A highly effective protocol for palladium-catalyzed selective arylation and heteroarylation of indolizines at C-3 has been developed. Mechanistic studies unambiguously support an electrophilic substitution pathway for this transformation.Indolizines substituted at C-3 are very attractive heterocyclic units, as a number of representatives of this class 1 and, especially their partially or completely reduced analogues, indolizidine alkaloids 2 and related unnatural compounds, 3 exhibit important biological properties. As a part of our program on developing efficient methods toward differently substituted indolizines, we were particularly interested in elaborating efficient approaches toward C-3-arylated indolizines. To date, no general convenient methods for synthesis of C-3-arylated indolizines exist. 4 Scattered reports on synthesis of C-3-arylated indolizines via various modes of Chichibabin-type cyclocondensation 5 are limited to particular substitution patterns 6 and generally provide low to moderate yields of the products. 7 Other methods such as dipolar cycloaddition of pyridinium ylides with alkynes, 8 cyclopropenones, 9 or cyclopropenes 10 are limited to use of symmetric substrates, producing indolizines with two identical substituents at the pyrrole ring. Finally, synthesis of indolizines via cycloisomerization of alkynyl pyridines, 11,12 a method recently developed within our group, potentially can give access to 3-arylindolizines; however, it would require employment of rather expensive 3-aryl-1-propynes and, in this case, is unlikely to be considered as synthetically useful. Thus, we decided to explore the possibility of direct, last-stage methods of arylation of the indolizine core. Although analogous reactions on various heterocyclic systems such as furans, 13 thiophenes, 13a,14 pyrroles, 15 and indoles 16 have long been known, no reports on arylations involving indolizines have been reported to date. 17 Herein we report efficient palladium-catalyzed arylation and heteroarylation of indolizines, which allows for selective incorporation of aromatic or heteroaromatic substituent at the C-3 position of this heterocycle.Initial experiments revealed that indolizine 1a indeed underwent arylation in the presence of different palladium sources in DMSO; however, a 3-fold excess with respect to arylbromide © 2004 American Chemical Society Correspondence to: Vladimir Gevorgyan, vlad@uic.edu. Supporting Information Available: Detailed experimental procedures for preparation of and spectroscopic data for compounds 3aa-ed and 10. This material is available free of charge via the Internet at http://pubs.acs.org.
NIH Public Access Author ManuscriptOrg Lett. Author manuscript; available in PMC 2013 July 11.
NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript was required to attain high yields, and the reaction was rather sluggish, as it took 2 days for completion. Optimization experiments 18 indicated that arylation of indolizines proceeded much faster (1-3 h) in the presence of cataly...
