Structures and Tropicity of Hydrogenation Products of a Bisdehydro[13]annulenone and a Bisdehydro[15]annulenone

The catalytic partial hydrogenation of the 5,10-dimethyl-6,8-bisdehydro[13]annulenone using Pd–BaSO4 afforded the 5,10-dimethyl[13]annulenone together with two isomeric 5,10-dimethyl-6-dehydro[13]annulenones. The dimethyl[13]annulenone, which is the first example of a monocyclic non-dehydro annulenone derivative larger than tropone, was found to be atropic on the basis of 1H NMR spectra. Molecular mechanics studies indicate the extensive deviation of the molecular skeleton from planarity and help to explain the lack of peripheral conjugation. Attempts to obtain the [15]annulenone and the [17]annulenone from the bisdehydro[15]annulenone and the bisdehydro[17]annulenone, respectively, were unsuccessful.