Intramolecular Diels–Alder reactions of oxazole–olefins: synthesis of the Rauwolfia alkaloids suaveoline and norsuaveoline

A full account of the highly stereoselective total synthesis of two indole alkaloids, suaveoline (4) and norsuaveoline (5), is presented. Central features of the synthetic strategy include the conversion of l-tryptophan methyl ester (12) into the oxazole derivative 11 and the intramolecular Diels–Alder reaction of the oxazole–olefin 19 leading to the pentacyclic pyridine derivative 21.