Ring compounds dipolar reagents

Recently, efficient synthetic methods for this class of compounds have been developed so that a variety of such compounds are now readily available. The nucleophilicity of the imino or amino nitrogen, the electrophilicity of the heteroaromatic ring, and the dipolar character permit them to react in a number of ways, depending upon the nature of the heteroaromatic ring, substituents, the reagent, and the conditions. 

Potts and co-workers have used phosphorus pentasulfide and pyridine as a standard reagent for constructing annelated diphenylthiophene rings from o-dibenzoyl compounds. Many of these annelated thiophenes can be written only in the tetracovalent sulfur or ylid dipolar form (96) and are thus termed nonclassical 123 other examples, particularly of the synthesis of non-classical thienoisothiazoles, have been reported by Gotthardt.124... 

Reactions of ascorbic acid have been intensively studied in relation to the behavior of this familiar compound in biological systems. Most of the studies treated ascorbic acid as an simple outersphere reducing reagent, until Creutz published an article concerning the complexity of the reaction pathways of ascorbic acid and related radicals. The authors recently demonstrated that the oxidation reactions of ascorbic acid in acidic aqueous solutions are not adiabatic from the volume analysis of the reactions. As the non-adiabaticity of the ascorbate reactions implies the involvement of the proton dissociation and/or ring-closure processes at the rate-determining step, it is expected that Ae reactions of ascorbic acid in dipolar aprotic solvents such as dimethyl sulfoxide (DMSO) are adiabatic. ... 

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