Convenient preparations of azo-dye labeled amino acids and amines

Article abstract of DOI:10.1039/b802846j

N-(4-Arylazobenzoyl)-1H-benzotriazoles 3 react with amino acids 4 and amines 6 to give azo-dye labeled amino acids (5a-m) and amines (7a-n) in high yields (74-100%) with retention of chirality.

Full text of DOI:10.1039/b802846j

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Convenient preparations of azo-dye labeled amino acids and amines†
Alan Roy Katritzky,* Qi-Yin Chen and Srinivasa Rao Tala
Received 19th February 2008, Accepted 26th March 2008
First published as an Advance Article on the web 6th May 2008
DOI: 10.1039/b802846j
N-(4-Arylazobenzoyl)-1H-benzotriazoles 3 react with amino acids 4 and amines 6 to give azo-dye
labeled amino acids (5a–m) and amines (7a–n) in high yields (74–100%) with retention of chirality.
Introduction
Results and discussion
Azobenzenes and their photochemistry are receiving increasing
attention.1 Azo-dye carboxylic acids have been widely used in
the materials and life sciences fields as molecular switches based
on their photoisomerization and FRET (fluorescence resonance
energy transfer):1c,d (i) to construct photoresponsive reporters
to monitor, regulate or control the activity of enzymes, with
potential as inhibitors for proteases;2 (ii) as quenchers to la-
bel oligonucleotides and phosphoramidites for the construc-
tion of molecular beacons or probes to detect specific nucleic
acids, DNA or glycoconjugates/saccaride;3 (iii) as fluorescence
quenchers/reporters of various biological reactions;4 (iv) in
the photoregulation of duplex formation of DNA and ODNs
(oligodeoxynucleotides);5 and (v) as photoisomerization units in
photoresponsive biomaterials.6
Connecting azo-dye carboxylic acids to host molecules is a key
step in the synthesis of azo-photoresponsive systems. In the synthe-
sis of photobiological switches or bioprobes, amino acids/peptides
or amines are common linkages between azo-dye acyl groups and
host molecules. Many azo-photoresponsive systems incorporate
azo-dye labeled a(x)-amino acids/peptides,2a,h,4a,f,5a,6a–e or x-amino
alcohols/amines.3a–j,4d,5b–e
Published synthetic methods to link azo-dye carboxylic acids
to bio-moieties (Scheme 1) have used (i) coupling reagents
such as DCC, EDCI, HOBT, HBTU, HATU;2f ,3f ,i,5b,c,e,7 (ii) acyl
chlorides;3a,j,8a–c and (iii) other activated azo-dye carboxylic acid
intermediates, including N-hydroxysuccinimidoester,3h,4b,c 4-[(4-
dimethylamino)phenylazo]benzoyl-1H-imidazole.4d
Utilization of these methods has encountered com-
plex procedures,2f ,3a,4b,c,7 harsh reaction conditions,4b,c,7 low
yields4c,5c,7,8b,c and difficulties in product purification.4b,c,f Thus mild
and efficient methods to label amino acids/peptides and amines
with azo-dye carboxylic acids are desired.
1. Preparation of N-(4-arylazobenzoyl)-1H-benzotriazoles (3)
N -[[4-(p-Dimethylaminophenylazo)]benzoyl]-benzotriazole 3a
and N-(4-phenylazobenzoyl)-benzotriazole 3b were prepared by a
standard method (Scheme 2).9c,d Treatment of 4-(arylazo)benzoic
acids 1 with 1-(methylsulfonyl)-1H-benzotriazole 29c,d under reflux
for 5 h gave products N-(4-arylazobenzoyl)-1H-benzotriazole 3a–
b in 85–86% yield.
2. Preparation of azo-dye carboxylic acid labeled amino acid
derivatives
Azo-dye carboxylic acid labeled amino acid derivatives were
obtained in high yields by treating N-(4-arylazobenzoyl)-1H-
benzotriazole 3 with appropriate amino acids in DMF–H2O (3 :
1, v/v) mixture in the presence of triethylamine (Scheme 3). These
reactions were completed within 24 h at room temperature (mon-
itored by TLC). Novel products were characterized by 1H-NMR,
13C-NMR and elemental analysis. In the preparation and purifi-
cation of 5a–m, the mild conditions retained the original chirality
of the amino acids and amines, as confirmed by optical rotation
and HPLC experiments. The results are summarized in Table 1.
