C-O Ring Construction: Ophiofuranone A (Schobert), Galanthamine (Brown), Spirooliganone A (Morita), Microansamycin H (Sittihan), Gymnothelignan N (Lee), 3(11)-Epoxyhypoestenone (Wang/Chen)

Ophiofuranone A (3) was isolated from the endolichenic fungus Ophiospaerella korrae. Rainer Schobert of the University Bayreuth prepared the 3-furanone of 3 by the cyclization of the β-keto ester 1 to 2 (J. Org. Chem. 2022, 87, 6520. DOI: 10.1021/acs.joc.2c00521).

Galanthamine (6), isolated from the Caucasian snow drop Galanthus caucasicus, is used clinically for the treatment of the early stages of Alzheimer's disease. Richard C. D. Brown of the University of Southampton established the central dihydrobenzofuran of 6 by the intramolecular Heck cyclization of the cyclohexenyl ether 4 to 5 (J. Org. Chem. 2022, 87, 1325. DOI: 10.1021/acs.joc.1c02638).

Spirooliganone A (10), isolated from Illicium oligandrum, an anise of Southwest China, showed potent anti-viral activity. Masao Morita of ITSUU Laboratory constructed the pentacyclic cyclohexadienone 9, having nearly the full carbon skeleton of 10, by the hetero Diels-Alder combination of the enone 7 with commercial sabinene 8 (Synlett 2022, 33, 581. DOI: 10.1055/s-0040-1719904).

Microansamycin H (13) was isolated from the overexpression of the transcriptional regulator gene mas13 in the bacterium Micromonospora sp. En route to 13, Satapanawat Sittihan of the Chulabhorn Graduate Institute optimized the selective anti-Baldwin intramolecular opening of the Sharpless-derived epoxide 11, leading to the benzooxepane 12 (Org. Lett. 2022, 24, 4470. DOI: 10.1021/acs.orglett.2c01760).

Gymnothelignan N (17) was isolated from the rainbow plant Gymnotheca chinensis Decne, a widely used perennial Chinese herb. Kiyoun Lee of the Catholic University of Korea assembled the tetrahydrofuran ring of 16 by the addition of the aryl lithium 15 to the lactone 14, followed by diastereoselective reduction (J. Org. Chem. 2022, 87, 4316. DOI: 10.1021/acs.joc.1c03167).

3(11)-Epoxyhypoestenone (20), isolated from Hypoestes forsskaolii, a shrub of Arabia and tropical Africa that is extremely toxic to all livestock, showed strong Hsp90 binding affinity. Liang Wang and Yue Chen of Nankai University devised the tandem base-mediated epoxide opening-transannular oxa-Michael that transformed the enone 18 into the bridged ether 19 (Angew. Chem. Int. Ed. 2022, 61, e202117476. DOI: 10.1002/anie.202117476).