Preparation of Condensed Aromatics by Superacidic Dehydrative Cyclization of Aryl Pinacols and Epoxides¹a

Aryl pinacols and epoxides, respectively, are cleanly and in high yield converted via superacidic dehydrative cyclization to the corresponding condensed aromatics. Dehydrative cyclization of benzopinacol ( 1a ), triphenylacetophenone ( 2 ), and tetraphenylethylene oxide ( 9 ) give 9,10-diphenylphenanthrene ( 3a ) as the major product in acidic media stronger than H o = −11. Aryl pinacol 12a forms the condensed aromatic 13a as the major product in acidic media stronger than H o = −13.5. It is proposed that the dehydrative cyclizations to provide aromatics 3a and 13a occurs through dicationic intermediates. Substituted benzopinacols 1f, 1g, and 1j are prepared and give the corresponding phenanthrenes ( 3f, 3g, and 3j ) in high yields. The regiochemistry of the cyclization of substituted benzopinacols is controlled by deactivating substituents on the aryl rings. Aryl pinacols ( 12a−d ) derived from acenaphthenequinone and pinacol 15 also give condensed aromatics ( 13a−d and 16, repectively) with superacidic triflic acid.