A new route to C-glycosyl compounds. Wittig-type reaction promoted by zinc

Abstract

In recent years, methods for the preparation of C-glycosyl compounds have become increasingly important in synthetic organic chemistry. These compounds are useful as subunits for the synthesis of biological active products’ and as potential enzyme inhibitors’. Significant attention has been focused on the development of new routes to prepare functionalized C-glycosyl compounds that are synthetic precursors of more complex C-glycosyl compounds. The starting materials can be furanose or pyranose carbohydrates, the main problem being the stereoselective introduction of a functionalized carbon atom at C-l. A widely used procedure to obtain carbon-carbon bonding at the anomeric position of carbohydrates was first reported by Zhdanov ef al. 3 This two-step procedure involves the formation of an unsaturated, open-chain intermediate, followed by a cyclization leading to the expected C-glycosyl compound. The first step is a Wittig reaction with protected furanose and pyranose hemiacetals. When stabilized ylides are used4, subsequent cyclization of the unsaturated intermediate may occur by treatment with bases or, sometimes, spontaneously. But when the Wittig reaction takes place with unstabilized ylides, the cyclization can be obtained by the iodoor mercuro-cyclization process’. It should be noted that C-glycosyl compounds can be synthesized in the same way when phosphonates are used instead of phosphorane$. Lastly, although stereoselective routes exist for both C-aand /I-glycosyl compounds, they mainly lack a general application. Stereoselectivity indeed depends on carbohydrate structures, ylides, and solvents. In the case of 2,3,4,6-tetra-O-benzyl-a,/?-D-glucopyranose (l), Nicotra et al.’ have observed an abnormal reaction of (carbethoxymethylene)triphenylphosphorane leading solely to a diene. The same elimination by Wittig reaction was observed for 3,4,6-tri-O-acetyl-2-O-benzyl-a&D-glucopyranose and partial elimination for 2,3,4,6-tetra-U-acetyl-a&D-glucopyranose. These abnormal reactions led Allevi et af.6 to use a Wittig-Horner reaction with 1 and sodium triethylphosphonoacetate