1,2-Azoles and 1,3-Azoles 115Oxazoles can be converted into pyridines by reaction with, for example, acrylic acid, the initial adduct losing water to
form the pyridine.
Oxygen and amine substituents
The oxygen-substituted 1,3-azoles exist in their carbonyl tautomeric forms and are essentially non-aromatic. Similarly,
3- and 5-hydroxy-1,2-azoles are very much the minor tautomeric forms. 1,3-Azol-2-ones react, like 2-pyridones,
with phosphorus halides and thus can be converted into the 2-haloazoles. 2,4-Dibromothiazole can be accessed from
thiazolidine-2,4-dione.
Azolones will undergo aldol-type condensations with aldehydes to the carbonyl and will also couple at that position
with aryldiazonium cations giving coloured compounds that have been used as dyestuffs – tartrazine, a food and drink
colorant, is an example.
The Vilsmeier formylation of ‘Antipyrine’, once used as an analgesic, is again best understood as electrophilic sub-
stitution at the -position of the enamine unit.
Oxazol-5-ones are simply cyclic anhydrides of N-acyl-amino acids, and are constructed in the way that this implies;
they have been known for many years and are usually called ‘azlactones’. Azlactones undergo ready condensation with
aldehydes, and the products can be converted into -amino acids via double-bond reduction followed by hydrolysis.