PHOTOCHEMISTRY WITHOUT LIGHT AND STEREOCHEMISTRY OF TYPE A DIENONE REARRANGEMENT . ORGANIC PHOTOCHEMISTRY .38.

A means of generating the zwitterionic species postulated as reaction intermediates in dienone photochemistry was developed. This involved synthesis of 2-bromo-6,6-diphenylbicyclo[3.1.0]hexan-3-one and 2,4-di-bromo-6,6-diphenylbicyclo[3.1.0]hexan-3-one. Treatment of the monobromo ketone with potassium r-butoxide or the dibromo ketone with zinc afforded the Type A rearrangement product, 6,6-diphenylbicyclo[3.1.0]hex-3-en-2-one in a process postulated as proceeding via the zwitterionic species of the photochemical process. 6-endo-Phenyl-6-exo-p-bromophenylbicyclo[3.1.0]hexan-2-one, 6-exo-phenyl-6-endo-p-bromophenylbicyclo[3.1.0]hexan-2-one, 6-endo-phenyl-6-exo-p-bromophenylbicyclo[3.1.0]hexan-3-one, 6-exo-phenyl-6-endo-p-bromophenylbicyclo[3.1.0]-hexan-3-one were synthesized and their configurations were interrelated. Reaction of 2-bromo-6-endo-phenyl-6-exo-p)-bromophenylbicyclo[3.1,0]hexan-3-one with potassium t-butoxide in t-butyl alcohol afforded 6-endo-phenyl-6-exo-p-bromophenylbicyclo[3.1.0]hex-3-en-2-one. Similarly, under these reaction conditions, 2-bromo-6-exo-3-phenyl-6-endo-p-bromophenylbicycIo[3.1,0]hexan-3-one gave stereospecifically 6-exo-phenyI-6-endo-p-bromophen-ylbicyclo[3.1.0]hex-3-en-2-one. In analogy to the stereochemistry of the santonin to lumisantonin rearrangement, the dark reaction of the bromo ketones proceeded with inversion of configuration at the benzhydryl carbon which migrates by a “slither” rather than a pivot mechanism. The stereochemical course is rationalized on a molecular orbital basis. © 1969, American Chemical Society. All rights reserved.