Sydnones, mesoionic ring systems

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. 

Alkylation ofsydnones. Mesoionic ring systems with exocyclic oxygen atoms ordinarily do not undergo O-alkylation with alkyl halides or sulfates. However, alkylation occurs readily with triethyloxonium fluoroborate. Thus 3-phenyl-sydnone (1) in methylene chloride is alkylated at room temperature to give (2). 

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. 

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). 

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," 1,3,4-oxadiazoles" and 1,2,5-oxadiazoles are well known, but the 1,2,3-oxadiazole system, which calculations indicate to be unstable relative to its ring-open diazo-ketone tautomer," is known only as a benzo-fused derivative (in solution) and in mesoionic substances, known as sydnones ," which have been well investigated. Furoxans ," which are formed by the dimerisation of nitrile oxides, have also been extensively studied. 1,2,3-Thiadiazoles, 1,2,4-thiadiazoles, 1,3,4-thiadiazoles" and... 

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