SYNTHESIS OF A 2,3-DIDEOXY-2,3-DIFLUOROFURANOSE WITH THE D-LYXO CONFIGURATION - AN INTRAMOLECULAR REARRANGEMENT OF METHYL -BENZOYL-2,3-DIDEOXY-2,3-DIFLUORO-D-LYXOFURANOSIDE OBSERVED DURING THE ATTEMPTED SYNTHESIS OF -DIDEOXY-2,3-DIFLUORO-BETA-D-LYXOFURANOSYL)THYMINE

A new sugar, methyl -benzoyl-2,3-dideoxy-2,3-difluoro-D-lyxofuranoside (8), which features fluorine substituents on adjacent carbon position s above the plane of the tetrahydrofuran ring, was synthesized from 1, 2: 5,6-di-O-isopropylidene-alpha-D-allofuranose in seven steps and 22% overall yield. During the synthesis, introduction of the second fluor ine atom required conditions more forceful than those normally used wi th diethylaminosulfur trifluoride (DAST). An attempt to use 8 in the s ynthesis of the all-cis nucleoside, dideoxy-2,3-difluoro-beta-D-lyxofu ranosyl)thymine, failed to give the desired product, providing instead y-3-fluoro-2-O-methyl-beta-D-xylofuranosyl)thymine (11), the structur e of which was confirmed by an independent synthesis. Formation of the rearranged product occurred with the concurrent loss of fluorine and retention of the methoxy group which was transposed from the anomeric to the 2'-position. The present work highlights the reactive nature of this novel dideoxydifluoro sugar precursor.