Alternative general synthetic routes to [2.2]cyclophanes and [3,2]cyclophanes from [3.3]cyclophane-2,11-diones by photodecarbonylation, and a structural study of [3.2]metacydophanes

Photodecarbonylation of [3.3]cyclophane-2,11-diones, which are readily prep ared by TosMIC coupling, affords [2.2]cyclophanes in high yield. This metho d also provides a general synthetic method for [3.2]cyclophan-2-ones by tak ing advantage of the fact that this photochemical reaction proceeds in a st epwise manner through [3.2]cyclophan-2-ones. A series of [2.2]cyclophanes, [3.2]cyclophanes, and [3.3]cyclophanes can thus be made available from the common precursor [3.3]cyclophane-2,11-diones. In sharp contrast to the pref erred syn geometry of [3.3]metacyclophanes, [3.2]metacyclophanes adopt anti geometries and the aryl ring inversion process is observed by variable-tem perature H-1 NMR spectroscopy. In the crystalline state, the two aryl rings of anti-[3.2]metacyclophanes are almost parallel in spite of the unsymmetr ical bridge length; they overlap only at the C-9 and C-17 positions, and th e transannular distances are shorter than the corresponding distances in [3 .3]metacyclophane-2,11-dione.