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 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).