Starting with 3,4-O- [(R)-2,2,2-trichloroethylidene]-1,2-O-isopropylidene- beta-D- tagatopyranose 2 obtained from 1,2-O-isopropylidene-beta-D-fructopyranose 1 by a non-classical one-step acetalization with chloral/DC~...Starting with 3,4-O- [(R)-2,2,2-trichloroethylidene]-1,2-O-isopropylidene- beta-D- tagatopyranose 2 obtained from 1,2-O-isopropylidene-beta-D-fructopyranose 1 by a non-classical one-step acetalization with chloral/DC~, the fluoroalkylated glyeosyl donors 15 and 17 were synthesised in 3~4 steps. By this sequence, one stereogenic center was inverted, one new chiral center was introduced, and one stereogenic center, for the time being eliminated, was later re-introduced. The glycals 11 and 12, key intermediates of the synthesis sequence, were accessible from triflate precursors (e. g., 10) by treatment with DBU. Corresponding halogeno-(6,7), tosyl-(5, 8), or mesyl-(9) precursors were unsuitable. The stereaselective introduction of a chlorodifluoromethyl group was realised by dithionite-mediated CF2C1Br-addition to the glycal double bond. Subsequently, either the chlorodifluoromethylated glyeosyl bromide (13) or the corresponding pyraneses (14 and 16) were isolated. The latter were still acetylated to the 1-O-acetyl derivatives 15 and 17, respectively. An x-ray analysis is given for the 5-O-tosylate 8.展开更多
文摘Starting with 3,4-O- [(R)-2,2,2-trichloroethylidene]-1,2-O-isopropylidene- beta-D- tagatopyranose 2 obtained from 1,2-O-isopropylidene-beta-D-fructopyranose 1 by a non-classical one-step acetalization with chloral/DC~, the fluoroalkylated glyeosyl donors 15 and 17 were synthesised in 3~4 steps. By this sequence, one stereogenic center was inverted, one new chiral center was introduced, and one stereogenic center, for the time being eliminated, was later re-introduced. The glycals 11 and 12, key intermediates of the synthesis sequence, were accessible from triflate precursors (e. g., 10) by treatment with DBU. Corresponding halogeno-(6,7), tosyl-(5, 8), or mesyl-(9) precursors were unsuitable. The stereaselective introduction of a chlorodifluoromethyl group was realised by dithionite-mediated CF2C1Br-addition to the glycal double bond. Subsequently, either the chlorodifluoromethylated glyeosyl bromide (13) or the corresponding pyraneses (14 and 16) were isolated. The latter were still acetylated to the 1-O-acetyl derivatives 15 and 17, respectively. An x-ray analysis is given for the 5-O-tosylate 8.
