2- furo quinoxalin-9-one

AA and lAA can be easily detected nsing UV or ED. Compared to AA, DHAA has a weak UV absorption and no response on ED. Therefore, derivatization prior to or after the chromatographic separation is needed to increase sensitivity. Different strategies can be applied (a) DHAA may be rednced to AA, prior to HPLC by L-cysteine [547] or dithiothreitol [548], or using a postanalyti-cal colnmn for a solid-state redaction [549] (b) DHAA may be derivatized with o-phenyldiamine to form the flnorophore 3(l,2-dihydroxy-ethyl)furo[3,4- ]quinoxaline-l-one (DFQ) [550] (c) AA may be oxidized to DHAA by an on-line postcolnmn oxidation with Cn + or Hg + followed by derivatization with o-phenyldiamine [551,552]. 

Reaction of 2-aminopyridine and 3-(jV, Af-dimethylaminocarbony 1) furo[2,3-6]quinoxaline 69 in boiling butanol gave 2-(2-oxo-2//-pyrido [ 1,2-a]pyrimidin-3-yl)-3,4-dihydroquinoxalin-2-one 70 (80CPB3537). 

The tosyloxylation of suitable ketones followed by heterocyclization has been utilized in the syntheses of the following heterocyclic systems 2-aroylbenzo[fe]furans [194], 3-aryl-5,6-dihydroimidazo[2,l- ][l,3]-thiazoles [194], 6-arylimidazo[2,l- ]thiazoles [195], (15 ,2/ )-indene oxide [196], 2-mercaptothiazoles [197], triazolo-[3,4-fc]-l,3,4-thiadiazines [198], dihydroindeno[l,2-e][l,2,4]triazolo[3,4- ][l,3,4]thiadiazines [199], furo[3,2-c]coumarins [200], 4,5-diarylisoxazoles [201], 2-substituted 4,5-diphenyloxazoles [202], quinoxaline [203], 3-carbomethoxy-4-arylfuran-2-(5//)-ones [204], thiazol-2(3//)-imine-linked glycoconju-gates [205] and other important heterocycles. 

It is apparent that the formation of the furo[2,3-l>]quinoxalin-2-one derivative 108 proceeds via the initial formation of compound A according to the first stage of the aliphatic Vilsmeier reaction, performed on the methylene group of the substituent at position 3 of quinoxalin-2(lfl)-one 103. The latter undergoes intramolecular cyclization with the subsequent elimination of EtOH and HCl results in the final product 108 (Scheme 6.36). 

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