Naked-eye bead property estimation using a red safety-catch linker

Article abstract of DOI:10.1071/CH04262

The attachment of linker molecules to polymer beads used as insoluble supports for organic synthesis is a frequent requirement. Defined immobilization of these tinker molecules before loading selected building blocks is crucial for subsequent transformati

Full text of DOI:10.1071/CH04262

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2005, 58, 182–187
www.publish.csiro.au/journals/ajc
Naked-Eye Bead Property Estimation Using a Red
Safety-Catch Linker
Philipp HeidlerA and Andreas LinkA,B
A Institut für Pharmazeutische Chemie, Fachbereich Pharmazie, Philipps-Universität Marburg,
35032 Marburg, Germany.
B Corresponding author. Email: link@mailer.uni-marburg.de
The attachment of linker molecules to polymer beads used as insoluble supports for organic synthesis is a frequent
requirement. Defined immobilization of these linker molecules before loading selected building blocks is crucial for
subsequent transformations. Therefore, the control of the linker attachment is a central task. Because the molecular
bodies of linkers are not incorporated in the final molecules, they can often be replaced or modified without affecting
the structure of the products that are finally released. Consequently, it seems straightforward to look for coloured
substitutes to established linker molecules. By using coloured linkers, visual inspection of the beads enables fast
property estimation after attachment and monitoring of losses during synthesis. This very simple estimation does
not have a validated loading determination, but is a useful element of straightforward and non-destructive reaction
control that has general applicability. Here we present a red azo dye as an alternative to the Kenner safety-catch linker.
Manuscript received: 9 November 2004.
Final version: 16 December 2004.
Introduction
cappingandwashingsteps. Concurrently, lossinloadinglevel
encounteredattheverybeginningofasyntheticsequencewill
lower the yield of final products obtained to the same degree
as the incompleteness of much more delicate reaction steps
down the line in a multi-step process.
On-bead reaction monitoring suffers from the notorious
limitations associated with the analysis of insoluble polymer-
bound intermediates. Many methods suited to gain the nec-
essary information developed so far are either destructive,
time consuming, or require expensive analytical hardware.
On-resin NMR,[1,2] IR,[3–5] MS,[6,7] and electrochemical
impedance spectroscopy (EIS)[8] experiments are valuable
tools, but tend to be elaborate, especially when quantita-
tive information is sought. On the other hand, colorimetric
techniques often offer fast, cheap, and simple access to bead
property estimation. Thus, colorimetric approaches can rep-
resent practical tools to qualitatively or quantitatively monitor
solid-phase reactions.[9–11] If reactions are performed in
robotic or semi-automated systems in parallel fashion, it is a
considerable advantage to be able to estimate the complete-
ness of a reaction by simply looking into the reactor block
or on an array of glass reactors instead of taking samples
from each vessel. Therefore, further additions to the exist-
ing repertoire of colorimetric on-bead reaction monitoring
methodology are in high demand.
Support-bound primary amines present in aminomethy-
lated polystyrene have commonly been detected with
ninhydrin[12] and other classical reagents (recently
reviewed[9]). The still widely used destructive Kaiser nin-
hydrin test is suited to indicate the absence of amino groups,
and can thus serve to monitor the degree of formation of
amide bonds between carboxylic acids and aminomethylated
polystyrene resins. Similarly, addition of the pH indicator
bromophenol blue allows the course of amide bond forma-
tion to be followed by continuous visual examination.[13] The
change in colour of the coupling slurry from deep violet to
blue and finally to green/yellow indicates the consumption of
free amino groups and can serve as a signal that the reaction
has proceeded as anticipated.This simple yet elegant standard
procedure widely used for reaction control during syntheses
using construct 1 (Scheme 1) requires no further bases other
than the amino group of the polymer taking part in the bond
formation. Unfortunately, novel supports like the recently
proposed ULTRA resins, invented by Rademan et al.,[14]
contain minor amounts of tertiary amines within their func-
tional backbone, and these cannot be subjected to amide bond
formation in a preceding capping step. Therefore, they are
likely to obstruct the use of pH-indicating dyes such as bro-
mophenol blue in the monitoring of reactions. As this newly
Results and Discussion
Given the fundamental importance of linker molecules for the
synthetic sequence, it is essential to verify completeness of
the initial attachment to the beaded resin. Residual polymer
bead functional groups might disturb the ongoing reaction
sequence and often necessitate additional time-consuming
© CSIRO 2005
10.1071/CH04262
0004-9425/05/030182

Naked-Eye Bead Property Estimation
183
developed type of high loading resin will almost certainly
gain wide spread use in the near future, novel techniques for
visual reaction monitoring are needed.
