Unusual spontaneous α→β isomerization of unsymmetrical benzoins. Products and their structure
The article describes reactions of a series of arylglyoxals with 2-methylfuran and
furfural N,N-dimethylhydrazone. These interactions lead selectively to unsymmetrical benzoins. It was found that some of the benzoins underwent spontaneous thermal α→β benzoin isomerization in situ. 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 para-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 S-isomer of 2-hydroxy-2-(4’’-chlorophenyl)-1-(5’-N,N-dimethylhydrazonylfuryl-2’)-ethanone-1 was isolated by crystallization, and its structure was confirmed by the X-ray crystallography.
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