The Di-TT-Methane Photorearrangement

The first reported use of ionic chiral auxiliaries in asymmetric synthesis was the work of Gudmundsdottir, Scheffer, and Trotter on the di-Tt-methane photorearrangement of the dibenzobarrelene system 33 [31]. Of a number of chiral auxiliaries tested,best results were obtained with prohne fert-butyl ester, which led to dibenzosemibullvalene derivative 34 in an enantiomeric excess of 95% at 40% conversion following diazomethane workup. By determining the ab-... 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

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