0.249 g of (S)-4-Benzyl-3-methoxyacetyl-oxazolidin-2-one (1.00 mmol) was dissolved under an argon atmosphere in 2.5 ml of abs. CH2Cl2 and treated with 0.167 ml of triethylamine (1.2 eq.). After cooling to −78°, nBu2BOTf was added slowly (1.1 ml of 1M solution in CH2Cl2) and enolborinate formation allowed to proceed for 50 Min. at −78° and for 50 Min. at 0°. After recooling, a solution of 0.363 g of 4-[2-(5-methyl-2-phenyl-oxazol-4-yl)-ethoxy]-benzo[b]thiophene-7-carbaldehyde (1.0 mmol) in 3.5 ml of abs. CH2Cl2 was added via dropping funnel and the mixture kept for 30 Min. at −78° and for 60 Min. at 0°. Pouring onto crashed ice, twofold extraction with AcOEt, washing with brine and water, drying over magnesium sulfate, and evaporation of the solvents, followed by flash chromatography (silica gel, hexane/AcOEt=7/3) left finally 0.259 g of the title compound as light yellow foam. According to NMR, one of the four isomers is strongly predominating. The configuration was tentatively assigned as 2S, 3R according to Tetrahedron Asymmetry 1999, 1353.
