Highly Stereocontrolled Synthesis of <i>g</i><i>em</i>-Difluoromethylenated Azasugars: d- and l-1,4,6-Trideoxy-4,4-difluoronojirimycin

d-1,4,6-Trideoxy-4,4-difluoronojirimycin and l-1,4,6-trideoxy-4,4-difluoronojirimycin, a novel series of gem-4,4-difluoromethylenated azasugars, were synthesized from CF3CH2OH in 10 steps. A key step was the highly diastereoselective construction of the piperidine ring via reductive amination.