C=C Bond Formation
2.6
357
⊡ Scheme 21
⊡ Scheme 22
Synthesis of unsaturated sugars with the strongly electron-withdrawing trifluoromethyl
group was initiated also from the tri-O-acetyl-D-glucal (23). Radical addition of the difluorochloromethyl fragment to a double bond afforded two products 44 and 45; reaction of the former with cesium fluoride furnished 2-trifluromethyl-2-eno-saccharide 46 ( > Scheme 23) [47].
The transposition of the exocyclic double bond into the C2 –C3 position leading to unsaturated nucleosides is presented also in > Scheme 24. The S N 2 reaction of 47 with lithium azide
led to 2 -azidomethylene derivative 48, while rearrangement of phosphinate 50 (prepared from
49 and Ph 2 PCl) caused by iodine afforded 52 (via transition state 51) [6].
The selected examples (both older and newer ones) shown in this part should emphasize to
the reader the problem of planning and execution of the synthesis of unsaturated sugars via
⊡ Scheme 23
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357
⊡ Scheme 21
⊡ Scheme 22
Synthesis of unsaturated sugars with the strongly electron-withdrawing trifluoromethyl
group was initiated also from the tri-O-acetyl-D-glucal (23). Radical addition of the difluorochloromethyl fragment to a double bond afforded two products 44 and 45; reaction of the former with cesium fluoride furnished 2-trifluromethyl-2-eno-saccharide 46 ( > Scheme 23) [47].
The transposition of the exocyclic double bond into the C2 –C3 position leading to unsaturated nucleosides is presented also in > Scheme 24. The S N 2 reaction of 47 with lithium azide
led to 2 -azidomethylene derivative 48, while rearrangement of phosphinate 50 (prepared from
49 and Ph 2 PCl) caused by iodine afforded 52 (via transition state 51) [6].
The selected examples (both older and newer ones) shown in this part should emphasize to
the reader the problem of planning and execution of the synthesis of unsaturated sugars via
⊡ Scheme 23