C-O Ring Construction: Burseran (Tanaka), Hyperione A (Dai), Spirotenuipesine A (Imagawa), Stachyodin A (Ito), Alstilobanine C Zhu), Biselide A (Britton)

The sap from the elephant tree of northern Mexico, Bursera microphylla, was used locally as a cure-all for diseases, particularly those affecting the skin. Hiroshi Tanaka of the Tokyo University of Technology prepared burseran (2), isolated from the sap, by the direct cyclization of the acetonide 1 (Chem. Eur. J. 2021, 27, 9422. DOI: 10.1002/chem.202100804).

Hyperione A (4) was isolated from the Chinese medicinal herb Hypericum chinense. Mingji Dai of Purdue University assembled the cyclopropane 3 via the Kulinkovich reaction, then cyclized it directly to 4 (Chem. Sci. 2021, 12, 1311. DOI: 10.1039/D0SC05556E).

Spirotenuipesine A (7), isolated from the entomopathogenic fungus Paecilomyces tenuipes, promoted neurotrophic factor biosynthesis in glial cells. En route to 7, Hiroshi Imagawa of Tokushima Bunri University converted the epoxide 5 to the tricyclic bis ether 6 (Tetrahedron Lett. 2021, 64, 152723. DOI: 10.1016/j.tetlet.2020.152723).

Stachyodin A (9), isolated from the roots of the Chinese medicinal shrub Indigofera stachyode, showed anti-inflammatory activity. Hisanaka Ito of the Tokyo University of Pharmacy and Life Sciences cyclized the phenol 8 directly to 9 (Org. Lett. 2021, 23, 3864. DOI: 10.1021/acs.orglett.1c00998).

Alstilobanine C (12), isolated from the Malaysian tree Alstonia angustiloba, is a selective inhibitor of butyrylcholinesterase. Jieping Zhu of the Ecole Polytechnique Fédérale de Lausanne constructed the oxepane ring of 12 by the cyclization of 10 to 11 (Angew. Chem. Int. Ed. 2021, 60, 12392. DOI: 10.1002/anie.202103580).

Biselide A (15), isolated from the Okinawan ascidian Didemnidae sp., showed significant preferential toxicity toward cancer cell lines. Robert Britton of Simon Fraser University assembled 15 via the intramolecular Reformatsky cyclization of the bromacetate 13 to the macrolide 14 (Chem. Sci. 2021, 12, 5534. DOI: 10.1039/D0SC06223E).