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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, acyloins or α-hydroxy ketones are a class of
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s of the general form , composed of 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 In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl group (e. ...
acyl groups (). They are one of the two main classes of hydroxy ketones, distinguished by the position of the hydroxy group relative to the ketone; in this form, the hydroxy is on the
alpha carbon In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of n ...
, explaining the secondary name of ''α-hydroxy ketone''.


Synthesis

Classic organic reactions exist for the synthesis of acyloins. * The acyloin condensation is a reductive coupling of esters * The
benzoin condensation In organic chemistry, the benzoin addition is an addition reaction involving two aldehydes (). The reaction generally occurs between aromatic compound, aromatic aldehydes or glyoxals (), and results in formation of an acyloin (). In the classic ex ...
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 with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such 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 * MoOPH oxidation of
carbonyl In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
s is a system with
molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...
,
pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, 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, weak ...
and
hexamethylphosphoramide Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (an amide of phosphoric acid) with the formula This colorless liquid is used as a solvent in organic synthesis. Structure and reactivity HMPA is the oxide of tris(dimethyla ...
. * A family of stereoselective syntheses oxidizes ''in situ''
enol In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene ...
s with nitroso equivalents. In the simplest version, nitrosobenzene oxidizes a carbonyl to an αhydroxylamine, with a proline organocatalyst for enantioselection. In a more elaborate version, enolates can be oxidized by sulfonyloxaziridines.


Oxidation with Sulfonyloxaridines

When sulfonyloxaziridines oxidize enol(ate)s, the latter reacts by
nucleophilic displacement In chemistry, a nucleophilic substitution (SN) 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). ...
at the electron deficient oxygen of the oxaziridine ring. : 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 language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...
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 kapu ...
(camphorsulfonyl oxaziridine). Each isomer gives exclusive access to one of the two possible
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
s. This modification is applied in the Holton taxol total synthesis. : In the enolate oxidation of the cyclopentadienone 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. :


Reactions

* Reduction of acyloins give
diol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol may also be called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. They are used as protecting gro ...
s. *
Oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of acyloins give diones. * α-hydroxy ketones give positive Tollens' and Fehling's test. * 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 organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
and
phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula Phosphorus, P4Oxygen, 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 desic ...
to an α-keto amine: : * Indole synthesis, compare Bischler–Möhlau


See also

*
Glycolaldehyde Glycolaldehyde is the organic compound with the formula . It is the smallest possible molecule that contains both an aldehyde group () and a hydroxyl, hydroxyl group (). It is a highly Reactivity (chemistry), reactive molecule that occurs both ...
, a related molecule equivalent to an acyloin with both R groups as hydrogen (and thus an aldehyde not a ketone)


References

{{Authority control Functional groups Hydroxyketones