Diastereoselective Synthesis of Tetrahydrofurans by Lewis Acid Catalyzed Intermolecular Carbenoid-Carbonyl Reaction-Cycloaddition Sequences: Unusual Diastereoselectivity of Lewis Acid Catalyzed Cycloadditions

The effects of including metal salts for three. component reactions involving alpha-alkyl-alpha-diazo esters, aromatic aldehydes, and 3-(2-alkenoyl)-2-oxazolidinones are described, in terms of yields and diastereoselectivities. Metal tetrafluorborates (10-30 mol %) such as Co(BF4)(2)center dot 6H(2)O, Ni(BF4)(2)center dot 6H(2)O, 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)center dot 6H(2)O or Co(ClO4)(2)center dot 6H(2)O (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).