Syntheses and NMR, MS and X-ray investigations of homoadamantane-fused pyridopyrimidinones

The reactions of ethyl 5-oxotricyclo[4.3.1.13,8]undecane-4-carboxylate (2) with methyl-substituted 2-aminopyridines in polyphosphoric acid (PPA) gave two products, linearly-condensed pyridopyrimidinones 4a-c and 2-pyridylcarboxamides 5a-c, whereas the reactions with amino, hydroxy and nitro derivatives of 2-aminopyridine furnished only linearly-condensed pyridopyrimidinones (4g-j). Use of a mixture of PPA and phosphorus oxychloride as solvent afforded both linearly- (4a-c,e,f) and angularly-condensed (6a–c,e,f) pyridopyrimidinones. In toluene, with p-toluenesulfonic acid as catalyst, 2-pyridylcarboxamides 5a-f were obtained. In a mixture of PPA and phosphorus oxychloride at 80–120 °C, 5a-f yielded angularly-condensed pyridopyrimidinones 6a-f. All the products exhibited characteristic features, as determined by NMR and electron ionization mass spectrometry and X-ray crystallography.