Self-indicating amine scavenger resins

Article abstract of DOI:10.1039/b315426b

Self-indicating methylisocyanate resin, which functions as both a scavenger and an indicator for amines, was used for in-situ reaction monitoring and purification of a urea based library.

Full text of DOI:10.1039/b315426b

Self-indicating amine scavenger resins
Jin Ku Cho,a Peter D. White,b Wolfgang Klute,c Tony W. Deand and Mark Bradley*a
a Combinatorial Centre of Excellence, School of Chemistry, University of Southampton, Highfield,
Southampton, UK SO17 1BJ. E-mail: mb14@soton.ac.uk; Fax: +44 (0)23 8059 6766; Tel: +44 (0)23 8059
3598
b Merck Biosciences Inc., Boulevard Industrial Park, Padge Road, Nottingham, UK NG9 2JR
c
Pfizer Global Research & Development, Ramsgate Road, Sandwich, UK CT13 9NJ
d GlaxoSmithKline, Discovery Research, Gunnels Wood Road, Stevenage, UK SG1 2NY
Received (in Cambridge, UK) 28th November 2003, Accepted 15th January 2004
First published as an Advance Article on the web 27th January 2004
Self-indicating methylisocyanate resin, which functions as both
a scavenger and an indicator for amines, was used for in-situ
reaction monitoring and purification of a urea based library.
and in each row was added isocyanate (0.20 mmol). After shaking
for 2 h, the self-indicating scavenger resin (0.10 mmol each) was
added to each well. The resins immediately became blue due to
excess amine but gradually turned yellow as the amines were
scavenged by isocyanate resin. After 1 h, about two thirds of the 24
reaction wells had turned a yellowish color (Fig. 2b). Although the
blue color of most reaction wells disappeared after about 3 h, the
colors of three wells (entries 2c, 3b, 3d) still showed a reddish blue
(Fig. 2c). To these three wells were added additional self-indicating
scavenger resin (0.10 mmol) and they were shaken for 2 h to
completely eliminate the blue color (Fig. 2d). After filtration and
evaporation, the yields and purities of the 24 ureas were
determined. The results are summarized in Table 1. All the
reactions gave high purities (91–99%) even in cases where yields
were not so high. In this experiment, the character of the amine had
little influence upon purification/scavenging kinetics but the three
One the major advantages of solid-phase synthesis is that it avoids
much of the effort necessary for purification that follows conven-
tional solution phase synthesis. However, a great many more
reactions have been optimized and documented for solution-phase
synthesis, while solution-phase chemistry avoids resin attachment
and cleavage steps (although these can be simply considered as
purely protection and deprotection steps of an insoluble protecting
group!). In addition, reaction progress as well as the identity and
purity of products may be analyzed and established by standard
chromatographic and spectroscopic means. Parallel combinatorial
methods have therefore been widely applied in the solution-phase
synthesis of compound libraries.1 This includes solution-phase
libraries prepared using a variety of resin-based systems such as
polymer-immobilized reagents, catalysts or scavengers.2 Many
scavenger resins have been employed to remove excess reagents
and by-products from crude reaction products,3 and one of the most
popular is methylisocyanate resin which is commonly utilized for
the sequestration of primary and secondary amines. This amine
scavenger resin has for example been applied for the purification of
urea-based libraries.4 In general, a 1.5–3.0 fold excess of the resin
is used for 1–18 h to remove excess amines. In order to efficiently
use the expensive resin and conveniently determine the completion
of each different reaction in a solution-phase parallel combinatorial
library, simple and reliable monitoring methods are required as
conventional chromatographic analyses are simply too laborious to
be applied to large libraries.
