Quinoxaline-2-carboxaldehydes preparations

Quinoxaline-2-carboxaldehydes are conveniently prepared either from the corresponding methyl- or polyhydroxyalkylquinoxalines. The oxidation of methylquinoxalines to quinoxalinecarboxaldehydes is usually carried out with selenium dioxide, and sodium metaperiodate (NaI04) is the reagent recommended for the oxidation of tetrahydroxybutyl-quinoxalines. An indirect method for the conversion of methylquinoxalines into quinoxalinecarboxaldehydes is illustrated in Scheme 1. 

Quinoxaline-2-carboxaldehyde has been converted into the 2-carboxylic acid by oxidation with potassium permanganate in acetone and reduced to the 2-hydroxymethyl compound by treatment with formalin and potassium hydroxide. It also undergoes other typical reactions of aromatic aldehydes such as benzoin formation on reaction with potassium cyanide - and condensation reactions with malonic acid and its diethyl ester and Schiff base formation. Acid-catalyzed reaction of quinoxaline-2-carboxaldehyde with ethylene glycol gives the cyclic acetal the diethylacetal has been prepared by reaction of 2-dibromomethylquinoxaline with sodium ethoxide. " An indirect preparation of the oxime 11 is achieved by treatment of 2-nitromethyl-quinoxaline (10) with diazomethane followed by thermolysis of the resulting nitronic ester. 

Diazomethane methylation of quinoxaline-2-carboxaldehyde yields 2-acetylquinoxaline in excellent yield,and a number of 2-benzoyl-quinoxalines have been prepared by chromium trioxide oxidation of the corresponding 2-benzylquinoxalines Reaction of the quinoxaline ester 2 with acetone and sodium hydride yields the acetoacetylquinoxaline 3 in excellent yield. ... 

Selenium dioxide oxidation of 2-methylquinoxaline yields quinoxaline-2-carboxaldehyde, and 3-methylquinoxaline-2-carboxaldehyde has been similarly prepared from 2,3-dimethylquinoxaline. Dimethylsulfoxide oxidation of 2,3-bis(bromomethyl)quinoxaline yields the furoquinoxaline 45, and this on sublimation gives quinoxaline-2,3-dicarboxaldehyde. 3-Dibromomethylquinoxaline-2-carboxaldehyde (46) is a by-product of the oxidation reaction. Direct selenium dioxide oxidation of 2,3-dimethylquinoxaline appears to be a less satisfactory method of preparing the dialdehyde. ... 

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