Use of the polymer-bound 4-(4-sulfamoylphenylazo)
benzoic acid 2 as a surrogate for the Kenner safety-
catch linker 4-sulfamoylbenzoic acid 1 proposed by Ellman
et al.[15] (Scheme 1) gives rise to deeply coloured resin beads.
Moreover, the depth of the resin colour directly allows for the
fast and easy, although approximate estimation of the loading
level by naked-eye inspection (Fig. 1).
It is obvious that the incorporation of an azo functionality
requires special precautions and that the novel red linker has
limitations with respect to possible chemical transformations,
especially its stability towards reducing agents. However, it
is not the focus of this report to present a novel linker with
broad applicability. Instead, we intend to demonstrate the
astonishingly simple but useful novel concept of dying the
beads by using coloured linker molecules. Other techniques
using polymer-bound dyes to monitor pH or reaction pro-
cesses attach surplus dyes on sites distant from the linker
groups, and thus, are only distantly related to this new sug-
gestion. We propose to replace existing linkers with coloured
analogues that are able to yield the same products that would
have been obtained using the established, colourless linker
groups. This point could be proven and is described in the
following paragraphs.
reactor-vessels, could especially profit from use of coloured
beads since the user gains the possibility of an external visual
bead property control, as well as the possibility of taking
photographs for process documentation purposes. A very
recent example of a highly original practical application is the
development of a self-indicating amine scavenging reagent
reported by Bradley and coworkers.[16]
Although the required 4-(4-sulfamoylphenylazo)benzoic
acid 5 is known, it is not commercially available. Therefore,
a practical synthesis starting from 4-aminobenzoic acid 3
was developed (Scheme 2). Originally, to obtain the corre-
sponding nitroso compound 4, we envisioned the oxidation
of 4-aminobenzoic acid 3 with oxone in either methanol/
water[17] or a two-phase system. The rationale for the selec-
tion of oxone as an inexpensive oxidizing agent was the need
to access the azo linker molecule 5 on a gram scale. The use
of oxone is known to lead to various side reactions, such as
the formation of polymerization products, and the separa-
tion of the mixtures obtained turned out to be laborious and
inefficient. Thus, oxidation was undertaken with hydrogen
peroxide and the slightly more expensive sodium tungstate
in a two-phase system.[18] This route furnished the desired
4-nitrosobenzoic acid 4 in satisfying quality and amounts.
4-Nitrosobenzoic acid 4 was then coupled with sul-
fanilamide in boiling acetic acid to give azo linker 5,[19]
the purification of which was undertaken by two different
routes. Desired compound 5 could be obtained by extraction
with ethyl acetate upon systematic lowering of the reac-
tion pH. In particular, the fractions extracted at pH 6–7
were found to contain the protonated acid, but the low
solubility of 5 rendered this approach time-consuming.Alter-
natively, we isolated the sodium salt of target compound 5 at
pH 13 in high purity and yield, and furnished the desired
4-(4-sulfamoylphenylazo)benzoic acid 5 upon subsequent
treatment with boiling hydrochloric acid.
Synthetic strategies using, for example, the Vanguard
(AdvancedChemTech) series with Teflon reactor blocks
(Fig. 2) or the Chemspeed automated synthesis work-
stations ASW1000 2000 (Chemspeed) with their glass
O
S
O
O
O
S
N
NH2
N
H
N
H
N
NH2
O
O
1
2
Scheme 1. Ellman’s Kenner safety-catch linker variant 1 and polymer-
bound 4-(4-sulfamoylphenylazo)benzoic acid 2.
Fig. 2. Resin 2 with different loadings in the Vanguard synthesizer
(AdvancedChemTech, Ares Block). Resins were not exhaustively dried.
