Synthetic Studies of the Daphniphyllum Alkaloids: A Cooperative Reaction of Proximal Functional Groups Forming a Tetracyclic System.

During the course of synthetic studies of the Daphniphyllum alkaloids, an unusual reaction of a rhodium carbenoid was observed. The bicyclic substrate, in which an α-diazo-β-ketoester moiety was present at the 3-position of a 1,4-diene moiety, was treated with rhodium pivalate to produce an intermediate having diene and ketene moieties. This intermediate underwent an intramolecular [4 + 2] cycloaddition reaction to form a tetracyclic compound.