Combining two-directional synthesis and tandem reactions. Part 4: A concise approach to the spirocyclic core of halichlorine and the pinnaic acids

A synthesis of the spirocyclic core structure of halichlorine, an inhibitor of the induced expression of VCAM-1 and the pinnaic acids, inhibitors of PLA2, is presented. A two-directional strategy is employed, in conjunction with a tandem oxime formation/Michael addition/cycloaddition to form the key azaspirocyclic skeleton in a very direct manner.