218
2
General Synthetic Methods
⊡ Scheme 31
several hours to a solution of the glycosyl halide, the concentration of the hydrogen donor
will be low enough to ensure rearrangement of the glycosyl radical before the reduction takes
place [213]. Under these conditions the acyloxy group at C2 will migrate to the anomeric
center and the resulting C2 centered radical will then abstract hydrogen to afford a peracylated
2-deoxyaldose ( > Scheme 32) [255]. The slow addition can be avoided if tributyltin hydride
is replaced by tris(trimethylsilyl)silane since the latter is a less effective hydrogen donor and
therefore allows the rearrangement to take place before the hydrogen abstraction [256].
2-Deoxyaldoses can also be prepared from partially protected glycals by addition of water or
acetic acid [257]. A special reaction for synthesis of a 2-deoxysugar employs dibenzylidenemannoside 88 ( > Scheme 32) [258]. Reaction of 88 with butyl lithium leads to a selective
⊡ Scheme 32
2
General Synthetic Methods
⊡ Scheme 31
several hours to a solution of the glycosyl halide, the concentration of the hydrogen donor
will be low enough to ensure rearrangement of the glycosyl radical before the reduction takes
place [213]. Under these conditions the acyloxy group at C2 will migrate to the anomeric
center and the resulting C2 centered radical will then abstract hydrogen to afford a peracylated
2-deoxyaldose ( > Scheme 32) [255]. The slow addition can be avoided if tributyltin hydride
is replaced by tris(trimethylsilyl)silane since the latter is a less effective hydrogen donor and
therefore allows the rearrangement to take place before the hydrogen abstraction [256].
2-Deoxyaldoses can also be prepared from partially protected glycals by addition of water or
acetic acid [257]. A special reaction for synthesis of a 2-deoxysugar employs dibenzylidenemannoside 88 ( > Scheme 32) [258]. Reaction of 88 with butyl lithium leads to a selective
⊡ Scheme 32