Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. Engl. 14/1994)
The cover picture shows the chemical formula of the tetraprotonated form of turcasarin, to date the largest expanded porphyrin. This class of compounds includes both macrocyclic porphyrin analogues and ring systems with more than four pyrrole units. The 40 π-electron annulene turcasarin exists as a pair of enantiomers in solution and in the solid state; the individual enantiomers adopt a nearly C2-symmetric, twisted “figure-eight” conformation. The interconversion of the two conformers via an open “circular” conformation (shown in the center of the picture) is slow on the NMR timescale at room temperature. More about this cyclic oligopyrrole, which in the tetraprotonated form, in analogy to several other expanded porphyrins, could serve as an anion binding and transport agent, is reported by J. L. Sessleret al. on p. 1509 ff.