From the chiral starting materials 4a–g and 4j,k, the corresponding
products 5a–g and 5j–m had the optical rotation stated; and all
showed single peaks in HPLC analysis. In the case of racemic
compounds 5h and 5i, the measured optical rotation was zero and
two peaks of equal intensity were observed in HPLC analysis.
For example, a single peak was obtained for the enantiomer 4-[4-
(dimethylamino)phenylazo]benzoyl-L-phenylalaine 5c at 3.04 min;
but the racemic mixture 4-[4-(dimethylamino)phenylazo]benzoyl-
DL-phenylalaine 5h gave two peaks at 3.04 and 3.37 min.
3. Preparation of azo-dye carboxylic acid labeled amine
derivatives
N-Acylbenzotriazoles are advantageous for N-, O-, C-, S-
acylation,9 especially where the corresponding acid chlorides
are unstable or difficult to prepare.10a,b We now report useful
reactions of amino acids and amines with N-(4-arylazobenzoyl)-
1H-benzotriazoles 3a and 3b.
Azo-dye labeled amine derivatives were also obtained in high
yields by treating N-(4-arylazobenzoyl)-1H-benzotriazole 3 with
appropriate amines. Optimum conditions for the reaction of 3
with amines 6 (Scheme 4) differ according to the type of amine
used, as shown in Table 2. Compound 3 and the amine 6 were
mixed in the appropriate solvent (dry THF or DMF) and stirred
at room temperature or with heating. Most of the products 7
were easily purified by washing with MeOH; others were obtained
by chromatography on silica gel; all the novel products were
characterized by 1H-NMR, 13C-NMR and elemental analysis. The
Center for Heterocyclic Compounds, Department of Chemistry, Univer-
sity of Florida, Gainesville, FL 32611-7200, USA. E-mail: katritzky@
chem.ufl.edu; Fax: +1 352-392-9199; Tel: +1 352-392-0554
† Electronic supplementary information (ESI) available: General & exper-
imental procedures with all spectral data. See DOI: 10.1039/b802846j
2400 | Org. Biomol. Chem., 2008, 6, 2400–2404
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Scheme 1 Literature methods of labeling amines and amino acids with azo-dye carboxylic acids.
Scheme 2
Scheme 3
Table 1 Preparation of azo-dye carboxylic acid labeled amino acids
Entry
3
Amino acid 4
5, yield (%)
5, mp/◦C
5, retention time/ming
5 [a]2D5
1
2
3
4
5
6
7
8
9
10
11
12
13
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
Glycine 4a
5aa, 99
5b, 86
5c, 82
5d, 90
5e, 87
5f, 81
5g, 88
5h, 82
5ib, 88
5jc, 95
5kd, 86
5le, 95
5mf, 87
238–240
205–207
202–203
220–222
230–235
205–207
205–207
203–204
189–191
208–210
120–122
185–186
222–224f
3.34
3.04
3.42
3.31
3.42
3.52
3.04,
3.37h
2.97,
3.56h
—
—
L-Alanine 4b
+73.3
+119.4
+112.1
+52.2
+31.1
+60.0
0.0
L-Phenylalanine 4c
L-Tryptophan 4d
L-Isoleucine 4e
L-Methionine 4f
L-Serine 4g
DL-Phenylalanine 4h
DL-Phenylglycine 4i
6-Aminocaproic acid 4j
L-Leucine 4k
0.0
—
+9.0
+103.0
+39.3
L-Phenylalanine 4c
L-Alanine 4b
3.19
3.23
a Lit.,4c,11 mp 232–233 ◦C, yield 43%. b Lit.,12 mp 188–190 ◦C. c Lit.,4b yield 77%. d Lit.,2f ,8c yield 95%, mp 173 ◦C. e Lit.,7 mp 183–184 ◦C, yield 65%. f Lit.,8c
mp 220 ◦C, yield 24%. g Single peak unless otherwise stated. h Two peaks of equal intensity.
results are summarized in Table 2, the products 7f, 7g from the
chiral starting materials 6f, 6g had certain optical rotation, and
showed a single peak in HPLC analysis. In contrast, for racemic
compound 7n, the optical rotation value was zero and two peaks
of equal intensity were observed in HPLC analysis.
