Polyaromatic scavenger reagents (PAHSR): A new methodology for rapid purification in solution-phase combinatorial synthesis

Article abstract of DOI:10.1021/ol005822f

(Formula presented) A new method of purification of solution-phase combinatorial libraries has been developed. Development of a chemically inert polyaromatic anchor with a reactive "scavenger reagent" (PAHSR) allows unreacted reagents and impurities to be removed from a reaction by absorption of the PAHSR to charcoal and simple filtration.

Full text of DOI:10.1021/ol005822f

ORGANIC
LETTERS
2000
Vol. 2, No. 13
1807-1809
Polyaromatic Scavenger Reagents
(PAHSR): A New Methodology for Rapid
Purification in Solution-Phase
Combinatorial Synthesis
Joseph S. Warmus* and Marianne I. da Silva†
Department of Chemistry, Parke-DaVis Pharmaceutical Research, A DiVision of
Warner-Lambert, 2800 Plymouth Road, Ann Arbor, Michigan 48105
joseph.warmus@wl.com
Received March 16, 2000
ABSTRACT
A new method of purification of solution-phase combinatorial libraries has been developed. Development of a chemically inert polyaromatic
anchor with a reactive “scavenger reagent” (PAHSR) allows unreacted reagents and impurities to be removed from a reaction by absorption
of the PAHSR to charcoal and simple filtration.
Combinatorial synthesis has become commonplace in in-
dustry today. Increasingly, solution-phase parallel synthesis
has proven useful in the preparation of both lead generation
libraries and in lead optimization in the pharmaceutical
industry.1-4 Inherent in the solution-phase synthetic strategy
is the need for rapid purification of large numbers of
compounds.5 A number of methods are used routinely,
including liquid-phase6 and fluorous-phase7 reactions, ion
exchange chromatography,8 and more recently the use of
automated HPLC systems.9-11
In fact, a number of scavenger resins are now com-
mercially available.18 This technique allows the removal of
excess reagents and byproducts by removal of resin via
filtration. This strategy has recently been extended to the
use of magnetic beads, thus facilitating the removal of the
resin.19
This technique has drawbacks, namely, difficulty in
automation of resin addition and removal, longer reaction
(7) (a) Curran, D. P. Med. Res. ReV. 1999, 19, 432-438. (b) Studer, A.;
Hadida, S.; Ferritto, R.; Kim, S.-Y.; Jeger, P.; Wipf, P.; Curran, D. P. Science
1997, 275, 823-826. (c) Curran, D. P.; Hadida, S. J. Am. Chem. Soc. 1996,
118, 2531-2532.
(8) Siegel, M. G.; Hahn, P. J.; Dressman, B. A.; Fritz, J. E.; Grunwell,
J. R.; Kaldor, S. W. Tetrahedron Lett. 1997, 38, 3357-3360.
(9) Kibbey, C. E. Lab. Rob. Autom. 1997, 9, 309-321.
(10) Goetzinger, W. K.; Kyranos, J. N. In PCT Int. Appl. (Arqule, Inc.,
USA). Wo, 1999; Vol. G01N030-34, p 36 pp.
(11) Zeng, L.; Kassel, D. B. Anal. Chem. 1998, 70, 4380-4388.
(12) Booth, R. J.; Hodges, J. C. J. Am. Chem. Soc. 1997, 119, 4882-
4886.
(13) Kaldor, S. W.; Siegel, M. G.; Fritz, J. E.; Dressman, B. A.; Hahn,
P. J. Tetrahedron Lett. 1996, 37, 7193-7196.
One of the more utilized strategies for parallel reaction
purification has been the use of polymer-supported scavenger
reagents12,13 or the PASP (polymer-assisted solution-phase)
strategy.14-17
† Present address: Department of Chemistry, Imperial College of Science,
Technology and Medicine, Exhibition Road, London SW7 2AZ, U.K.
(1) Fukase, K. Yuki Gosei Kagaku Kyokaishi 1997, 55, 474-479.
(2) Fruechtel, J. S.; Jung, G. Angew. Chem., Int. Ed. Engl. 1996, 35,
17-42.
(3) Lam, K. S. Anti-Cancer Drug Des. 1997, 12, 145-167.
(4) Storer, R. Drug DiscoVery Today 1996, 1, 248-254.
(5) Ferritto, R.; Seneci, P. Drugs Future 1998, 23, 643-654.
(6) (a) Wentworth, P. Trends Biotechnol. 1999, 17, 448-452. (b) Perrier,
H. l. n.; Labelle, M. J. Org. Chem. 1999, 64, 2110-2113.
(14) (a) Flynn, D. L. Med. Res. ReV. 1999, 19, 408-431. (b) Weidner,
J. J.; Parlow, J. J.; Flynn, D. L. Tetrahedron Lett. 1999, 40, 239-242. (c)
Starkey, G. W.; Parlow, J. J.; Flynn, D. L. Bioorg. Med. Chem. Lett. 1998,
8, 2385-2390.
10.1021/ol005822f CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/01/2000

