Intramolecular Diels-Alder Cycloaddition: 7-Isocyanoamphilecta-11(20),15-diene (Miyaoka), (-)-Scabronine G (Kanoh), Basiliolide B (Stoltz), Hirsutellone B (Uchiro), Echinopine A (Chen)

The amphilectane diterpenes, exemplified by 7-Isocyanoamphilecta-11(20),15-diene (3), have been little investigated. In the course of a synthesis of 3, Hiroaki Miyaoka of the Tokyo University of Pharmacy and Life Sciences took advantage (Synlett 2011, 547. DOI: 10.1055/s-0030-1259514) of the kinetic enolization and silylation of 1 to convert it into a trienone, that spontaneously cyclized to 2.

Scabronine G (6), isolated from the mushroom Sarcodon scabrosus, was found to enhance the secretion of neurotrophic factors from 1321N1 astrocytoma cells. To set the absolute configuration of the two quaternary centers that are 1,4 on the cyclohexane ring of 6, Naoki Kanoh and Yoshiharu Iwabuchi of Tohoku University cyclized (Org. Lett. 2011, 13, 2864. DOI: 10.1021/ol200873y) 4 to 5. Although described by the authors as a double Michael addition, this transformation has the same connectivity as an intramolecular Diels-Alder cycloaddition.

The diterpenes isolated from the genus Thapsia, represented by Basiliolide B (9), induce rapid mobilization of intracellular Ca²⁺ stores. Brian M. Stoltz of Caltech effected (Angew. Chem. Int. Ed. 2011, 50, 3688. DOI: 10.1002/anie.201008003) Claisen rearrangement of 7, to give an intermediate that cyclized to 8 as a mixture of diastereomers. A significant challenge in the synthesis was the assembly of the delicate enol ether/lactone of 9.

Hirsutellone B (12), isolated from Hirsutella nivea, shows significant antituberculosis activity. Hiromi Uchiro of the Tokyo University of Science found it useful (Org. Lett. 2011, 13, 6268. DOI: 10.1021/ol202748e) to protect the intermediate unsaturated keto ester by intermolecular cycloaddition with pentamethylcyclopentadiene before constructing the triene of 10. Simple thermolysis reversed the intermolecular addition, opening the way to intramolecular cycloaddition to give 11.

The tetracyclic ring system of the diterpene Echinopine A (15) represents a substantial synthetic challenge. David Y.-K. Chen of Seoul National University approached this problem (Org. Lett. 2011, 13, 5724. DOI: 10.1021/ol202053m) by Pd-mediated cyclization of 13 to the diene, that then underwent intramolecular Diels-Alder cycloaddition to give 14, with control of the relative configuration of two of the three ternary centers of 15. Double bond migration followed by oxidative cleavage of the resulting cyclohexenone then set the stage for the intramolecular cyclopropanation that completed the synthesis of 15.