Preparation of Tautomeric Quinoxalinones

Most tautomeric quinoxalinones have been made either by primary synthesis (see Chapter 1) or by direct or indirect hydrolysis of halogenoquinoxalines (see Section 3.2.2). The remaining preparative routes are illustrated in the following classified examples. 

Note Both 2- and 3-alkoxyquinoxalines are amenable to reasonably gentle hydrolysis, but 5- to 8-alkoxyquinoxalines require more vigorous procedures. 5- [A-( 1 -Dimethoxyphosphinylethyl)-A-ethylaminomethyl] -2,3-dimethoxyquino-xaline (15) gave 5- [A-(l-dihydroxyphosphinylethyl)-A-ethylaminomethyl]- 

The kinetics for alkaline hydrolysis of 2-(o-, m-, or -nitrophenoxy)quinoxaline to 2(171)-quinoxalinone have been measured in some detail. 6,7-Dimethoxy-2-phenylquinoxaline (17) gave 2-phenyl-6,7(l//,4//)-quinoxaline-dione (18) (48% HBr, reflux, 23 h 53% as hydrobromide). 

Note This transformation is often done indirectly, for example, by a Meissen-heimer reaction and subsequent hydrolysis, but it can be done at least semidirectly as exemplibed here. 

2-Azido-6-chloroquinoxaline 4-oxide (22, R = N3) gave 3-azido-7-chloro-2(lT0-quinoxalinone (23, R = N3) (AC2O, AcOH, reflux, 4 h 46%) ° 6-chloro-2-piperidinoquinoxaline 4-oxide [22, R = N(CH2)s] gave 7-chloro-3-piperidino-2(177)-quinoxalinone [23, R = N(CH2)5](likewise 58%). ° 

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