A New Access to Dihydrotropones through Ring Expansion of Spirocyclohexadienones: Synthesis and Mechanism^†

In this paper we report the rearrangement of spirocyclohexadienones into dihydrotropones in basic
conditions as a new method for the preparation of seven-membered ring ketones, which are key building
blocks for the synthesis of tropoloalkaloids. DFT calculations and deuterium labeling studies support the
mechanism we propose for this rearrangement, involving the ring opening of a spirocyclopropane
intermediate followed by successive base-catalyzed 1,3-hydrogen shifts. The X-ray structure of the resulting
dihydrotropone shows near-perfect planarity and the conjugation gain is likely to be the driving force of
the reaction.