Asymmetric cyclopalladation in the ferrocene series as a tool for enantioselective synthesis. Preparation of some analogues of natural products

The previously described asymmetric cyclopalladation has been applied to the esters of 7-dimethylamino-7-ferrocenylenanthic acid, I, to afford the optically active palladium derivative, II. The absolute configuration of the chiral plane is determined by the configuration of the inductor: the acylamino acid salt. The palladium atom in II was then substituted by a σ-ketovinyl group using the reaction with pentyl vinyl ketone, to yield III. The latter substance undergoes full reduction when treated with Et₃SiH + CF₃COOH (the ionic hydrogenation reaction) resulting in the optically active prostanoic acid analogue, IV. On the other hand, III in the form of the methiodide is reduced by NaBH₄ with the elimination of the amine group and the and the formation of allyl alcohol V. This kind of side chain is characteristic of many prostaglandins. Bromination of II followed by the repeated cyclopalladation opens the way to trisubstitute derivatives. The pathway outlined provides a rapid synthesis of optically active compounds which are the ferrocene analogues of natural prostaglandins.