Synthesis and NMR Spectroscopic Data of Pyrazolo[4’,3’:5,6]pyrano[2,3-b]pyrazin -4(1H)-ones: Derivatives of a Novel Tricyclic Ring System

Derivatives of a novel tricyclic ring system, pyrazolo[4’,3’:5,6]pyrano[2,3-b]pyrazin-4(1H)-one, were prepared by reaction of either 1-substituted or 1,3-disubstituted 2-pyrazolin-5-ones and 3-chloro-2-pyrazinecarbonyl chloride in the presence of Ca(OH)2 in refluxing 1,4-dioxane. In some cases the corresponding title compounds thus were obtained directly due to a spontaneous intramolecular nucleophilic substitution reaction of the intermediate 4-pyrazinoylpyrazol-5- ols. In other cases mixtures of the latter intermediates and the target compounds were obtained, which were completely converted into the desired tricycles upon treatment with HCl in a chloroform/methanol mixture. The parent system carrying no substituent in positions 1 and 3 was prepared by treatment of the 1-PMB (p-methoxybenzyl) protected congener with trifluoroacetic acid. Detailed NMR spectroscopic data (1H, 13C, 15N) are presented for the title compounds.