Reporter resins for solid-phase chemistry

Article abstract of DOI:10.1021/ol006751n

(Matrix presented) An analytical construct resin, designed to aid the analysis of solid-phase chemistry, has been mixed in a small proportion with a conventional resin. The analytical construct ("reporter resin") contains two orthogonal linkers that allow

Full text of DOI:10.1021/ol006751n

ORGANIC
LETTERS
2001
Vol. 3, No. 4
507-510
Reporter Resins for Solid-Phase
Chemistry
,†
Miles S. Congreve,* Mark Ladlow,† Peter Marshall,‡ Nigel Parr,‡
Jan J. Scicinski,† Tom Sheppard,† Emma Vickerstaffe,† and Robin A. E. Carr‡
Glaxo Wellcome Cambridge Chemistry Laboratory, UniVersity Chemical Laboratories,
Lensfield Road, Cambridge, CB2 1EW, U.K., and Glaxo Wellcome Medicines Research
Centre, Gunnels Wood Lane, SteVenage, Hertfordshire, SG1 2NY, U.K.
mc0067@glaxowellcome.co.uk
Received October 19, 2000
ABSTRACT
An analytical construct resin, designed to aid the analysis of solid-phase chemistry, has been mixed in a small proportion with a conventional
resin. The analytical construct (“reporter resin”) contains two orthogonal linkers that allow cleavage of either the synthetic intermediates (at
linker 2) or their analytically enhanced derivatives (at linker 1). The convenient and rapid monitoring of each step in the syntheses of representative
library compounds was possible using small resin aliquots.
Development of new chemistries and synthetic sequences
directly on solid supports can be a difficult and time-
consuming process.1 In particular, assessment of product
mixtures formed on a resin can be problematical. Usually
cleavage of the synthetic intermediates from the linking group
prior to analysis is necessary because current methods, such
as IR or NMR, for directly analyzing polymer-supported
compounds are limited and often require expensive instru-
mentation.2 Without the ability to directly monitor reactions
as they proceed by HPLC or mass spectrometry, or to easily
identify byproducts formed during each step, reaction
optimization can be difficult. In an effort to overcome these
problems we have developed a number of analytical construct
resins that incorporate features which greatly simplify the
analysis of materials synthesized on solid supports.3 These
constructs are multidetachable resins4 (Figure 1) which have
an additional linker that allows cleavage either of the
substrates themselves (cleavage at linker 2) or of an
analytically enhanced fragment (cleavage at linker 1).
Typically, linker 1 will be an amine-releasing linker so that
each fragment will contain an amine functionality greatly
improving the ionization properties of the materials and thus
allowing easy detection by electrospray mass spectrometry
in the ESI+ mode (a MS sensitizer).5 Introduction of an
isotopic label between the two linkers, usually as a 1:1
† Cambridge Chemistry Laboratory.
‡ Medicines Research Centre.
(3) (a) Geysen, H. M.; Wagner, C. D.; Bodnar, W. M.; Markworth, C.
J.; Parke, G. J.; Schoenen, F. J.; Wagner, D. D.; Kinder, D. S. Chem. Biol.
1996, 3, 679. (b) McKeown, S. C.; Watson, S. P.; Carr, R. A. E.; Marshall,
P. Tetrahedron Lett. 1999, 40, 2407. (c) Murray, P. J.; Kay, C.; Scicinski,
J. J.; McKeown, S. C.; Watson, S. P.; Carr, R. A. E. Tetrahedron Lett.
1999, 40, 5609.
(4) Tam, J. P.; Tjoeng, F. S.; Merrifield, R. B. J. Am. Chem. Soc. 1980,
102, 6117.
(5) Carrasco, M. R.; Fitzgerald, M. C.; Oda, Y.; Kent, S. B. H.
Tetrahedron Lett. 1997, 38, 6331.
(1) (a) Lebl, M. J. Comb. Chem. 1999, 1, 3. (b) Czarnik. A. W. Anal.
Chem. 1998, 70, 378A.
(2) (a) Yan, B.; Gremlich, H.-U.; Moss, S.; Coppola, G. M.; Sun, Q.;
Liu, L. J. Comb. Chem. 1999, 1, 46. (b) Fitch, W. L. Mol. DiVersity 1999,
4, 39. (c) Loo, L. A. Eur. Mass Spectrom. 1997, 3, 93. (d) Kyranos, J. N.;
Hogan, J. C. Anal. Chem. 1998, 70, A389. (e) Sussmuth, R. D.; Jung, G.
J. J. Chromatogr. B 1999, 725, 49. (f) Swali, V.; Langley, G. J.; Bradley,
M. Curr. Opin. Chem. Biol. 1999, 3, 337. (g) Szwartz, M. E. Analytical
Techniques in Combinatorial Chemistry; Marcel Dekker: New York, 2000.
10.1021/ol006751n CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/19/2001

