Heterocycles with More than Two Heteroatoms: Higher Azoles (5-Membered) and Higher Azines (6-Membered) 141Oxadiazinones are good ‘dienes’ for Diels–Alder reactions, as shown below. It is interesting that nitrogen rather than
carbon dioxide is eliminated from the initial adduct.
Enamines are often used as acetylene equivalents, the amine fragment being lost at a late stage to generate the aromatic
heterocycle.
The best equivalent of ethyne itself for these reactions is norbornadiene, cyclopentadiene as well as nitrogen being lost
in the fi nal stages.
Ring synthesis of higher azines
1,2,3-Triazine is prepared by oxidation of 1-aminopyrazole. Other triazines are usually prepared by cyclocondensation
reactions. Symmetrically substituted 1,2,4,5-tetrazines are prepared by dimerisation–condensation reactions, followed
by oxidative aromatisation. Most unsymmetrically-substituted tetrazines are prepared by manipulation of the sub-
stituents of symmetrical compounds.