Orthoquinone monoketal chemistry. Experimental and density functional theory studies on orthoquinol acetate rearrangements

The non-dimerizing orthoquinone monoketal, 6-acetoxy-6-methoxy-3-methoxycar bonylcyclohexa-2,4-dienone, conveniently prepared from oxidative acetoxylat ion of its parent phenol with PhI(OAc)(2) in CH2Cl2-AcOH (3:1), cleanly und ergoes 1,3-acetoxy migrations in the presence of silica gel at room tempera ture to furnish a 60:40 product mixture conceivably derived from [3,5] and [3,3] sigmatropic rearrangements. Density functional theory calculations in dicate that the [3,5] shift is pseudopericyclic, has a remarkably low activ ation energy of 20.1 kcal/mol, and is favored by 5.4 kcal/mol over the peri cyclic [3,3] shift, in qualitative agreement with the experimental observat ions. (C) 1999 Elsevier Science Ltd. All rights reserved.