Indirect approach to C-3 branched 1,2- cis-glycofuranosides: synthesis of aceric acid glycoside analogues

Abstract

Aceric acid (3- C-carboxy-5-deoxy-α- l-xylofuranose) residues are present in pectic polysaccharide rhamnogalacturonan II (RG II) in the form of synthetically challenging 1,2- cis-glycofuranosides. To access synthetic fragments of RG II incorporating aceric acid, a four-step procedure based on C-2 epimerisation of initially prepared 1,2- trans-glycofuranoside was developed. Readily available derivatives of branched-chain l-lyxofuranose bearing a 3- C-vinyl group as a masked 3- C-carboxyl group were investigated as potential precursors of aceric acid units. In the first step of the procedure, installation of a participating group at C-2 of the furanose ring ensured stereocontrol of the O-glycosylation, which was carried out with the thioglycoside of 2- O-acetyl-3,5-di- O-benzyl-3- C-vinyl- l-lyxofuranose. After the glycosylation step, the 2- O-acetyl group was removed, the free 2-OH group was oxidised and the resulting ketone was finally reduced to form the C-3-vinyl- l-xylofuranoside. The use of L-Selectride in the key reduction reaction was essential to achieve the required stereoselectivity to generate 1,2- cis-furanoside.