58 Diazines
Diazine diazonium salts are even less stable than those in the pyridine series and are only synthetically useful for the
conversion of amino-diazines into diazinones.
Ring synthesis – disconnections
Only the most frequently used routes are discussed here. The disconnections for pyridazines, pyrimidines and pyra-
zines are shown together with the new bonds formed in each route. Pyrimidines can also be prepared via cycload-
dition reactions on 1,3,5-triazines (see page 141) and pyridazines via cycloaddition reactions with 1,2,4-triazines or
1,2,4,5-tetrazines (see page 141).
Synthesis of pyridazines from 1,4-dicarbonyl compounds
(2,3- and 1,6-bonds made)
Pyridazines are most commonly prepared by condensation of 1,4-dicarbonyl compounds with hydrazine. When a
saturated 1,4-dicarbonyl component is used, there has to be a fi nal oxidative step to bring about aromatisation. A pre-
ferred option is the use of a dicarbonyl component already having a carbon–carbon double bond located between the
carbonyl groups; this leads to a fully aromatic pyridazine directly.
Synthesis of pyrimidines from 1,3-dicarbonyl compounds
(3,4- and 1,6-bonds made)
Pyrimidines can be prepared by condensation of a 1,3-dicarbonyl compound with an N-C-N unit, usually a urea,
thiourea, amidine RC(=NH)NH 2 , or guanidine RHNC(=NH)NH 2. The urea products have a 2-carbonyl (thiocar-
bonyl), the amidine products have a 2-R substitutent and the guanidine products have a 2-RHN substituent. These
cyclocondensations must start with an amino-to-carbonyl addition (or amino-to-carbon of the enolised form of
the 1,3-dicarbonyl system) but one cannot say in which order the subsequent bonds are made and eliminations of
water occur.