Described here are
tandem photoelectrocyclization and [1,5]-hydride
shift reactions of heteroaryl-containing bis-aryl cyclohexenone derivatives
that give heteroaryl-substituted dihydrophenanthrenes. This Letter
demonstrates that electrocyclization intermediates can be trapped
with acid when the [1,5]-hydride shift is relatively slow. From a
practical perspective, the observation that the acid-mediated reaction
gives a divergent stereochemical outcome when compared with the reaction
run under neutral conditions makes these transformations powerful.
Outlined
here are studies exploring the scope of the sequential
photoelectrocyclization, [1,5]-hydride shift of conjugated bis-aryl
cycloalkenone substrates. We have found not only that the cyclization
precursors can be synthesized in a modular fashion but also that the
cyclization is efficient and amenable to the presence of a range of
cycloalkenones and aromatic systems. Among the interesting discoveries
from this work is that the electrocyclization intermediate can be
competitively captured with protons and that the nature of the excited
state (singlet vs triplet) is dependent on aromatic substitution.
In
this work, we demonstrate that readily available conjugated
bis-aryl cyclobutenones undergo photoelectrocyclization reactions
to give the corresponding dihydrophenanthrene cyclobutanones when
exposed to 350 nm light, TFA, and TMSCl. We have also found that cyclobutenone
electrocyclizations and cycloreversions are in equilibrium.
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