Other Methods for Carbocyclic Construction: Aspidodispermine (Heretsch), Lepadiformine A (Tokuyama), Farnesin (Gao), Ganocin B (Zhao), Dankasterone B (Heretsch), Canataxpropellane (Gaich)
The Ramberg-Bäcklund alkene construction is usually carried out on sulfones. In the course of a synthesis of aspidodispermine (3), Philipp Heretsch of the Freie Universität Berlin showed that the preparation of the cyclobutene 2 was better carried out starting with the sulfoxide 1 (Org. Lett. 2020, 22, 3956. DOI: 10.1021/acs.orglett.0c01242).
Lepadiformine A (6), isolated from the marine tunicate Clavelina lepidaformis, is a cardiac K+-channel blocker. A key step in the route to 6 devised by Hidetoshi Tokuyama of Tohoku University was the reductive radical cyclization of 4 to 5 (Org. Lett. 2020, 22, 3313. DOI: 10.1021/acs.orglett.0c00474).
Farnesin (9) was isolated from the seeds of Acacia farnesiana. En route to 9, Shuanhu Gao of East China Normal University prepared the key cyclopentanone 8 by the photochemical Nazarov cyclization of 7 (Angew. Chem. Int. Ed. 2020, 59, 7444. DOI: 10.1002/anie.202001350).
Ganocin B (12) was isolated from the fruiting bodies of Ganoderma cochlear, a famous medicinal mushroom of China. Yu-Ming Zhao of Shaanxi Normal University assembled almost the entire structure of 12 by the cyclization of 10 to 11 (Angew. Chem. Int. Ed. 2020, 59, 7419. DOI: 10.1002/anie.202000677).
Dankasterone B (15), isolated from the sponge-derived fungus Gymnascella dankaliensis, has a massively rearranged steroid skeleton. Professor Heretsch replicated this rearrangement, oxidizing 13, then reducing the α-iodo ketone so produced to deliver 14 (J. Am. Chem. Soc. 2020, 142, 104. DOI: 10.1021/jacs.9b12899).
Canataxpropellane (18), isolated from Taxus canadensis, has a particularly challenging carbon skeleton. Tanja Gaich of the University of Konstanz assembled 18 via the photochemical cyclization of 16 to 17 (Science 2020, 367, 676. DOI: 10.1126/science.aay9173).
We note with regret the passing of our friend and colleague Victor Snieckus, 1937-2020.