Acyloins or α-hydroxy ketones are a class of

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...

s which all possess a

hydroxy group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

adjacent to a ketone group. The name acyloin is derived from the fact that they are formally derived from reductive coupling of carboxylic acyl groups.

Synthesis

Classic organic reactions exist for the synthesis of acyloins. * The

acyloin condensation Acyloin condensation is a reductive coupling of two carboxylic esters using metallic sodium to yield an α-hydroxyketone, also known as an acyloin. The reaction is most successful when ''R'' is aliphatic and saturated. The reaction is performe ...

is a reductive coupling of esters * The

benzoin condensation The benzoin addition is an addition reaction involving two aldehydes. The reaction generally occurs between aromatic aldehydes or glyoxals, and results in formation of an acyloin Acyloins or α-hydroxy ketones are a class of organic compounds w ...

is condensation reaction between aldehydes catalyzed by a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

* Oxidation of

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...

s is possible with molecular oxygen but not selective * Better alternative is oxidation of corresponding silyl enol ethers with ''m''CPBA in the

Rubottom oxidation The Rubottom oxidation is a useful, high-yielding chemical reaction between silyl enol ethers and peroxyacids to give the corresponding α-hydroxy carbonyl product.Kürti, pp. 388–389.

MoOPH MoOPH, also known as oxodiperoxymolybdenum(pyridine)-(hexamethylphosphoric triamide), is a reagent used in organic synthesis. It contains a molybdenum(VI) center with multiple oxygen ligands, coordinated with pyridine and HMPA ligands. It is an el ...

oxidation of

s is a system with

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...

and

hexamethylphosphoramide Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (an amide of phosphoric acid) with the formula This colorless liquid is a useful reagent in organic synthesis. Structure and reactivity HMPA is the oxide of the highly basic t ...

Enolate oxidation by sulfonyloxaziridines

Enolate In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds. Bonding and structure Enolate anions are electr ...

s can be oxidized by sulfonyloxaziridines. The enolate reacts by

nucleophilic displacement In chemistry, a nucleophilic substitution is a class of chemical reactions in which an electron-rich chemical species (known as a nucleophile) replaces a functional group within another electron-deficient molecule (known as the electrophile). The ...

at the electron deficient oxygen of the oxaziridine ring. : Enolate oxidation sulfonyloxazirines

This reaction type is extended to

asymmetric synthesis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecul ...

by the use of

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

oxaziridines derived from

camphor Camphor () is a waxy, colorless solid with a strong aroma. It is classified as a terpenoid and a cyclic ketone. It is found in the wood of the camphor laurel ('' Cinnamomum camphora''), a large evergreen tree found in East Asia; and in the k ...

(camphorsulfonyl oxaziridine). Each isomer gives exclusive access to one of the two possible

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

s. This modification is applied in the Holton taxol total synthesis. : Camphorsulfonyl oxaziridine

In the enolate oxidation of the cyclopentaenone below with either camphor enantiomer, the ''trans'' isomer is obtained because access for the hydroxyl group in the ''cis'' position is limited. The use of the standard oxaziridine did not result in an acyloin. : Acyloin example Hughes

Reactions

* Reduction of acyloins give

diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...

s. *

Oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

of acyloins give diones. * α-hydroxy ketones give positive Tollens' and

Fehling's test In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone () functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' r ...

. * Some acyloins rearrange with positions swapped under the influence of base in the Lobry–de Bruyn–van Ekenstein transformation * A similar reaction is the so-called Voigt amination where an acyloin reacts with a primary

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...

and

phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...

to an α-keto amine: : VoightAmination

* Indole synthesis, compare Bischler–Möhlau

References

{{Reflist Functional groups Hydroxyketones

Synthesis

Enolate oxidation by sulfonyloxaziridines

Reactions

See also

References