Heterocycle Construction: The Chang Synthesis of Louisianin C
It has been known for some time that an acid chloride 1 can be added to an alkyne 2 to give the β-chloro enone. Yasushi Tsuji of Kyoto University found (J. Am. Chem. Soc. 2009, 131, 6668. ) that with an Ir catalyst, the condensation of 1 with 2 could be directed to the furan 3. Huanfeng Jiang of the South China University of Technology described (Org. Lett. 2009, 11, 1931. ) a complementary route to furans, Cu-mediated condensation of a propargyl alcohol 4 with the diester 5 to give 6.
Bruce A. Arndtsen of McGill University developed (Org. Lett. 2009, 11, 1369. ) an approach to pyrroles such as 9, by condensation of an α,β-unsaturated α-cyano imine 7 with the acid chloride 8. Thomas J. J. Müller of Heinrich-Heine-Universität Düsseldorf observed (Org. Lett. 2009, 11, 2269. ) the condensation of an acid chloride 11 with a propargyl amine 10, leading to the iodo pyrrole 12.
John A. Murphy of the University of Strathclyde uncovered (Tetrahedron Lett. 2009, 50, 3290. ) a new entry to the Fischer indole synthesis, by Petasis homologation of a hydrazide 13. Dali Yin of Peking Union Medical College took advantage (Org. Lett. 2009, 11, 637. ) of the easy sequential displacement of the fluorides of 15, leading, after acid-catalyzed cyclization, to the indole 17. Kang Zhao of Tianjin University extended (Org. Lett. 2009, 11, 2417, ; Org. Lett. 2009, 11, 2643, ) his studies of oxidation of an enamine 18 to the 2H-azirine, that on heating cyclized to the indole 19. Peter Wipf of the University of Pittsburgh established (Chem. Commun. 2009, 104. ) a microwave-promoted indole synthesis, illustrated by the intramolecular Diels-Alder cyclization of 20 to 21. A review delineating all nine types of indole syntheses will appear shortly in Angewandte Chemie.
Fushun Liang and Qun Liu of Northeast Normal University demonstrated (J. Org. Chem. 2009, 74, 899. ) that the readily-prepared ketene thioacetal 22 condensed with NH3 to give the pyridine 23. Sundaresan Prabhakar and Ana M. Lobo of the New University of Lisbon observed (Tetrahedron Lett. 2009, 50, 3446. ) that the addition of the alkoxy propargyl amine to the alkyne 25 gave a Z alkene, that on warming rearranged to the pyridine 26. Yan-Guang Wang of Zhejiang University, Hangzhou developed (J. Org. Chem. 2009, 74, 903. ) a pyridine synthesis based on the Wolff rearrangement of a diazo ketone such as 28. The iminophosphorane derived from 27 added to the resulting ketene, leading, after by electrocyclic rearrangement, to the pyridine 29.
A great deal of work has been done on the selective metalation of pyridines. Ching-Yao Chang of Asia University, Taichung used this (Tetrahedron 2009, 65, 748, ); for another report on pyridine metalation, see Tetrahedron Lett. 2009, 50, 1768, ) to advantage in developing a synthetic route to the Streptomyces-derived Louisianin alkaloids. Lithiation of 4-cyanopyridine was followed by bromination. The product was again lithiated, then iodinated to give 30. A selective Heck reaction on 30 gave 32, that was carried on to Louisianin C (33).