Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Whereas pyrylium ions are well investigated isoelectronic 1,2-azapyrylium ions are a rather elusive species. This review describes methods for generation of 1,2-azapyrylium ions as well as some of the physical properties of these highly electrophilic species. Synthetic applications of 1,2-azapyrylium ions - generally prepared by ionization of 6-alkoxy-6H-1,2-oxazines - involve additions of various nucleophiles to C-6 giving new 6H-1,2-oxazines. Alcohols, electronrich (hetero) arenes, allyl silanes and silyl enol ethers are particularly suitable nucleophiles. Employing azidotrimethylsilane, cyanotrimethylsilane, trimethyl phosphite or propargyl trimethylsilane as nucleophilic components caused 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 hetero diene components and alkynes as dienophiles in a Diels-Alder reaction with inverse electron demand as crucial step.
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