Fig. 1. Prepared test row with increasing loading level. Loading level
begins on the left side (0 up to 0.93 mmol g−1). Resins dried from THF.
O
NH2
S
NH2
N
O
O
(a)
(b)
N
HO
HO
N
HO
O
O
3
4
5
O
Scheme 2. Synthetic pathway towards 4-(4-sulfamoylphenylazo)benzoic acid 5: (a) H2O2, Na2WO4,
Bu4N+Br−, H3PO4, 35–40◦C, 3 h; (b) sulfanilamide, acetic acid, 110–120◦C, 4 h.

184
P. Heidler and A. Link
In the next step, azo dye 5 was coupled onto aminomethyl-
red colour could easily be destroyed by treating the beads with
tin(ii) chloride hydrate in N,N-dimethylformamide (DMF).
Therefore, should additional colour tests further down the
reaction sequence appear necessary, a small aliquot of the
beads can be decolourized by this widely used reducing
reagent. Upon reduction, the azo functionality is destroyed,
but the linker bound construct remains attached to the poly-
mer bead. Subsequent colour tests can thereby be performed
without being hampered by the strong colour of the beads.
Despite all this, the capability of the new linker to transfer
carboxylic acid moieties onto amines still had to be demon-
strated. For the preparation of the polymer-bound acylating
reagents 7a–7j, ten commercially available carboxylic acids
were immobilized onto construct 2 by in situ anhydride
formation (Scheme 4).[15] Independently, a natural product
derived amine scaffold 9 was obtained by classical solution-
phase techniques as recently reported by Lerner et al.[21]
After activation of the safety-catch linker with bromoace-
tonitrile (Scheme 5),[15] the carboxylic acid residues a–j were
successfully transferred onto previously synthesized adeno-
sine template 9 to form the corresponding amides 10a–10j
(Scheme 6). Complete conversion to the desired products
10a–10j could be observed by thin-layer chromatography
(TLC) after 16 hours. Thus, ten novel adenosine deriva-
tives with putative biological activity[22,23] could easily be
made available in good to excellent yields and purities. The
novelcompoundsreportedhereareintendedforongoingdrug
design studies pertaining to molecular recognition in ATP-
binding proteins and are of importance for understanding
enzymatic mechanisms.
ated polystyrene beads 6 under standard conditions[15,20]
usingN,N-diisopropylcarbodiimide(DIC), N-hydroxybenzo-
triazole (HOBt), and N,N-diisopropylethylamine (DIPEA)
(Scheme 3).
On-bead synthesis of polymer-bound linker 2 by acylation
of aminomethylated polystyrene with 4-nitrosobenzoic acid
4 followed by condensation with sulfanilamide was not envi-
sioned in this project, but could be an interesting topic to be
studied in the future.
By tuning the amount of azo dye 5 applied in the coupling
step from 0.07 to 4.0 equivalents, a row of resin samples with
varying loading levels was successfully prepared (Fig. 1),
and elemental analysis of the sulfur content indicated that
the loading values ranged from 0 to 0.93 mmol g−1. We
could easily observe the desired increase in colour, starting
with white through to orange and then to a deep red. A test
sample with unknown loading level was then prepared, and
based upon a visual comparison with the previously com-
posed test row, we predicted a loading level of 0.74 mmol g−1
.
To verify our prediction, the sample was subjected to ele-
mental analysis, whereby the sulfur content turned out to
be 0.71 mmol g−1. It was also determined that the red bead
colour is not pH-dependent. Neither addition of 2% trifluoro-
acetic acid (TFA) nor 2% DIPEA to the swollen polymer
resulted in a shift of colour or colour depth. However, the deep
O
H
N
(a)
N
NH2
S
NH2
N
O
All samples were routinely purified by MPLC to remove
particulates and undetectable impurities as recently recom-
mended by Yan et al.[24] Products 10a–10j (Scheme 7) were
also characterized by HPLC, 1H NMR spectroscopy, and high
O
6
2
Scheme 3. Coupling of 4-(4-sulfamoylphenylazo)benzoic acid 5 onto
aminomethylated polystyrene 6: (a) DIC, HOBt, DIPEA, DMF.
