Preparation of Heteroaromatics: The Movassaghi Synthesis of (+)-Chimonanthine
There has been a recent surge of enthusiasm for pyrrole synthesis. Stephen L. Buchwald of MIT has reported (Org. Lett. 2007, 9, 973. ) a clever application of his Cu-catalyzed N-vinylation chemistry, leading to hydrazides such as 4, which on heating rearrange to the pyrroles. Mitsuro Shindo of Kyushu University has expanded (Org. Lett. 2007, 9, 1963. ) his studies of ynolate chemistry to include addition to α-amido ketones such as 7, leading to the pyrrole 8. Jonathan T. Reeves of Boehringer Ingelheim, Connecticut has found (Org. Lett. 2007, 9, 1875. ) that on dehydration, aldol products such as 9 from formyl oxazole rearrange to the pyrrole. And, Jae Nyoung Kim of Chonnam National University, Gwangju has condensed (Tetrahedron Lett. 2007, 48, 4119. ) Baylis-Hillman adducts such as 11 with halo ketones to give pyrroles.
There has also been substantial interest in the synthesis of pyridines. C. Oliver Kappe of the Karl-Franzens-University, Graz has found (J. Org. Chem. 2007, 72, 4440. ) that thioamides such as 14 couple smoothly under Pd catalysis with areneboronic acids, even more rapidly than the usually reactive alkenyl bromide. Hans-Dieter Arndt of the Universitšt Dortmund has developed (J. Org. Chem. 2007, 72, 4205. ) a hetero Diels-Alder approach to pyridines, based on the addition of alkynes such as 16 to the diene 17. Richmond Sarpong of the University of California, Berkeley, has devised (Org. Lett. 2007, 9, 2167. ) an elegant Pt-catalyzed rearrangement of alkynyl aziridines such as 19, giving, after sulfinate elimination, pyridines such as 20.
A variety of approaches to indoles have been put forward. Koichi Narasaka, now at Nanyang Technical University, Singapore, has extended the utility of the aziridine approach to indoles, finding (Chemistry Lett. 2007, 36, 52. ) that the rearrangement is catalyzed by Rh2(tfa)4. Simple thermolysis of 21 led to a complex mixture that did not include 22. In a complementary approach, Akio Saito and Yuji Hanazawa of Showa Pharmaceutical University have found (Angew. Chem. Int. Ed. 2007, 46, 3931. ) a Rh catalyst that converts propargyl amines such as 23 to indoles such as 24. In a Diels-Alder route, Pedro Mancini of the Universidad Nacional del Litoral, Santa Fe, Argentina, has shown (Tetrahedron Lett. 2007, 48, 1435. ) that dienes add to nitro pyrroles such as 26 to give indoles.
Many indole-derived natural products, exemplified by (+)-chimonanthine (31), are dimers. Mohammad Movassaghi of MIT has devised (Angew. Chem. Int. Ed. 2007, 46, 3725. ) an effective route to such dimers, bromination followed by metal-mediated one-electron reduction.