| Member | Definition | Role |
|---|
| 1-cyano-3,4-epithiobutane | | Thiiranepropanenitrile |
| 1,4,7-triazacyclononane | | 1,4,7-triazonane |
| 12-crown-4 | A crown ether that is cyclododecane in which the carbon atoms at positions 1, 4, 7 and 10 have been replaced by oxygen atoms. | 12-crown-4 |
| 15-crown-5 | A saturated organic heteromonocyclic parent that is cyclopentadecane in which the carbon atoms at positions 1, 4, 7, 10 and 13 have been replaced by oxygen atoms to give a crown ether. | 15-crown-5 |
| 18-crown-6 | A crown ether that is cyclooctadecane in which the carbon atoms at positions 1, 4, 7, 10, 13 and 16 have been replaced by oxygen atoms. | 18-crown-6 |
| azetidine | | azetidine |
| aziridine | | aziridine |
| cyclam | | 1,4,8,11-tetraazacyclotetradecane |
| cyclen | An azacycloalkane that is cyclododecane in which the carbon atoms at positions 1, 4, 7 and 10 are replaced by nitrogen atoms. | 1,4,7,10-tetraazacyclododecane |
| ethylene oxide | A saturated organic heteromonocyclic parent that is a three-membered heterocycle of two carbon atoms and one oxygen atom. | oxirane |
| ethylene sulfide | | thiirane |
| hexahydroazepine | An azacycloalkane that is cycloheptane in which one of the carbon atoms is replaced by a nitrogen atom. | azepane |
| imidazolidines | | imidazolidine |
| morpholine | An organic heteromonocyclic compound whose six-membered ring contains four carbon atoms and one nitrogen atom and one oxygen atom that lies opposite to each other; the parent compound of the morpholine family. | morpholine |
| oxetane | A saturated organic heteromonocyclic parent that is a four-membered ring comprising of three carbon atoms and an oxygen atom. | oxetane |
| piperazine | An azacycloalkane that consists of a six-membered ring containing two nitrogen atoms at opposite positions. | piperazine |
| piperidine | An azacycloalkane that is cyclohexane in which one of the carbons is replaced by a nitrogen. It is a metabolite of cadaverine, a polyamine found in the human intestine. | piperidine |
| pyrrolidine | A cyclic amine whose five-membered ring contains four carbon atoms and one nitrogen atom; the parent compound of the pyrrolidine family. | pyrrolidine |
| s-trioxane | A saturated organic heteromonocyclic parent that is cyclohexane in which the carbon atoms at positions 1, 3 and 5 are replaced by oxygen atoms. | 1,3,5-trioxane |
| tetrahydrofuran | A cyclic ether that is butane in which one hydrogen from each methyl group is substituted by an oxygen. | oxolane |
| thiamorpholine | A saturated organic heteromonocyclic parent that is an analogue of morpholine where the oxygen atom is replaced by sulfur. | thiomorpholine |
| thiophane | | tetrahydrothiophene |