C-O Natural Products: Leupyrrin B₁ (Menche), Polycavernoside A (Sasaki), Intricenyne (Suh), Toxicodenane A (Ito), Orientalol F (Gong/Yang), Phorbaketal A (Tong)

Dirk Menche of the Universität Bonn achieved (Chem. Eur. J. 2017, 23, 3300. DOI: 10.1002/chem.201604445) high regioselectivity in the intramolecular cyclozirconation/bromination of the diyne 1 to give 2. This set the stage for the synthesis of leupyrrin B₁ (3).

Polycavernoside A (7) is the lethal toxin of the red alga Gracilaria edulis. Makoto Sasaki of Tohoku University established (J. Org. Chem. 2017, 82, 13204. DOI: 10.1021/acs.joc.7b02293) the central core of 7 by the hetero Diels-Alder cycloaddition of 4 with 5 to give 6.

The construction of medium rings is a continuing challenge. Young-Ger Suh of Seoul National University found (Org. Lett. 2017, 19, 6642. DOI: 10.1021/acs.orglett.7b03370) that Pd-catalyzed cyclization of 8 to 9 proceeded efficiently, setting the stage for the synthesis of intricenne (10).

Toxicodenane A (13) was isolated from the resin of the Chinese lacquer tree Toxicodendron vernicifluum, a traditional Chinese medicine. Hisanaka Ito of the Tokyo University of Pharmacy and Life Sciences observed (Eur. J. Org. Chem. 2017, 6693. DOI: 10.1002/ejoc.201701219) that the pendant pivalate of 11 played a critical role in directing the cylization to 12.

For the synthesis of orientalol F (16), it was necessary to assemble both the seven-membered ring and the bridging ether. Jianxian Gong of the Peking University Shenzhen Graduate School and Zhen Yang of Peking University accomplished (Org. Chem. Front. 2017, 4, 2296. DOI: 10.1039/C7QO00654C) both tasks in a single step, the gold-catalyzed hydration and rearrangement of 14 to 15.

Phorbaketal A (19), isolated from the marine sponge Phorbas sp., stimulates osteoblast differentiation. Rongbiao Tong of the Hong Kong University of Science and Technology set (Angew. Chem. Int. Ed. 2017, 56, 9096. DOI: 10.1002/anie.201704628 ) four new stereocenters in the selective epoxidation and spirocyclization of 17 to 18.