Dmitry L. Lipilin

A New Cyclization Involving the Diazonium and ortho-(tert-Butyl)-NNO-azoxy Groups − Synthesis of 1,2,3,4-Benzotetrazine 1-Oxides

The synthesis of 1,2,3,4-benzotetrazine 1-oxides (BTOs) is described. Bromo-BTOs 4b−d were prepared by the intramolecular cyclization of diazonium salts bearing an ortho-(tert-butyl)-NNO-azoxy group. BTOs bearing electron-releasing substituents were obtained by nucleophilic displacements of bromine in 4b−d. The formation of the BTO cyclic system involves the intermediate 2-(tert-butyl)-1,2,3,4-benzotetrazinium 4-oxides, which arise from an N,N-[1,2]-shift of the tert-butyl group. Decomposition of BTOs involves opening of the tetrazine ring to afford ortho-azidonitroso derivatives, followed by their cyclization with the evolution of the N2 molecule to give benzofurazans. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

2‐Alkyl‐1,2,3,4‐benzotetrazinium Salts: Synthesis and NMR Studies of the Novel 2‐Alkyl‐1,2,3,4‐tetrazinium/ortho‐(Alkylazo)diazonium Equilibrium

The diazotization of anilines bearing the ortho-alkylazo group resulted in benzenediazonium salts 3, which exist in equilibrium with the 2-alkyl-1,2,3,4-benzotetrazinium salt cyclic isomers 4. The structures of 3 and 4 were confirmed by 1H, 13C, and 15N NMR studies. The relative proportions of the cyclic and open-chain forms were determined by a 1H NMR study. The 3 ⇄ 4 equilibrium is fast on the NMR timescale, and only one set of signals is observed in the 1H and 13C NMR spectra. The equilibrium strongly depends on the substituents on the aromatic ring, while being practically unaffected by the substituents (methyl or tert-butyl) on the azo group. Some of the investigated salts exist only in the cyclic form (4f, 4g), others only in the open-chain form (3k). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

The cyclopropyliminium rearrangement of 2-cyclopropyl-4-nitrobenzimidazoles

Abstract2-Cyclopropyl-4(7)-nitrobenzimidazoles undergo the cyclopropyliminium rearrangement into two isomeric 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazoles, one isomer considerably dominating over the other. This reaction is more regioselective than those involving 5- or 6-substituted analogs, which is due to the electronic and steric effects of the nitro group as well as to stabilization of one intermediate by hydrogen bonding between the adjacent NH and NO2 fragments.

Synthesis and transformations of 3(5)-(3-methylfurazan-4-yl)-4-nitro-1 Н -pyrazole-5(3)-carboxylic acid

Methyl 3-(4-methylfurazan-3-yl)-1Н-pyrazole-5-carboxylate was obtained by condensation of 3-acetyl-4-methylfurazan with diethyl oxalate with subsequent treatment of the β-diketoester intermediate with hydrazine, and was nitrated to produce 3(5)-(3-methylfurazan-4-yl)-4-nitro-1Н-pyrazole-5(3)-carboxylic acid. The amide of this acid was used in a Hofmann rearrangement, providing 3-amino-5-(4-methylfurazan-3-yl)-4-nitro-1Н-pyrazole, nitration of which yielded the respective nitramine.

Synthesis and tautomeric transformations of 2 -(tert-butyl ) -1,2,4 -benzotriazine -3 (2H ) -thiones

Treatment of 2-(tert-butyl)-1,2,3,4-benzotetrazinium tetrafluoroborates with sodium thiocyanate afforded 2-(tert-butylazo)phenyl isothiocyanates 3, which exist in equilibrium with 2-(tert-butyl)-1,2,4-benzotriazine-3(2H)-thiones 3′. The equilibrium depends on the substituents R in the benzene ring: the percentage of the open isomer 3 is about 20% for R = H or Me; for R = Cl or Br, the equilibrium is completely shifted to cyclic isomer 3′. The equilibrium is slow on the time scale of the 1H and 13C NMR experiments. For compounds 3a/3′a (R = H), the spectra at 24 °C show two sets of signals, while those at 0 °C contain only signals for isomer 3′a.