An unusual aromatisation of dihydropyrimidines facilitated by reduction of the nitro group

Design of fused systems based on σH-adducts of 6-nitro-1,2,4-triazolo[1,5-a]pyrimidine with π-excessive heteroaromatic compounds

“…The nitro group activates the heterocycle for the nucleophilic attack and stabilizes σ H adducts, and the reduction of the nitro group results in the initiation of the aromatization of the ring apparently through the in termediate formation of hydroxylamine A. 4 Finally, the transformation of the nitro group into the amino group provides structural prerequisites for cyclocondensation reactions. …”

“…As a continuation of the research on the reductive dehydrogenation of compounds 6, 4 we studied a series of σ H adducts 7-10. These dihydro compounds are formed in high yields as stable bright colored crystalline sub stances.…”

Synthesis of Condensed Heterocycles, Containing Partially Saturated Pyrimidine Nuclei, from Aromatic Pyrimidine Derivatives

“…The reaction of 53 and 54 with some π‐excessive aromatic and heterocyclic compounds to proceedes smoothly at reflux temperature giving the corresponding σ‐adducts 56 and 57 . These fused dihydropyrimidine derivatives 55 – 57 are formed in high yields as stable crystalline substances.…”

“…The aforementioned examples describe the condensed heterocycles where the pyrimidine ring is fused via C5-C6 linked centers. A pyrimidine, which is incorporated in 1,2,4-triazolo-and 1,2,3,4-tetrazolo-6-nitropyrimidine The reaction of 53 and 54 with some π-excessive aromatic and heterocyclic compounds to proceedes smoothly at reflux temperature giving the corresponding σ-adducts 56 and 57 [62,63]. These fused dihydropyrimidine derivatives 55-57 are formed in high yields as stable crystalline substances.…”

10.1007/s11178-008-1017-8

“…We briefly reported previously that σ H -adducts derived from 6-nitro [1,2,4] VIIa, VIIb dine ring in the course of reduction of the nitro group [10]. It is known [5] that oxidative aromatization of σ H -adducts formed from dihydroazines, by contrast, follows the S N H (AO) mechanism.…”