Ring-Chain Equilibrium between an [18]Cyclacene Derivative and a Ladder Oligomer

The synthesis of the AB-type Diels-Alder (DA) monomer 12 is reported whose self-condensation leads to the formation of the linear ladder oligomer 17 and its cyclic congener 18. 18 is the largest [n]cyclacene derivative known (n = 18) and can be obtained by thermal treatment of 17 in yields of up to 45% utilizing the reversibility of the DA cyclization involved. 17 can be considered a “storage form” for 18.