AN IMPROVED SYNTHESIS OF 4-O-BENZOYL-2,2-DIFLUOROOLEANDROSE FROM L-RHAMNOSE - FACTORS DETERMINING THE SYNTHESIS OF 2,2-DIFLUOROCARBOHYDRATES FROM 2-ULOSES

4-O-Benzoyl-2,2-difluorooleandrose (16) has been synthesized from L-rh amnose. The key steps are the formation of a difluoromethylene group i n 12 by reacting ulose 11 with DAST and the chemoselective methylation of diol 13 to give compound 14. To obtain the 2,2-difluoro compound 1 2, the substituents in the neighborhood of the carbonyl group in ulose 11 must be equatorial and the sugar ring must have restricted conform ational mobility. This was done using 1,1,2,2-tetramethoxycyclohexane (TMC) to protect the hydroxyls at positions 3 and 4 in the sugar ring.