Sydnones, reactions with

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... 

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>. 

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>. 

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 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>. 

Dumitrascu and co-workers (52) transformed 4-halosydnones into 5-halopyr-azoles by cycloaddition with DMAD and methyl propiolate followed by retro-Diels-Alder loss of CO2. Turnbull and co-workers (194) reported that the cycloadditions of 3-phenylsydnone with DMAD and diethyl acetylenedicarboxylate to form pyrazoles can be achieved in supercritical carbon dioxide. Nan ya et al. (195) studied this sydnone in its reaction with 2-methylbenzoquinone to afford the expected isomeric indazole-4,7-diones. Interestingly, Sasaki et al. (196) found that 3-phenylsydnone effects the conversion of l,4-dihydronaphthalene-l,4-imines to isoindoles, presumably by consecutive loss of carbon dioxide and A-phenylpyrazole from the primary cycloadduct. Ranganathan et al. (197-199) studied dipolar cycloadditions with the sydnone 298 derived from A-nitrosoproline (Scheme 10.43). Both acetylenic and olefinic dipolarophiles react with 298. In... 

Destro et al. (201) examined the reactions of several sydnones with triazoles to give pyrazoles, and the stable bicyclic sydnone 302 has been prepared by Storr and co-workers (202). Reaction with DM AD affords the pyrazolothiazole 303. 

Sydnones behave as 1,3-dipolar systems and undergo addition reactions with various dipolarophiles. Huisgen, Grashey, Gotthardt, and Schmidt503,504 were the first to react acetylenic esters with sydnones and obtained pyrazoles e.g., 71 with DMAD (1 hour, 120°, in xylene) gave 99% of the pyrazole 73. These reactions have also been carried out with propiolic and phenylpropiolic esters, and their kinetics have been studied.505... 

Dihydrodiazepines can also be obtained in the reactions with unsaturated ketones if one of the amino groups of the diamine in the heterocycle is endocyclic (e.g., amine 37, Scheme 4.11) [48]. This reaction is achieved under rather severe conditions (long-duration heating in dimethylformamide). An interesting case using sydnones 40 in the synthesis of 1,5-dihydrodiazepines 41 was described by Kavali and Badami [49] (Scheme 4.12). 

Sydnones can be regarded as cyclic azomethine imines and as such they undergo thermal cycloaddition reactions with a range of dipolarophiles. Thus, reaction with phenyl isocyanate converts 401 into 1,2,4-triazole 402. On photolysis, 3,4-diarylsydnones lose carbon dioxide and give nitrile imines, which can also be intercepted by dipolarophiles. Thermal reactions with acetylenic dipolarophiles lead to the formation of pyrazoles (Scheme 88) however, these reactions are rarely completely regioselective with unsymmetrical alkynes, e.g., <2000BKC761, 2000TL1687>. 

The special effects of a mesoionic system as a substituent have been noted in the reactions with nucleophiles of 3-7V (4-chloro-3-nitrophenyl)sydnone. A synthesis, using two amino-debromination reactions, has been used to prepare phenothiazines analogous to methylene blue (23). An imusual susceptibihty to the nature of the counteranion has been observed in the kinetics of the reaction of iV-(2,4-dinitrophenyl)-4-dimethylaminopyridinium salts (24) with piperidine in acetonitrile, and may indicate participation of the anion in stabilising the intermediate. An ANRORC mechanism is imphcated in the reaction of A-(2,4-dinitrophenyl)-4-(4-pyridyl)pyridinium cations with arylamines. Ring-opening and ring-closure reactions... 

Oxadiazoles. - 1,2,3-Oxadiazoles. Irradiation of the sydnone (466) affords the betaine iV-oxide (467) in low yield " the reaction with dimethyl acetylenedicarboxylate gives a mixture of the pyrrole (468) and the oxazinone (469) both originate from the intermediate cyclo-adduct... 

Cycloadditions were found to be first-order reactions with respect to both 1,3-dipole and dipolarophile, in all cases so far investigated. There are some limits to kinetic studies of these reactions, as many 1,3-dipoles are very reactive substances. While aryl azides, diazoalkanes, some classes of azomethine imines (for instance sydnones), and some classes of azomethine oxides (nitrones) are stable and isolable, azomethine ylides are usually unstable, an exception being represented by a mesoionic oxazolone that has been used for kinetic investigations benzonitrile oxide has a very limited stability, although some substituted derivatives are stable for long periods nitrile imines are not commonly isolable because of their strong tendency to dimerise. 1,3-Dipoles of... 

It is possible to compare the reactivity of some 1,3-dipoles inserted in cyclic structures with that of the same dipoles in open chains. This comparison, recalling that between cyclic and open-chain dienes (Section 4.1.3), is limited to two classes of 1,3-dipoles without a double bond , namely azomethine imines and oxides. For the former, there are data in Table 12 for the reaction with dimethyl acetylene dicarboxylate (iii) the cyclic azomethine imine (sydnone) is about 300 times less reactive than the open-chain cyanoazo-methine imine. In the case of nitrones, a typical comparison is. for reaction with ethyl crotonate in toluene at 100°C "... 

At a Gordon Research Conference on Polymers, I learned from Charlie Overberger that he was engaged in nitrile oxide polymerization, so we limited our research to the bis-nitrile imine emd sydnone dipoles which afforded polypyrazoles in reactions with diethylbenzenes. 1,58,60,61,71,73,9 nitrileimine-producing monomers which... 

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). 

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