Microwave-assisted synthesis of aryl amide bonds on solid phase

Article abstract of DOI:10.1080/00397910903370618

A novel microwave-assisted synthesis of a library of triarylamides has been undertaken on the solid-phase. Copyright Taylor & Francis Group, LLC.

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Microwave-Assisted Synthesis of Aryl
Amide Bonds on Solid Phase
Philip A. Helliwell a , Ruth E. Fake a , Kevin G. Kerr b , Barbara
Santry a , Austen Speakman a & Anne Routledge a
a Department of Chemistry , University of York , Heslington, York, UK
b Department of Microbiology , Harrogate District Hospital ,
Harrogate, UK
Published online: 13 Sep 2010.
To cite this article: Philip A. Helliwell , Ruth E. Fake , Kevin G. Kerr , Barbara Santry , Austen
Speakman & Anne Routledge (2010) Microwave-Assisted Synthesis of Aryl Amide Bonds on Solid Phase,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 40:20, 3058-3066, DOI: 10.1080/00397910903370618
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Synthetic Communications1, 40: 3058–3066, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903370618
MICROWAVE-ASSISTED SYNTHESIS OF ARYL AMIDE
BONDS ON SOLID PHASE
Philip A. Helliwell,1 Ruth E. Fake,1 Kevin G. Kerr,2
Barbara Santry,1 Austen Speakman,1 and Anne Routledge1
1Department of Chemistry, University of York, Heslington, York, UK
2Department of Microbiology, Harrogate District Hospital, Harrogate, UK
A novel microwave-assisted synthesis of a library of triarylamides has been undertaken on
the solid-phase.
Keywords: Arylamide; microwave; solid-phase synthesis
Compounds that promote cleavage of cellular DNA have been shown to moderate
cell proliferation and as such have the potential to be used as therapeutic agents.[1]
It had been demonstrated that simple triarylamide-bridged compounds (Fig. 1)
could induce strand scission in DNA. The enhanced cleavage observed at pH
6.5–7.3 implied that proton transfer from aniline initiated the cleavage.[2] To probe
this phenomenon, we proposed to synthesize a library of triarylamides containing a
variety of functional groups (X, Y) on solid phase (Fig. 2).
The triarylamide scaffold has been used extensively as a platform for the
solution-phase synthesis of mesogenic,[3] organogelating,[3] and dendrimeric[4] struc-
tures. The most common approach has utilized an amidation reaction between
an acid chloride and a 3,5-diaminobenzoic ester, giving a symmetrical triarylamide
scaffold. Our approach required a synthetic route that would allow us to rapidly
access both symmetrical and nonsymmetrical triarylamide structures without the
need to synthesize activated acid chlorides. The three nonsymmetrical triarylamide
structures previously synthesized by Warner et al. in the initial study into DNA
cleavage were furnished via sequential coupling (Scheme 1), which gave a moderate
overall yield (28–40%) of final product.[2]
Our aim was to access a library of substituted triarylamides by performing only
deprotection and coupling chemistry on the solid phase. This would be achieved by
immobilizing an orthogonally protected centroid scaffold onto hydroxymethyl poly-
styrene and deprotecting one amine with subsequent coupling of a suitably functio-
nalized carboxylic acid. The process would be repeated to furnish a resin-bound
triarylamide (Scheme 2).
Received August 3, 2009.
Address correspondence to Anne Routledge, Department of Chemistry, University of York,
Heslington, York YO10 5DD, UK. E-mail: ar30@york.ac.uk
3058

MICROWAVE-ASSISTED SYNTHESIS OF ARYL AMIDE BONDS
3059
Figure 1. Triarylamide-bridged compounds inducing DNA strand scission.
Figure 2. Proposed solid-phase library.
Scheme 1. (i) Fmoc-Cl, Na2CO3; (ii) cyclohexene, Pd-C; (iii) 3-NO2-C6H4COCl, pyridine; (iv) piperidine,
DMF; (v) PyBOP, DIPEA, DMF HOOC-Ar; (vi) piperidine, DMF.

