Substituent effects in the transannular cyclizations of germacranes. Synthesis of 6-epi-costunolide and five natural steiractinolides

The occurrence and orientation of substituents in the 10-membered ring characteristic of germacranes is determinant in the stereochemical outcome of the acid-promoted transannular cyclization of these metabolites. An explanation of the synthesis of eudesmanolides and guaianolides produced by the Umbelliferae family of plants is advanced. The syntheses of the natural products 6-epi-costunolide and five steiractinolides are reported.