The reporter resin was synthesized as illustrated in Scheme
1. The amines 1a and 1b10 were reacted with sulfonyl
Scheme 1. Synthesis of the Reporter Resina
Figure 1. The principle of the “reporter resin”. Introduction of an
analytical enhancer between the linker used in the solid-phase
synthesis (linker 2) and another chemically orthogonal linker (linker
1) in 5% of the beads enables monitoring of each chemical step by
cleavage of linker 1 followed by LC/MS analysis.
a (a) 2 (1 equiv), DIPEA (1.5 equiv), CH2Cl2, 0 to 20 °C, 17 h,
3a 69%, 3b 63%; (b) PS-BEMP12 (1.3 equiv), 9-(3-bromopropyl)
anthracene (1.1 equiv), DMF, 20 °C, 14 h, 4a 98%, 4b 86%; (c)
aqueous sodium hydroxide (1.2 equiv), methanol:THF (1:1), 20 °C,
3 h, 90%; (d) ArgoGel-NH2 (0.75 equiv), DIC (2 equiv), HOBt (2
equiv), DMF, 20 °C, 16 h; (e) TFA: CH2Cl2 (1:1), 20 °C, 2 × 30
min.
mixture of hydrogen and deuterium isotopes, produces a
doublet in the mass spectrum of the analytical fragment (peak
splitter).6 This ensures that all analytical fragments cleaved
from the resin can be easily recognized in the mass spectrum.
Such construct systems have been successfully applied to
detection of materials by MS after “split-mix-pool” synthesis
from single beads.7 We recently reported a construct system
that additionally includes an anthracene moiety. The chro-
mophore enables the direct monitoring of resin-derived
products by HPLC-UV at a wavelength that is remote from
extraneous reagents and reaction substrates (386 nm) with
single bead sensitivity.8
In this Letter we now describe how such a construct can
be used as a “reporter resin” to predict the course of reactions
occurring on conventional resins when mixed in the same
chemical reaction. This is achieved simply by mixing the
analytical construct resin in a small proportion, typically 5%
weight for weight, with the conventional resin before
commencing the synthetic sequence (Figure 1).9 In order for
an analytical construct resin (the “reporter”) to be generally
predictive of the chemistry occurring on the conventional
resin we considered that it must be derived from the same
polymeric support to minimize differences in the chemistry
due to variations in resin solvation. We then sought to
determine whether the reporter resin, possessing construct
functionality that might potentially modify the properties of
the material, was indeed predictive of the chemistry of a
conventional resin throughout a sequence of reactions.
chloride 211 to give sulfonamides 3a and 3b. N-Alkylation
with 9-(3-bromopropyl)anthracene, saponification of an
equimolar mixture of the methyl esters 4a and 4b, coupling
to ArgoGel-NH2 resin, and treatment with trifluoroacetic acid
(TFA) to remove the Boc-protecting group gave amino-resin
5.
The reporter resin 5 was then mixed 1 part in 20 with
commercial ArgoGel-NH2 resin (0.4 mmol g-1 initial load-
ing). The mixture of resins was then used to synthesize model
combinatorial library products (Scheme 2). At each stage
the progress of the reactions was monitored by HPLC and
LC/MS analysis of the secondary amine-containing analytical
fragments. Selective cleavage at the sulfonamide linker was
achieved by treatment with a thiol,13 and the results are
displayed in Table 1. A 10% solution of the sodium salt of
mercaptoethanol in methanol was found to be a suitable
protocol, effecting very rapid cleavage (5-10 min) and
giving a good signal-to-noise ratio using 2 mg of resin (which
equates to approximately 50 reporter beads).14 The amide
coupling of the mixed resins with linker 615 gave the
dimethoxyaldehyde resins 7. These were subjected to reduc-
(6) Gray, W. R.; Del Valle, U. E. Biochemistry 1970, 9, 2134.
(7) (a) Lorthioir, O.; McKeown S. C.; Parr, N. J.; Watson, S. P.;
Congreve, M. S.; Scicinski J. J.; Kay, C.; Marshall, P.; Carr, R. A. E.;
Geysen, M. H. Anal. Chem. In press. (b) Lorthioir, O.; McKeown S. C.;
Parr, N. J.; Washington, M.; Watson, S. P. Tetrahedron Lett. 2000, 41,
8609.
(8) Williams, G. M.; Carr R. A. E.; Congreve, M. S.; Kay, C.; McKeown
S. C.; Murray, P. J.; Scicinski, J. J.; Watson, S. P. Angew. Chem., Int. Ed.
2000, 18, 3293.
(10) The monoprotected diamines were purchased from AstaTech, Inc.,
Philadelphia.
(11) Kay, C.; Murray, P. J.; Sandow, L.; Holmes, A. B. Tetrahedron
Lett. 1997, 38, 6941.
(12) 2-tert-Butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-dia-
zophosphorine on polystyrene resin from Fluka.
(13) Fukuyama, T.; Jow, C.-K.; Cheung, M. Tetrahedron Lett. 1995,
36, 6373.
(14) ArgoGel is reported as 130-230 µm diameter beads, which equates
to approximately 500 beads per mg of resin.
(15) Commercially available from Peakdale Fine Chemicals, Glossop,
U.K. Khehyong, N.; Patel, D. V. J. Org. Chem. 1997, 62, 7088.
(9) The concept of using one resin as an analytical tool to predict the
course of chemical reactions on another conventional resin is protected in
an unpublished US patent application by H. M. Geysen.
508
Org. Lett., Vol. 3, No. 4, 2001

