On the absolute stereochemistry of the photochemical rearrangement of 2,5-cyclohexadienones to lumiketones

Abstract Specification of the absolute configurations of starting material and product in the photochemical rearrangement of a 2,5-cyclohexadienone to a bicyclo[3.1.0]hexen-2-one (lumiketone) permits unambiguous determination of the stereochemistry at two crucial stages in the generally accepted reaction mechanism, namely the electrocyclic ring closure to give a 3,5-bonded intermediate, and the subsequent [1,4]-sigmatropic shift. The stereochemical course of the latter process in turn allows specification of whether the rearrangement is occurring directly from an electronically excited intermediate or a ground state zwitterion. An unconstrained monocyclic chiral cyclohexadienone has been studied in which steric constraints are minimal, and in which the system should have free choice of available pathways. The absolute configuration of the resolved dienone was determined by X-ray crystallographic analysis of a derivative containing a chiral center of known configuration. The absolute configuration of the resultant optically active diastereomeric lumiketones has been assigned from the Cotton effects observed on the corresponding dihydrolumiketones, assuming these chiroptical effects are governed by a normal Octant Rule instead of the Inverse Octant Rule which empirically correlates such data for cyclopropyl ketones lacking polarizable substituents. The limitations on these assignments are discussed, as well as alternative efforts to obtain completely unambiguous stereochemical assignments for the lumiketones. On the basis of these assignments, it is concluded that the sigmatropic shift proceeds with inversion of configuration at the migrating carbon, consistent with reaction via a ground state zwitterion.