Unusual spontaneous α→β isomerization of unsymmetrical benzoins. Products and their structure

• Main Authors: Andrey A. Anishchenko, Vasiliy G. Shtamburg, Viktor V. Shtamburg, Vitaliy V. Volosyuk, Remir G. Kostyanovskiy
• Format: Article
• Language: English
• Published: Oles Honchar Dnipropetrovsk National University 2014-03-01
• Series: Vìsnik Dnìpropetrovsʹkogo Unìversitetu: Serìâ Hìmìâ
• Subjects: benzoins; arylglyoxals; isomerization
• Online Access: http://chemistry.dnu.dp.ua/article/view/22865
• ISSN: 2306-871X; 2313-4984

<p>The article describes reactions of a series of arylglyoxals with 2-methylfuran and<em> <br /> </em>furfural<em> N,N</em>-dimethylhydrazone. These interactions lead selectively to unsymmetrical benzoins. It was found that some of the benzoins underwent spontaneous thermal α→β benzoin isomerization <em>in situ</em>. The rearrangement took place in the absence of bases, which could be explained by two structural factors: (a) the presence of a halogen atom in the <em>para</em>-position of the aryl moiety, and (b) the presence of the Me₂NN=CH-substituent in the 5-position of the furan ring. The proposed mechanism of the thermal rearrangement starts with an intramolecular protonation of the carbonyl oxygen by the hydroxyl. This leads to the 1,2-hydride shift onto the carbonyl group, finally yielding β-benzoins. The <em>S</em>-isomer of 2-hydroxy-2-(4’’-chlorophenyl)-1-(5’-<em>N,N</em>-dimethylhydrazonylfuryl-2’)-ethanone-1 was isolated by crystallization, and its structure was confirmed by the X-ray crystallography.</p>