Intramolecular Dehydro Diels−Alder Reactions of Diarylacetylenes:  Switching between Benzo[b]- and Benzo[c]fluorenones as Products by Controlling the Rearrangement of Cyclic Allene Intermediates

Thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-trimethylsilylpropynones affords a mixture of benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied between 100:0 and 0:100 by introducing substituents with appropriate electronic and steric properties on the aryl rings and using an appropriate solvent.