O
O
H
N
H
N
H
N
(a)
NH2
N
N
S
S
N
N
R
O
O
O
O
2
7a–7j
Scheme 4. Coupling of the carboxylic acid building blocks a–j onto construct 2: (a) DIC, DMAP, DIPEA, THF.
N
O
S
O
S
H
N
H
N
H
N
(a)
N
N
N
R
N
N
R
O
O
O
O
7a–7j
8a–8j
Scheme 5. Activation of the polymer-bound carboxylic acid building blocks 7a–7j with bromoacetonitrile: (a) DIPEA, NMP, bromoacetonitrile.
R
H2N
NH
N
N
O
O
NH2
NH2
N
(a)
N
ϩ
8a–8j
N
N
N
N
HO
OH
HO
OH
9
10a–10j
Scheme 6. Transfer of the activated polymer bound building blocks 8a–8j onto amine 9: (a) THF/NMP.

Naked-Eye Bead Property Estimation
185
R
NH
N
O
NH2
N
N
N
HO
OH
R
F
O
F
H
N
O
O
O
O
O
O
10a
10b
10c
10d
10i
10e
10j
O
O
O
Cl
O
F
Cl
O
O
Cl
F
Cl
10f
10g
10h
Scheme 7. Structure of compounds 10a–10j.
Table 1. HPLC purities and HRMS (ESI) results for compounds 10a–10j
Compound
Purity [%]A
Purity [%]B
Yield [%]C
HRMS (ESI) (Calc.)
HRMS (ESI) (Found)
10a
10b
10c
10d
10e
10f
10g
10h
10i
98.9
93.9
92.1
97.9
68.2
99.2
92.5
97.2
78.5
97.7
93.5
95.1
91.9
99.7
86.2
99.8
95.6
92.9
83.5
94.5
87
96
96
100
100
99
98
93
95
87
375.1803
433.1440
450.1913
439.2102
485.2130
479.0990
545.1102
433.1460
399.1784D
401.1961
375.1781
433.1436
450.1890
439.2094
485.2146
479.1001
545.1083
433.1436
399.1757D
401.1937
10j
A Relative purity was determined by HPLC using the 100% method, UV detection at 220 nm.
B Relative purity was determined by HPLC using the 100% method, UV detection at 254 nm.
C Yield was determined through the conversion rate detec•ted at 254 nm.
D Calculated and detected mass in this case is [M + Na]+
.
resolution mass spectrometry (HRMS (ESI)). The results are
shown in Table 1.
using tetramethylsilane as internal standard. Test samples prepared in
mg quantities were evaluated by HRMS. Purity of the latter compounds
was deduced from 1H NMR and LC data. HRMS data were obtained
on a Micromass Autospec (ESI, methanol/water 1 : 1 v/v, infusion at
10 µL min−1 with polyethylene glycol as reference) instrument. Prepar-
ative column chromatography was performed on silica gel 100–200
active, 60 Å, from ICN, or Dowex OH− (1 × 2–200) using glass columns
(4.5 × 15 cm). TLC reaction control was performed on Macherey–
Nagel Polygram Sil G/UV254 precoated microplates with spots being
visualized under UV illumination at 254 nm.
Conclusions
The limited possibilities of straightforward non-destructive
bead property analysis still hamper the use of polymer-bound
reagents. Through insertion of a phenylazo moiety into the
Kenner linker modified according to Ellman et al.,[15] a novel
red linker was obtained that did not affect the functionality
of the safety-catch principle. Thus, we have contributed to
currently known techniques for visual bead property estima-
tion for the first step of every polymer related synthesis step,
namely attachment of the linker onto the resin. The depth of
colourallowsforanaked-eyeapproximationoftheresinload-
ing, but if a validated and exact determination was desired,
other analytical methods would be necessary. Use of the novel
linker for high yielding syntheses of demanding nucleoside
derivatives was successfully demonstrated.
