Cover Picture (Angew. Chem. Int. Ed. Engl. 8/1991)

The cover picture (center) shows a CPK model of the structure of a hemicarcerand—a global minimum calculated with the MM2 force field (with the phenylethyl groups omitted). Quite astounding transformations have been carried out in its cavity. Reactions that occur within the host are gold‐colored and the one that occurs outside is blue. The golden orb in the middle shows the locations of the guests within the hemicarcerand and highlights the small portal through which the guests enter and exit the interior of the host. “Incarcerated” cyclobutadiene is stable at room temperature and above in the absence of oxygen, but reacts with oxygen that enters the “container” to give incarcerated maleinaldehyde. This new type of chemistry promises to open a whole new field involving the study of electronically and sterically unperturbed reactive species. Further details are reported by D. J. Cram et al. on p. 1024.