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18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is a cyclic polyether and a strong complexing agent. The 18 in its name indicates the number of atoms in the ring; the 6 denotes the number of oxygen atoms.

In 1967, Charles J. Pedersen at DuPont (Wilmington, DE) reported the , including 18-crown-6. To synthesize almost all of the crown ethers, he began with vicinal aromatic diols, which he oligomerized by treating them with sodium hydroxide and a dichloroether compound.

The exception was the synthesis of unsubstituted 18-crown-6, which Pedersen made via the reaction of 17-chloro-3,6,9,12,15-pentaoxaheptadecanol, 1,2-dimethoxyethane¹, and potassium t-butoxide², albeit with a yield of only â‰�2%. The author did not provide much information about 18-crown-6, concentrating predominantly on dibenzo-18-crown-6³ and its hydrogenated derivative, dicyclohexyl-18-crown-6⁴. Pedersen shared the 1987 Nobel Prize in Chemistry for his pioneering work in crown ethers.

In 1974, two reviews of crown ether chemistry appeared. The first was by Pedersen’s fellow Nobel Prize winner Donald J. Cram* and Jane M. Cram at the University of California, Los Angeles, titled . In the second, James M. Christensen*, Delbert J. Eatough, and Reed M. Izatt at Brigham Young University (Provo, UT) concentrated on the .

In 1972, R. N. Greene at DuPont reported synthetic methods for making 18-crown-6 in yields as high as 93%. Greene also demonstrated that 18-crown-6 is a , with stability constants for its sodium and potassium complexes greater than those measured by Pedersen for dicyclohexyl-18-crown-6.

In 1987, Jerry L. Atwood and co-workers at the University of Alabama (Tuscaloosa) reported the (H₃O⁺) by 18-crown-6 via its reaction with water and hydrogen chloride. When the reaction takes place in the presence of aromatic solvents such as benzene or toluene, the complex forms a liquid clathrate with the solvent.

And in 2021, Timofey P. Martyanov at the Russian Academy of Sciences (Chernogolovka), Sergey P. Gromov at the Russian Academy of Sciences (Moscow) and M. V. Lomonosov Moscow State University, and colleagues at these and other Russian institutions described more intricate complexes of 18-crown-6. They synthesized a stilbene derivative of 18-crown-6, complexed it with styryl dyes that contained an alkylammonium group, and then irradiated the complex to produce a 2 + 2 cross-photocycloaddition reaction that formed an unusual combination of a .

1. CAS Reg. No. 110-71-4.
2. CAS Reg. No. 865-47-4.
3. CAS Reg. No. 14187-32-7.
4. CAS Reg. No. 16069-36-6.