Functional Group Protection: The Pohl Synthesis of β-1,4-Mannuronate Oligomers
D. Srinivasa Reddy of the National Chemical Laboratory converted (Org. Lett. 2015, 17, 2090. DOI: 10.1021/acs.orglett.5b00637) the selenide 1 to the alkene 2 under ozonolysis conditions. Takamitsu Hosoya of the Tokyo Medical and Dental University found (Chem. Commun. 2015, 51, 8745. DOI: 10.1039/C5CC01784J) that even highly strained alkynes such as 4 can be generated from a sulfinyl vinyl triflate 3.
An alkyne can be protected as the dicobalt hexacarbonyl complex. Joe B. Gilroy and Mark S. Workentin of the University of Western Ontario found (Chem. Commun. 2015, 51, 6647. DOI: 10.1039/C5CC01522G) that following click chemistry on a non-protected distal alkyne, deprotection of 5 to 6 could be effected by exposure to TMNO. Stefan Bräse of the Karlsruhe Institute of Technology and Irina A. Balova of Saint Petersburg State University showed (J. Org. Chem. 2015, 80, 5546. DOI: 10.1021/acs.joc.5b00409) that the bend of the Co complex of 7 enabled ring-closing metathesis, leading after deprotection to 8.
Morten Meldal of the University of Copenhagen devised (Eur. J. Org. Chem. 2015, 1433. DOI: 10.1002/ejoc.201403643) 9, the base-labile protected form of the aldehyde 10. Nicholas Gathergood of Dublin City University and Stephen J. Connon of the University of Dublin developed (Eur. J. Org. Chem. 2015, 188. DOI: 10.1002/ejoc.201402947) a imidazolium catalyst for the exchange deprotection of dithiane 11 to 13, with the inexpensive aldehyde 12 as the acceptor. Peter J. Lindsay-Scott of Eli Lilly demonstrated (Org. Lett. 2015, 17, 476. DOI: 10.1021/ol503479g) that on exposure to KF, the isoxazole 14 unraveled to the nitrile 15. Masato Kitamura of Nagoya University observed (Tetrahedron 2015, 71, 6559. DOI: 10.1016/j.tet.2015.04.088) that the allyl ester of 16 could be removed to give 17, with the other alkene not affected.
Benzyl ethers are among the most common of alcohol protecting groups. Yongxiang Liu and Maosheng Cheng of Shenyang Pharmaceutical University showed (Adv. Synth. Catal. 2015, 357, 1029. DOI: 10.1002/adsc.201401097) that 18 could be converted to 19 simply by exposure to benzyl alcohol in the presence of a gold catalyst. Reko Leino of Åbo Akademi University developed (Synthesis 2015, 47, 1749. DOI: 10.1055/s-0034-1380155) an iron catalyst for the reductive benzylation of 20 to 21. Related results (not illustrated) were reported (Org. Lett. 2015, 17, 1778. DOI: 10.1021/acs.orglett.5b00553) by Chae S. Yi of Marquette University.
Hidetoshi Ohta and Yutaka Watanabe of Ehime University used (Tetrahedron Lett. 2015, 56, 2910. DOI: 10.1016/j.tetlet.2015.04.069) a nickel catalyst to prepare 23 by the silylation of 22. Qi Zhang and Yonghai Chai of Shaanxi Normal University desilylated (Synthesis 2015, 47, 55. DOI: 10.1055/s-0034-1379032) 24 to 25 using a gold catalyst.
There are many technical challenges to iterative oligosaccharide synthesis. Nicola L. B. Pohl of Indiana University developed (Org. Lett. 2015, 17, 2642. DOI: 10.1021/acs.orglett.5b01013) the perfluoroalkyl group of 26 to enable efficient machine-based assembly, as illustrated by the preparation of the β-1,5-Mannuronate six-mer 28 by repeated exposure to near-stoichiometric amounts of 27. The perfluoro group was removed by exchange metathesis, leading to an allyl ether that was reduced in the subsequent debenzylation step.