Aza-crown ether



In organic chemistry, an aza-crown ether is an aza analogue of a crown ether (cyclic polyether). That is, it has a nitrogen atom (amine linkage, \sNH\s or >N\s) in place of each oxygen atom (ether linkage, \sO\s) 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), and hexaaza-18-crown-6 (n = 6).

Synthesis
The synthesis of aza crown ethers are subject to the challenges associated with the preparation of macrocycles. The 18-membered ring in (CH2CH2NH)6 can be synthesized by combining two triamine components. 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. Aza crowns often feature trimethylene ((CH2)3) as well as ethylene ((CH2)2) linkages. One example is cyclam (1,4,8,11-tetraazacyclotetradecane).
 * Variable length linkers

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.
 * Tertiary amines

Another large class of macrocyclic ligands feature both ether and amines.. One example is the diaza-18-crown-6, [(CH2CH2O)2(CH2CH2NH)]2.
 * Mixed ether-amine ligands

The presence of the amine allows the formation of Lariat crown ethers, which feature sidearms that augment complexation of cation.
 * Lariate crowns