3060
P. A. HELLIWELL ET AL.
Scheme 2. Synthesis of resin-bound triarylamide library.
RESULTS AND DISCUSSION
The orthogonally protected centroid aromatic scaffold 1 was synthesized
from commercially available 3-amino-5-nitrobenzoic acid via Fmoc protection to
give 2, with reduction to 3[5] and subsequent Boc protection (Scheme 3).
To test the robustness of the proposed synthetic methodology for library syn-
thesis, a simple symmetrical triarylamide 4 was the initial target molecule. Scaffold 1
was coupled to hydroxymethyl polystyrene (Advanced ChemTech, 1.1 mmol gꢀ1
)
using standard dicyclohexylcarbodiimide (DIC)-mediated coupling[6] (Scheme 4).
Scheme 3. (i) Fmoc chloroformate, Na2CO3, H2O=dioxane (1:1), 18 h; (ii) SnCl2 ꢁ 2H2O, EtOH, 70 ꢂC, 2 h;
and (iii) Boc anhydride, DIPEA, 4 h.

MICROWAVE-ASSISTED SYNTHESIS OF ARYL AMIDE BONDS
3061
Scheme 4. Coupling of centroid scaffold 1 to hydroxymethyl polystyrene.
The coupling reaction was judged to have gone to completion by fluorescent staining
of the derivatized beads 5 with N-methylisatoic anhydride.[7] A negative test indi-
cated complete consumption of the resin-bound hydroxyl moiety.
One amino group of the derivatized resin 5 was revealed using 20% piperidine
in dimethylformamide[8] to remove the Fmoc protecting group. Attempts to couple
the next building block (benzoic acid) using DIC-mediated coupling resulted in an
incomplete reaction even after prolonged reaction times and repeated couplings.
Alternative coupling reagents (HATU,[9] HBTU,[10] and EDCI[11]) failed to improve
the coupling efficiency.
It has been shown that p-nitrobenzenesulfonyl chloride could be used to
synthesize aromatic amides in solution via an activated mixed sulfonic–carboxylic
anhydride.[12] Using this approach, we synthesized in situ activated building block
6 (Scheme 5) in tetrahydrofuran (THF). (The in situ solutions could be scaled up
and stored at 0 ꢂC for 24 h.)
The THF solution containing 6 was added to deprotected resin 7 (Scheme 6);
the coupling was monitored by cleavage of a small portion of product resin 8 with
high-performance liquid-chromatographic (HPLC) analysis of the cleavage mixture.
The reaction went to completion after 18 h at ambient temperature. [Reacted resin
(ꢃ5 mg) was swelled in THF=MeOH (4:1, 0.5 ml), NaOMe (5 mg) was added,
and the reaction was agitated for 2 h. The resin was removed by filtration, and the
filtrate was concentrated under reduced pressure. The residue was filtered through
a plug of silica and eluted with ethyl acetate (5 ml); the ethyl acetate was removed
in vacuo for subsequent HPLC analysis.]
Scheme 5. In situ synthesis of activated building block 6.

3062
P. A. HELLIWELL ET AL.
Scheme 6. On-bead formation of biarylamide.
Microwave-assisted solid-phase synthesis (MASS) is a rapidly expanding area
in combinatorial synthesis[13] for both peptide[14] and small molecule[15] libraries. To
explore whether the formation of the biarylamide link could be accelerated,
the coupling was undertaken using microwave irradiation. Complete coupling could
be achieved in minutes.
After successful attachment of the first benzoic acid group, removal of the Boc
protecting group using 50% trifluoroacetic acid (TFA) in dichloromethane (DCM)[16]
and microwave-accelerated coupling with 6 was repeated to furnish the resin-bound
symmetrical triarylamide 9. The triarylamide was cleaved from the resin with excess
sodium methoxide in MeOH=THF (1:4) to give the methyl ester 4. With the chemical
methodology in hand, a small library of triarylamides was synthesized (Table 1).
Table 1. Library of triarylamides
Compound
Xa
Ya
Crude yield (%)b
Purity (%)c
Isolated yield (%)d
1
2
3
4
5
6
7
8
9
H
H
H
Cl
74
79
72
62
82
79
87
55
86
97
69
57
98
94
68
63
30
68
—
51
39
—
—
45
48
11
42
H
NO2
OCH3
OCH3
NO2
Cl
H
NO2
NO2
Cl
Cl
Cl
NO2
OCH3
apara-Substitution.
bMass recovery based on the loading of resin (1.1 mmol gꢀ1).
cHPLC analysis of the crude product at 280 nm UV detection.
dTriarylamides with >90% crude purity were not purified further. Other examples were purified using
PLC (2:1 ethyl acetate=petroleum ether, 40–60 ꢂC). Isolated yields were based on the loading of resin
(1.1 mmol gꢀ1).

