·6员杂环 吡啶 吡喃 密啶 pyridine pyran pyrimidine 必秦 吡嗪 pyridazine pyrazine 稠杂环 IN 8 2 3 喋啶 purine pteridine 6 N N N O N N N N pyridine pyran pyrimidine pyridazine pyrazine 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 N N N N N N N N purine pteridine 1 2 3 4 5 6 7 8 9 H