Tandem Cu-catalyzed ketenimine formation and intramolecular nucleophile capture: Synthesis of 1,2-dihydro-2-iminoquinolines from 1-(o-acetamidophenyl) propargyl alcohols

Article abstract of DOI:10.3762/bjoc.10.125

The copper-catalyzed ketenimine formation reaction of 1-(o-acetamidophenyl) propargyl alcohols with various sulfonyl azides is found to undergo a concomitant intramolecular nucleophile attack to generate 1,2-dihydro-2- iminoquinolines after aromatization (via elimination of acetyl and hydroxy groups) and tautomerization. The reaction produces 4-substituted and 3,4-unsubstituted title compounds in moderate to good yields under mild reaction conditions.

Full text of DOI:10.3762/bjoc.10.125

Tandem Cu-catalyzed ketenimine formation and
intramolecular nucleophile capture: Synthesis of
1,2-dihydro-2-iminoquinolines from
1-(o-acetamidophenyl)propargyl alcohols
Gadi Ranjith Kumar‡1,2, Yalla Kiran Kumar‡2, Ruchir Kant3
and Maddi Sridhar Reddy*1,2,§
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1255–1260.
1Medicinal & Process Chemistry Division, CSIR-Central Drug
Research Institute, BS-10/1, Sector 10, Jankipuram extension,
Sitapur Road, P.O. Box 173, Lucknow 226031, India, 2Academy of
Scientific and Innovative Research, New Delhi 110001, India and
3Molecular & Structural Biology Division, CSIR-Central Drug
Research Institute,BS-10/1, Sector 10, Jankipuram extension, Sitapur
Road, P.O. Box 173, Lucknow 226031, India
doi:10.3762/bjoc.10.125
Received: 12 February 2014
Accepted: 30 April 2014
Published: 28 May 2014
Associate Editor: J. P. Wolfe
Email:
© 2014 Kumar et al; licensee Beilstein-Institut.
License and terms: see end of document.
Maddi Sridhar Reddy* - msreddy@cdri.res.in
* Corresponding author ‡ Equal contributors
§ Tel +91-7897582222.
Keywords:
alkyne; azide; cycloaddition; cyclization; quinoline
Abstract
The copper-catalyzed ketenimine formation reaction of 1-(o-acetamidophenyl)propargyl alcohols with various sulfonyl azides is
found to undergo a concomitant intramolecular nucleophile attack to generate 1,2-dihydro-2-iminoquinolines after aromatization
(via elimination of acetyl and hydroxy groups) and tautomerization. The reaction produces 4-substituted and 3,4-unsubstituted title
compounds in moderate to good yields under mild reaction conditions.
Introduction
The synthesis of N-sulfonylketenimines via CuAAC (copper- reaction for the synthesis of diverse chemical compounds by
catalyzed azide–alkyne cycloaddition) between terminal alkynes trapping the thus formed ketenimine inter- and intramolecularly.
and sulfonyl azides has staged for a conceptually novel class of Various multicomponent reactions using external nucleophiles
approaches for the synthesis of various nitrogen-containing like H2O, alcohols, amines, imines, etc., have been reported
cyclic and acyclic motifs of pharmaceutical and material inter- while very few recent methods [16-23], including ours [16],
ests [1-23]. After its discovery by Chang and co-workers [1-6], disclosed the utilization of internal nucleophiles for trapping the
various groups (including the discoverers) have utilized the ketenimines successfully. In continuation of our interest in the
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Beilstein J. Org. Chem. 2014, 10, 1255–1260.
functionalization of alkynes through electrophilic activation
[24-29], we herein report a synthesis of 1,2-dihydro-2-imino-
quinolines (tauromerized form of 2-sulfonylaminoquinolines)
from 1-(o-acetamidophenyl)propargyl alcohols via copper-
catalyzed ketenimine formation with various sulfonyl azides.
