A Modern Comprehensive Treatise. Heterocyclic compounds. Part IJ.. 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 heterocylic compounds with three or more hetero-atoms. Vol. 4