In its original form, Koenigs and Knorr treated acetobromoglucose with alcohols in the presence of silver carbonate.[1] Shortly afterwards Fischer and Armstrong reported very similar findings.[2]
In the above example, the stereochemical outcome is determined by the presence of the neighboring group at C2 that lends anchimeric assistance, resulting in the formation of a 1,2-trans stereochemical arrangement. Esters (e.g. acetyl, benzoyl, pivalyl) generally provide good anchimeric assistance, whereas ethers (e.g. benzyl, methyl etc.) do not, leading to mixtures of stereoisomers.
Mechanism
In the first step of the mechanism, the glycosyl bromide reacts with silver carbonate upon elimination of silver bromide and the silver carbonate anion to the oxocarbenium ion. From this structure a dioxolanium ring is formed, which is attacked by methanol via an SN 2 mechanism at the carbonyl carbon atom. This attack leads to the inversion. After deprotonation of the intermediate oxonium, the product glycoside is formed.[3]
The reaction can also be applied to carbohydrates with other protecting groups. In the oligosaccharide synthesis in place of the methanol other carbohydrates are used, which have been modified with protective groups in such a way that only one hydroxyl group is accessible.
History
The method was later transferred by Emil Fischer and Burckhardt Helferich to other chloro-substituted purines and produced thus for the first time synthetic nucleosides. It was later improved and modified by numerous chemists.
^Kürti, László; Czakó, Barbara (2005). Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms. Elsevier. p. 246–7. ISBN978-0-12-429785-2.
^Helferich, B.; Zirner, J. (1962). "Zur Synthese von Tetraacetyl-hexosen mit freiem 2-Hydroxyl. Synthese einiger Disaccharide". Chem. Ber.95 (11): 2604. doi:10.1002/cber.19620951103.