MICROWAVE-ASSISTED SYNTHESIS OF ARYL AMIDE BONDS
3063
The purity of the cleaved triarylamides proved variable. Entries 1, 4, and 5
(Table 1) gave triarylamides with excellent purity, suitable for screening. Other
entries required further purification, but the overall isolated yields were comparable
to those obtained by Warner[2] but were obtained in less time.
In conclusion, we have demonstrated a novel microwave-assisted solid-phase
aryl amide synthesis that may find wide application in solid-phase library synthesis.
The methodology was exemplified with the synthesis of a small triarylamide library.
A larger library will be synthesized using the developed methodology, and
DNA cleavage will be explored. The results from this study will be reported in
due course.
EXPERIMENTAL
Solution-phase reagents were obtained from Aldrich Chemical Co., Fluka,
or Lancaster Chemicals Ltd. Resins were supplied from Advanced ChemTech.
All reagents were used as supplied unless otherwise stated. Prior to use all, resins were
allowed to stand in anhydrous DCM or THF for 1 h. After each synthetic step and
prior to cleavage, the solid support was exhaustively washed with a range of solvents
to remove reactants from the resin matrix. A typical washing procedure used DCM
(5 ꢄ 5 ml), DMF (5 ꢄ 5 ml), THF (5 ꢄ 5 ml), and DCM (5 ꢄ 5 ml). Resin washing
was carried out using a commercially available vacuum manifold for parallel washing
and product isolation. Solid-phase organic synthesis (SPOS) reactions were carried
out in capped filtration columns of various capacities (10–25 ml) or in sealed test
tubes. Reactions were agitated on a J-KEM Scientific orbital shaker (model 310).
Reactions utilizing microwave techniques took place in a CEM Discover LabMate
Microwave reactor in 10-ml CEM pressure vials. HPLC analysis was undertaken
on a Phenomenex Prodigy 5 ODS-2 column with a Gilson 321 pump, 170 DAD,
and Unipoint system software. Gradients were run using solvents (A) acetonitrile
and (B) 0.1% (v=v) TFA=water, with 280-nm UV detection.
3-Amino-5-(9H-fluoren-9-ylmethoxycarbonylamino) Benzoic Acid 3
3-Nitro-5-(9H-fluoren-9-ylmethoxycarbonylamino) benzoic acid (1.61 g,
4 mmol) and tin dichloride dihydrate (4.51 g, 20 mmol) were dissolved in ethanol
(60 ml) and heated under N2 to 70 ꢂC. After 2 h, the reaction mixture was poured
onto ice, and the pH was adjusted to 7 by addition of 5% NaHCO3. The reaction
was extracted with ethyl acetate (3 ꢄ 50 ml). The organic phase was washed with
brine and dried over MgSO4, and the solvent was removed in vacuo to yield 3 as
a yellow solid (1.20 g, 81%).
1H NMR dH (400 MHz, DMSO-d6): 9.62 (1H, s, NH), 7.91 (2H, d, J 8.0 Hz,
Ar-H), 7.76 (2H, d, J 8.0 Hz, Ar-H), 7.44 (2H, t, J 7.5 Hz, Ar-H), 7.36 (2H, t, J
7.5 Hz, Ar-H), 7.25 (1H, s, Ar-H) 7.02 (1H, s, Ar-H), 6.86 (1H, s, Ar-H), 5.31
(2H, br s, NH2), 4.42 (2H, d, J 7.0 Hz, CH2), 4.30 (1H, t, J 7.0 Hz, CH); 13C
NMR dC (100.6 MHz, DMSO-d6): 170.21 (COOH), 160.30 (NH-CO-O-R), 144.52
(ipso-Ar linked to COOH), 140.39 (ipso-Ar linked to NH-CO-O-R), 140.02 (ipso-Ar
linked to NH2), 136.79 (CH Ar), 134.61 (CH Ar), 128.51 (CH Ar), 127.88 (ipso-Ar of
Fmoc), 127.69 (ipso-Ar of Fmoc), 124.96 (CH Ar), 120.04 (CH Ar), 116.08 (CH Ar),

