Tenth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-10), http://www.mdpi.net/ecsoc-10, November 1-30, 2006


[Cxxx]

Synthesis and Fluorescent Properties of 2-Amino Substituted 6,7-Dimethoxy-4-trifluoromethyl-quinolines

Wolfgang STADLBAUER *, Appasaheb B. AVHALE, Naresh S. BADGUJAR and Georg URAY

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Institut für Chemie, Organic Synthesis Group,
Karl-Franzens-University of Graz
Heinrichstrasse 28, A-8010 Graz (Austria)

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Received: 9th October 2006

Contents:

  1. Abstract
  2. Introduction
  3. N-2 Functionalized 2-Amino-4-trifluoromethylquinolines
  4. Synthesis of 2-Chloro Substituted Quinoline
  5. Synthesis of 2-Aminosubstituted Quinolines
  6. Synthesis of the Active OSu Ester
  7. Linking of the Fluorescent Heterocycle to Amino-Biomolecules and Amino-Biopolymers
  8. Fluorescence Properties and Conclusion
  9. Experimental
  10. Acknowledgement
  11. References

Abstract

2-Amino substituted 6,7-dimethoxy-4-trifluoromethyl-quinolines were synthesized from the 2-oxo derivatives by chlorination with phosphoryl chloride and aminated with 4-aminobenzoate to give a highly fluorescent molecule suitable for linking to appropriate biomolecules and biopolymers. The linking was achieved by conversion of the benzoate to reactive O-succinimide esters, which react easily with amino groups of biomolecules such as amino acids, aminosugars and the related biopolymers. The investigation of the fluorescent properties show pH independent properties and large Stokes shifts, however a rather low quantum yield.

Introduction
abstract scheme

Coumarins with push-pull systems (indicated with black and white arrows) are studied extensively as fluorescent dyes [e.g. 1] whereas quinoline derivatives are rarely described [e.g. 2].

N-2 Functionalized 2-Amino-4-trifluoromethylquinolines
formula scheme

Recently we described the synthesis of highly fluorescent push-pull substituted carbostyrils with donor substituents (dark blue arrows) and acceptor substituents (light blue arrows) [3] as fluorescent dyes, which have been found to have largely pH independent properties, large Stokes shifts and medium to high quantum yields.
We described the functionalization with O-succinimidyl (OSu) esters at N-1 and 6-O (green arrows) which allowed labelling of biopolymers such as peptides and carbohydrates [4] to give long-wavelength absorbing and emitting derivatives with potential applications in biochemistry and medicine.
In this presentation we report about the investigation on the synthesis and properties of 2-aminoquinolines as fluorescence labelling compounds (functionalization indicated by the pink arrow).

Synthesis of 2-Chloro Substituted Quinoline
formula scheme

The synthesis of the carbostyril ring system was achieved by cyclocondensation of dimethyoxaniline with trifluoromethyl-acetoacetate as described recently [3,4].

The conversion of the 2-oxo group to the reactive 2-chloro moiety could be performed by reaction with phosphoryl chloride.

Synthesis of 2-Aminosubstituted Quinolines
formula scheme

The conversion of the 2-chloro group to an amino group was achieved in a 3 step reaction by exchange of the chloro moiety against the azido group, reaction with triphenylphosphane in a Staudinger reaction to the corresponding phosphazene followed by acid catalyzed hydrolysis to the free amine. The free amine did not show sufficient fluorescent properties, so this way was not followed further.

Arylamino and benzylamino compounds, which were obtained from the chloro derivative in a one step reaction, showed excellent fluorescence properties (probably deriving from the tautomer imino structure). Using suitable derivatives such as 4-aminobenzoate (R = COOEt, n = 0) allowed the construction of linker groups attached at the position 2.

Synthesis of the Active O-Succinimide Esters
formula scheme

A reactive O-succinimide ester (OSu-ester) was synthesized by saponification of the ethyl ester group and re-esterification with N-hydroxy-succinimide in the presence of carbodiimide. This transformation gave the active OSu ester in good yields.

OSu-esters attached to heterocycles are known to be easily available, stable and useful to link to biological samples.

Linking of the Fluorescent Heterocycle to Amino-Biomolecules and Amino-Biopolymers
formula scheme

The OSu ester reacts smoothly under biological conditions with biomolecules such as aminoacids and aminosugars, and with biopolymers such as peptides, proteins or chitosan to give fluorescence marked molecules.

Fluorescence Properties and Conclusion
formula scheme diagram

Comparison of unlinked and linked derivatives show that there is no difference in the absorption and emission maxima after linkage (similar as recently observed with N-1 and 6-O linked derivatives). However, similar as observed with 6-O-derivatives: the fluorescence quantum yield, which is high for any N-1-linked analyte (0.3-0.5), decreased also in the case of the 2-amino analogues (similar to the findings of the 6-O derivatives) to 0.03 to 0.06, compared with quinine.

These findings show again, that only at N-1 high quantum yields are obtained, which are necessary for use in analytical tasks, whereas 6-O and 2-N linked derivatives gave similar low values.

Experimental

Synthesis of 2-aminoquinolines: Amination of 2-chloro-6,7-dimethoxy-4-trifluoromethyl-quinoline was performed by reaction with sodium azide in dimethylformamide at temperatures below 90 °C; the azide was transformed via a Staudinger reaction with triphenylphosphane to the phosphazene and then cleaved with diluted acetic acid to the amino compound. Reaction of 2-chloro-6,7-dimethoxy-4-trifluoromethyl-quinoline with anilines or benzylamines was performed at temperatures between 100 and 180 °C. The ester group of the benzoate derivative was hydrolyzed by heating in aqueous sodium hydroxide solution and the acid reacted with N-hydroxysuccinimde in the presence of carbodiimide at room temperature to give O-succinimidyl (OSu) esters. The labeling reactions were performed with aqueous solutions of amino acids, peptides, proteins, amino sugars in slightly basic medium.

UV/vis spectra: Shimadzu UV/VIS scanning spectrophotometer UV-2101 PC; concentration: 0.01 mg/mL.

Excitation and emission spectra: Shimadzu RF-5001 PC spectrofluorometer (150-W Xe lamp, 6 selectable slits: 1.5, 3, 5, 10, 15, 20 nm, R452-01 photomultiplier; monochromator: ion-blazed holographic concave grating F/2.5); concentration: 0.001 mg/mL.

Determination of quantum yields: Emission signals were set in relation to the known signal of quinine under the same conditions.

Acknowledgement

This work was supported by 1-year-scholarships of the Austrian Academic Exchange Service for A.B.A. and N.S.B.

References

[1] M.-S. Schiedel, C. A. Briehn, P. Bäuerle, Angew. Chem. Int. Ed., 40 (2001) 4677
[2] K. M. Solntsev, E. N. Sullivan, L. M. Tolbert, S. Ashkenazi, P. Leiderman, D. Huppert, J. Am. Chem. Soc., 128 (2004) 12701
[3] G. A. Strohmeier, W. M. F.Fabian, G. Uray, Helv. Chem. Act., 87 (2004) 215
[4] N. S. Badgujar, M. Pazicky, P. Traar, A. Terek, G. Uray, W. Stadlbauer, Eur. J. Org. Chem., (2006) 2715