4. Utilities and advantages of our method over previous methods
In comparison with literature data, our method comprises:
(1) mild and simple reaction and work-up conditions: we iso-
lated most of the products under milder reaction conditions
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Table 2 Preparation of azo-dye carboxylic acid labeled amino acid esters
Entry
3
Amine 6
7, yield (%)
7, mp/◦C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
Morpholine 6a
7a, 100
7b, 93
7c, 85
7da, 94
7eb, 93
7fc, 88
7g, 91
7h, 86
7i, 74
212–215
212–214
228–230
176–178
150–152
222–224
156–158
263–265
185–187
190–192
204–206
192–193
168–169
131–132
n-Butylamine 6b
t-Butylamine 6c
2-(Ethylamino)-ethanol 6d
6-Amino-1-hexanol 6e
L-a-Methylbenzylamine 6f
L-Valine methyl ester hydrochloride 6g
p-Toluidine 6h
N-Methylaniline 6i
2-Aminopyridine 6j
Carbazole 6k
n-Benzylamine 6l
m-Toluidine 6m
DL-Valine methyl ester hydrochloride 6n
7j, 84
7k, 81
7ld, 91
7me, 86
7nf, 91
a Lit.,3i yield 98%. b Lit.,3a yield 80%. c Lit.,13 yield 70%. d Lit.,8b mp 194–194.5 ◦C, yield 50%. e Lit.,8b mp 168.5–170.0 ◦C, yield 42%. f Lit.,14 mp130–131 ◦C,
yield 56%.
Scheme 4
and just by washing or chromatography on silica gel, whereas
reported methods gave side-products (for example, DCU in DCC
Experimental
method,5c phenylazobenzic acids in acyl chloride method,8b HOSu
in NHS method4b,c) and also they needed complex purification
procedures2f ,3a,i,7,8b,c,11,13,14 (entries 1, 11–13 in Table 1 and entry 4,
5, 9, 12–14 in Table 2); (2) high yields: most of our products were
obtained in high yield (74–100%); whereas some old reported
methods7,8b,c,11,14 provided low yields for some compounds (for
example, entries 1, 12, 13 in Table 1 (yield range 24–43%) and entry
12, 13, 14 in Table 2 (yield range 42–56%)); (3) no racemization
occurred for chiral compounds in our method, but in reported
methods some coupling reagents reduced the ee value of products
in the coupling method7; (4) very high selectivity to amine over
alcohol groups, which is important for the synthesis of some
probes.3a (for example, entry 7 in Table 1 and entries 4, 5 in Table 2);
(5) cost effective: our methodology is cost effective because Bt–
H is cheaper than coupling reagents, such as HBTU, HATU,
EDCI.
General Methods
Melting points were determined on Fisher melting apparatus. 1H-
NMR (300 MHz) and 13C-NMR (75 MHz) spectra were recorded
on a 300 MHz NMR spectrometer in CDCl3 or DMSO-d6. HPLC
analyses were performed on Beckman system gold programmable
solvent module 126, using Chirobiotic T column (4.6 × 250 mm),
detection at 254 nm, flow rate of 1.0 mL min−1 and MeOH as
eluting solvent. Elemental analyses were performed on a Carlo
Erba-1106 instrument. Optical rotation values were measured with
the use of sodium D line. Arylazobenzoic acids, amino acids,
and amines were purchased from Fisher or Aldrich chemical
companies.
General procedure for preparation of N-(4-arylazo)benzoyl-
benzotriazoles (3a,b). Arylazobenzoic acid 1 (7.43 mmol, 2.0 g
for 1a, 1.68 g for 1b), 1-(methylsulfonyl)-1H-benzotriazole 29d
(1.46 g, 7.43 mmol) and Et3N (1.5 mL, 10.4 mmol) were mixed
in THF at room temperature. After refluxing for 5h, the reaction
mixture was cooled to room temperature and kept overnight at
room temperature, then solid was precipitated. After filtration
and drying under vacuum, the corresponding product, N-(4-
arylazo)benzoyl-benzotriazoles 3a–b, were obtained.
