Glucose cyclizes to form asix- membered pyranose ring whilst five –carbon surgars and six-carbon ketose sugars (ketohexoses) such as fructnose rings.

环状结构 四碳糖和更大的糖能环化，醛基或酮基与糖的另一碳原子上的羟基反应。

Method Treatment of 8,9 anhydroerythromycin 6,9 hemiketal with iodine in the alkaline solution provided corresponding N monodemethylated 12 membered ring erythromycin derivative.

方法8,9脱水红霉素6,9半缩酮与碘在碱性溶剂中反应得到相应的N去甲基十二元环红霉素衍生物。