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In organic chemistry, a hemiacetal is a functional group the general formula R 1 R 2 C(OH)OR, where R 1, R 2 is a hydrogen atom or an organic substituent. They generally result from the nucleophilic addition of an alcohol (a compound with at least one hydroxy group ) to an aldehyde ( R−CH=O ) or a ketone ( R 2 C=O ) under acidic conditions.
Cis-3-hexenal is generated by conversion of linolenic acid to the hydroperoxide by the action of a lipoxygenase followed by the lyase-induced formation of the hemiacetal. [ 5 ] It must be noted, however, that this enzyme catalyzed path follows a different mechanism from the usual Schenck ene reaction.
D-ribose in itself is a hemiacetal and in equilibrium with the pyranose 3. In aqueous solution ribose is 75% pyranose and 25% furanose and a different acetal 4 is formed. Selective acetalization of carbohydrate and formation of acetals possessing atypical properties is achieved by using arylsulfonyl acetals.
The whole mechanism of acetal formation from hemiacetal is drawn below. Acid catalyzed acetal formation from the corresponding hemiacetal Acetals, as already pointed out, are stable tetrahedral intermediates so they can be used as protective groups in organic synthesis.
In contrast to variations of R, both R' groups are organic fragments. If one R' is a hydrogen, the functional group is instead a hemiacetal, while if both are H, the functional group is a ketone hydrate or aldehyde hydrate. Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water.
This forms an intramolecular hemiacetal. If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1] The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution. [2]
DERA has also been used to perform tandem aldol reactions with three aldehyde substrates, with reaction equilibrium driven by the formation of the six-membered cyclic hemiacetal. [10] This intermediate has been used in the synthesis of statin drugs, such as atorvastatin, [11] rosuvastatin and mevastatin. [12]
The acid catalyzed furan synthesis proceeds by protonation of one carbonyl which is attacked by the forming enol of the other carbonyl. Dehydration of the hemiacetal gives the resultant furan. [6] Paal-Knorr furan synthesis mechanism. The mechanism of the Paal–Knorr furan synthesis was elucidated in 1995 by V. Amarnath et al. [3]