C-O Ring Containing Natural Products: (+)-Isatisine A (Panek), Cephalasporolide E (Sartillo-Piscil), (+)-Xestodecalactone (Jennings), Colchilomycin B (Banwell), Lactimidomycin (Georg), 5,6-Dihydrocineromycin B (Fürstner)

(+)-Isatisine A (4) was isolated from Isatis indigotica, long used in traditional Asian medicine for the treatment of viral diseases. James S. Panek of Boston University set the stage (J. Org. Chem. 2015, 80, 2959. DOI: 10.1021/acs.joc.5b00051) for the synthesis of 4 by the addition of the allyl silane 2 to the aldehyde 1 to give the highly substituted tetrahydrofuran 3.

Fernando Sartillo-Piscil of Benemérita Universidad Autónoma de Puebla devised (J. Org. Chem. 2015, 80, 2601. DOI: 10.1021/jo502757c) an H-atom abstraction/fragmentation/cyclization cascade that converted 5 into the spiroketal Cephalosporolide E (7). Cephalosporolide F, the unstable kinetic product from the cyclization, could be observed in the NMR spectrum of the crude product when a base was added.

(+)-Xestodecalactone A (10) was isolated from the fungus Pencillium cf. montanese, that was secured from the marine sponge Xestospongia exigua. Michael P. Jennings of the University of Alabama constructed (Eur. J. Org. Chem. 2015, 3303. DOI: 10.1002/ejoc.201500287) the macrolactone of 10 by cyclizing the carboxylic acid 8 to 9 under Friedel-Crafts conditions.

Cochliomycin B (13), isolated (without acetone!) from the marine fungus Cochliobolus lunatus, is a naturally occuring acetonide. Martin G. Banwell of the Australian National University showed (J. Org. Chem. 2015, 80, 460. DOI: 10.1021/jo5024602) that the Nozaki-Hiyama-Kishi cyclization of 11 to 12 proceeded with high diastereoselectivity.

Lactimidomycin (16) binds to and so blocks the tRNA E-site of the 80S ribosome. Gunda I. Georg of the University of Minnesota assembled (Chem. Commun. 2015, 51, 8634. DOI: 10.1039/C5CC02571K) the macrolide triene of 16 by the cyclization of 14 to 15.

Alois Fürstner of the Max-Planck-Institut für Kohlenforschung, who developed effective catalysts for alkyne metathesis, has been exporing (Angew. Chem. Int. Ed. 2015, 54, 6241. DOI: 10.1002/anie.201501608) the conversion of the product cyclic alkynes to trisubstituted alkenes. In the synthesis of 5,6-Dihydrocineromycin B (19), he took advantage of the directing propargylic alcohol for the conversion of 17 to 18.