2-Aminoquinolines and their derivatives have been found to be
important constituents in pharmaceutical chemistry [30-42] and
also found to play a crucial role in molecular recognition
processes [43-46]. Due to its vast-spread importance, several
methods have been developed to construct this useful frame-
work. Apart from the classical Chichibabin reaction [47], there
Scheme 1: Synthesis of unsaturated amides via ketenimine formation.
has been considerable interest shown for the synthesis of these Results and Discussion
compounds with varying substitution patterns [48-64]. Most of Thus, we synthesized a substrate 3 (from 1 via 2a) through
the methods used presynthesized quinolines for the 2-amination Grignard reaction followed by acetylation (Scheme 2). Sub-
via either creating initially a leaving group and its subsequent strate 3 contains two nucleophiles in the form of acetyl and
substitution or via activation of the nitrogen atom and direct acetamide groups at equal distance. When we treated 3 with
amination at C-2 while few methods afforded the direct similar reaction conditions (TsN3, 10 mol % CuI, 1.2 equiv
2-aminoquinoline synthesis from the acyclic precursors. K2CO3, 0.1 M CH3CN) as used in the previous method, surpris-
ingly, only α,β-unsaturated amide 5 was obtained as a single
Most recently, we have disclosed a method for the synthesis of product through acetyl migration on the intermediate 4. No
E-α,β-unsaturated amides from propargyl acetates via copper trace of product through acetamide attack was obtained, which
catalyzed ketenimine formation with sulfonyl azides was probably due to the weaker nucleophilic nature of the latter
(Scheme 1) [16]. The reaction was thought to proceed through nucleophile. We then wanted to test the method on 2a which
intramolecular nucleophilic attack on the in situ generated contains a hydroxy group as a nucleophile for a relatively
ketenimine. We became curious to know the fate of the reac- strained 4-membered attack and the acetamide group for the
tion in the presence of another internal nucleophile in the same 6-membered attack. In our earlier study, the hydroxy group was
distance (6-membered) as a competitor.
found to be a less effective nucleophile compared to the acetyl
Scheme 2: Intramolecular nucleophilic capture by ketenimines.
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Beilstein J. Org. Chem. 2014, 10, 1255–1260.
group in such a reaction, as the earlier requires higher energy alkyne partner to produce 3,4-unsubstituted adducts. We herein
for its strained 4-membered cyclization. In light of this, we report the intramolecular version of Wang's protocol for the
speculated that the acetamide group might have more chances synthesis of differently substituted products.
for the cyclization in the tentative intermediate 6. As expected,
the reaction on 2a proceeded through a 6-membered cycliza- To explore the practicality of the above reaction, a series of
tion to form 7 which underwent aromatization via elimination starting materials were selected with the variation in substitu-
of acetic acid to produce 8. Product 8 exists in tautomerized tion on propargyl alcohols as well as sulfonyl azides. It was
form of 9 (vide infra, X-ray structure of 9j). In fact, an elegant found that the cascade process was applicable for a wide range
synthesis of such compounds was earlier achieved by Wang et of substrates providing the dihydroquinoline derivatives 9 in
al. through three component coupling reaction under similar yields ranging from 36% to 77% (Scheme 3). Initially, we
conditions [13]. The method was shown to be not working for screened 2-propargyl alcohols prepared from aminobenzalde-
the synthesis of 4-unsubstituted adducts as the 2-aminoben- hydes as they were unreactive in Wang's protocol. Thus, 2a was
zaldehyde was found to be unreactive in the given conditions treated with various sulfonyl azides. Trifluoromethanesulfonyl
and, moreover, the reaction is not possible with acetylene as the azides reacted similar to toluenesulfonyl azide to afford the
Scheme 3: Synthesis of 1,2-dihydro-2-tosyliminoquinolines. Reaction conditions: Sulfonyl azide (1.2 mmol), 2 (1.0 mmol), base (1.5 mmol),
CuI (0.1 mmol), DCE (5 mL, 0.2 M solution), 50 °C, open air. aIsolated yield.
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Beilstein J. Org. Chem. 2014, 10, 1255–1260.
products 9b and 9c in 71% and 55% yields. The lower yield in method is overall an intramolecular version of the Wang's
case of 9c can be attributed to the steric hindrance created by protocol for the synthesis of differently substituted 1,2-dihydro-
the bulky triflruoromethyl group at the ortho position. Benzene- 2-tosyliminoquinolines which cannot be synthesized by the
and methanesulfonyl azides produced the corresponding prod- latter method. The reaction was performed under mild condi-
ucts (9d and 9e) in somewhat lower yields (46% and 36%, res- tions to obtain the products in good yields and with a broad
pectively). Fluorinated 2o-propargyl alcohol 2b was then spectrum of substitution patterns.
reacted with various aryl sulfonyl azides to get the corres-
ponding products 9f–i in yields ranging from 45–50%.
