60 Diazines
‘pteridine’ – the pteridine ring system is present in a number of important natural products (pages 159–160).
Pteridines show the expected reactivity for the individual diazine rings and can be prepared using standard pyrazine
or pyrimidine ring syntheses, as in the example.)
Synthesis of pyrazines from -amino-carbonyl compounds
(1,2- and 4,5-bonds made)
-Amino-ketones, which are stable only as their salts, are usually prepared in situ by the reduction of 2-diazo-,
-oximino- or -azido-ketones. Self-condensations of such amino-ketones give symmetrically substituted dihydro-
pyrazines that are very easily aromatised, sometimes just by heating. (Similar processes also occur in nature and
during cooking (see pages 186–187).)
-Amino-esters are more stable than -amino-ketones but nonetheless easily self-condense to give stable heterocycles,
generally known as 2,5-diketopiperazines (a misleading name as the carbonyl groups are amide not ketone carbonyls).
Hexahydropyrazine is called piperazine.
Benzodiazines
Reactions in the heterocyclic ring are principally electrophilic additions on nitrogen and nucleophilic addition to
carbon (there are no pyridine-like -carbon positions), with an increase in reactivity comparable to the pyridine–
quinoline/isoquinoline relationship. The heterocyclic rings are resistant to oxidation and the heterocyclic core is
retained on strong oxidation.