4-Nitrosobenzoic Acid 4
This was prepared according to the method of Melnikov et al.[18]
4-Aminobenzoic acid 3 (2.75 g, 0.02 mmol) was dissolved in a three-
neck round bottom flask in 1 : 1 dichloromethane/diethyl ether (250 mL)
and disodium tungstate dehydrate (0.66 g, 2.0 mmol), phosphoric acid
(1.0 mL), tetrabutylammonium bromide (0.20 g, 0.62 mmol), and 30%
hydrogen peroxide (20 mL, 0.2 mol) were added. The suspension was
then stirred at 35–40◦C for 180 min. The organic layer was separated,
washed with 0.01 M HCl (100 mL) and water (100 mL), dried over
sodium sulfate, and evaporated under vacuum to give 4 (86%). (Found:
C 55.5, H 3.5, N 9.3. C7H5NO3 requires C 55.6, H 3.3, N 9.3%).
Experimental
4-(4-Sulfamoylphenylazo)benzoic Acid 5
The structures of all compounds were assigned by NMR spectroscopy.
Glacial acetic acid (100 mL) was added to a mixture of 4 (3.02 g,
NMR spectra were recorded on a JEOL ECLIPSE+500 spectrometer
0.02 mol) and sulfanilamide (3.42 g, 0.02 mol). The resultant mixture

186
P. Heidler and A. Link
was heated at reflux for 4 h and allowed to cool to room tempera-
ture. Small portions of toluene were then added to aid in removing the
acetic acid under vacuum. Purification was performed by two different
methods.
skin contact) over 12–48 h in the presence of Hünig’s base (170 µL,
1.0 mmol). The dark brown slurry was then washed exhaustively with
dry dimethyl sulfoxide (5 × 5 mL) and THF (3 × 10 mL).
2-(3-Aminopropenyl)-5-(6-aminopurin-9-yl)tetrahydrofuran-
3,4-diol 9
Method A
The brownish powder was suspended in water (150 mL), concen-
trated sodium hydroxide solution was added until the solid dissolved,
and the solution was washed with ethyl acetate (3×). The resultant red
solid that formed in the aqueous layer was then separated by suction fil-
tration, washed with water, and dried under vacuum to afford the sodium
salt, mp 385◦C (dec.). δH (500 MHz; [D6]DMSO) 8.03 (6H, m), 7.82
(2H, d, J 8.5), 7.51 (2H, br s).
The sodium salt was subsequently heated at reflux with 1 M HCl for
10 min and the resultant red solid was separated by filtration and dried
under vacuum.
This was prepared according to the method of Lerner et al.[21] δH
(500 MHz; [D6]DMSO + 1 drop D2O) 8.30 (1H, br s), 8.16 (1H, br s),
5.89 (1H, m), 5.79 (2H, m), 4.65 (1H, m), 4.31 (1H, m), 4.10 (1H, m),
3.59 (1H, m), 3.20 (1H, m). δC (126 MHz; [D6]DMSO) 156.1, 153.1,
149.7, 140.1, 132.4, 129.2, 119.2, 84.4, 77.8, 74.1, 73.2, 41.9.
General Procedure for the Synthesis of Compounds 10a–10j
The activated polymer-supported acids 8a–8j (approx. 0.2 mmol)
described above were transferred to the amino group of the appropri-
ate amino template 9 (6.0 mg, 0.02 mmol) dissolved in NMP (1 mL) by
being shaken at room temperature in THF (4 mL). The reaction mixture
was monitored by TLC and terminated upon the quenching of the start-
ing material 9 (12–48 h). Polymer beads and particulates were removed
by filtration and the beads were extracted exhaustively with dry THF
and methanol. The combined fractions were evaporated to furnish the
target compounds 10a–10j.
Method B
The brownish powder was suspended in water (150 mL), concen-
trated sodium hydroxide solution was added until the solid dissolved,
and the solution was washed with ethyl acetate (3×). The pH of the
solution was then adjusted to 6 and the aqueous layer was extracted with
ethyl acetate (3×) (because of poor solubility, several fractions had to
be collected, separately). The combined organic layers were dried and
the solvent was removed under vacuum to afford 5, mp 321◦C (dec.).
(Found: C 51.1, H 3.6, N 14.1. C13H11N3O4S requires C 51.1, H 3.6,
N 13.8%). δH (500 MHz; [D6]DMSO) 13.48 (1H, br s), 8.17 (2H, d, J
8.5), 8.07 (4H, d, J 8.7), 8.02 (2H, d, J 8.5), 7.54 (2H, br s).