Here, we wish to report on a novel scavenger resin, which not
only scavenges the amine from solution but also indicates its
presence. The dye used was bromophenol blue, which is blue in the
presence of amines, and resin-bound bromophenol blue has been
successfully employed in monitoring amines in solution.5 This dye
was derivatized via a Suzuki reaction between bromophenol blue
and 4-carboxyphenylboronic acid† and then coupled with mono
tert-Boc protected diaminopentane with 1 using TFFH (tetra-
methylfluoroformamidinium hexafluorophosphate) to give 2 in
85% yield. After deprotection of the tert-Boc group with 50% TFA
in DCM,‡ the compound was attached to methylisocyanate
polystyrene resin with ~ 5% of total sites loaded (Scheme 1).§,¶
When self-indicating scavenger resin (0.20 mmol of resin-bound
isocyanate) was added to remove amine after urea formation with
n-butylamine (0.25 mmol) and phenylisocyanate (0.20 mmol), the
resin immediately turned blue due to excess amine (Fig. 1a). As the
amine in solution was removed (quenched) by the resin-bound
isocyanate, the resin’s color gradually changed to yellow (Fig. 1b–
1d). After filtration and evaporation of solvent, the crude product
was obtained in 98% yield and 99% purity.
Scheme 1 Preparation of self-indicating methylisocyanate resin. Reagents
and conditions: (i) 4-carboxyphenylboronic acid, Pd(OAc)2, K3PO4, (n-
Bu)4NBr, DMF, 100 °C, 33%; (ii) Boc-NH-(CH2)5-NH2, TFFH, TEA,
DMF, 85%; (iii) TFA, DCM; (iv) methylisocyanate polystyrene resin, TEA,
DMF.
t
A small solution-phase library was prepared using a 24 well
microplate. Six amines including one secondary amine and three
isocyanates and one isothiocyanate were used as building blocks. In
each column of the plate was placed the same amine (0.25 mmol)
Fig. 1 Color change of self-indicating methylisocyanate polystyrene resin
used during the purification of urea: (a) t = 0 h, (b) t = 1 h, (c) t = 2 h, (d)
t = 3 h.
502
C h e m . C o m m u n . , 2 0 0 4 , 5 0 2 – 5 0 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Biochem/Merck, Amersham/Nycomed Amersham and Evotec OAI
and EPSRC (JIF initiative).
Notes and references
† Synthesis of 1: Bromophenol blue (2.0 g, 3.0 mmol), 4-carboxy-
phenylboronic acid (3.6 mmol, 1.2 equiv.), potassium phosphate (9.0 mmol,
3.0 equiv.), tetrabutylammonium bromide (3.0 mmol, 1.0 equiv.), and
palladium(II) acetate (2.0 mol%) were placed in a reactor capped with a
septum and gently sparged with argon for 10 min. DMF (10 mL) was added
and the reaction mixture was stirred under argon at 110 °C for 18 h. The
resulting mixture was treated with saturated KHSO4, extracted with EtOAc,
dried over Na2SO4, and evaporated under reduced pressure to afford a
reddish oil. The product was isolated by column chromatography on silica
gel (chloroform : methanol : acetic acid = 5 : 1 : 0.06) to give 1 in 33%
yield: 1H NMR (400 MHz, 5% CF3CO2D in CD3CN): d = 8.03 (d, J = 8.4
Hz, 2H), 7.95 (d, J = 6.8 Hz, 1H), 7.82 (t, J = 7.6 Hz, 1H), 7.74 (t, J = 7.8
Hz, 1H), 7.58 (d, J = 7.6 Hz, 1H), 7.54 (d, J = 8.4 Hz, 2H), 7.47 (d, J =
2.4 Hz, 1H), 7.45 (s, 2H), 7.17 (d, J = 2.0 Hz, 1H); calculated mass for
C26H15Br3O7S: 711.2, found: ESI-MS, m/z, negative [M 2 H]2: 708.4,
710.6.
‡ Synthesis of 3: 1 (360 mg, 0.5 mmol) was dissolved in DMF (5 mL).
Triethylamine (5 mmol, 10 equiv.), mono tert-Boc protected diamino-
pentane (0.75 mmol, 1.5 equiv.), and TFFH (1.0 mmol, 2.0 equiv.) were
added and the reaction mixture was stirred for 18 h. The resulting mixture
was treated with saturated KHSO4, extracted with EtOAc, dried over
Na2SO4, and evaporated under reduced pressure to afford a reddish oil. The
product was isolated by column chromatography on silica gel (chloroform
Fig. 2 Color change of library according to time: (a) position of each
compound in 24 well microplate, (b) t = 1 h, (c) t = 3 h, (d) more scavenger
added.
