Isomerization of (Het)arylbenzoins in Basic Media.

Mass-spectral behavior and thermal stability of hetaryl analogs of unsymmetrical benzoins

“…The yield is not reduced by increasing the heating time to 30 min. This indicates that whether thermal isomerization occurs does not depend on the structure of the benzoin, unlike α→β isomerization in a basic medium, where the transformation time depends strongly on the electronic effect of the hetaryl group [16]. Consequently, the thermal transformation takes place through the enediol form as a result of keto-enol tautomerism.…”

“…Earlier during investigation of the isomerization of α-benzoins obtained by electrophilic hydroxymethylation of π-excessive heterocycles [15] we demonstrated differences in the 1 H NMR spectral characteristics of the α-and β-isomers [16]. In the present work we studied the differences in the mass spectral characteristics of the obtained isomeric benzoins.…”

Interaction of N,N-dimethylhydrazonomethyl and α-hydroxyketone groups in hetaryl analogs of unsymmetrical benzoins

“…The furan hydrazone 1 is comparable in reactivity with pyrroles 7 and 2 in the isomerization reaction ( Activation of the hetaryl residue leads to the situation that in the case of the hydrazones isomerization proceeds successfully in nonpolar benzene, unlike the isomerization of α-benzoins 7 and 8, which was carried out in ethyl alcohol [29]. In the more polar alcohol α-benzoins 1, 2 were completely oxidized by the oxygen of the air to benzils 5 and 6 respectively, while the more stable β-benzoins 3, 4 were stable in boiling alcohol.…”

“…It was shown previously in the example of derivatives of π-excess heterocycles, that an increase in the electron-donating properties of the hetaryl residue assists α→β-isomerization of unsymmetrical benzoins [29]. Activation of the hetaryl residue in α-benzoins 1, 2 by the introduction of a N,N-dimethylhydrazonomethyl group reduces the isomerization time compared with the unfunctionalized hetaryl-α-benzoins 7, 8 correspondingly (in the Scheme the hetaryl residues are placed in increasing order of isomerization time).…”

“…1, 2). Assignment of the isomers was made on the basis of the signals of the protons of the unsubstituted phenyl ring in the 1 H NMR spectra of the benzoins and on the rules for the position and form of the signals of these protons depending on the isomer, which was proposed by us previously in [29]. In the 1 H NMR spectra of β-benzoins 3, 4 the signals were displaced towards high field in comparison with those in the spectra of α-benzoins 1, 2.…”