Note pubs.acs.org/joc
Access to Quinolines through Gold-Catalyzed Intermolecular Cycloaddition of 2-Aminoaryl Carbonyls and Internal Alkynes Shuting Cai, Jing Zeng, Yaguang Bai, and Xue-Wei Liu* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 S Supporting Information *
ABSTRACT: A facile and general method leading to polyfunctionalized quinolines was developed. In the presence of a highly efficient combination encompassing (PPh)3AuCl and AgOTf, the reactions between 2-aminocarbonyls and an array of internal alkynes proceeded smoothly to afford quinoline derivatives in good to excellent yields (up to 93%).
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udging from the extensive reports surrounding quinolines,1 the generation of an efficient platform for their expedient synthesis has, indubitably, been of great interest for decades. Apart from their wide presence in various natural products (Figure 1) and pharmaceutical compounds, 2 quinoline
as they provide a convenient approach to the acquirement of cyclic compounds.10 Our group has been actively developing new synthetic routes to heterocycles through efficacious methodologies.11 One current example is the microwaveassisted three-component synthesis of pyrimidinones, catalyzed by copper(II) triflate.12 Inspired by the immense opportunities presented by alkyne moieties and our growing interests in the assembly of heterocycles, we envisioned the use of alkyne in the construction of an array of quinoline derivatives. Although there have been various reports on the synthesis of quinolines using alkynes,9,13 the strategies often employ terminal alkynes with a rather limited substrate applicability. Internal alkynes, in contrast to terminal alkynes, allow integration of more functional groups into quinolines, increasing their possible applications and employability. Encouraged by the seemingly sparse report on using internal alkynes to construct quinolines, we sought to develop a novel method for the reaction of internal ynoates report successful applications of a variety of 2-aminoaryl carbonyls and internal alkynes in the synthesis of quinoline derivatives. Preliminary studies were conducted with the aim to locate a suitable catalyst to promote the reaction between 2-aminobenzaldehyde 1a and ethyl 3-phenylpropiolate 2a. Ni(cod)2 was chosen as the initial experimental catalyst due to reports demonstrating nickel’s successful applications in such addition reactions.14 However, the catalyst failed to produce the desired product. On top of that, no reaction was observed for Sc(OTf)3 after heating at 100 °C for 24 h (Table 1, entries 1 and 2). Following the failure of preceding catalysts, attention was then directed to gold catalysts. Gold catalysts provide huge possibilities in organic reactions and ubiquitous reports have acknowledged their potentials, especially in cycloaddition reactions of alkyne as they preferentially coordinates to alkyne bonds.15 In addition, gold catalysts have also been reported to aid substantially to the synthesis of quinolines16 and isoquino-
Figure 1. Examples of quinoline-containing natural products.
derivatives have been reported to possess biological properties such as antibacterial,3a anti-inflammatory,3b and antimicrobial activities.3c Therefore, the benefits associated with them justify the continual search for strategies to develop their synthesis. Pioneering work on the construction of quinolines by Koenigs4 in 1879 drew the attention of researchers and prompted a widespread search to increase the preparative value as the initial method required harsh conditions.5 Among the methods described over the years, prominent conventional examples include Friedländer synthesis, Skarup reactions, as well as Doebner−Miller reactions.6 However, a recent upsurge of reports on synthesizing quinolines has demonstrated that new cycloadditions of suitable precursors through acid catalysis7 and novel metal-catalyzed coupling− cyclization reactions8 could compete and surpass conventional strategies in terms of celerity and efficacy. One prominent example is the recent work done by Li and co-workers, in which they reported an elegant work on the use of terminal ynones with aminoaryls to synthesize 3-carbonyl quinolines through Fe-catalyzed Michael addition−cycloaddition approach.9 Alkyne moieties have been ubiquitously employed in many organic syntheses. In particular, metal-catalyzed intermolecular alkyne reactions have been extremely popular in cycloadditions © 2011 American Chemical Society
Received: November 14, 2011 Published: December 12, 2011 801
dx.doi.org/10.1021/jo202281x | J. Org. Chem. 2012, 77, 801−807
The Journal of Organic Chemistry
Note
Table 1. Optimization of Quinoline 3a
entrya
catalyst
1 2 3 4 5b 6c 7 8 9 10c 11 12d 13e
Ni(cod)2 Sc(OTf)3 AuCl3 (Ph3P)AuCl RAuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl (Ph3P)AuCl
ligand/additive PPh3
AgOTf AgOTf AgNO3 Ag2CO3 AgOTf AgOTf AgOTf AgOTf AgOTf AgOTf
solvent
temp (°C)
time (h)
yield (%)
DMF DMF DMF DMF DMF DMF DMF ACN THF CH2Cl2 DMF DMF DMF
100 100 100 100 100 100 100 reflux 60 reflux 50 100 100
24 24 4 2 3 24 24 12 12 24 5 8 1
nr 30 88 72
