Pyrans, thiopyrans, and selenopyrans

This chapter presents the literature for pyrans, thiopyrans, and selenopyrans. Pyran structure refers to a six-membered ring possessing one oxygen, sulfur, selenium, or telurium heteroatom singly bonded in a cyclic system of two double bonds and one tetrahedral atomic center. The pyrans, thiopyrans, and selenopyrans can be synthesized using acyclic or cyclic precursors that indicate if a pyran ring closure does or does not occur during synthesis. Reactions such as (1) oxidation, (2) disproportionation, (3) hydrogenation and reduction, (4) desulfurization of thiopyrans, (5) isomerization, (6) ring-opening reactions, (7) substitution reactions, and (8) conversion to carbocyclic systems (outline the chemical properties of pyrans, thiopyrans, and selenopyrans. Pyrans are very rare as natural products. Only some 2H-pyrans have been isolated from plants. The chapter finally describes the physical properties of triazolopyridines such as electronic spectra, infrared spectra, nuclear magnetic resonance, mass spectra, and some other miscellaneous properties.