Conclusion
In conclusion, a convenient and an efficient method for the
preparation of azo-dye labeled amino acids and amines has been
developed by reacting N-(4-arylazobenzoyl)-1H-benzotriazole
with amino acids and amines. All the azo-dye labeled products
were obtained under mild and simple reaction conditions in
high yields with no detectable racemization for chiral com-
pounds. For substrates having amine and alcohol functionality,
this methodology has shown high selectivity for amines to
alcohol.
N-[[4-(p-Dimethylaminophenylazo)]benzoyl]-benzotriazole (3a).
(2.24 g, 81%). Red microcrystal; mp 210.0–212.0 ◦C (from THF);
(found: C, 68.27; H, 4.77; N, 22.59. Calc. for C21H18N6O: C, 68.09;
H, 4.90; N, 22.69%); dH(300 MHz; CDCl3) 8.42 (1H, d, J 8.4,
ArH), 8.38 (2H, d, J 8.7, ArH), 8.19 (1H, d, J 8.1, ArH), 7.97
(4H, dd, J1 = J2 8.4, ArH), 7.72 (1H, t, J 7.2, ArH), 7.56 (1H,
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t, J 8.4, ArH), 6.77 (2H, d, J 9.0, ArH) and 3.13 (6H, s, 2 ×
NCH3,); dC(75 MHz, CDCl3) 170.0, 166.3, 156.4, 153.2, 145.9,
143.9, 133.1, 132.6, 131.1, 130.5, 126.5, 125.8, 122.1, 120.3, 115.0,
111.6 and 40.4.
172.8, 167.0, 155.4, 152.9, 143.7, 134.2, 128.1, 125.6, 122.4, 111.6,
57.6, 52.4, 40.4, 31.8, 19.2 and 18.2.
General procedure for the preparation of 7i–k. Procedure C.
N-[[4-(p-Dimethylaminophenylazo)]benzoyl]-benzotriazole (3a)
(200 mg, 0.54 mmol) and corresponding amine (2.0–3.0 eq.), Et3N
(0.23mL, 1.62 mmol) were mixed in DMF (10mL). The mixture
was heated at 150 ◦C for 5–10 h. After the evaporation of solvent
under reduced pressure, the residue was worked up with MeOH
and the products were obtained as red solids.
General method for the preparation of carboxylic azo-dye labeled
amino acids 5a–m. N-(4-Arylazo)benzoyl-benzotriazole
3
(200 mg, 0.54 mmol for 3a and 0.611 mmol for 3b, 1eq.) and
amino acid 4 (1eq.) were added to a mixture of DMF and water
(3 : 1,v/v), and stirred at room temperature for 24 h. After the
evaporation of solvent, washed with CH2Cl2 and drying under
vacuum, the corresponding pure products 5 were obtained with
high yield of 81–99%; For 5k–m, after the evaporation of solvent,
the residue was dissolved in CH2Cl2 and washed with 4 N HCl.
4-[4-(Dimethylamino)phenylazo]benzoyl-N-methylaniline (7i).
3 eq. 6i; worked up with MeOH to give 7i as red microcrystal
(140 mg, 74%); mp 185.0–187.0 ◦C (from MeOH); found: C,
73.38; H, 6.35; N, 15.71. Calc. for C22H22N4O: C, 73.72; H, 6.19;
N, 15.63%; dH(300 MHz; DMSO-d6; Me4Si) 7.75 (2H, d, J 7.5,
ArH), 7.57 (2H, d, J 6.9, ArH), 7.38 (2H, d, J 7.8, ArH), 7.27
(2H, d, J 6.6, ArH), 7. 20 (3H, d, J 6.3, ArH), 6.82 (2H, d, J
7.8, ArH), 3.40 (3H, s, CONCH3) and 3.06 (6H, s, 2 × NCH3);
dC(75 MHz; DMSO-d6; Me4Si) 168.9, 152.7, 152.4, 144.4, 142.6,
136.8, 129.3, 129.1, 127.1, 126.5, 125.0, 121.0, 111.5, 39.8 and
37.8.
