Koenigs-Knorr reaction
The Koenigs–Knorr reaction in organic chemistry is the substitution reaction of a glycosyl halide with an alcohol to give a glycoside. It is one of the oldest and simplest glycosylation reactions. It is named after Wilhelm Koenigs (1851–1906), student of von Bayer and fellow student with Hermann Emil Fischer, and Edward Knorr, student of Koenigs.
In its original form, Koenigs and Knorr treated acetobromoglucose with alcohols in the presence of silver carbonate. Shortly afterwards Fischer and Armstrong reported very similar findings.
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.
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 dioxolaniumnium is formed, which is attacked by methanol via an SN2 mechanism at the carbonyl carbon atom. This attack leads to the inversion. After cleavage of a hydrogen atom, the glycoside is formed.
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