time due to solid-liquid interface, and difficulty in prepara-
tion of new or specialized reagents.
of the core was dissolved in 2 mL of a 0.05 M solution of
p-bromoanisole (an internal standard) in CDCl3. A 1 mL
aliquot was treated with 250 mg of charcoal, and the solution
was stirred for 30 min. The solutions were filtered and
analyzed by NMR.
As shown in Table 1, pyrene appeared to be an ideal core,
as both the amine hydrochloride and the alcohol were >95%
absorbed onto charcoal. In the case of the alcohol, the
experiment was repeated, but carried out to 12 h, at which
time the solution was filtered and analyzed by NMR,
showing no trace of pyrene alcohol.
On the basis of this simple experiment, we decided to use
the pyrene alcohol as a starting point for investigation of
this strategy.
We set out to prepare a tris-amine scavenger reagent
similar to the available polymer-supported tris-amine resin.18
Pyrene methyl alcohol 1 was converted to chloride 2 with
thionyl chloride (Scheme 1). The bromide could also be
We sought to develop a reagent scavenger system that
would avoid these limitations. To this end, we were intrigued
by the work of Ramage’s group using the polyaromatic
tetrabenzo[a,c,g,i]fluorene (Tbf) group for purification of
peptides.20-22 The TBF group is absorbed onto charcoal in
polar solvents and can be desorbed using a nonpolar solvent.
Recently, Ramage has shown that this strategy can be used
for parallel synthesis.23 In this strategy, the substrate was
attached to the Tbf group via a linker and chemistry carried
out in solution. The TBF substrate was absorbed onto
charcoal, allowing excess reagents to be removed by filtra-
tion. The Tbf substrate could then be desorbed and further
chemistry carried out.
In contrast, we envisioned using the PAH as an anchor
for a scavenging functionality. This would allow a homo-
geneous reaction, thus allowing simple robotic addition and
rapid reaction time. After scavenging of unreacted starting
materials with the PAH scavenger reagent (PAHSR), simple
addition of charcoal and filtration would provide pure
material. In addition, this strategy would allow simple
modification of the scavenging moiety, thus allowing rapid
synthesis of specialized scavenger reagents. To be cost-
effective, we looked for a commercially available, inexpen-
sive polyaromatic hydrocarbon that could be easily modified.
We tested five simple substrates for the ability to be
absorbed onto charcoal (Table 1). In each case, 0.1 mmol
Scheme 1. Preparation of PAHSR Reagent
Table 1. Survey of PAH Cores
prepared using phosphorus tribromide. Displacement of the
benzylic chloride with bis-BOC triamine 3 24 went smoothly
to the protected PAHSR 4.
(15) 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, 4874-4881.
(16) Flynn, D. L.; Devraj, R. V.; Parlow, J. J. Curr. Opin. Drug DiscoVery
DeV. 1998, 1, 41-50.
(17) Parlow, J. J.; Devraj, R. V.; South, M. S. Curr. Opin. Chem. Biol.
1999, 3, 320-336.
(18) Calbiochem-Novabiochem, 800 228-9622.
(19) Rana, S.; White, P.; Bradley, M. Tetrahedron Lett. 1999, 40, 8137-
8140.
(20) Ramage, R.; Swenson, H. R.; Shaw, K. T. Tetrahedron Lett. 1998,
39, 8715-8718
(21) Brown, A. R.; Irving, S. L.; Ramage, R.; Raphy, G. Tetrahedron
1995, 51, 11815-30.
1808
Org. Lett., Vol. 2, No. 13, 2000

This could be deprotected in acidic methanol. We found
salt 5 to be a convenient way to store and handle the PAHSR.
Prior to use, the desired quantity was taken up in CH2Cl2
and Na2CO3 added. Simple filtration provided a solution of
free amine 6 for use.25
reaction stirred overnight. Charcoal (500 mg) was added and
the reaction monitored by MS and TLC for disappearance
of the bis-amide of the pyrene reagent.
The reaction was filtered and concentrated. NMR indicated
pure product in both reactions. No trace of the PAH reagent,
or products from the scavenger reagent, was detected.
These experiments showed that a PAHSR could be used
for parallel synthesis. During the course of this work, it was
noted that the researchers became sensitized to the pyrene
reagent, in particular the pyrene chloride or bromide. This
caused repeated cases of allergic dermatitis that were
exacerbated by sunlight. It was therefore deemed prudent to
discontinue research with this reagent. Work has continued
with another PAH anchor group that is devoid of these toxic
effects. This research will be reported in due course.
To test the methodology, two simple acylation reactions
were chosen (Scheme 2) In both, amine (0.4 mmol), Et3N
Scheme 2. Reaction Purification by PAHSR Reagent
Acknowledgment. The authors thank Dennis M. Down-
ing for his work on this project. We are also indebted to
Professor Robert Ramage for insightful discussions.
OL005822F
(22) Brown, A. R.; Irving, S. L.; Ramage, R. Tetrahedron Lett. 1993,
34, 7129-32.
(23) Hay, A. M.; Hobbs-Dewitt, S.; MacDonald, A. A.; Ramage, R.
Synthesis 1999, 11, 1979-1985.
(24) Adamczyk, M.; Fishpaugh, J. R.; Heuser, K. J. Org. Prep. Proced.
Int. 1998, 30, 339-348.
(3 mmol), and acyl chloride (0.64 mmol) were combined in
CH2Cl2 and stirred for 5 h. At this point, a solution of the
PAH scavenger reagent 6 (0.7 equiv) was added and the
(25) Data for PAHSR reagent 6: 1H NMR (CDCl3) δ 8.15 (d, 1H, J )
9.1 Hz), 7.97(m, 8H), 4.2 (s, 2H), 2.67 (t, 2H, J ) 5.9 Hz), 2.55 (t, 2H, 5.9
Hz); MS (CI) M(+1) 486; melting point of the HCl salt, 157 °C.
Org. Lett., Vol. 2, No. 13, 2000
1809