1,2-AZAPYRYLIUM IONS - PROPERTIES AND SYNTHETIC APPLICATIONS

Whereas pyrylium ions are well investigated isoelectronic 1,2-azapyryl ium ions are a rather elusive species. This review describes methods f or generation of 1,2-azapyrylium ions as well as some of the physical properties of these highly electrophilic species. Synthetic applicatio ns of 1,2-azapyrylium ions - generally prepared by ionization of 6-alk oxy-bH-1,2-oxazines - involve additions of various nucleophiles to C-6 giving new 6H-1,2-oxazines. Alcohols, electron-rich (hetero)arenes, a llyl silanes and silyl enol ethers are particularly suitable nucleophi les. Employing azidotrimethylsilane, cyanotrimethylsilane, trimethyl p hosphite or propargyl trimethylsilane as nucleophilic components cause d formation of rather surprising reaction products being the result of several mechanistically intriguing reaction cascades. This led to the discovery of a new pyridine synthesis using 1,2-azapyrylium ions as h etero diene components and alkynes as dienophiles in a Diels-Alder rea ction with inverse electron demand as crucial step.