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
3,3-dimethylacrylamide 10a
Relative purity by HPLC: 98.8% (220 nm), 93.5% (•250 nm). (Found
•
(HRESI): [M + H]+ 375.1781. Calc. for [M + H]+ 375.1803). δH
General Procedure for the Coupling of 4-(4-Sulfamoylphenylazo)
benzoic Acid 5 onto Aminomethylated Polystyrene 6 To Yield
Polymer-Bound Linker 2
(500 MHz; [D6]DMSO) 8.28 (1H, br s), 8.14 (1H, br s), 7.85 (1H, t,
J 5.7), 7.22 (2H, br s), 5.88 (1H, d, J 5.1), 5.77 (1H, m), 5.66 (1H, m),
5.47 (1H, d, J 5.8), 5.25 (1H, d, J 5.9), 4.67 (1H, q, J 5.2), 4.31 (1H, t,
J 6.1), 4.10 (1H, q, J 5.3), 3.72 (2H, q, J 4.9), 3.32 (1H, m, overlapping
H2O), 2.07 (3H, d, J 1.1), 1.78 (3H, d, J 1.1).
N,N-Diisopropylcarbodiimide (480 µL, 3.1 mmol) was added to a sus-
pension of HOBt (416 mg, 3.1 mmol) and 4-(4-sulfamoylphenylazo)
benzoic acid 5 (935 mg, 3.1 mmol) in THF (20 mL) and the resultant
mixture was activated for 4 h (CAUTION: N,N-diisopropylcarbodiimide
may lead to severe allergic reactions, strictly avoid skin con-
tact). Aminomethylated polystyrene 6 (1.0 g, initial loading level
1.40 mmol g−1, 2% cross-linked, purchased from Novabiochem, batch
number A25711), pre-swollen in THF (9 mL), was then added to the
reaction mixture, which was subsequently shaken for 24 h. The resin
was washed with DMF (3×), dichloromethane (3×), and THF (3×),
and was then dried under vacuum. Combustion analysis of the sulfur
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
2,6-difluorobenzamide 10b
Relative purity by HPLC: 93.9% (220 nm), 95.1% (•250 nm). (Found
•
(HRESI): [M + H]+ 433.1436. Calc. for [M + H]+ 433.1440). δH
(500 MHz; [D6]DMSO) 8.85 (1H, m), 8.26 (1H, br s), 8.14 (1H, br s),
7.50 (1H, m), 7.22 (2H, br s), 7.14 (2H, m), 5.91 (1H, m), 5.87 (1H, m),
5.77 (1H, m), 5.51 (1H, m), 5.29 (1H, m), 4.63 (1H, m), 4.34 (1H, m),
4.11 (1H, m), 3.90 (2H, m).
content revealed a loading level of 0.93 mmol g−1
.
General Procedure for the Synthesis of Polymer-Supported
Acids 7a–7j
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
2-(1H-indol-3-yl)acetamide 10c
N,N-Diisopropylethylamine(150 µL, 0.88 mmol)and4-dimethylamino-
pyridine (DMAP) (10 mg, 0.08 mmol) were added to a suspension of dry
4-(4-sulfamoylphenylazo)benzoylaminomethyl polystyrene 2 (500 mg
with an initial loading level of 0.93 mmol g−1 as determined by elemen-
tal analysis) in THF (6 mL), and the resin was allowed to swell at room
temperature for 2 h. The appropriate acid a–j (1.2 mmol, 2.6 equiv.) in a
minimum amount of THF was treated with DIC (93 µL, 0.6 mmol) for
4 h before being added to the resin mixture, and the resultant reaction
was agitated at room temperature for 24 h. The resin beads were filtered
off and washed with DMF (3 × 5 mL), dichloromethane (3 × 5 mL),
and THF (3 × 5 mL). After careful drying, the increase in weight was
determined. The progress of the reaction could also be followed by IR
spectroscopy as acylation of the sulfonamide linker led to a decrease in
the intensity of the sulfonamide absorption at 3398 cm−1 concurrently
Relative purity by HPLC: 92.1% (220 nm), 91.9% (•250 nm). (Found
•
(HRESI): [M + H]+ 450.1890. Calc. for [M + H]+ 450.1913). δH
(500 MHz; [D6]DMSO) 10.80 (1H, br s), 8.26 (1H, br s), 8.15 (1H, br s),
7.99 (1H, t, J 5.7), 7.53 (1H, d, J 8.4), 7.32 (1H, d, J 8.4), 7.23 (2H,
br s), 7.17 (1H, s), 7.04 (1H, m), 6.94 (1H, t, J 7.8), 5.88 (1H, d, J 5.3),
5.76 (1H, m), 5.69 (1H, m), 5.47 (1H, m), 5.24 (1H, m), 4.63 (1H, m),
4.29 (1H, m), 4.07 (1H, m), 3.71 (2H, m), 3.52 (2H, br s).
