Approaches to heterocyclic natural product systems

Part I describes the successful utilization of the 1,3-dipolar cycloaddition reaction of an activated thiazolium ylide to create highly stereospecific tricyclic systems which can be modified to produce substituted pyrrolidines and (DELTA)(\u271)- or (DELTA)(\u272)-pyrrolines. The key steps are the cycloaddition reaction to activated olefins and cleavage of the products by either silver ions or tri-n-butyltin hydride followed by acid;Part II describes the successful modification of one of the pyrrolidines (from Part I) into a biologically interesting natural product: (alpha)-allokainic acid. Two of the three asymmetric centers are correctly set by the 1,3-dipolar addition; the last step in the synthesis involves an epimerization with base at an elevated temperature to obtain the correct stereochemistry of the natural product;Part III describes the additions of the carbene thermally generated from two 1,2-benzoquinonediazides followed by in situ rearrangements to give 1,2-dihydrobenzofurans. Two tricyclic systems are rapidly and elegantly created which are major subunits of compounds of the highly biologically active aflatoxin and versicolorin family