Sydnone

Sydnones are neutral, highly crystalline, stable compounds, soluble in most organic solvents. N-Arylsydnones typically are obtained by treating N-nitroso-N-aryl-glycines with etha-noic anhydride. The parent glycine is regenerated when the sydnone is heated with dilute alkali. 

A general method for the preparation of 2JT-isoindol-4,7-diones, eg (97) [72726-02-4] involves 1,3-dipolar addition of oxa2ohum-5-oxides (sydnones) (98) to 2-meth5l-l,4-ben2oquinone (96). Yield of (97) is 37%. 

C2N2O 0 N N — — fV-(p-Bromophenyl)sydnone 4,4 -dichloro-3,3 -ethylenebis(sydnone)... 

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

As expected, nitration is facilitated by activating groups such as an amino group for example, nitration of (115) occurs at about 20 °C (HNO3/H2SO4). Sydnones (116) are nitrated readily. The pyrazolinone (117) is nitrated as indicated, and 1,2,4-triazolinones have also been ring nitrated. 

Enamines and enolate anions react with benzofuroxan to give quinoxaline di-A -oxides (Scheme 38) (69AHC(10)1). Sydnones (274) with phenyl isocyanate give 1,2,4-triazoles (275) (76AHC(19)l), and from (276) the intermediate adduct (277) can be isolated (73JA8452). This is one of the few instances in which such primary cycloadducts have been isolated in the oxazole series of mesoionic compounds. 

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). 

Use of mesoionic ring systems for the synthesis of five-membered heterocycles with two or more heteroatoms is relatively restricted because of the few readily accessible systems containing two heteroatoms in the 1,3-dipole. They are particularly suited for the unambiguous synthesis of pyrazoles as the azomethine imine is contained as a masked 1,3-dipole in the sydnone system. An attractive feature of their use is that the precursor to the mesoionic system may be used in the presence of the cyclodehydration agent and the dipolarophile, avoiding the necessity for isolating the mesoionic system. 

A-Substituted polypyrazoles can also be obtained by using A-alkylhydrazines, and it should be noted that these polymers consist of a random mixture of head-to-head and head-to-tail structures. Other syntheses of polypyrazoles have been described in the literature. Thus polyphenylene pyrazoles (742) and (743) occurred when m- or p-diethynyl-benzene (DEB) reacted with 1,3-dipoles such as sydnones or bis(nitrilimines) (Scheme 64). 

Tetrazoles (744), as bis(nitrilimine) (745) generators (Section 4.04.3.1.2(ii)), afford polypyrazoles when reacted with diynes. Benzoquinone has also been condensed with bis-sydnones to incorporate a fused pyrazole nucleus (746). 

Sydnone, 3-p-chlorophenyl-dipole moment, 6, 368 (48JCS2269, 49JCS746) Sydnone, 4-cyano-3-dimethylamino-IR, 6, 369 (79LA63)... 

Sydnone, 3-methyl-conductance, 6, 371 dielectric constants, 6, 368 dipole moments, 6, 368 Sydnone, 3-methyl-4-phenyl-nitration, 6, 372... 

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... 

The term mesoionic was suggested originally to describe compounds of the sydnone (129) or sydnone imine (130) type (49JCS307, 50JCS1542). Since then this term has been applied somewhat indiscriminately to a variety of heterocyclic molecules which can be represented formally as zwitterions. The original, rather restrictive definition put forward by Baker and Ollis (55CI(L)910) was later made even more so by Ollis and Ramsden (76AHC(19)1), and it is this most recent definition which now appears to be the most useful. 

Pertluoroallene is also quite reactive in its additions to nitrones [20], diazoalkanes [20] and sydnones [19] With sydnones, the isolated product derives from a fluoride ion rearrangement of the primary adduct (equation 21)... 

The possibility of the equilibrium 298a/298b for sydnones was considered [76AHC(S1), p. 498], but not yet firmly established (Scheme 110). 

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