Functional Group Protection: The Kraus Synthesis of Bauhinoxepin J
Amos B. Smith III of the University of Pennsylvania found (Synlett 2009, 3131. ) that the advanced SAMP intermediate 1 could be deprotected to 2 without racemization under mild oxidative conditions. Akihiko Ouchi of the National Institute of Advanced Industrial Science and Technology, Tsukuba, showed (Org. Lett. 2009, 11, 4870. ) that the C-Te of 3 was easily oxidized to the aldehyde 4. Secondary C-Te bonds were converted to ketones. Asit K. Chakraborti of NIPER prepared (J. Org. Chem. 2009, 74, 5967. ) esters by warming an acid 5 with an alcohol 6 in the presence of acidic silica gel. Gilles Quéléver of Aix-Marseille Université established (Tetrahedron Lett. 2009, 50, 4346. ) that a cyanomethyl ester 8, readily prepared from the acid, efficiently exchanged with an alcohol 9 to give the ester 10.
Martin J. Lear of the National University of Singapore protected (Tetrahedron Lett. 2009, 50, 5267. ) an alcohol 11 as the p-methoxybenzyl ether 13 under mild conditions (AgOTf/DTBMP) with with the new reagent 12. Isao Kadota of Okayama University selectively (Tetrahedron Lett. 2009, 50, 4552. ) removed the primary PMB ether from the 14 to give 15. Hiromishi Fujioka of Osaka University, starting (Org. Lett. 2009, 11, 5138. ) from 16, was able to selectively prepare either the primary protected 18 or the secondary protected 19. In other developments, not pictured, Mattie S. M. Timmer and Brendan A. Burkett of the Victoria University of Wellington devised (Tetrahedron Lett. 2009, 50, 7199. ) a convenient preparation for azulene-containing α-keto esters. The distinctively-colored protecting group was conveniently removed in the presence of other esters by treatment with o-phenylenediamine. Scott D. Taylor of the University of Waterloo established (J. Org. Chem. 2009, 74, 9406. ) a robust protocol for converting alcohols to the corresponding protected sulfates.
P. Shanthan Rao of the Indian Institute of Chemical Technology, Hyderabad, showed (Tetrahedron Lett. 2009, 50, 7099. ) that an amine 20 was formylated by warming with formic acid in the presence of ZnCl2. The easily-hydrolyzed formamide 21 is readily converted to the corresponding isonitrile. Shiyue Fang of Michigan Technological University selectively (Tetrahedron Lett. 2009, 50, 5741. ) monoacylated the symmetrical diamine 22 using phenyl esters. Guy C. Lloyd-Jones and Kevin I. Booker-Milburn of the University of Bristol observed (Angew. Chem. Int. Ed. 2009, 48, 8721. ) that on gentle warming, the congested urea 24 released phenyl isocyanate. Rolf Breinbauer of the Graz University of Technology established (Angew. Chem. Int. Ed. 2009, 48, 5841. ) that polystyrene sulfonyl chloride is a robust and easily removable protecting group.
George A. Kraus of Iowa State University showed (Tetrahedron Lett. 2009, 50, 5303. ) that the phenol of 29 could be selectively protected by reaction with the bromoquinone 30. Although the protecting group could be removed, in this case the coupled product 31 was carried on to the antimalarial dibenz[bf]oxepin Bauhinoxepin J (32).