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
2-(1H-indol-3-yl)-4-phenylbutyramide 10d
with the appearance of a new carbonyl stretch at 1729 cm−1
.
Relative purity by HPLC: 97.9% (220 nm), 99.7% (•250 nm). (Found
•
(HRESI): [M + H]+ 439.2094. Calc. for [M + H]+ 439.2102). δH
General Procedure for Activation of the Polymer-Bound
Carboxylic Acids 7a–7j and 8a–8j
(500 MHz; [D6]DMSO) 8.28 (1H, br s), 8.14 (1H, br s), 7.92 (1H, t,
J 5.5), 7.27 (2H, m), 7.22 (2H, be s), 7.16 (3H, m), 5.88 (1H, d, J 5.0),
5.78 (1H, m), 5.69 (1H, m), 5.47 (1H, br s), 5.25 (1H, br s), 4.65 (1H, t,
J 5.0), 4.30 (1H, m), 4.1 (1H, t, J 4.9), 3.70 (2H, m), 2.55 (2H, t, J 7.7),
2.10 (2H, t, J 7.5), 1.79 (2H, m, J 7.6).
The appropriate resin 7a–7j (approx. 0.2 mmol) in N-methylpyrrolidone
(NMP) (2 mL) was activated for cleavage by alkylation with bromoace-
tonitrile (320 µL, 4.5 mmol) (CAUTION: alkylating agent, strictly avoid

Naked-Eye Bead Property Estimation
187
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
2-(1H-indol-3-yl)-3,4-diethoxybenzamide 10e
Acknowledgments
This work was supported by the Fonds der Chemischen
Industrie. Technical assistance of trainee Andreas Schmidt
is acknowledged.
Relative purity by HPLC: 68.2% (220 nm), 86.2% (•250 nm). (Found
•
(HRESI): [M + H]+ 485.2146. Calc. for [M + H]+ 485.2130). δH
(500 MHz; [D6]DMSO) 8.47 (1H, t, J 5.4), 8.29 (1H, br s), 8.08 (1H,
br s), 7.44 (2H, m), 7.21 (2H, br s), 6.99 (1H, d, J 8.7), 5.89 (1H, d,
J 5.5), 5.81 (2H, m), 5.48 (1H, m), 5.26 (2H, m), 4.68 (1H, m), 4.32
(1H, m), 4.10 (1H, m), 4.06 (4H, m), 3.90 (2H, m), 3.65 (1H, m), 1.33
(6H, t, J 6.7).
References
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N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
2-(3,4-dichlorophenyl)acetamide 10f
Relative purity by HPLC: 99.2% (220 nm), 99.8% (•250 nm). (Found
•
(HRESI): [M + H]+ 479.1001. Calc. for [M + H]+ 479.0990). δH
(500 MHz; [D6]DMSO) 8.27 (1H, s), 8.21 (1H, t, J 5.7), 8.16 (1H,
br s), 7.44 (1H, s), 7.42 (1H, s), 7.29 (1H, t, J 8.4), 7.23 (2H, br s), 5.89
(1H, d, J 5.0), 5.82 (1H, m), 5.72 (1H, m), 5.49 (1H, d, J 5.9), 5.25 (1H,
d, J 5.5), 4.65 (1H, m), 4.32 (1H, m), 4.11 (1H, m), 3.82 (2H, s), 3.73
(2H, m).
