Adenine isosteres with bridgehead nitrogen. Part 1. Two independent syntheses of the [1,2,4]triazolo[1,5-a][1,3,5]triazine ring system leading to a range of substituents in the 2, 5 and 7 positions
Condensation of a 3-substituted 5-amino-1,2,4-triazole with an N-cyanocarbonimidate (RX)2CNCN, yields the title compounds, in most cases as a mixture of isomers; separation and elaboration then gives [1,2,4]triazolo[1,5-a][1,3,5]triazines bearing a range of substituents in the 5 and 7 positions. Under milder reaction conditions, less stable isomeric products (e.g., the [1,2,4]triazolo[4,3-a][1,3,5]triazine ring system or an uncyclised N-cyano intermediate) can be isolated and shown to rearrange to the [1,5-a] isomer. A second synthesis is described in which a suitably substituted 2-hydrazido-1,3,5-triazine is cyclodehydrated to give a single isomer with identical substituents in the 5 and 7 positions; the large difference in reactivity between these allows selective replacement at the 7-position with nucleophile. Additionally, the second synthesis is more readily adaptable to the introduction of substitutents at the 2 position of the bicyclic nucleus. 7 Amino-2-(2-furyl)-5-phenoxy[1,2,4]triazolo[1,5-a][1,3,5]triazine 12a is prepared independently by both synthetic sequences and its structure verified by X-ray crystallographic analysis; 1H NMR, 13C NMR and MS data are also presented in support of the proposed structures.