3064
P. A. HELLIWELL ET AL.
81.71 (CH2 Fmoc), 62.86 (CH Fmoc). HR-MS (ESI) C22H18N2O4Na requires
397.1164; found 397.1164.
3-tButoxycarbonylamino-5-(9H-fluoren-9-ylmethoxycarbonylamino)
Benzoic Acid 1
DIPEA (0.8 ml, 4.64 mmol) was added to 3 (870 mg, 2.32 mmol) in 1,4
dioxane (20 ml) and H2O (10 ml) followed by (Boc)2O (506 mg, 2.32 mmol).
The reaction was stirred at rt for 4 h. The solvent was removed in vacuo, and
H2O (20 ml) added. The pH was adjusted to 3 with 1 M HCl, the aqueous layer
was extracted with EtOAc (3 ꢄ 50 ml), and the organic layers were washed with
brine, dried over MgSO4, and concentrated. Column chromatography (70:30
EtOAc=petroleum ether 40–60 ꢂC) gave 1 as an off-white solid (554 mg, 51%).
1H NMR dH (400 MHz, DMSO-d6): 9.89 (1H, s, NH), 9.54 (1H, s, NH),
7.96–7.72 (7H, m, Ar-H), 7.46–7.34 (4H, m, Ar-H), 4.44 (2H, d, J 7.0 Hz, CH2),
4.31 (1H, t, J 7.0 Hz, CH), 1.48 (9H, s, CH3); 13C NMR dC (100.6 MHz,
DMSO-d6): 180.47 (COOH), 177.16 (2 ꢄ NH-CO-O-R), 153.72 (ipso-Ar linked to
COOH), 143.76 (2 ꢄ ipso-Ar linked to NH), 140.82 (2 ꢄ Ar CH), 139.96 (Ar
CH), 127.90 (ipso-Ar of Fmoc), 127.28 (ipso-Ar of Fmoc), 125.72 (Ar CH),
125.61 (Ar CH), 120.36 (Ar CH), 99.72 [O-C-(CH3)3 Boc], 81.28 (CH2 Fmoc),
46.86 (CH Fmoc), 28.37 (3 ꢄ CH3 Boc). HR-MS (ESI) C27H26N2O6Na requires
497.1692; found 497.1688.
In Situ Synthesis of Benzoic 4-Nitrobenzenesulfonic Anhydride 6
Benzoic acid (61 mg, 0.5 mmol), 4-nitrobenzenesulfonyl chloride (110 mg,
0.5 mmol), dimethylaminopyridine (DMAP; 3 mg, 0.025 mmol), and diisopropy-
lethyl amine (DIPEA; 174 ml, 1.0 mmol) were added to anhydrous THF (5 ml).
The reaction mixture was agitated until no nitrobenzenesulfonyl chloride was
detected by tlc (ca. 30 min.), forming 6 in situ.
Microwave-Accelerated Arylamide Synthesis
A solution of benzoic 4-nitrobenzenesulfonic anhydride 6 (1 ml) was added to
deprotected resin (40–60 mg) and irradiated for 2 min (100 W, 80 ꢂC) in a CEM 10-ml
pressure vial. The resin was washed with THF (3 ꢄ 3 ml), and the microwave
coupling was repeated.
Cleavage of Triarylamides from Resin
Reacted resin was swelled in THF=MeOH (4:1, 1 ml), NaOMe (10 mg) was
added, and the reaction was agitated for 18 h. The resin was removed by filtration,
and the filtrate was concentrated under reduced pressure. The residue was filtered
through a plug of silica and eluted with ethyl acetate (5 ml). The ethyl acetate
was removed in vacuo to give crude triarylamide, for example, 4. HR-MS (ESI)
C22H19N2O4 requires 375.1339; found 375.1345.

MICROWAVE-ASSISTED SYNTHESIS OF ARYL AMIDE BONDS
3065
ACKNOWLEDGMENTS
We thank the British Society of Antimicrobial Chemotherapy (BSAC),
Engineering and Physical Sciences Research Council (EPSRC), and the University
of York for financial support.
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