Alternative base matrices for solid phase quenching reagents

Article abstract of DOI:10.1016/S0040-4039(00)00192-1

In this paper we describe the use of macroporous, highly cross-linked poly(styrene-divinylbenzene) as a base matrix for solid phase scavengers. We demonstrate that this matrix is superior to the more traditional Merrifield resins when non-swelling solvents like acetonitrile are used. A small library of sulfonamides is generated in high purity as an example. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00192-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2323–2326
Alternative base matrices for solid phase quenching reagents
Robert B. Nicewonger,∗ Lori Ditto and Laszlo Varady
Applications Development Group, ArQule, Inc., 19 Presidential Way, Woburn, MA 01801, USA
Received 22 November 1999; revised 20 January 2000; accepted 24 January 2000
Abstract
In this paper we describe the use of macroporous, highly cross-linked poly(styrene-divinylbenzene) as a base
matrix for solid phase scavengers. We demonstrate that this matrix is superior to the more traditional Merrifield
resins when non-swelling solvents like acetonitrile are used. A small library of sulfonamides is generated in high
purity as an example. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: polymer support; solid-supported reactions; supported reagents; sulfonamides.
Introduced in this decade, combinatorial chemistry is widely practiced in the pharmaceutical and
biotechnology industry and is regarded as an important component of the drug discovery process. While
traditional combinatorial chemistry is carried out on solid supports, the use of solution phase techniques
for library generation is gaining momentum. Approximately 30–40% of all published libraries in 1998
utilized this technique.1 The introduction of solid-supported scavengers facilitated this trend2,3 and
now several types are commercially available. Reactive groups (e.g. amine, aldehyde, thiol, hydrazine,
isocyanate) linked to poly(styrene-divinylbenzene) beads (typically 1 or 2% crosslinked) are used to
quench excess or unreacted starting materials of complementary reactivity in solution. These quenching
reagents are particularly useful in combinatorial chemistry, where the purification of large numbers of
compounds is difficult to achieve using traditional methods such as recrystallization or flash chromato-
graphy. However, the commercially available quenching reagents do have some drawbacks. First, they
must be used in solvents with good swelling properties, such as DMF, methylene chloride, or THF.
These solvents are undesirable when running thousands of reactions — DMF is difficult to remove from
the final product, methylene chloride is toxic, and THF may contain peroxides. Secondly, according to
most manufacturers’ instructions, an excess of quenching reagent is required for a few hours to overnight
to completely remove impurities. This translates into longer synthesis times when running large numbers
of reactions. Kaldor et al. reported that polystyrene linked benzaldehyde could be used to quench excess
primary amines in methanol with dichloromethane as a cosolvent.4 The reaction used two equivalents of
quenching resin and required an overnight reaction time to completely remove all of the excess reagents.
Our goal was to find alternative base matrices for solid phase quenching reagents that would rapidly
∗
Correspondence author. Tel: (781) 994-0496; fax: (781) 376-6019; e-mail: rnicewonger@arqule.com (R. B. Nicewonger)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00192-1
tetl 16472

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(15 minutes or less) remove impurities in solvents more suited for parallel solution phase combinatorial
chemistry, such as acetonitrile.
Macroporous, highly cross-linked polystyrenes are the base matrices of choice for solid phase organic
synthesis in acetonitrile because they swell equally in polar and non-polar solvents as long as the solvent
wets the surface.5 In addition, macroporous resins are widely used as chromatography matrices, where
good mass transfer between the pores and the bulk solvents can overcome the slow diffusion kinetics
experienced when 1–2% crosslinked gels are used.6 Based on these facts, we investigated the use of
macroporous, highly cross-linked resins as matrices for quenching reagents in non-swelling solvents.
We chose the following resins for our investigation: ArgoPore-Cl (Argonaut Technologies, 190-micron
beads, pore size of 90 Å) and two highly cross-linked chloromethyl resins (Polymer Laboratories,
60-micron beads, pore size of 300 Å and 1000 Å). For comparison, we included PS-Tris (Argonaut
Technologies) — this is a 1% DVB crosslinked poly(styrene-divinylbenzene) which represents a typical
base matrix used for quenching reagents. The various halo-resins were reacted with an excess of tris(2-
aminoethyl)amine to yield the aminated polystyrene.7 The total quenching capacity in acetonitrile of each
resin was measured by reacting overnight with an excess of trans-β-styrene sulfonyl chloride (Scheme
1).8 Results are shown in Table 1 below. It is clearly seen that the scavenging activity is inversely
proportional to the pore size and consequently, directly proportional to the surface area. The measured
activity of PS-Tris in acetonitrile is significantly lower than that in methylene chloride (3.8 mmol/g as
provided by the manufacturer). This is most likely due to the low level of swelling in acetonitrile.
Scheme 1. Synthesis of macroporous Tris-resin and subsequent quenching of a sulfonyl chloride
Table 1
Amount of trans-β-styrene sulfonyl chloride that could be quenched in an overnight reaction in
acetonitrile
Since we wanted to limit our quench time to 15 minutes, we performed ‘rapid quenching experiments’
on each of the matrices. For each sample four different amounts of resin (approximately 20, 40, 60, and
80 mg) were allowed to react with 50 µmol of trans-β-styrene sulfonyl chloride for 15 minutes. Then the
supernatants were analyzed by reversed-phase HPLC. The amount of resin was plotted against the peak
area of the unquenched sulfonyl chloride. Extrapolating the curves to 0 µmol sulfonyl chloride allowed
us to calculate how much resin was needed to quench 50 µmol. These values are shown in Table 2. There
is a good correlation between the total activity and the ‘rapid quenching’ activity. The higher the total
activity the less resin is needed for rapid quenching. It should be noted that the traditional Merrifield type
PS-Tris was unable to quench any sulfonyl chloride in this short time presumably due to the non-swelling.

