Aza-crown ether
Ring molecule with several amine (–N– or >N–) groups
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In organic chemistry, an aza-crown ether is an aza analogue of a crown ether (cyclic polyether).[2][3] That is, it has a nitrogen atom (amine linkage, −NH− or >N−) in place of each oxygen atom (ether linkage, −O−) around the ring. While the parent crown ethers have the formulae (CH2CH2O)n, the parent aza-crown ethers have the formulae (CH2CH2NH)n, where n = 3, 4, 5, 6. Well-studied aza crowns include triazacyclononane (n = 3), cyclen (n = 4),[4] and hexaaza-18-crown-6 (n = 6).[5]
- Selected aza-crowns and their complexes
1,4,7-Trimethyl-1,4,7-triazacyclononane, a tridentate ligand used in coordination chemistry.
Plerixafor, a derivative of cyclam, is used to mobilize blood stem cells as a part of blood cancer treatment.[6]
2.2.2-Cryptand is an aza-crown of the mixed ether-amine variety.
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Synthesis
The synthesis of aza crown ethers are subject to the challenges associated with the preparation of macrocycles.[7] The 18-membered ring in (CH2CH2NH)6 can be synthesized by combining two triamine components.[5] By reaction with tosyl chloride, diethylene triamine is converted to a derivative with two secondary sulfonamides. This compound serves as a building block for macrocyclizations.
Variants
Many kinds of aza crown ethers exist.
- Variable length linkers
- Aza crowns often feature trimethylene ((CH2)3) as well as ethylene ((CH2)2) linkages. One example is cyclam (1,4,8,11-tetraazacyclotetradecane).
- Tertiary amines
- In many aza-crown ethers some or all of the amines are tertiary. One example is the tri(tertiary amine) (CH2CH2NCH3)3, known as trimethyltriazacyclononane. Cryptands, three-dimensional aza crowns, feature tertiary amines.