Pyrazoles, 3-azido

The most important chemistry of azidoazoles is the fragmentation of derived nitrenes of which the prototypes are (453) (454) and (455) (456). Thus 5-azido-l,4-diphenyltriazole (457) evolves nitrogen at 50 °C (70JOC2215). 4-Azido-pyrazoles and -1,2,3-triazoles (458) undergo fragmentation with formation of unsaturated nitriles (8lAHC(28)23l). 

The azido/tetrazole rearrangement of pyrazole and indazole derivatives will be discussed in the corresponding section (4.04.2.3.4(v)). 

The azido-3(5)-phenyl-4-pyrazole isomer 317 is the sole form in the solid state by X-ray study (74CSC713).The azidoazole isomers also predominate in the ease of 3(5)-azido-s-triazoles 318, 4(5)-azido-v-triazoles 319, azido-tetrazole 320 [77MI545, 86JCS(CC)959], and azidoisoxazoles 321 (77CJC 1728,77JHC1299). 

Balli and Felder (1978) and Balli and Ritter (1981) showed that diazo transfer can be applied advantageously to the diazotization of sufficiently nucleophilic heteroaromatic compounds such as 5-hydroxy- and 5-amino-3-methyl-l-phenyl-pyrazole if 3-ethyl-2-azido-benzthiazolium tetrafluoroborate (2.50) is used as diazo transfer reagent (for other applications of this diazo transfer reagent see Zollinger, 1995, Secs. 2.6-2.8). The diazonio group is introduced in the 4-position (2.51). 

A useful method for obtaining indazoles in high yields involves treatment of ortho-azido phenyl ketones or aldehydes with hydrazine hydrate. When this reaction was applied to 3-azido-2-formylselenophene, selenolo[3,2-c] pyrazole (111) was obtained in low yield. The yield could not be improved when the amino derivative was prepared as an intermediate142 (Scheme 13). 2-Dialkylamino-5-formylselenophenes react with diazonium salts under deformylation conditions to give azo dyes (Eq. 39).143 Another nitrogen-containing derivative of selenophene is compound U2.144 ... 

Thermolysis reactions have also been investigated as methods for preparing pyrazoles. Thermolysis of azido imines 18 led to 2-substituted-4,6-dinitro-2//-indazoles 19 <00S1474>. High temperature intramolecular cyclization of AA-diethyl-A -(4-substituted-2-... 

Pyrazole 174 has been formed by electrocyclization of transient azido-substituted pyrazole 193 generated from chloropyrazoles upon treatment with sodium azide in refluxing aqueous DMF, but yields of isolated bicycles are moderate (Equation 21) <1995M1265>. 

Many N-unsubstituted pyrazoles can be obtained from A-allyl-TV-nitrosamides (90H(30)789), e.g. (56) — (57). Reactions of l-azido-3-chloropropane with Grignard reagents afford intermediates (58) that cyclize into azimines of A -pyrazolines (59) (92TL4683). 

Five-membered ring heterocyclic azides are readily reduced (H2S, LiAlH4) to the corresponding amines. On thermolysis they lose nitrogen, thereby generating nitrenes. For example, 3-azido-2-vinyl derivatives in xylene at 120130C yield [3,2-A]-fused pyrroles 430 nitrene insertion into an ortho-disposed imino function similarly yields [3,2-A]-fused pyrazoles 431. 

Deprotonation can occur at the -GH of pyrazole A-alkyl groups for example 1-methylpyrazole with -BuLi. Such proton loss is facilitated in cationic azido rings, and the ylides so formed sometimes undergo rearrangement. Thus, quaternized 1,2-benzisoxazoles 796 lose a proton and then rearrange to 1,3-benzoxazines, e.g., 797. Quaternized derivatives of benzofuroxan formed in situ undergo rearrangement to 1-hydroxybenzimidazole A-oxides 798. Reactions of this type are also known for A-alkylazolinones. 

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