Tetrazoles from azido-1,2,3-triazoles

Most transformations of other azoles into tetrazoles involve azido substituents for example, 5-azido-l,2,3-triazoles 753 with a C02Me group at C(4) rearrange at 5070 C in organic solvents into the 5-diazoester substituted tetrazoles 754 (Scheme 325) <1998J(P2)785>. For a comprehensive review on the preparation of tetrazoles from transformations of other heterocycles see <1998JPR687>. 

Other studies (see, e.g., ref. 187c and references cited therein) have revealed that, in systems containing only O and N as heteroatoms, the azide form 184 is usually the only isomer observable in solution or in the solid state, whatever the position of the heteroatoms. Recent work has shown that the tetrazole forms are predominant in systems containing only nitrogen as the heteroatom in two special cases. If a proton is removed from the azide (e.g., 305), the anion 306 cyclizes to the tetrazole 306b. The cyclized form is likewise formed from the anions of azido-indazoles, -imidazoles, and -benzimidazoles but cycliz-ation is only partial with s-triazoles and negligible with tetrazoles. It is possible to trap the cyclized anions (e.g., 306b) by methylation - (Section IV,C,2) and obtain stable pyrazolo[l,5-rf]tetrazoles, 307 and 308. 

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