Other Methods for Carbocyclic Construction: Isoscopariusin A (Li/Puno), Pleosporol A (Xu), Altemicidin (Maimone), Dysideanone B (Lu), Stephadiamine (Nagasawa), Shikoccin (Ding)
Léon Ghosez demonstrated that [2+2] cycloaddition was particularly efficient with keteniminum salts. Ang Li of the Shanghai Institute of Organic Chemistry and Pema-Tenzin Puno of the Kunmin Institute of Botany extended this to the dehydrative addition of the amide 2 to the congested alkene 1, leading, after hydrolysis, to the cyclobutanone 3, that they carried on to isoscopariusin A (4) (Angew. Chem. Int. Ed. 2021, 60, 12859. ).
Pleosporol A (7), isolated from the marine fungus Pleospora sp, showed marked antimicrobial activity. Jinzhong Xu of Zhejiang University, Zhoushan assembled the highly substituted cyclohexane core 6 by the combination of the lactol 5 with N-methylhydroxylamine followed by heating (Tetrahedron 2021, 81, 131913. ).
Thomas J. Maimone of the University of California, Berkeley also used intramolecular dipolar cycloaddition to construct the carbocyclic core of altemicidin 10. Heating of the oxime 8 followed by direct methanolysis yielded the crystalline nitrile 9 (J. Am. Chem. Soc. 2021, 143, 7935. ).
Dysideanone B (13), isolated from the South China Sea sponge Dysidea avara, showed marked cytotoxicity. In the course of a synthesis of 13, Zhaoyong Lu of Nankai University observed high diastereoselectivity in the radical cyclization of the bromoarene 11 to the ketone 12 (Angew. Chem. Int. Ed. 2021, 60, 13807. ).
Stephadiamine (16) was isolated from the Stephania japonica, the snake vine of Southeast Asia. En route to 16, Minami Odagi and Kazuo Nagasawa of the Tokyo University of Science and Technology achieved high diastereoselectivity in the oxidative cyclization of the secondary silyl ether 14 to the cyclohexadienone 15 (J. Am. Chem. Soc. 2021, 143, 2699. ).
Shikoccin (19) was isolated from the Chinese medicinal herb Rabdosia shikokiana. Hanfeng Ding of Zhejiang University constructed the tetracyclic intermediate 18 by the oxidative electrolysis of the alkynyl arene 17 (Angew. Chem. Int. Ed. 2021, 60, 14892. ).