Mesoionic ring systems synthesis

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. 

Another nonclassical heterocycle, thienol3,4-cJpyrazole, was synthesized, utilizing the ability of mesoionic ring systems to act as 1,3 dipoles in cycloadditions. Condensation of IV-phenylsydnone (162) with dibenzoylacetylene formed 3,4-dibenzoyl-1-phenylpyrazole (163) (85%) with phosphorus pentasulfide in refluxing pyridine, this gave 85% of 2,4,6-triphenyIthieno[3,4-c]pyrazole (164) [Eq. (44)]. The synthesis of 5-methyl-l,3,4,6-tetraphenylthieno[3,4-c]pyrrole is also described. ... 

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