Steric Activation in the Nazarov Cyclization of Fully Substituted Divinyl Ketones.

Fully substituted divinyl ketones (fsDVKs) have long been regarded as resistant to Nazarov cyclization (NC) unless they contain strategically positioned electronically activating substituents. Here, however, we show that fsDVKs bearing only electronically neutral alkyl or aryl groups actually undergo facile NC due to steric crowding in the pentadienyl cation intermediate, which raises its energy and reduces the barrier height to cyclization. Strongly ionizing and suitably bulky acid moieties further increase the energy of this intermediate cation, favoring cyclization.