A Modern Comprehensive Treatise. Heterocyclic Compounds. Six-membered heterocyclic compounds with two hetero-atoms from Group V of the Periodic Table: the pyridazine and pyrimidine groups, the pyrazine group. Phenoxazine, phenothiazine, phenazine and sulphur dyes. Six-membered heterocyclic compounds with three or more hetero-atoms. 4:i/j,