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 generally provide good anchimeric assistance, whereas ethers (eg benzyl, methyl etc) do not, leading to mixtures of stereoisomers.

Generally, the Koenigs–Knorr reaction refers to the use of glycosyl chlorides, bromides and more recently iodides as glycosyl donors.

The Koenigs–Knorr reaction can be performed with alternative promoters such as various heavy metal salts including mercuric bromide/mercuric oxide, mercuric cyanide and silver triflate.

Other glycosidation methods are Fischer glycosidation and the Helferich method.

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