Organofluorine compounds and fluorinating agents, Part 33: Regioselective chlorodifluoromethylations in the 1-position of 1,2-unsaturated monosaccharide derivatives

The dithionite-mediated chlorodifluoromethylations of 3,-di-O-acetyl-1,5-anhydro-2,6-dideoxy-L-arabino-hex-1-enitol (1), 1,5-anhydro-2,3,4-tri-O-pivaloyl-D-erythro-pent-1-enitol (13) 2,3,4-tri-O-acetyl-1,5-anhydro-6-deoxy-L-lyxo-hex-1-enitol (15), and 2,3,4,6-tetra-O-acetyl-1,5-anhydro-D-arabino-hex-1-enitol (18) with CBrClF2 in acetonitrile-water and methanol, respectively, are described. Sodium dithionite serves as radical initiator and reducing reagent. Whereas, starting with 1, the chlorodifluoromethyl group was predominantly introduced into position 2, the 1-chlorodifluoromethyl substituted C-glycosides 1,5-anhydro-1-(S)-chlorodifluoromethyl-2,3,4-tri-O-pivaloyl-D-ribitol (14), 2,3,4-tri-O-acetyl-1,5-anhydro-1-(R)-chlorodifluoromethyl-6-deoxy-L-galacti-tol (16), and 2,3,4,6-tetra-O-acetyl-1,5-anhydro-1-(S)-chlorodifluoromethyl-D-glucitol (19) could be synthesized in moderate to good yields from 13, 15, and 18, respectively. X-ray analyses are given for the products 3,4-di-O-acetyl-1,5-anhydro-2-chlorodifluoromethyl-2,6-dideoxy-L-glucitol, 1,3,4-tri-O-acetyl-2-chlorodifluoromethyl-2,6-dideoxy-alpha-L-mannopyranose, 1,5-anhydro-1-(S)chlorodifluoromethyl-2,3,4-tri-O-pivaloyl-D-ribitol, and 2,3,4,6-tetra-O-acetyl-1,5-anhydro-1-(S)-chlorodifluoromethyl-D-glucitol.