C-O Ring Construction: Karrinkinolide (Hosokawa), Sanctis B (Liu/Tan), Caesalpinnone A (Zheng), Cochlearol A (Ishigami), Alstoscholarisine E (Martin), Aranorosinol A (Xu)
Karrinkinolide (2), isolated from plant smoke, has potent seed-germination activity. Seijiro Hosokawa of Waseda University cyclized the phosphonate derived from 1 to give an intermediate that was then cyclized with acid to the pyran lactone 2 (J. Org. Chem. 2020, 85, 3936. DOI: 10.1021/acs.joc.9b03195).
Sanctis B (6) was isolated from the lichen Parmotrema sancti-angelii. Hongxin Liu of the Guangdong Institute of Microbiology and Haibo Tan of the South China Botanical Garden cyclized 3 with acid, then condensed that intermediate with orcinol (4) to give the bis-dihydrobenzopyran 5 (Org. Lett. 2020, 22, 934. DOI: 10.1021/acs.orglett.9b04486).
Caesalpinnone A (10), isolated from the Chinese tree Caesalpinnea enneaphylla, shows significant cytotoxicity. Huaiji Zheng of the Northwest A&F University showed that on deallylation with 8 the cyclohexadienone 7 cyclized to 9, that could be carried on to 10 (Org. Lett. 2020, 22, 520. DOI: 10.1021/acs.orglett.9b04276).
Cochlearol A (13), isolated from Ganoderma cochlear, a mushroom used in Chinese medicine, inhibits the fibrogenic activity of TGF-β1. Ken Ishigami of the Tokyo University of Agriculture assembled the bridged lactone 12 by the acid catalyzed cyclization of the aldehyde 11 (Tetrahedron Lett. 2020, 61, 151845. DOI: 10.1016/j.tetlet.2020.151845).
The indole alkaloid alstoscholarisine E (16), isolated from the devil's tree Alstonia scholaris, promotes neural stem cell proliferation. En route to 16, Stephen F. Martin of the University of Texas achieved high diastereoselectivity in the acid catalyzed cyclization of the lactam 14 to 15 (Org. Lett. 2020, 22, 786. DOI: 10.1021/acs.orglett.9b04093).
Tao Xu of the Ocean University of China prepared 17 via the oxidative dearomatization of Cbz-protected tyrosine. Hydroxyl-directed bis epoxidation delivered 18, that was carried on to aronorosinol A (19) (Chem. Sci. 2020, 11, 656; DOI: 10.1039/C9SC05252F, J. Org. Chem. 2020, 85, 4335; DOI: 10.1021/acs.joc.0c00028).