Experimental
General information: All reagents and solvents were purchased
Next, 3o-propargyl alcohols prepared from aminoacetophe- from commercial sources and used without purification. NMR
nones and aminobenzophenones were subjected to the title reac- spectra were recorded with a 300 or 400 MHz spectrometer for
tion for the synthesis of 4-methyl/phenyl-1,2-dihydro-2-imino- 1H NMR, 75 or 100 MHz for 13C NMR spectroscopy. Chem-
quinolines. It was found that these 3o-propargyl alcohols were ical shifts are reported relative to tetramethylsilane in CDCl3 or
more productive compared to the above 2o-propargyl alcohols. to residual signals of undeuterasted solvent CDCl3/DMSO-d6
Thus 2c–f were subjected to the standard reaction conditions for 1H and 13C NMR spectroscopy. Multiplicities are reported
with various sulfonyl azides to get the corresponding 4-alkyl- as follows: singlet (s), doublet (d), broad singlet (bs), doublet of
substituted quinoline derivatives 9j-o in 48–77% yields. In doublets (dd), doublet of triplets (dt), triplet (t), quartet (q),
further exploration, 4-phenyl-substituted adducts 9p and 9q multiplet (m). HRMS spectra were recorded by using a QTof
were synthesized from 2g and 2h in 61% and 72% yields, res- mass spectrometer. Column chromatography was performed
pectively. Halogen groups (chloro and fluoro) on the core part with silica gel (100–200 mesh) as the stationary phase. All reac-
as well as the pendant phenyl ring survived well in the reaction tions were monitored by using TLC. The purity and characteri-
to produce the products (9r–t) in yields ranging from 55% to zation of compounds were further established by using HRMS.
69%. Electron rich phenyl substrate 2l produced the corres-
ponding product 9u in 65% yield while electron poor substrate General procedure for the synthesis of 9 from 2 taking syn-
2m was found to be unreactive in the reaction.
thesis of 9a as an example: To the substrate 2a (190 mg,
1 mmol) dissolved in dichloroethane (5 mL) was added tosyl
The structures of the products were confirmed by 1H NMR, azide (236 mg, 1.2 mmol), copper iodide (19 mg, 0.1 mmol),
13C NMR and High Resolution Mass Spectrometry (HRMS). and K2CO3 (207 mg, 1.5 mmol). The mixture was stirred at 50
X-ray crystallography of one of the compounds, 9j, gave unam- °C for 12 h. The reaction mixture was then added water (5 mL)
biguous structure confirmation (Figure 1). See also Supporting followed by brine solution (5 mL) and extracted with ethyl
Information File 1, pages S49–S57 and Supporting Information acetate (3 × 10 mL). The combined extracts were dried over
File 2.
Na2SO4, filtered and concentrated. The crude solid product was
purified by column chromatography (silicagel, 20–30% EtOAc
in hexanes) to get the pure product 9 (180 mg, 62% yield).
Supporting Information
Supporting Information File 1
Analytical data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-125-S1.pdf]
Supporting Information File 2
X-ray structure.
Figure 1: X-ray Structure of 9j (CCDC number 971729).
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-125-S2.cif]
Conclusion
We have described a method for the synthesis of 1,2-dihydro-2-
iminoquinolines, tautomerized products of 2-tosylaminoquino-
lines, from 1-(o-acetamidophenyl)propargyl alcohols through
copper-catalyzed ketenimine formation with various sulfonyl
azides and concomitant intramolecular nucleophilic attack. The
Acknowledgements
GRK and YKK thank CSIR for the PhD fellowships. We thank
SAIF division CSIR-CDRI for the analytical support. We thank
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Beilstein J. Org. Chem. 2014, 10, 1255–1260.
25.Reddy, M. S.; Kumar, Y. K.; Thirupathi, N. Org. Lett. 2012, 14,
824–827. doi:10.1021/ol2033493
Dr. Tejender S. Thakur of Molecular and Structural Biology
Division, CSIR-Central Drug Research Institute for supervising
the X-ray data collection and structure determination of 9j
reported in this paper. We gratefully acknowledge the financial
support by CSIR-Network project 'BSC0102' (CSIR-CDRI-
THUNDER) and by DST under the 'Fast Track Proposal
Scheme for Young Scientists'. CDRI Communication No. 8678.
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