Pyrazoles, 4-diazo-, decomposition

Protected 6-amino-hexahydro-l,7-dioxopyrazolo[l,2-4]pyrazole-2-carboxylic acid 274 is available by a thermolytic decomposition of diazo compound 273 via the Wolff rearrangement. The starting compound is simply available by alkylation of racemic 272 with the corresponding bromoacetoacetate and subsequent diazo transfer reaction (Scheme 35) <1996TL4891>. 

AIkynyl(diisopropylsilyl)oxy-diazoacetates (295) undergo intramolecular 1,3-dipolar cycloaddition in good yield when R = H (isolation of silver pyrazolide 296 was possible) and R, R = Me,Me or (CH2)s, but no reaction occurred when R = H, R = H or R =Me (340). The silicon substimtion is apparently crucial. Replacement of the Si(i-Pr)2 in 295 (R = R = H) by Si(f-Bu)2 allowed an uncatalyzed intramolecular [3 + 2] cycloaddition to take place [xylene, 140-160 °C, 11% yield (340)], while Ag(I) catalysis led to decomposition. A diazoacetic acid (2-propyn-l-yl)oxysilyl ester also produced a bicyclic pyrazole, but in low yield. On the other hand, the same diazo compound 295, which reacted intm-molecularly under silver ion catalysis, underwent dimerization by an /nfermolecular... 

Diazomethane and its simple analogs undergo cycloaddition to unsaturated compounds both directly and after conversion to carbenes. The direct cycloadditions are 1,3-dipolar for the most part and provide access to pyrazolines and pyrazoles. Intramolecular cyclizations were recognized as early as 1965 95 The two main methods used in generation of diazo compounds for subsequent intramolecular cycloaddition include thermolysis of tosylhydrazone salts and thermolysis of iminoaziridines. Decomposition of nitros-amines has also been employed. 

The most common source of a,j5-unsaturated carbenes for cyclopropene synthesis is from the decomposition of 3H-pyrazoles which are, in turn, available from the addition of a diazo compound to an alkyne. On photolysis these compounds suffer ring cleavage to an a>diazoalkene prior to nitrogen loss and carbene formation. In this way cyclopropene 27 is available from either 25 or 26 by way of common diazo and carbene intermediates. ... 

Decomposition of 9-diazo-97/-fluorene by thermolysis in the presence of an alkene may involve a stable 4,5-dihydro-3//-pyrazole intermediate, which, in some cases, can be isolated, instead of a carbene species. Thus, when a benzene solution of 9-diazo-9//-fluorene and nor-... 

Although unintended, it was found that the a-diazobenzamide 49 readily participates in the Pechmann pyrazole synthesis with dimethyl acetylene dicarboxylate (41). While studying the reactivity of carbenes generated at the benzylic position of 49, the use of Cu(acac)2 did not effectively catalyze the decomposition of the diazo functionality, thereby allowing the Pechmann pyrazole synthesis to proceed efficiently upon treatment with dimethyl acetylene dicarboxylate. 

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