Sydnones reactions

Substitution of the nitrogen atom in (289) and subsequent ring closure of (293) under acid cyclodehydration conditions gave the mesoionic system anhydro-5-hydroxythiazoIium hydroxide (294). These reactions are analogous to the cyclodehydration of the A-nitrosogly-cines (295) with acetic anhydride to give the sydnones (296) (see Chapter 4.21). 

Sydnone, 3-p-nitrophenyl-4-phenyl-mass spectra, 6, 370 Sydnone, 3-phenyl-acetylation, 5, 58 6, 373 carbonylation, 6, 373 dipole moments, 6, 368 nitration, 6, 372 reduction, 6, 371 Vilsmeier reaction, 6, 373 Sydnone, 4-phenyl-3-p-tolyl-photolysis, 6, 374 Sydnone, 3-(3-pyridyl)-photolysis, 6, 375 Sydnone, 3-p-tolyl-dipole moments, 6, 368 Sydnones... 

Alkoxy)alkynylcarbene complexes have been shown to react with nitrones to give dihydroisoxazole derivatives [47]. Masked 1,3-dipoles such as 1,3-thia-zolium-4-olates also react with alkynylcarbene complexes to yield thiophene derivatives. The initial cycloadducts formed in this reaction are not isolated and they evolve by elimination of isocyanate to give the final products [48]. The analogous reaction with munchnones or sydnones as synthetic equivalents of... 

Irradiation of 3,4-diarylsydnones 329 possessing an allyl or alkenyloxy substituent provided fused dihydropyrazoles 331 presumably via decarboxylation of sydnones 329 to nitrilimine 330 and the latter s INIC reaction (Eq. 38) [94]. 

Sydnones undergo a range of useful reactions despite their susceptibility to acid-catalyzed hydrolysis and instability in basic media. Since the last review of the subject <1996CHEC-II(4)165>, some novel methods have appeared and these are presented in the following sections. Table 3 provides a short summary of some recent transformations applied to 3-arylsydnones. 

Metallation reactions at C4 of sydnones and sydnonimines are used as a means of achieving electrophilic substitution. Examples that have appeared since the last review of the subject <1996CHEC-II(4)165> are given in Section 5.03.7.1.1. 

Phenylsydnone 89 is not restricted to [3+2] cycloaddition. Reaction of sydnone 89 and its derivatives with the substituted azete 90 gives isomeric l//-triazepines after extrusion of carbon dioxide (Equation 8). 

Photolysis of the triazepine products produces 2,2-dimethylpropanenitrile and the corresponding pyrazole in quantitative yield <1997BSF927>. Reaction of sydnone 89 with fulvene 91 proceeds by [ji4s + jt6s]-cycloaddition followed by spontaneous loss of carbon dioxide and a molecule of dimethylamine or acetic acid from the pseudo-azulene , cyclopentaMpyridazine 92 (Equation 9) <1996CC1011, 1997T9921>. 

The Weintraub reaction was revisited to form additional members of the series of diazatetracycloundecanes by tandem 1,3-dipolar cycloaddition of sydnones and 1,5-cyclooctadiene (Equation 11) <1996JHC719>. 

Diazotization of 3-(4-aminophenyl)sydnones followed by reaction with 1- or 2-hydroxynaphthalene provides azo dyestuff materials <1998MI209>. A new type of reaction between 4-acetyl-3-arylsydnones and hydrazine yields substituted pyrrolidinones by a cycloaddition process involving loss of nitric oxide (Equation 20) <1999H(51)95, 2001AHC73>. 

Sydnonimimes are prepared in a similar way to sydnones (Section 5.03.9.2) but rely on the availability of the appropriate aminonitrile (Scheme 8) <2002CRV1091>. Substituted dihydro- and tetrahydrophthalazine 122 and 124, formed from phthalazine by modification of the Reissert reaction, were converted to the novel sydnonimines 123 and 125 (Equation 22) <1995JHC643>. 

This procedure is a modification of preparations of 3-phen)d, sydnone described earlier. The dehydration of N-nitroso-N phenylglycine has also been effected by the use of thionyl chloride and pyridine in dioxane, thionyl chloride in ethertrifluoroacetic anhydride in etherand diisopropylcarbodiimide in water or by reaction of the alkali metal salts of N-nitroso-N-phenylglycine with phosgene or benzenesulfonyl chloride in water or with acetyl chloride in benzene. ... 

The sydnone ring has also been used as an ortho-director of lithiation. Thus, on reaction with Bu Li-TMEDA, 3-phenylsydnone has been found to form a dilithio species which can be regiospecifically acylated at the ort/io-position by a weak electrophile. 

The preparation and reactions of 1,2,3-oxadiazoles have been reviewed by Meier and Hanold <94HOU(E8c)397). 1,2,3-Oxadiazoles were described by Clapp in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-I(6)365). The chemistry of sydnones is described in two other reviews <82X2965,91MI403-01). For this chapter the literature published between 1982 and mid-1995 has been surveyed. 

The chemistry of the 1,2,3-oxadiazole ring system is confined almost entirely to the mesoionic sydnones and related compounds. Sydnones are sensitive to hydrolysis, especially in basic media where they are rapidly cleaved. Nevertheless a range of useful reactions of sydnones, involving both ring cleavage and substitution with retention of the ring system, is known. Many of the publications that have appeared in the period 1982-1995 represent modifications or extensions to known reactions of sydnones. The subjects covered include the following. 

Substituents can be introduced into the 4-position of sydnones by conventional electrophilic substitution but there are also several examples of electrophilic substitution following metallation at C-4 <95H(41)1525). Standard transformations of functional groups at C-4 of sydnones have also been extensively investigated. In particular, these reactions have been used to synthesize sydnones bearing a variety of heterocyclic substituents at the 4-position <92Mi 403-01). Sydnones can act as... 

Metallation reactions at C-4 of sydnones have been reviewed <95H(4l)i525>. Typical conditions for 3-arylsydnones involve reaction of the sydnone with butyllithium at — 50°C <86BCJ483> methyl-magnesium bromide has also been used as the base <92MI 403-02>. These reactions are invariably used as a means of effecting electrophilic substitution at C-4 as described in Section 4.03.7. 

The addition to alkenes normally leads to unstable adducts that lose carbon dioxide under the reaction conditions. The intramolecular cycloaddition of the sydnone (30) takes place at room temperature, however (Equation (5)) and the cycloadduct (31) has been characterized <86HCA927>. The unstable species formed by the loss of carbon dioxide are also azomethine ylides. It is therefore possible for a second 1,3-dipolar addition to take place, as illustrated in Scheme 6 for the reaction of 3-phenylsydnone with Al-phenylmaleimide <86TL317,92JA8414>. This 2 1 addition has been used as the basis of a synthesis of polyimides. Imides of the type (32) were used as the dipolarophiles and their reaction with 3-phenylsydnone gave linear polymers <87MM726>. 

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