Functional Group Protection: The Xu Synthesis of Caldaphnidine J
Wei Han of Nanjing Normal University used oxidative conditions to protect the alcohol of 1 as the mixed acetal 2 (Synlett 2020, 31, 1400. DOI: 10.1055/s-0040-1707162). Alexei V. Demchenko of the University of Missouri, St. Louis devloped a protocol for removing the picoloyl ester from 3, leading to 4 (Org. Biomol. Chem. 2020, 18, 4863. DOI: 10.1039/D0OB00803F). Dawen Niu of Sichuan University demonstrated that with the addition of the appropriate additive, the triol 5 could be coupled with allyl acetate 6 to give each of the three possible monoallyl ethers, illustrated by 7 (Nature Commun. 2020, 11, 5681. DOI: 10.1038/s41467-020-19348-x). Chidambaram Gunanathan of the National Institute of Science Education and Research devised conditions for converting the methyl ether 8 selectively to the boronate ester 9, with loss of methane (ACS Catal. 2020, 10, 14390. DOI: 10.1021/acscatal.0c04269).
Kenward Vong and Katsunori Tanaka of RIKEN designed the 2-alkynylbenzamide 10, that could be deprotected to the amine 11 by exposure to a gold catalyst (Chem. Sci. 2020, 11, 10933. DOI: 10.1039/D0SC04329J). Paul Knochel of Ludwig-Maximilians-Universität München ozonized the N-alkyl pyrrole 12 to give the formamide 13 (Chem. Eur. J. 2020, 26, 8951. DOI: 10.1002/chem.202000870). Wubing Yao of Taizhou University reduced the amide 14 with HBpin, leading to the amine 15 (Org. Lett. 2020, 22, 8086. DOI: 10.1021/acs.orglett.0c03033). Klaus Banert of the Chemnitz University of Technology, who unfortunately passed away in 2020, and Paul R. Rablen of Swarthmore College found that the highly congested tertiary amine 16 underwent Hofmann-like elimination to the corresonding secondary amine 17 at only slightly elevated temperature (J. Org. Chem. 2020, 85, 13630. DOI: 10.1021/acs.joc.0c01790).
Mitsuru Kitamura of the Kyushu Institute of Technology used the α-diazoketone 19 to protect the aldehyde 18 as the acetal 20 (Eur. J. Org. Chem. 2020, 5319. DOI: 10.1002/ejoc.202000315). Shohei Hamada of the Kyoto Pharmaceutical University oxidized the primary p-methoxybenzyl ether 21 directly to the aldehyde 18 (Org. Lett. 2020, 22, 5486. DOI: 10.1021/acs.orglett.0c01839). Kenji Watanabe and Takamitsu Hosoya, also of RIKEN, used long wavelength visible light to convert the aryl ketone 22 to the carboxylic acid 23 (Org. Lett. 2020, 22, 5434. DOI: 10.1021/acs.orglett.0c01799). Naoki Ishida and Masahiro Murakami of Kyoto University also used visible light to mediate the oxidative coupling of the aldehyde 24 with the phenol 25, leading to the aryl ester 26 (Angew. Chem. Int. Ed. 2020, 59, 18267. DOI: 10.1002/anie.202008897).
Caldaphnidine J (29) is the first of the yuzurimine subgroup of the Daphniphyllum alkaloids to be prepared. In the course of this synthesis, Jing Xu of the Southern University of Science and Technology converted the α-sulfinyl dithiane 27 to the methyl ester 28 (Nature Commun. 2020, 11, 3538. DOI: 10.1038/s41467-020-17350-x).