MAOS of Quinoxalines, Conjugated Pyrazolylquinoxalines and Fused Pyrazoloquinoxalines from l‐Ascorbic and d‐Isoascorbic Acid

Abstract

Microwave‐assisted organic synthesis (MAOS) has been used to accelerate the conversion of L‐ascorbic acid (1) and D‐isoascorbic acid (2) to the title heterocycles by conversion to 3‐(L‐threo‐ or D‐erythro‐glycerol‐1‐yl)quinoxaline‐2‐carboxylic acid o‐aminoanilides (7 and 8), which were transformed to lactones 9 and 10, respectively, under acidic condition. The acetylation of compounds 9 and 10 afforded 3‐(L‐threo‐ or D‐erythro‐2,3‐di‐O‐acetyl‐glycerol‐1‐yl)quinoxaline‐2‐carboxylic‐γ‐lactones (11 and 12). Treatment of 10 with phenylhydrazine gave the hydrazides 13. 3‐[1‐Phenylhydrazono‐L‐threo‐2,3,4‐trihydroxybutyl]‐1H‐quinoxalin‐2‐one (14) and its D‐erythro‐analog 15 were prepared from 1 and 2. Subsequent cyclizations gave the respective pyrazolylquinoxalines 16 and 17 and pyrazolo[3,4‐b]quinoxalines 26 and 27. The regioselectivities of allylation and epoxypropylation of 16 and 17 were investigated and could be interpreted by the semiempirical AM1 method. Degradation of 26 or 27 gave 1‐phenylpyrazolo[3,4‐b]quinoxaline‐3‐carboxaldehyde (28). Degradation of 14 or 15 gave aldehyde 29. The combination of using microwave (MW) and bentonite, as a support, has improved the yields in less reaction times in addition to performing the reactions under environmentally clean conditions