Azido-tetrazolo tautomerizations

This procedure is also efficient for the synthesis of other fused tetrazoles, such as ethyl l,5-dihydro-5-oxo-7-phcnyl-8-(phenylsulfonyl)tetrazolo[ 1,5- pyridine-6-carboxy late <2000J(P1)3686>. At the same time, the unexpected azido-tetrazolo tautomerization and reversible tetrazolo transformation of 3,6-diazido-l,2,4,5-tetrazine are remarkable compared to all other polyazido heteroaromatic high-nitrogen C-N compounds <2005JA12537>. 

Unexpected azido-tetrazolo tautomerizations and irreversible tetrazolo transformation have been studied in a report dealing with 3,6-diazido-l,2,4,5-tetrazine (DiAT) for which an improved synthesis pathway is also provided. DiAT undergoes azido-tetrazolo equilibria in CD3OD, (CD3)2C0 and CD3CN and transforms to tetrazolo isomer 53 in DMSO. The... 

The azido-tetrazolo tautomerization and the tetrazolo transformation have been investigated by NMR and cyclic voltammetry studies. Results of the latter suggested that the conversion of the azido form to the tetrazolo structure is electronically favored (see Scheme 11). However, the experimental data revealed that the last transformation presented in... 

The second peculiarity is characteristic of heterocyclic azides with -position of azido group relative to endocyclic nitrogen atom and is connected with the azido-tetrazolo tautomerism which can be defined as a 1,5-dipolar cyclization [9]. 

Since the excellent 1973 summary of azido—tetrazolo valence-tautomerism in nitrogenous heterocycles,1713 little has been added to our knowledge of factors governing such tautomerism (227 228) in the pyrazine series. However, it has been shown by NMR studies that 2-azidopyrazine 4-oxide (229) exists as such in chloroform, as tetrazolo[l,5-a]pyrazine 7-oxide (230) in dimethyl sulfoxide, and as a mixture in acetone.272 For obvious pragmatic reasons, all such compounds are named as azidopyrazines in this book, irrespective of their predominant structures. 

Since the excellent 1973 summary of azido—tetrazolo valence-tautomerism in nitrogenous heterocycles, little has been added to our knowledge of factors governing such tautomerism (227 228) in the pyrazine series. However, it has been... 

Smalley et al. reported the synthesis of the cyano-containing keto ester 98 by reaction of o-azidobenzoyl chloride 97 with cyanoacetic ester in the presence of triethylamine. This keto ester was then heated in acetonitrile for 30 min and gave the ring closed product 99 which was isolated in the fully aromatic tautomeric form 100 <1997S773>. A similar approach to tetrazolo[l,5- ]quinolines has been applied by a Korean research group in this case a reflux of the cyano-azido compound 101 for a longer period was needed in order to accomplish the cyclization to 4-acetoxymethyl-tetrazolo[l,5- ]quinoline 102 <2003JHC1103>. 

Chloro- (33, R = C1) gave 2-azido-7-ethoxy-5-phenyl-l, 8-naphthyridine (33, R = N3), formulated as the tautomeric tetrazolo[l,5-a][l,8]naphthyridine (33a) (NaN3, Me2NCHO, reflux -80%).718... 

Bromination of (47) led to 3-azido-4-bromoisoquinoline (261). On the other hand, trifluoroacetic acid caused only the tetrazolo tautomer (262) to exist in the equilibrium of (47) with its azido partner. This is due to elimination of the unfavored o- quinoid structure (47) from the tautomeric equilibrium and formation of the more stable naphthalenic structure (262) which, consequently, is preferred over the azido form (8UOC843). To extend this equilibrium study further, NMR investigations on substituted derivatives of (47) indicated that electron-releasing groups on the isoquinoline favor the tetrazolo tautomer whereas electron-attracting substituents favor the azido tautomer. Furthermore, the tetrazole was found to be preferred in DMSO solution relative to chloroform solution (81JOC843). 

Reaction of the hydrazino group with nitrous acid affords the azido group or the valence tautomeric tetrazolo-l,2,4-triazine,324 e.g. 17 and 18.303 The hydrazino group can also be acylated, formylated, and sulfonylated and reacts with aldehydes to give hydrazones.356... 

Tetrazolo azido tautomerism is exemplified by the 2-azidopyrazine-tetrazolo[l,5-a]pyrazine equi-... 

Ring-chain tautomerism was also studied in the system azido-l,2,4-benzotriazine (51)/tetra-zolo[5,l-c][l,2,4]benzotriazine (52)/tetrazolo[l,5-ft][l,2,4]benzotriazine (53). It was found that electron-donating groups and polar solvents stabilize the tetrazole tautomers over the azide forms. A... 

---