Table 1 Urea library using self-indicating scavenger resin
: methanol
= 3 : 1) to give 2 in 85% yield: calculated mass for
C36H35Br3N2O8S: 895.5, found: ESI-MS, m/z, negative [M 2 H]2: 893.5,
895.0. To deprotect tert-Boc group, 2 was treated with 50% TFA in DCM
for 4 h. TFA and DCM were removed by nitrogen bubbling and the resulting
residue was precipitated with cold diethyl ether and filtered to give 3
(orange solid): 1H NMR (400 MHz, 5% CF3CO2D in CD3CN): d = 7.95 (d,
J = 7.2 Hz, 1H), 7.84–7.80 (m, 3H), 7.75 (t, J = 7.6 Hz, 1H), 7.59–7.54 (m,
3H), 7.47–7.45 (m, 3H), 7.18 (d, J = 2.0 Hz, 1H), 3.42 (t, J = 6.0 Hz, 2H),
2.94 (t, J = 6.0 Hz, 2H), 1.71–1.60 (m, 4H), 1.41 (p, J = 8.0 Hz, 2H);
calculated mass of C31H27Br3N2O6S: 795.4, found: ESI-MS, m/z, positive
[M + H]+: 795.0, 797.4, negative [M 2 H]2: 792.6, 794.4.
§ Preparation of 4: Methylisocyanate polystyrene resin (3.0 g, 4.5 mmol,
0.15 mmol of isocyanate/g resin) was added to 3 (0.23 mmol, 5% of total
loading capacity of methylisocyanate resin), triethylamine (0.69 mmol, 3.0
equiv.) in DMF (20 mL) and the suspension was shaken for 1 h. The resin
was filtered and washed with 5% acetic acid in DCM, 5% triethylamine in
DCM, and DMF by turns until no blue color was present in the washing.
¶ IR absorption at 2260 cm21 designates the CO stretching peak of resin-
bound isocyanate. No change was found between methylisocyanate resin
and indicator–methylisocyanate resin.
Yield
(%)
Puritya
(%)
Entry
R1, R1A
R2
X
1a
2a
3a
4a
5a
6a
1b
2b
3b
4b
5b
6b
1c
2c
3c
4c
5c
6c
1d
2d
3d
4d
5d
6d
Isopentyl, H
Bn, H
4-MeOBn, H
4-CF3Bn, H
Ph2CH, H
CH2(CH2)3CH2
Isopentyl, H
Bn, H
4-MeOBn, H
4-CF3Bn, H
Ph2CH, H
CH2(CH2)3CH2
Isopentyl, H
Bn, H
4-MeOBn, H
4-CF3Bn, H
Ph2CH, H
CH2(CH2)3CH2
Isopentyl, H
Bn, H
4-MeOBn, H
4-CF3Bn, H
Ph2CH, H
Bn
Bn
Bn
Bn
Bn
Bn
4-MeOBn
4-MeOBn
4-MeOBn
4-MeOBn
4-MeOBn
4-MeOBn
2-ClBn
2-ClBn
2-ClBn
2-ClBn
2-ClBn
2-ClBn
Bn
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
99
97
99
98
98
97
95
87
48
90
99
87
100
66
99
100
99
99
99
83
62
75
70
87
99
95
96
94
95
99
99
96
91
95
95
98
99
93
96
96
94
99
99
96
94
98
95
99
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Bn
Bn
Bn
Bn
S
S
S
S
CH2(CH2)3CH2
Bn
S
a Purities were determined by integration (l = 250 nm) of the crude
products by HPLC and confirmed by LC/MS.
reactions that required more scavenger resin showed lowest
yields.
In summary, self-indicating scavenger resins were successfully
used for in-situ reaction monitoring and purification during the
synthesis of a library of ureas and provide an ideal way of visually
following arrays of reactions.
4 B. A. Dressman, U. Singh and S. W. Kaldor, Tetrahedron Lett., 1998, 39,
3631.
We thank the Combinatorial Centre of Excellence partners:
GSK, AstraZeneca, Pfizer, Roche, Eli-Lilly, Organon, Nova-
5 J. K. Cho, P. D. White, W. Klute, T. W. Dean and M. Bradley, J. Comb.
Chem., 2003, 5, 632.
C h e m . C o m m u n . , 2 0 0 4 , 5 0 2 – 5 0 3
503