Quinoxalines. Part 12: Synthesis and structural study of 1-(thiazol-2-yl)-1 H-pyrazolo[3,4- b]quinoxalines—the dehydrogenative cyclization with hydroxylamine hydrochloride

Abstract

Starting with 2-acetylquinoxaline a novel class of heterocyclic compounds, the 1-(thiazol-2-yl)-1 H-pyrazolo[3,4- b]quinoxalines 4 , were prepared by following two different synthetic procedures: 2-acetylquinoxaline reacted with thiosemicarbazide to the thiosemicarbazones 1a which was (i) cyclized with α-halogeno ketones to the thiazoles 3 . These compounds were dehydrogenated in acidic medium to the title compounds 4 . (ii) The thiosemicarbazone 1a could be also dehydrogenated using NH 2OH·HCl to the thioamide 5a and these, finally, were cyclized with α-halogeno ketones to the title compounds 4 . Only thiazole 3a was isolated, the other thiazoles 3 were dehydrogenated in a one-pot procedure. From the thioamide 5a also both the compounds 9 , by reacting with dibromodiacetyl, and 10 , by treatment with dimethyl acetylenedicarboxylate, were obtained. The analysis of both the 1H and 13C NMR spectra was not straightforward but could be attained finally by employing the whole arsenal of 1D and 2D NMR spectroscopy.