Abstract
Spirocyclic scaffolds are embedded in many biologically active natural compounds, including candidates for medicines, perfumes and agricultural chemicals. Therefore, a development of efficient synthetic methods directed at those structures is strongly demanded. To this end, we have recently developed synthetic methodologies for two different spirocyclic frameworks, spiro[4.5]decanes and spirocyclic oxindoles, based on Claisen rearrangement. We have developed the Claisen rearrangement protocol by which bicyclic 2-(alkenyl) dihydropy-rans with functionality at C 4 can be transformed to spiro[4.5]decanes in good-to-excellent yields with excellent stereoselectivities. We applied this method to a concise total synthesis of several biologically active spirocyclic sesquiterpenes. Related Claisen rearrangement in (alkenyl)pyranoindole systems can be also achieved. Thus, a one-pot intramolecular Ullmann coupling/Claisen rearrangement sequence from 2-iodoindoles was found to provide spirocyclic oxindoles in good yields with excellent stereoselectivities. We applied this sequence to the synthesis of pyrrolidinoindoline alkaloids.
