Cover Picture (Angew. Chem. Int. Ed. Engl. 3/1988)

The cover shows the oxygen analogue of porphyrin, the tetraoxaporphyrin dication distinguished by its D 4 h symmetry. The red, black, and green contours symbolize the three possible 18π major conjugation paths, as a consequence of which the molecule has both pyrylium ion as well as carbenium ion character. That the synthesis of this conceptually very simple porphyrin structural variant—a sort of “umgepoltem porphyrin”, and thus potential host for small anions—has never been reported until now seems difficult to understand, especially since furanoid macrocycles with the ring skeleton of the dication, the so‐called tetraoxaquaterenes, have long been known and have been the subject of detailed studies for a number of years under the heading of crown ethers. More about the perchlorate of the tetraoxaporphyrin dication, isolated as magnificent violet crystals, is reported by E. Vogel et al. on p. 406 ff.