Cyclic hemiaminal formation in some N-unsubstituted azole aldehydes

Abstract

Cyclic hemiaminal formation in solid X-unsubstituted azole aldehydes seems to be a general phenomenon when stabilization factors and position of substitution are favourable. Previously reported for 5- substituted 1,2,4-triazole-3-carbaldehydes and benzimidazole-2- carbaldehyde, it has been found to occur also in some previously undescribed aldehydes of pyrazole, imidazole, 3H-imidazo[4,5- b]pyridine, 3H-imidazo[4,5-c]pyridine, and purine. In purine-8- carbaldehyde the hemiaminal may be accompanied by the aldehyde hydrate. P.m.r. studies of the dimers, and of the dipiperidino cyclic aminals derived from them, indicate that in some cases more than one isomeric form is present.

1,3-dipolar cycloaddition of trimethylsilyl azide to propynals and dimerization of 1H-1,2,3-triazole-5-carbaldehydes to tricyclic bis-hemiaminals

“…The 1 H NMR spectrum (CD 3 CN) of the reaction mixture obtained from propynal I and trimethylsilyl azide (III) contained signals belonging to triazole IV and a signal at δ 7.33 ppm (2H, d), which was assigned to CH proton in the pyrazine ring (monomerdimer ratio 1 : 1). In keeping with published data, signal from the CH(OH) fragment in cyclic hemiaminals derived from NH-azolecarbaldehydes is observed at δ 6.3-7.5 ppm [4]. Likewise, the 1 H NMR spectrum (CD 3 CN) of the reaction mixture obtained from propynal (II) and trimethylsilyl azide (III) contained signals from protons of triazole V, methine proton in the pyrazine ring of tricyclic hemiaminal VII at δ 6.46 ppm (2H, d), and methine proton in the triazole ring at δ 8.64 ppm (2H, d), the ratio monomerdimer being 1 : 1.…”

“…Depending on the propynal structure, the reactions with methyl diazoacetate follow 1,3-dipolar cycloaddition pattern with formation of isomeric pyrazoles and products of dimerization of 5-formylpyrazoles to tricyclic bis-hemiaminals or/and aldol-like condensation to give α-diazo-β-hydroxyalkynes (in the absence of initiating species). It was also found that in the absence of a solvent 5-formylpyrazoles exist mainly as the corresponding dimers, 3,8-disubstituted dimethyl 4,9-dihydroxy-4H,9H-dipyrazolo[1,5-a:1',5'-d]pyrazine-2,7dicarboxylates, in a way similar to other fromylazoles (e.g., imidazoles and their fused derivatives and 1,2,4triazoles) in which the formyl group is attached to a carbon atom neighboring to the endocyclic NH group [4]. It should be noted that methyl 5-formyl-4-phenylpyrazole-3-carboxylate, unlike silicon-, germanium-, and alkyl-containing analogs, does not undergo dimerization under the same conditions [2].…”

“…Taking into account that, unlike many NH-azolecarbaldehydes which exist as dimeric bis-hemiaminals in the absence of a solvent [1][2][3][4], compounds IV and V in crystal retain their formyltriazole structure, we tried to find conditions promoting their dimerization to heterocyclic bis-hemiaminas. For this purpose, we examined the effects of the polarity of the medium and temperature by 1 H NMR and IR spectroscopy.…”

“…In the 1 H NMR spectrum of the product in acetone-d 6 we observed a multiplet at δ 6.80 ppm (CH, monomerdimer ratio 1 : 11). Splitting of the CH proton signal in the 1 H NMR spectra of dimers derived from some azolecarbaldehydes was explained in [4] by the presence of isomeric structures. Raising the temperature to 50°C changes the intensity ratio of the CHO and CH-O proton signals to 1 : 140, and this ratio increases to 1 : 40 on cooling to 28°C (after 24 h).…”

Structure cristalline et moléculaire de l'aminal du dimère de la carbométhoxy-3 méthyl-5 formyl-5 pyrazoline-Δ2, C16H20N4O6

“…Celle-ci peut ~tre obtenue notamment par la dim6-risation d'h&6rocycles azot6s porteurs d'un NH en d'une fonction ald6hydique (Browne, 1971). Cependant, l'6tude de ce type de fonction a 6t6 rendue difficile jusqu'& pr6sent par l'obtention de m61anges d'isom~res mal d6finis et par le fait d'un 6quilibre monom~re (ald6hyde)-dim~re (h6miaminal) en solution.…”