Heterocyclic synthesis 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. 

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. ... 

This chapter deals with the synthesis, reactivity, and characterization of five-membered heterocycles containing two ring sulfur atoms (1,3-dithiole derivatives) and is a review of the literature in the period 1995-2006. Previous reviews covered the literature till 1982 (CHEC(1984)) <1984CHEC(6)813> and 1995 (CHEC-II(1996)) <1996CHEC-11(3)607>. Among 1,3-dithiole compounds, 1,3-dithiolylium ions 1, mesoionic l,3-dithiol-4-ones 2, mesoionic 1,3-dithiole-4-thiones 3, 1,3-dithioles 4, 1,3-dithiolanes 5, and the tetrathiafulv alene (TTF) system 6 as a special class of compounds are discussed. 7l-Extended tetrathiafulvalenes (7t-exTTFs) containing more than one conjugated multiple bond between two 1,3-dithiole moieties are also discussed, in conjunction with 6. 

Cycloaddition Reactions.— Acetylenes undergo many types of cycloaddition reaction and provide valuable starting materials for the synthesis of carbo-and hetero-cyclic systems. Many acetylenes are dipolarophiles, and dimethyl acetylenedicarboxylate (DMAD), because of its ready availability, often plays this role in the formation of a variety of heterocycles. Mesoionic systems often function as the 1,3-dipoles, as in the formation of pyrrole (99) from the A -oxazolium-5-olates (munchnones) (100) and DMAD. The procedure can be extended to fuse pyrrole rings on to the piperidine rings of (101) and (102) in moderate yields. Further examples of... 

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