Flash Vacuum Pyrolysis in the Synthesis of O- and N- Heterocycles

GLORIA I. YRANZO; ELIZABETH L. MOYANO

Modern Approaches to the Síntesis of O- and N- Heterocycles

Research Singpost

Lugar: India; Año: 2007; p. 231 - 300

Flash vacuum pyrolysis (FVP) consists in submitting a molecule to the application of high temperature during a short period of time (~10-2 s). This important feature allows getting kinetic as well as unstable products. This is an interesting methodology to study reaction mechanisms and for synthesis. Flash vacuum pyrolysis is used as a single step synthetic procedure as well as a complement of a multistep route; these reactions are normally clean, without solvent, so they normally involve carbenes, nitrenes, radicals and concerted reactions. The main principles and applications of FVP as well as different reactions affording O- and N- heterocyclic compounds, are described, according their ring size and the number of heteroatoms present in the molecules involving monocyclic and fused rings. The effect of different substituents and some kinetic results are also discussed. The most common reactions that afford heterocycles are cyclisations of unsaturated compounds, ring fragmentations including retro Diels-Alder reactions, isomerisations and extrusion of small neutral molecules (SO2, CO2, N2). Flash vacuum pyrolysis of Meldrum´s acid have a special attention as this is one of the most important routes to heterocycles.