First Friedel-Crafts diacylation of a phenanthrene as the basis for an efficient synthesis of nonracemic [7]helicenes

Reported are the first examples of Friedel-Crafts reactions used to prepare 3,6-diacylphenanthrenes. 9,10-Dimethoxyphenanthrene gives its 3,6-diacetyl derivative in good yield and in large amounts. The ketone's triisopropylsi lyl enol ether when combined with 1,4-benzoquinone forms a [7]helicenebisqu inone. This bisquinone's reduction product, a bishydroquinone, when combine d with methanolic HCl gives the [7]helicene whose peripheral side chains ar e all methoxyls but whose interior hydroxyls remain. The diastereomeric (1S )-(-)-camphanates can be separated by crystallization. Their structures, an alyzed by X-ray diffraction, demonstrate that the camphanates' lactone func tions point away from the ring system when the helicene has the (P) configu ration and toward it when the helicene has the (M) configuration. This is b ecause the camphanates' O=C-C-O dihedral angles are, as expected, close to 0 degrees in the former and close to 180 degrees in the latter. Other deriv atives of 3,6-diacetylphenanthrene and of [7]helicenebisquinone are prepare d, and the crystal structure of one of the latter is analyzed.