Classification Nomenclature e Six-membered ring heterocycles 5 2 6 O idi ne 2H-pyran 吡啶 2H-吡喃 5 5 N 3 N pyrimidine Pyridazine Pyrazine 嘧啶 哒嗪 吡嗪 Six-membered ring heterocycles Classification & Nomenclature pyridine 吡啶 pyrimidine 嘧啶 1 2 3 4 5 6 N     2H-pyran 2H- 吡喃 1 2 3 4 5 O 6 1 2 3 4 5 6 N N 1 2 3 4 5 6 N N 1 2 3 4 5 6 N N Pyridazine 哒嗪 Pyrazine 吡嗪