Diazines 55Mesoionic pyrazinium-3-olates undergo cycloadditions similar to those known for pyridinium-3-olates (page 40)
and pyrylium-3-olates (page 73). 3,5-Dihalo-2(1H)-pyrazinones and their derivatives also undergo cycloadditions,
followed by cycloreversions to give pyridones.
The photocatalysed dimerisation of uracil may have relevance to mutagenesis mechanisms.
Oxygen substituents (see also pages 48 and 164)
The majority of oxydiazines exist in the keto form and are named as such – diazinones. Notable exceptions are the
phenol-like 5-hydroxypyrimidine and 6-hydroxypyridazin-3(2H)-one (‘maleic hydrazide’). The latter exists as the
mono-keto-mono-hydroxy form, thus avoiding the unfavourable juxtaposition of like charges on nitrogen (due to
amide mesomerism) in the dicarbonyl form. The family of 5,5-dialkylated drugs known as ‘barbiturates’ exist neces-
sarily as tricarbonyl structures, but so does the parent, unsubstituted barbituric acid.
N-Deprotonation, N-alkylation and N-arylation
As with pyridones, diazinones are relatively acidic and can be N-H-deprotonated under mild conditions, the resulting
anions reacting with electrophiles on either nitrogen (usually), oxygen or carbon.