Scheme 2. Synthesis of Model Library Compoundsa
a (a) 6 (5 equiv), PyBOP (5 equiv), DIPEA (10 equiv), CH2Cl2:DMF (1:4), 20 °C, 8 h; (b) (i) sec-BuNH2 (5 equiv), AcOH (5 equiv),
NMP, 20 °C, 1.5 h, (ii) Bu4NBH4 (5 equiv), AcOH (5 equiv), NMP, 20 °C, 2 h; (c) 9 (10 equiv), PyBOP (10 equiv), DIPEA (20 equiv),
CH2Cl2: DMF (1:4), 20 °C, 30 min; (d) piperidine:DMF (1:4), 20 °C, 30 min; (e) p-toluenesulfonyl chloride (2 equiv), Et3N (4 equiv),
MeCN, 20 °C, 2 h; (f) TFA:CH2Cl2 (1:1), 20 °C, 1 h; (g) 13 (6 equiv) 14 (3 equiv), PyBOP (10 equiv), DIPEA (20 equiv), CH2Cl2: DMF
(1:4), 20 °C, 3 h.
tive amination with sec-butylamine to give the resin-bound
secondary amines 8. Analytical cleavage of 8 indicated that
the reaction had gone to completion in less than 4 h. Next,
monitoring of the amide coupling of resins 8 with amino
acid 9 indicated that the reaction was complete after only
30 min, as assessed by disappearance of the starting material
analytical fragment by HPLC. Typically a hindered coupling
of this type would be left for at least 18 h, but the reporter
indicated that this was unnecessary in this case. Fmoc
deprotection of resins 10a using a mixture of piperidine in
DMF gave the amino-resins 10b. Fmoc quantification16
indicated a loading of 0.21 mmol g-1.
To further assess the predictive nature of the reporter, the
resin system 10b was condensed with a mixture of carboxylic
acids to give the resin-bound secondary carboxamides 15a
and 15b. The ratio of the products formed was assessed by
HPLC analysis of the amine fragments following sulfona-
mide linker cleavage (Table 1). The mixture of resins 15a
and 15b was then cleaved with TFA and the ratio of the
1
products 16 and 17 determined by H NMR. In both cases
close to a 1:1 ratio was obtained. This again indicated that
the reporter had been predictive of the chemistry taking place
on the conventional resin (HPLC ratio 48:52; 1H NMR ratio
49:51).
Reaction of 10b with tosyl chloride gave the resins 11 in
good conversion with minimal formation of resin-bound
byproducts. Using the reporter it was possible to make a
quantitative assessment by HPLC of the purity of the final
resin-bound product before cleavage and isolation as being
89% (at 386 nm). Treatment of the resins 11 with TFA in
CH2Cl2 gave the expected product 12 in a purity that was
consistent with that predicted by the reporter (90-95% by
1H NMR). This indicated that the chemistry monitored on
the 5% of beads bearing the reporter was representative of
the course of the reactions on the commercial ArgoGel
support. The yield of the isolated product 12 was 94% based
upon the observed loading of 10a (0.21 mmol g-1).
Throughout the syntheses described above, analysis of the
resins using magic angle spinning (MAS) NMR was also
undertaken.17 Due to the small quantity of reporter resin
present in each sample, resonances from these resins were
not clearly visible using the MAS NMR technique, and it
was, therefore, possible to assess the products present on
the conventional resin at each synthetic step. The NMR data
were fully consistent with the expected products in each case,
again indicating a good correlation of the chemistry occurring
on the ArgoGel support with that on the reporter resin. Trace
impurities that were obvious by use of the reporter analyses
could not be assessed by MAS NMR, as it is not a
sufficiently sensitive or quantitative technique.
(16) Atherton, E.; Sheppard, R. C. Solid-phase peptide synthesis: a
practical approach; IRL Press: Oxford, 1989.
(17) Fitch, W. L.; Detre, G.; Holmes, C. P.; Schoolery, J. N.; Keifer, P.
A. J. Org. Chem. 1994, 59, 7955.
Org. Lett., Vol. 3, No. 4, 2001
509