4-[(4-Dimethylamino)phenylazo]benzoyl-glycine (5a). (175 mg,
99%). Redmi◦crocrystal;mp 238.0–240.0 ◦C (from CH2Cl2) (lit.,4b,10
mp 232–233 C); dH(300 MHz, DMSO-d6; Me4Si) 8.94 (1H, t, J
5.7, NH), 8.02 (2H, d, J 8.7, ArH), 7.84 (2H, d, J 8.7, ArH), 7.83
(2H, d, J 9.3, ArH), 6.85 (2H, d, J 9.3, ArH), 3.95 (2H, d, J 6.0,
CH2) and 3.08 (6H, s, 2 × CH3); dC(75 MHz, DMSO-d6) 171.4,
165.9, 154.1, 152.9, 142.7, 134.1, 128.4, 125.1, 121.6, 111.6, 41.4
and 39.8.
Gerenal procedure for the preparation of 7a–f. Procedure A.
N-[[4-(p-Dimethylaminophenylazo)]benzoyl]-benzotriazole (3)
(200 mg, 0.54 mmol for 3a and 0.611 mmol for 3b) and
corresponding amines (1–3eq.) were mixed in THF (10mL) and
stirred for 1–48 h at room temperature (monitored by TLC). After
the evaporation of solvent, pure products 7 were obtained from
the residues after simple purification procedures with high yield
of 85–100%.
Acknowledgements
We thank Dr C. D. Hall for helpful discussions.
Notes and references
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4-[4-(Dimethylamino)phenylazo]benzoyl-morpholine (7a). 1eq.
6a; purified with column on silica gel eluting with ethyl acetate–
hexane (1 : 2, v/v) to give 7a as red microcrystal (184 mg, 100%);
mp 212.0–215.0 ◦C (from EtOAc–hexane); found: C, 67.57; H,
6.66; N, 16.57. Calc. for C19H22N4O2: C, 67.44; H, 6.55; N, 15.56%;
dH(300 MHz, CDCl3) 7.88 (4H, t, J 8.1, ArH), 7.52 (2H, d, J 8.1,
ArH), 6.76 (2H, d, J 9.0, ArH), 3.85–3.40 (8H, br s, CH2CH2) and
3.11 (6H, s, 2 × CH3); dC(75 MHz, CDCl3) 170.1, 154.0, 152.8,
143.6, 135.6, 128.1, 125.4, 122.3, 111.5, 66.9, 40.3 and 29.8.
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The preparation of 7g, 7h. Procedure B. N-[[4-(p-Dimethyl-
aminophenylazo)]benzoyl]-benzotriazole
(3a)
(200
mg,
0.54 mmol) was added to the solution of amine (1–3eq.)
and Et3N (3.0 eq.) in THF (10 ml) at room temperature. The
mixture was heated under reflux for 24 h. After filtration, the
solvent was evaporated under reduced pressure. Pure products
were obtained from the residues after simple purification
procedures.
4-[4-(Dimethylamino)phenylazo]benzoyl-L-valine methyl ester
(7g). 1.0eq. 6g; purified with column on silica gel eluting with
ethyl acetate–hexanes (1 : 3, v/v) to give 7g as red microcrystal
◦
(188 mg, 90%); mp 156.0–158.0 C (from EtOAc–hexane); [a]D25
+59.6 (c 2.8 in MeOH); retention time: 3.29; found: C, 66.05; H,
6.94; N, 14.37. Calc. for C21H26N4O3: C, 69.95; H, 6.85; N, 14.65%;
dH(300 MHz, CDCl3) 7.90 (6H, m, ArH), 6.76 (2H, d, J 9.0, ArH),
6.67 (1H, d, J 8.4, NH), 4.83–4.79 (1H, m, J 5.1, 3.6, NHCH),
3.79 (3H, s, OCH3), 3.12 (6H, s, 2 × NCH3), 2.33–2.27 (1H, m,
CHCH3) and 1.02 (6H, t, J 6.6, 2 × CHCH3); dC(75 MHz, CDCl3)
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2404 | Org. Biomol. Chem., 2008, 6, 2400–2404
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