Furoxan-furazan rearrangement

A third example to illustrate the construction of G graphs is the furoxan-furazan rearrangement (see Scheme L8), Based on the principles outlined previously it is evident that this rearrangement can be pictured by the Go, Gi, G2, and G3, graphs as presented in Scheme 1.12. They also show the mirror plane symmetry. 

Another example of this rearrangement has been used to prepare 1,2,3-triazole 146 from furazanic phenylhydrazone 147 (Scheme 84) [93JCS(P1)2491]. Interestingly, furoxanic Z-phenylhydrazones 150 underwent thermal recyclization to 1,2,3-triazole A-oxides 152, evidently through intermediate 151. Treatment of the hydrazone 150 with rerr-BuOK leads to the nitromethyl derivative 149 [OOOMIl] (Scheme 84). Lead tetraacetate oxidation of 147 with subsequent Lewis acid treatment of the initially formed intermediate afforded indazole 148 (Scheme 84) (85JHC29). 

As mentioned in CHEC-II(1996), three main routes have been reported for the formation of furazan rings (1) the dehydrative cyclisation of 1,2-dioxims (2) the deoxygenation of furoxans and (3) the Boulton-Katritzky rearrangement of other five-membered heterocyclic systems <1996CHEC-II(4)229>. In this section the recent publications on the synthesis of furazans published after 1996 are discussed. 

The side products of the reaction between benzoylnitromethane 279 and dipolarophiles (norbornene, styrene, and phenylacetylene) in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) were identified as furazan derivatives (Scheme 72). The evidence reported indicates that benzoylnitromethane gives the dibenzoylfuroxan as a key intermediate, which is the dimerization product of the nitrile oxide. The furoxan then undergoes addition to the dipolarophile, hydrolysis, and ring rearrangement to the final products (furazans and benzoic acid) <2006EJ03016>. 

A similar base-induced rearrangement has been reported to occur at room temperature with the Z-isomer of 3-methyl-4-benzoylfuroxan oxime (143), yielding the 3-(a-nitroethyl)-4-phenylfurazan (144) (82G181), and with 3,4-diformaldoxime furoxan (145), which gives 3-(a-nitroacetal-doxime)furazan (146) (Scheme IV.57) (81AHC(2G)251). 

The generation from furoxans of other heterocyclic systems, some of which show useful biological activity (see Section 4.22.5), has been the subject of intensive investigation. The following subsections summarize the conversion of benzofuroxans into quinoxaline and benzimidazole oxides, the rearrangement of 4-substituted benzofuroxans, and the transformation of monocyclic furoxans into isoxazoles and isoxazolines, furazans, and pyrazolines. More detailed discussion is to be found in recent comprehensive reviews (75S415, 76H(4)767, 81AHC(29)251>. 

Thermolysis of 3-anilino-4-nitroso-5-phenylisoxazole, obtained from aniline and dibenzoylfuroxan, produces 3-anilino-4-benzoylfurazan in high yield (69JHC317) other acylanilino derivatives have been prepared similarly. Further examples of furazan formation resulting from the rearrangement of furoxans are described in Section 4.22.3.2.5(ii). 

The N - S rearrangements of furazan and furoxan derivatives will be discussed in the appropriate section. 

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