C-nucleosides. 11. Synthesis of quinoxaline C-nucleosides through condensation of 1,2-diaminobenzenes with 6-hydroxy-6-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)-2,6-dihydropyran-3-one

Abstract

The synthesis of 6- and 7-substituted-2-(β-D-ribofuranosyl)quinoxaline and 7- and 8-substituted-1-(β-D-ribofuranosyl)pyrrolo[1,2-a]quinoxaline from 6-hydroxy-6-(2,3,5-tri-O-benzoyl-β-D- ribofuranosyl)-2,6-dihydropyran-3-one (1) is described. Treatment of (1) with 1,2-diamino-4-chlorobenzene (2a) afford three compounds, the 6- and 7-chloroquinoxalines (3a) and (3b) and the 7-chloropyrrolo[1,2-a]quinoxaline (4a) in 23, 43, and 9% yield, respectively. The position of the substituent in products (3a) and (3b) was determined by comparison of these ¹H n.m.r. spectra with those of the corresponding N-oxides (5a), (6a), and (5b), (6b), prepared by oxidation of compounds (3a) and (3b) with m-chloroperbenzoic acid. The position of the substituent in (4a) was confirmed by ¹H–¹³C long-range COSY experiment with corresponding deblocked pyrrolo[1,2-a]quinoxaline (4c). Treatment of compound (1) with 1,2-diamino-4-nitrobenzene (2b) afforded two compounds, the 6-nitroquinoxaline (3c) and the 8-nitropyrrolo[1,2-a]quinoxaline (4b). Deprotection of compounds (3a–c), (4a, b), (5a, b), and (6a, b) with methanolic sodium hydroxide afforded (3d–f), (4c, d), (5c, d), and (6c, d), respectively.