Five-Membered Ring Systems with Two or More Heteroatoms

Five-Membered Ring Systems with Two or More Heteroatoms 

Benzo[c]cinnoline (114) (3%) was separated from a complex mixture of colored products obtained from the oxidation of 9 with active manganese dioxide formation of 114 suggests the stepwise loss of nitrogen, 10- 113-+ 1, and 1,4-dipolar cycloaddition of benzyne to 113.16 Oxidation of 9 with (diacetoxyiodo)benzene gave azobenzene (4%), 1-phenylbenzotriazole (112) 

A series of adducts analogous to 132 has been prepared from benzyne and substituted 2,l,3-benzoselenadiazoles.84a The adduct 133, from 5,6-dimethyl-2,l,3-benzoselenadiazole (129), undergoes a further remarkable rearrangement either thermally or photochemically to give the selenocyanate 136. The corresponding compound 137 is obtained directly from the reaction of benzyne with 130, which implies that the adduct 134 is even more labile 

Mesoionic Compounds and Heterocycles without a Formal Diene System 

Recent developments in the chemistry of mesoionic compounds85 include cycloaddition-elimination reactions, which afford novel synthetic routes to a variety of heterocyclic systems. These reactions may be seen as involving 1,3-dipolar cycloadditions, following Huisgen,86 or alternatively as 1,4-cycloadditions to heterodiene systems,87 depending on the choice of canonical structure to represent the mesoionic compound. Benzyne has been employed in such reactions less frequently than more stable acetylenic or ethylenic dipolarophiles. 

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VI. Five-Membered Ring Systems with Two or More Heteroatoms.206... 

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