Preparation of Tautomeric Pyrazinones

Many such pyrazinones have been made by primary synthesis (see Chapters 1 and 2) or by hydrolysis of halogenopyrazines (Section 4.2.2). Other methods of preparation are illustrated in the following examples, classified according to the type of substrate  

5-Benzylthio-2-pyrazinamine gave 5-bcnzyl thio-2( I //)-pyrazinonc (15) (NaN02, AcOH—H20—dioxane, 5°C, 15 min 46%).1565 Also other examples.54,64,397 

Note This reaction can be done in several ways, as shown in these examples. 

Hydrolysis. 2-Methoxy-3-methyl-5-phenylpyrazine (16) gave 3-methyl-5-phenyl-2(l//)-pyrazinone (17) (6 M HC1, reflux, 3 h 97%) 1307 other products like 3,6-diisopropyl-2( I //(-pyrazinonc (18, R = Pr ) (97%)1311 and 3,6-diphenyl-2(l/7)-pyrazinone (18, R = Ph) (84%)82 were made similarly. Hydriodic acid may also be used.1307 

Trimethylsilyl iodide method. 2,5-Dimethoxy-3,6-dimethylpyrazine (19) gave 5-hydroxy-3,6-dimethyl-2(l//)-pyrazinonc (20) (Me3SiI, (CH2)4S02, N2, 40°C, 2 h then H20 [, 0 — 70°C, 30 min 84%) 1392 also other examples.57 

Ethyl 5-amino-2-pyrazinecarboxylate (12) gave ethyl 5-oxo-4,5-dihydro-2-pyrazinecarboxylate (13) (NaNO2, H2SO4, 3 - 45°C, 1 min 80% the use of concentrated H2SO4 ensured minimal hydrolysis of the ester grouping).  

3-Amino-2-pyrazinecarbonitrile gave 3-oxo-3,4-dihydro-2-pyrazinecarbonitrile (14) (NaNOi, dilute H2SO4, 0 20°C, 3 h 58%)32% 

Note This reaction can be done in several ways, as shown in these examples. Hydrolysis. 2-Methoxy-3-methyl-5-phenylpyrazine (16) gave 3-methyl-5- 

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