Synthesis and fluorescence of xanthone amino acids

Article abstract of DOI:10.1016/j.tetlet.2013.06.113

Fmoc- and Boc-protected α-amino acids bearing xanthone as side chain are easily accessible by palladium-catalyzed cross-coupling of a xanthone triflate with appropriate amino acid residues. The xanthone triflate was obtained in three steps taking advantag

Full text of DOI:10.1016/j.tetlet.2013.06.113

Tetrahedron Letters 54 (2013) 4585–4587
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Tetrahedron Letters
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Synthesis and fluorescence of xanthone amino acids
b
a
a,
Christian Hoppmann a, , Ulrike Alexiev , Elisabeth Irran , Karola Rück-Braun
⇑
⇑
a Technische Universität Berlin, Institut für Chemie, Strasse des 17. Juni 135, D-10623 Berlin, Germany
b Freie Universität Berlin, Department of Physics, Arnimallee 14, D-14195 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Fmoc- and Boc-protected
a-amino acids bearing xanthone as side chain are easily accessible by
Received 6 May 2013
Revised 18 June 2013
Accepted 22 June 2013
Available online 1 July 2013
palladium-catalyzed cross-coupling of a xanthone triflate with appropriate amino acid residues. The
xanthone triflate was obtained in three steps taking advantage of a high-yield and mild copper-catalyzed
diarylether formation. The new xanthone amino acid (Xan-aa) shows typical xanthone fluorescence
properties suitable for studying folding processes of polypeptides by triplet–triplet energy transfer
(TTET).
Keywords:
Xanthone
Ó 2013 Elsevier Ltd. All rights reserved.
Amino acids
Fluorescence
Cross-coupling
Photo-reactive unnatural amino acids (Uaas) embedded into
biomolecules (e.g., proteins or peptides) allow imaging a variety
of biological processes like protein–protein interactions, protein
localization, or folding mechanisms.1 Besides fluorescence trip-
let–triplet energy transfer (TTET) from a xanthone moiety (Xan)
to a naphthylalanine (NAla) allows folding mechanisms in poly-
peptides to be elucidated on an ultrafast timescale. Based on a
two-electron exchange reaction requiring the direct contact be-
tween the donor- and acceptor molecule TTET occurs on a ps time-
scale with a strong distance dependence that allows measuring
absolute rate constants for loop formation reactions slower than
5 ps.2
So far, for TTET studies the xanthone moiety has been attached
at the N-terminal part or at appropriate side chains of polypeptides
by means of xanthone carboxylic acid. In order to incorporate the
xanthone directly into a putative peptide or protein domain with-
out additional post synthesis modifications and protecting groups,
we have designed either Boc- or Fmoc-protected xanthone amino
acids (Xan-aa) amenable to peptide synthesis.
The coupling of protected iodoalanine derivatives 3a/b with aryl
halides to obtain non-natural amino acid building blocks was first
reported by Jackson and co-workers and has been applied to sev-
eral other examples successfully.3,4 However, because preparation
of bromoxanthone requires harsh reaction conditions and lacks
regioselectivity, we synthesized the xanthone triflate 2 for cross-
coupling reaction with 3. Although easily accessible and exten-
sively used in palladium catalyzed cross-coupling reactions only
a
few reports describe coupling of 3a with aryl triflates
(ArOSO2CF3).5,6
The xanthone triflate 2 was prepared starting from a mild
copper(I)-catalyzed coupling of 2-bromo-benzoic acid 4 with 1.5
equiv of 4-methoxyphenol 5 in the presence of 1.6–2.0 equiv of
Cs2CO3 yielding the diarylether 2-(4-methoxyphenoxy) benzoic
acid 6 in very good yields (89–91%) (Table 1). In comparison to
classical Ullmann reaction protocols7 using 2-chloro-benzoic acid
the procedure presented requires only small amounts of copper
catalyst and mild temperatures of 90–115 °C. As shown in Table
1, Buchwald’s Cu(OTf)2ꢀbenzene complex8 (2.5 mol %) is extremely
effective while using CuI resulted in only moderate yields (entry 1).
When working at a larger scale (ꢁ40 mmol) only 1 mol % of
copper catalyst was used without significant loss in yield (entry
4). Using 4-dimethylaminopyridine (4-DMAP) and Cu(OTf)2ꢀben-
zene as catalyst-ligand system gained the diarylether 6 in high
yields (ꢁ90%, entries 3 and 4) while N,N-dimethylglycine9 as ligand
resulted in lower yields (max. 66%, entries 1 and 2). The precursor
2-hydroxyxanthone 7 of the xanthone triflate 2 was obtained in
one step by cyclization of diarylether 6 and simultaneous cleavage
of the methyl ether in very good overall yield (64%) treating 6 with
a tenfold excess of concentrated sulfuric acid.10 Subsequently, the
obtained hydroxyxanthone 7 was treated with 1.1 equiv of triflic
Here, we describe the synthesis and the fluorescence properties
of novel xanthone a-amino acids (Xan-aa). Either Fmoc- or Boc-pro-
tected xanthone amino acid methyl esters 1 were synthesized by
palladium-catalyzed coupling of the xanthone triflate 2 with N-pro-
tected amino acid methyl esters 3 (Scheme 1). Following cleavage of
the methyl esters was realized using subtilisin Carlsberg or acidic
conditions, respectively.
⇑
Corresponding authors at present address: The Salk Institute for Biological
Studies, 10010 N. Torrey Pines Road, La Jolla, CA 92037-1099, USA.
E-mail addresses: choppmann@salk.edu (C. Hoppmann), karola.rueck-braun@tu-
berlin.de (K. Rück-Braun).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.113

