DEVELOPMENTS IN THE CHEMISTRY OF THIOPYRANS, SELENOPYRANS, AND TELUROPYRANS

This chapter discusses the developments in the chemistry of thiopyrans, selenopyrans, and teluropyrans from 1983 to 1992. The chapter specifically considers isolable or spectroscopically identifiable thio, seleno, and teluropyrans without exocyclic double bonds. With regard to nomenclature, depending on the position of the sp3 carbon atom, the heteropyrans are usually classified as 2H and 4H isomers. The expressions acyclic or cyclic precursors indicate whether a heteropyran ring does or does not occur during a given synthesis. The chapter describes the various methods of synthesis from acyclic and cyclic precursors. The chapter also describes the various reactions that these pyran compounds essentially undergo. These include oxidation, disproportionation, hydrogenation and reduction, isomerization, substitution, and ring expansion. The physical properties and theoretical chemistry of pyran compounds are also presented that includes studies concerning x-ray crystallography and molecular structure, molecular energy and electronic structure, nuclear magnetic resonance, and electronic and infrared spectra. © 1994 Academic Press Inc.