C-O Ring Construction: The Ito Synthesis of Callilongisin B
Young Ho Rhee of the Pohang University of Science and Technology assembled the dihydrofuran 3 by Pd-mediated coupling of the allene 1 with the keto acid 2, followed by ring-closing metathesis (Angew. Chem. Int. Ed. 2021, 60, 22166. ). Jack R. Norton of Columbia University used a Co catalyst followed by oxidation to convert the alkenyl borate ester 4 to the tetrahydrofuran 5 (Angew. Chem. Int. Ed. 2021, 60, 22678. ). Yu Du and Weiping Su of the Fujian Institute of Research on the Structure of Matter used a combination of Pd and Rh catalysis to construct the tetrahydrofuran 8 by the coupling of the enone 6 with the carbonate 7 (Chem. Eur. J. 2021, 27, 12742. ). Wei-Min Dai of the Hong Kong University of Science and Technology showed that the diol from Sharpless asymmetric dihydroxylation of the mesylate 9 selectively cyclized to the tetrahydrofuran 10 (Org. Chem. Front. 2021, 8, 6491. ).
Eric N. Jacobsen of Harvard University devised a thiourea catalyst that promoted enantioselective Prins cyclization of the aldehyde 11 to afford a mixture of the tetrahydropyran 12 and the corresponding tertiary chloride, the latter being converted to 12 in a subsequent elimination step (J. Am. Chem. Soc. 2021, 143, 20077. ). Mark S. Taylor of the University of Toronto showed that the photo-oxidation of the protected sugar 13 to the 3-ketone 14 proceeded via initial oxidation to the 2-ketone, followed by rearrangement (Chem. Commun. 2021, 57, 12135. ). Kazuhiko Sakaguchi of Osaka City University prepared the tetrahydropyranone 16 by combining the enantiomerically-pure allyl silane 15 with two equivalents of benzaldehyde (J. Org. Chem. 2021, 86, 11884. ). Wenjing Zhang of Zhengzhou University and Zhili Zuo and Liang-Liang Wang of the Kunming Institute of Botany used an enantiomerically-pure phosphoric acid to cyclize the dienyl alcohol 17 to the tricyclic spiroketal 18 (Nature Commun. 2021, 12, 7188. ).
Ilan Marek of Technion showed that Ir-mediated alkene migration of the diene epoxide 19 led, via Cope rearrangement, to the 4,5-dihydrooxepine 20 with high diastereoselectivity (Chem. Sci. 2021, 12, 9328. ). Jiarong Shi of Chongqing University and Yang Li of Jilin University used in situ-generated benzyne to cyclize the hydroxy acid 21 to the macrolactone 22 (Org. Lett. 2021, 23, 7274. ). In honor of the late Jiro Tsuji, it should be noted that he reported ring closing metathesis to form a macrolactone more than forty years ago (Tetrahedron Lett. 1980, 21, 2955. ).
Inés Pérez-Martín and Ernesto Suárez of the Instituto de Productos Naturales y Agrobiología del CSIC devised free radical conditions for cyclizing the N-alkoxyphthalimide 23 to the spiroketal 24 (J. Org. Chem. 2021, 86, 14508. ). Chuang-Chuang Li of the Southern University of Science and Technology assembled the tetracyclic ether 26 by the Rh-mediated [3+2] cycloaddition of the N-sulfonyltriazole 25 (Org. Lett. 2021, 23, 7771. ).
Callilongisin B (29), isolated from the Chinese flowering shrub Callicarpa longissima, showed significant cytotoxicity and anti-inflammatory activity. In the course of a synthesis of 29, Hisanaka Ito of the Tokyo University of Pharmacy and Life Sciences achieved high diastereoselectivity in the oxidative cyclization of the amide 27 to the lactone 28 (Org. Lett. 2021, 23, 6916. ).