C=C Bond Formation
2.6
363
⊡ Scheme 31
⊡ Scheme 32
⊡ Scheme 33
5 Acyclic Unsaturated Sugars
One of the first unsaturated open-chain sugars (observed for the first time by Emil Fischer)
was obtained by hydrolysis of tri-O-acetyl-D-glucal (23). Heating of this compound in water
results in smooth formation of the E-α,β-unsaturated open-chain aldose 76 ( > Scheme 34) [1].
The carbonyl group can be protected, which allows one to replace the acetate blocks into more
stable benzyl groupings thus providing compound 77 ( > Scheme 34) [71].
In 1979 Vasella introduced a convenient procedure for the preparation of unsaturated openchain sugars by reductive fragmentation of 6-deoxy-6-halogenopyranosides (and 5-deoxy5-halogeno-furanosides) with zinc in ethanol [72]; the same fragmentation is induced with
n-butyl-lithium. Fürstner proposed modification of the original procedure and in his approach
activated zinc on graphite was used [73]. The conditions of the Fürstner process are milder
and, for example, unsaturated acid 79 can be prepared in very high yield from iodolactone 78
( > Fig. 8).
2.6
363
⊡ Scheme 31
⊡ Scheme 32
⊡ Scheme 33
5 Acyclic Unsaturated Sugars
One of the first unsaturated open-chain sugars (observed for the first time by Emil Fischer)
was obtained by hydrolysis of tri-O-acetyl-D-glucal (23). Heating of this compound in water
results in smooth formation of the E-α,β-unsaturated open-chain aldose 76 ( > Scheme 34) [1].
The carbonyl group can be protected, which allows one to replace the acetate blocks into more
stable benzyl groupings thus providing compound 77 ( > Scheme 34) [71].
In 1979 Vasella introduced a convenient procedure for the preparation of unsaturated openchain sugars by reductive fragmentation of 6-deoxy-6-halogenopyranosides (and 5-deoxy5-halogeno-furanosides) with zinc in ethanol [72]; the same fragmentation is induced with
n-butyl-lithium. Fürstner proposed modification of the original procedure and in his approach
activated zinc on graphite was used [73]. The conditions of the Fürstner process are milder
and, for example, unsaturated acid 79 can be prepared in very high yield from iodolactone 78
( > Fig. 8).