Table 1. Analysis of Resins after Each Synthetic Step
a Analytical fragments are the result of cleavage of the sulfonamide linker with the sodium salt of mercaptoethanol in methanol to give derivatives for
analysis by LC/MS and by HPLC. b HPLC area was measured at 386 nm, the remote absorption of the anthracene chromophore; each peak at this wavelength
will be derived from a resin-bound intermediate and can be quantified by peak area. A selected region of the spectrum in each case is illustrated to indicate
the major product and observed byproducts and to give an indication of the signal-to-noise ratio. c Product molecular ions are indicated (MH+) and were
measured using an LC-MS instrument (Thermoquest LCQ in ESI+ mode). d Doubly charged ions from the diamine product. e Cleavage of resin 10a also
causes some removal of the Fmoc protecting group. f Sodium adducts. g 15a R′ ) t-Bu 15b, R′ ) CH2CHdCH2.
In summary, by the use of a mixture of a conventional
resin and a resin bearing an analytical construct as a reporter,
it has been possible to conveniently monitor each step in
the syntheses of model library products without the need to
cleave large quantities of resin at each stage for analysis. In
addition, it was possible to indirectly assess the purity of
the final products before cleavage and isolation with TFA.
This powerful analytical method will greatly assist the
development of new chemistry on solid supports and allow
for the facile and rapid assessment of the success of a given
library synthesis. The use of a small quantity of the reporter
resin in the synthesis maximizes the cost effectiveness of
the analytical resin without compromising the compound
loading and enables quality control of library products prior
to cleavage and isolation of the target molecules.
Acknowledgment. We thank Mr. Stephen Richards for
MAS-NMR studies and Prof. Steven Ley for his ongoing
support of the GW Cambridge Chemistry Laboratory.
Supporting Information Available: Experimental pro-
cedures for the synthesis of 9-(3-bromopropyl) anthracene,
compounds 3a, 3b, 4a, 4b, 5, and 7 and general condi-
tions for analytical cleavage of the reporter. This
material is available free of charge via the Internet at
http:/www.pubs.acs.org.
OL006751N
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Org. Lett., Vol. 3, No. 4, 2001