C-O Ring Construction: Vancosamine (Gademann), Scleropentaside A (Schützenmeister), Monocerin (Ghosh), Obolactone (Fernandes), Caseabalansin A (Sugita), Rhizoxin D (Fürstner)

In the course of a synthesis of mangrolide D, Karl Gademann of the University of Zurich prepared the vancosamine donor 3. The α-quaternary azide of 2 was established by intramolecular conjugate addition of the imidate derived from 1, followed by oxidation (Org. Lett. 2019, 21, 3456. DOI: 10.1021/acs.orglett.9b01256).

Scleropentaside A (6) was isolated from the Southeast Asian evergreen tree Scleropyrum pentandrum, widely used in traditional medicine. Nina Schützenmeister of the Universität Hamburg assembled 6 by coupling the iodide derived from 4 with the anion 5 (Angew. Chem. Int. Ed. 2019, 58, 5110. DOI: 10.1002/anie.201900995).

Moncerin (10), isolated from the fungus Exserohilum monoceras, shows potent anti-malarial activity. Arun K. Ghosh of Purdue University uncovered the one-pot coupling of 8 with 7 and subsequent cyclization to 9 (J. Org. Chem. 2019, 84, 6191. DOI: 10.1021/acs.joc.9b00414).

Obolactone (13) was isolated from the Australian laurel Cryptocarya obovata. En route to 13, Rodney A. Fernandes of the Indian Institute of Technology Bombay cyclized 11 to the 2,3-dihydro-4H-pyran-4-one 12 (J. Org. Chem. 2019, 84, 3537. DOI: 10.1021/acs.joc.8b03141).

Caseabalansin A (16), isolated from the leaves and twigs of Casearia balansae, shows modest cytotoxic activity. A key step in the synthesis of 16 described by Kazuyuki Sugita of Hoshi University was the cyclization of the ester 14 to the lactone 15 (Chem. Asian J. 2019, 14, 2077. DOI: 10.1002/asia.201900368).

Rhizoxin D (19), the biosynthetic precursor to the clinically-interesting bis-epoxide rhizoxin (not illustrated), can also be converted into the latter by chemical transformation. In the course of a formal synthesis of 19, Alois Fürstner of the Max-Planck-Institut für Kohlenforschung showed that a Mo alkyne metathesis catalyst could convert the free diol 17 to the macrolactone 18 (Angew. Chem. Int. Ed. 2019, 58, 248. DOI: 10.1002/anie.201812096).