Acyloin

Synthesis ==

Classic organic reactions exist for the synthesis of acyloins.

• The acyloin condensation is a reductive coupling of esters
• The benzoin condensation is condensation reaction between aldehydes catalyzed by a nucleophile
• Oxidation of carbonyls is possible with molecular oxygen but not selective
• Better alternative is oxidation of corresponding silyl enol ethers with mCPBA in the Rubottom oxidation
• MoOPH oxidation of carbonyls is a system with molybdenum peroxide, pyridine and hexamethylphosphoramide.
• A family of stereoselective syntheses oxidizes in situ enols 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 at the electron deficient oxygen of the oxaziridine ring. :[[File:Enolate oxidation sulfonyloxazirines.png|600px|Enolate oxidation by sulfonyloxaziridine]]

This reaction type is extended to asymmetric synthesis by the use of chiral oxaziridines derived from camphor (camphorsulfonyl oxaziridine). Each isomer gives exclusive access to one of the two possible enantiomers. This modification is applied in the Holton taxol total synthesis.

:[[File:Camphorsulfonyl oxaziridine.png|250px|two optical isomers of camphorsulfonyl oxaziridine]]

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.

:[[File:Acyloin example Hughes.png|500px|Enolate oxidation example]]

Reactions ==

• Reduction of acyloins give diols.
• Oxidation 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 and phosphorus pentoxide to an α-keto amine: :[[File:VoightAmination.svg|400px|Voigt amination]]
• Indole synthesis, compare Bischler–Möhlau

References

References

1. "acyloins".
2. Taber, Douglass F.. (26 Jan 2004). "Catalytic Enantioselective Synthesis".
3. (1984). "Synthesis of α-hydroxycarbonyl compounds (acyloins): direct oxidation of enolates using 2-sulfonyloxaziridines". [[J. Org. Chem.]].
4. (1986). "Asymmetric oxidation of ester and amide enolates using new (camphorylsulfonyl)oxaziridines". [[J. Org. Chem.]].
5. (2005). "An Electrochemical Approach to the Guanacastepenes". [[Organic Letters.
6. (1886). "Ueber die Einwirkung von primären aromatischen Aminen auf Benzoïn". [[Journal für Praktische Chemie.
7. Lawrence, Stephen A.. (2004). "Amines: Synthesis, Properties and Applications". [[Cambridge University Press]].
8. Roth, Lepke. (1972). "Synthese von Indol- und Carbazol-Derivaten aus α-Hydroxyketonen und aromatischen Aminen". Archiv der Pharmazie.