4586
C. Hoppmann et al. / Tetrahedron Letters 54 (2013) 4585–4587
Scheme 1. Retrosynthesis of protected xanthone amino acid methyl esters 1.
Table 1
Synthesis of the diarylether 2-(4-methoxyphenoxy) benzoic acid 6
Entry
Copper source (mol %)
Ligand (mol %)
Solvent
T (°C)
Yielda (%)
1b
2c
3d
4e
CuI (10)
N, N-dimethylglycine (30)
N, N-dimethylglycine (30)
4-DMAP (2.5)
Dioxane
Toluene
Dioxane
Toluene
90
115
90
54
66
91
89
(CuOTf)2ꢀPhH (2.5)
(CuOTf)2ꢀPhH (2.5)
(CuOTf)2ꢀPhH (1.0)
4-DMAP (5.0)
115
a
b
c
Isolated yields.
Reaction was carried out with 1.25 mmol of 4 and 1.5 equiv of 5.
Reaction conditions: 1.00 mmol of 4.
Reaction conditions: 4.00 mmol of 4.
d
e
Reaction was carried out with 39.80 mmol of 4 and 1.6 equiv of Cs2CO3.
anhydride in the presence of 1.1 equiv of pyridine yielding the xan-
thone triflate 2 in 83% (Scheme 2).
In contrast to a previously described method5a using Pd2(dba)3
and P(o-tol)3 as catalyst-ligand system the addition of 3 equiv of
LiCl was not necessary when employing Pd[P(o-tol)3]2Cl2 as cata-
lytic species or Pd(PPh3)2Cl2 in a more recent report from Jackson
and co-workers.5c Furthermore, complete conversion of the start-
ing material during the coupling reaction requires an elevated tem-
perature of 60 °C.
The prepared xanthonetriflate 2 readily undergoes palladium-
catalyzed cross-coupling with 1.3 equiv of 3 forming the amino
acid 1 using 5.4 mol % Pd[P(o-tol)3]2Cl2 as catalyst. Thereby, the
amino acid derived organozinc reagent 8 was generated in situ
by insertion of activated zinc (Scheme 2).11
Scheme 2. Synthetic route for protected xanthone amino acid 1.
Scheme 3. Enzymatic cleavage of xanthone amino acid methyl ester 1a.

C. Hoppmann et al. / Tetrahedron Letters 54 (2013) 4585–4587
4587
Scheme 4. Cleavage of protected xanthone amino acid methyl ester 1b.
Supplementary data
Supplementary data (Experimental procedures and spectral
data for all compounds including absorbance and fluorescence
spectra of 9. Crystallographic data (excluding structure factors)
for the structures in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication,
CCDC deposition number: 935192.) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.06.113.
References and notes
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Figure 1. Fluorescence spectra of 9 in ethanol (blue line) and water (green line).
Finally, the N-Boc-protected 1a {½a D20
ꢂ
+30.00 (c 1.00, MeOH)}
was obtained in 40% from 2 while the N-Fmoc-protected amino acid
methyl ester 1b {½a D20
ꢂ
+60.00 (c 0.50, CH2Cl2)} was gained in 42%
yield. Yields obtained are in good agreement to previous reports.5a,c
The X-ray crystal structure of 1a confirms the expected planar
configuration of the xanthone skeleton (see graphical abstract
and Cambridge Crystallographic Data Centre, CCDC deposition
number: 935192).
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Carlsberg (25 wt %) from Bacillus licheniforis in phosphate buffer
(pH 7.5) and MeCN (Scheme 3).12 The resulting N-Boc-protected
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Perez-Gonzalez, M.; Jackson, R. F. W. Org. Synth. 2005, 81, 77–85; (c) Jackson, R.
xanthone amino acid 9 {½a D20
ꢂ
+40.00 (c 0.50, MeOH)} was obtained
ˇ
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in 84% yield.
Contrarily, enzymatic cleavage of Fmoc-protected 1b failed,
possibly due to the poor solubility of 1b in aqueous solution and
the steric hindrance of the Fmoc-group. Therefore, the methyl ester
of 1b was hydrolyzed under acidic conditions yielding the N-Fmoc-
protected xanthone amino acid 10 {½a D20
ꢂ
+13.33 (c 0.75, DMF)} in
62% (Scheme 4).
Fluorescence measurements (Fig. 1) of 9 in ethanol (blue line)
and water (green line) reveal xanthone-specific features: The fluo-
rescence of 9 differs by two orders of magnitude in both solvents.
The unusual fluorescence of xanthone has been previously
described in detail by Gilch and co-workers.13
In summary, we have described
a concise synthesis of
Fmoc- and Boc-protected xanthone -amino acids with defined
a
stereochemistry. We prepared a xanthone triflate at a large scale
followed by cross-coupling with the amino acid derived organozinc
reagents yielding enantiopure products. The prepared xanthone
amino acid (Xan-aa) may serve as probe to map folding mecha-
nisms of polypeptides by triplet–triplet-energy-transfer (TTET).
Acknowledgments
We gratefully acknowledge financial support by the Volkswa-
gen Stiftung. We thank Christian Klammt and Innokentiy
Maslennikov for help with NMR spectra.