities (93:7 to 99:1) of the corresponding tetrahydrofuran derivatives while suppressing the competitive formation of 1,3-dioxolane. Using (S)-3-(2-alkenoyl)-4-isopropyl-2-oxazolidinones as the dipolarophiles in the three-component reactions, in the presence of Ni(BF4)2·6H2O or Co(ClO4)2·6H2O (10-30 mol %), optically active tetrahydrofurans that possess four successive asymmetric centers were synthesized in high diastereoselectivities (99:1). On the basis of the configuration of the cycloadduct using the X-ray analysis, the high diastereoselectivity could be explained by the unusual Re-face approach to the dipolarophile in the presence of the Lewis acid, proceeding through the s-cis conformer of the 3-(2-alkenoyl)-2-oxazolidinone with the two carbonyls in opposing directions (dipole-dipole interaction).
authors.affiliation: Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University , Wakasato, Nagano 380-8553, Japan and.
authors.forename: Yuta
authors.initials: Y
authors.lastname: Hashimoto
authors.forename: Kennosuke
authors.initials: K
authors.lastname: Itoh
authors.forename: Akikazu
authors.initials: A
authors.lastname: Kakehi
authors.forename: Motoo
authors.initials: M
authors.lastname: Shiro
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Suga