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3, 559. doi:10.1021/CC010019D
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N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}butyramide 10g
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Relative purity by HPLC: 92.5% (220 nm), 95.6% (•250 nm). (Found
•
(HRESI): [M + H]+ 545.1083. Calc. for [M + H]+ 545.1102). δH
(b) F. Gaggini, A. Porcheddu, G. Reginato, M. Rodriquez,
M.Taddei, J. Comb. Chem. 2004, 6, 805. doi:10.1021/CC049963A
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CC010060G
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(500 MHz; [D6]DMSO) 8.29 (1H, br s), 8.15 (1H, br s), 8.20 (1H, t,
J 5.6), 7.54 (1H, d, J 2.7), 7.33 (1H, dd, J 6.2 and 2.7), 7.25 (2H, br s),
7.14 (1H, d, J 9.0), 5.89 (1H, d, J 5.2), 5.79 (1H, m), 5.69 (1H, m), 5.50
(1H, m), 5.27 (1H, m), 4.66 (1H, m), 4.30 (1H, m), 4.10 (1H, m), 4.05
(2H, t, J 6.4), 3.71 (2H, q, J 5.2), 2.29 (2H, t, J 7.2), 1.95 (2H, m).
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
3,5-difluorobenzamide 10h
Relative purity by HPLC: 97.2% (220 nm), 92.9% (•250 nm). (Found
•
(HRESI): [M + H]+ 433.1436. Calc. for [M + H]+ 433.1460). δH
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41, 2975. doi:10.1002/1521-3773(20020816)41:16<2975::AID-
ANIE2975>3.0.CO;2-N
(500 MHz; [D6]DMSO) 8.81 (1H, m), 8.28 (1H, br s), 8.06 (1H, br s),
7.55 (1H, m), 7.43 (1H, t, J 4.4), 7.21 (2H, br s), 5.88 (2H, m), 5.79
(1H, m), 5.46 (1H, m), 5.26 (1H, m), 4.69 (1H, m), 4.33 (1H, m), 4.10
(1H, m), 3.92 (2H, m).
(b) M. Barth, J. Rademann, J. Comb. Chem. 2004, 6, 340.
doi:10.1021/CC034041D
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N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}-
3-methylbutyramide 10i
Relative purity by HPLC: 78.5% (220 nm), 83.5% (•250 nm). (Found
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Commun. 2001, 66, 897. doi:10.1135/CCCC20010897
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•
(HRESI): [M + H]+ 399.1757. Calc. for [M + H]+ 399.1784). δH
(500 MHz; [D6]DMSO) 8.25 (1H, br s), 8.13 (1H, br s), 7.89 (1H, t,
J 5.5), 7.21 (1H, br s), 5.87 (1H, d, J 4.8), 5.71 (2H, m), 4.63 (1H, t,
J 4.81), 4.29 (1H, t, J 5.7), 4.09 (1H, t, J 5.0), 3.68 (2H, m), 0.99 (2H,
d, J 6.4), 0.84 (7H, m).
N-{(E)-3-[(2R,3S,4R,5R)-5-(6-Amino-9H-purin-9-yl)-
3,4-dihydroxytetrahydrofuran-2-yl]prop-2-enyl}cyclohex-
2-ene-carboxylic Acid Amide 10j
Relative purity by HPLC: 97.7% (220 nm), 95.4% (•250 nm). (Found
•
(HRESI): [M + H]+ 401.1937. Calc. for [M + H]+ 401.1961). δH
(500 MHz; [D6]DMSO) 8.28 (1H, br s), 8.15 (1H, br s), 7.91 (1H, m),
7.26 (2H, br s), 5.89 (1H, d, J 5.0), 5.73 (2H, m), 5.65 (2H, br s), 5.47
(1H, m), 5.24 (1H, m), 4.66 (1H, m), 4.31 (1H, m), 4.11 (1H, m), 3.70
(2H, m), 2.34 (1H, m), 2.09 (1H, m), 2.03 (3H, m), 1.75 (1H, m), 1.51
(1H, m).
Accessory Materials
HPLC profiles of compounds 10a–10j at 250 and 220 nm
are available from the author or, until February 2010, the
Australian Journal of Chemistry.