C-O Ring Construction: Triciadolide C (Wang), Nesteretal A (Ito), Toxicodenane (Han), Dehydro-exo-Brevicomin (Kanomata), Diocollettines A (Houk/Smith), Heliannuol K (Green)
The maleic anhydride derivative triciadolide C (3) was isolated from the aquatic hyphomycete Tricladium castaneicola. Shaozhong Wang of Nanjing University developed a general route to such anhydrides, exemplified by the gold-catalyzed cyclization of the propargyl pyruvate 1 to the lactone 2 (J. Org. Chem. 2021, 86, 15318. ).
Nesteretal A (6) was isolated from the coral-derived actinomycete Nesterenkonia habia. Hisanaka Ito of the Tokyo University of Pharmacy and Life Sciences set the absolute configuration of the triol 4 by Sharpless asymmetric dihydroxylation of an acyclic precursor. Acid-mediated intramolecular ketalization of 4 led to 5, that was carried on to 6 (Org. Lett. 2021, 23, 7074. ).
Toxicodenane (9) was isolated from the Chinese lacquer tree Toxicodendron vernicifluum, used in traditional Chinese medicine. En route to 9, Fu-She Han of the Chinese Academy of Sciences, Changchun devised the transacetalization/Prins cyclization of the acetal 7 to the tricyclic ether 8 (Org. Lett. 2021, 23, 8570. ).
Dehydro-exo-brevicomin (11) is a pheromone of the mouse, Mus musculus. Nobuhiro Kanomata of Waseda University also used Sharpless asymmetric dihydroxylation to set the absolute configuration of the diol 10, that on irradiation cyclized to 11 (Org. Biomol. Chem. 2021, 19, 6897. ).
Diocollettines A (14), isolated from the Chinese perennial vine Dioscorea collettii, showed modest anti-cancer activity. K. N. Houk of UCLA and Amos B. Smith III of the University of Pennsylvania also used irradiation to promote the coupling of the enone 12 with dihydroxyacetone 13, leading to 14 (J. Org. Chem. 2021, 86, 13583. ).
Heliannuol K (17) was isolated from the sunflower, Helianthus annuus. James R. Green of the University of Windsor showed that the Nicholas cyclization of 15 proceeded smoothly, to give the benzo[b]oxocane 16 (Org. Biomol. Chem. 2021, 19, 7152. ).
We note the passing of Tomáš Hudlický, who in addition to his many other conributions made practical the enzymatic oxidation of benzene derivatives to enantiomerically-pure carbocyclic starting materials.