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Table 2
Amount of trans-β-styrene sulfonyl chloride that could be quenched in a 15 minute reaction in
acetonitrile
To demonstrate the usefulness of these resins in a parallel format, we synthesized an array of 12
sulfonamides by reacting four different amines with a two-fold excess of three sulfonyl chlorides.9 The
purity of each of the products is indicated in Table 3.10
Table 3
Percent purities (as judged by HPLC/ELSD) of sulfonamides derived from the corresponding amine
and sulfonyl chloride
In conclusion, we demonstrated for the first time that highly cross-linked macroporous poly(styrene-
divinylbenzene) is a very viable base matrix for generating solid phase scavenger resins. In less than
three hours we could generate an array of sulfonamides in extremely high purity using only acetonitrile
as a solvent. The quenching of excess sulfonyl chloride required no co-solvent and was complete within

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15 minutes. Synthesis of other scavengers and solid phase reagents, as well as further investigation of the
effect of particle and pore sizes is underway.
Acknowledgements
We thank Polymer Laboratories for the generous gift of highly cross-linked macroporous polystyrene
resins.
References
1. Dolle, R. E.; Nelson Jr., K. H. J. Comb. Chem. 1999, 1, 235.
2. Booth, R. J.; Hodges, J. C. J. Am. Chem. Soc. 1997, 119, 4882.
3. Flynn, D. L.; Crich, J. Z.; Devraj, R. V.; Hockerman, S. L.; Parlow, J. J.; South, M. S.; Woodard, S. J. Am. Chem. Soc. 1997,
119, 4874.
4. Kaldor, S.W.; Siegel, M. G.; Fritz, J. E.; Dressman, B. A.; Hahn, P. J. Tetrahedron Lett. 1996, 37, 7193.
5. Hudson, D. J. Comb. Chem. 1999, 1, 403 and references cited therein.
6. Doyle, C. A.; Dorsey, J. G. In Handbook of HPLC; Katz, E.; Eksteen, R.; Schoenmakers, P.; Miller, N., Eds.; Marcel Dekker:
New York, 1999; pp. 293–324 and references cited therein.
7. To a mixture of macroporous highly cross-linked polystyrene beads (1.75 g) in N-methyl pyrrolidine (20 mL) was added
tris(2-aminoethyl)amine (3 mL). The mixture was agitated overnight in a heater/shaker at 60°C. The beads were washed
three times each with DMF, methanol, methylene chloride, ether and then dried in vacuo.
8. To a sample of Tris resin (approximately 20 mg) was added a 0.025 M solution of trans-β-styrene sulfonyl chloride in
acetonitrile. The mixture was shaken overnight at ambient temperature, then spun down in a centrifuge. An aliquot (100 µL)
was removed and added to acetonitrile (900 µL). Reverse-phase HPLC was performed and the amount of quenched sulfonyl
chloride was calculated from a standard curve.
9. To a 0.025 M solution of amine (1 mL) was added a 0.050 M solution of sulfonyl chloride (1 mL) and triethyl amine (60
µL). The solution was shaken for two hours before the addition of the Tris resin derived from Argopore-Cl (1.2 equiv. based
on the ‘rapid quenching’ activity). The mixture was shaken for an additional 15 minutes and then spun down in a centrifuge.
An aliquot was removed and subjected to reverse-phase HPLC/ELSD to obtain the purity.
10. The purity of the reaction is in reference to only the amine and sulfonyl chloride. No attempt was made in these experiments
to remove the triethylamine and triethylammonium chloride.