Stereochemistry of carbohydrate derivatives. Part I. X-Ray analysis of the adduct formed by addition of dichloromethylene to 3-deoxy-1,2:5,6-di-O-isopropylidene-α-D-erythro-hex-3-enofuranose

Abstract

The product resulting from the addition of dichloromethylene to 3-deoxy-1,2:5,6-di-O-isopropylidene-α-D-erythro-hex-3-enofuranose (I) has been established by X-ray crystallographic analysis to be 3-deoxy-3,4-C-(dichloromethylene)-1,2:5,6-di-O-isopropylidene-α-D-galactofuranose (II). The trioxabicyclo[3,3,0]octane ring system is in a ‘twin-envelope’ conformation, but each ring is distinctly flattened compared with the envelope form normally adopted by single, five-membered rings. The crystals of the adduct are orthorhombic, space group P2₁2₁2₁, with four molecules of C₁₃H₁₈Cl₂O₅ in a cell of dimensions a 17·50, b 15·72, c 5·44 Å. The final value of R, the conventional discrepancy factor, is 16·8% over 778 observed reflections.