Fischer-Helferich glycosylation
The Fischer-Helferich glycosylation is a name reaction in organic chemistry . It is the acid-catalyzed synthesis of O- or S -glycosides .
The reaction was first published in 1893 by the German chemist Emil Fischer (1852-1919) and then modified by his pupil and later assistant Burckhardt Helferich (1887-1982).
Overview reaction
During the synthesis, monosaccharides like aldoses or ketoses react with an alcohol or thiol .
O glycosides
In the acid-catalyzed reaction with an alcohol - e.g. B. methanol - the hydroxyl group is exchanged in galactose and an O -glycoside is obtained as a mixture of anomers :
S glycosides
If a thiol (methyl mercaptan in the example) is used in the reaction instead of an alcohol, the product is the anomeric mixture of an S -glycoside:
Reaction mechanism
The following mechanism is described in the literature and exemplified using the reaction of galactose with methanol:
By adding an acid (e.g. HCl ), a proton attaches to the hydroxyl group of galactose 1 . This leads to the elimination of water in the next step and the formation of the mesomeric-stabilized carboxonium ion. The lone pair of electrons in methanol attach to the carbenium ion 2 , so that the oxonium ion 3 is formed as an intermediate . A proton is split off by electron rearrangement , and the product obtained is O- glycoside 4 as a mixture of anomers.
Individual evidence
- ↑ a b Zerong Wang: Comprehensive Organic Name Reactions and Reagents , Wiley, 2010, ISBN 9780470638859 , p. 1088, doi: 10.1002 / 9780470638859 .
- ↑ E. Fischer: About the glucosides of alcohols In: Reports of the German Chemical Society 26, 1893, p. 2400, doi: 10.1002 / cber.18930260327 .
- ↑ B. Helferich and W. Schäfer: Methyl-Glucoside In: Organic Synthesis Collective Vol. 1, 1941, p. 364.
- ↑ E. Broad Maier and G. Jung: Organic Chemistry. Georg Thieme, 2012, ISBN 9783135415079 , p. 872.