Heterocyclic Chemistry at a Glance

Diazines 59

With regard to the 1,3-dicarbonyl unit, when a carbonyl is at the acid/ester oxidation level, oxygenated pyrimidines
result. The use of a nitrile in lieu of a carbonyl leads to amino-pyrimidines.

Synthesis of pyrazines from 1,2-dicarbonyl compounds

(4,5- and 1,6-bonds made)

Pyrazines are usually prepared via condensation reactions of 1,2-dicarbonyl compounds with 1,2-diamines. When
saturated diamines are used, an additional oxidation (aromatisation) step is required. Aromatisation is not necessary
when one of the amines is an amide, the resulting product being a pyrazinone.

Unsaturated 1,2-diamines (ene-diamines), where an aromatisation step would not be needed, are generally not
stable, one exception being diaminomaleonitrile. 5,6-Diaminouracil can be used as a stable masked ene-diamine,
a fi nal hydrolysis revealing a 3-aminopyrazine-2-carboxylic acid. (Note: The intermediate bicyclic compound is a