Synthesis of a novel, recyclable, solid-phase acylating reagent.

Article abstract of DOI:10.1016/S0960-894X(02)00272-X

In this paper, we describe the synthesis of N-(6-cyano-1,3-dimethyl-2,4-dioxo-5-substituted-1,3-dihydropyridino[2,3-d] pyrimidin-7-yl)imides 1. We will show the synthesis of 1 using both conventional heating and microwave techniques. In addition, the imide was attached to polystyrene and this immobilized imide was equally as effective at acylating various primary and secondary amines as its solution-phase counterpart.

Full text of DOI:10.1016/S0960-894X(02)00272-X

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1799–1802
Synthesis of a Novel, Recyclable, Solid-Phase Acylating Reagent
Robert B. Nicewonger,* Lori Ditto, Doug Kerr and Laszlo Varady
Department of Chemistry ArQule, Inc., 19 Presidential Way, Woburn, MA 01801, USA
Received 30 November 2001; accepted 20 February 2002
Abstract—In this paper, we describe the synthesis of N-(6-cyano-1,3-dimethyl-2,4-dioxo-5-substituted-1,3-dihydropyridino[2,3-d]
pyrimidin-7-yl)imides 1. We will show the synthesis of 1 using both conventional heating and microwave techniques. In addition,
the imide was attached to polystyrene and this immobilized imide was equally as effective at acylating various primary and sec-
ondary amines as its solution-phase counterpart. # 2002 Elsevier Science Ltd. All rights reserved.
The acylation of amines is one of the most common
reactions in all of organic chemistry. Among the
numerous reagents available for this transformation are
acyl halides, anhydrides, and activated esters prepared
from an acid and coupling reagents such as EDC, DCC,
BOP, PyBOP, HOBt, CDI, HBTU, or HATU.1 Solid-
phase coupling agents have also been developed and are
available commercially. These include polymer sup-
ported EDC, HOBt, tetrafluorophenol, and a few dif-
ferent carbodiimides.
synthesis of the solid-phase imide and its reaction with
various amines.
Synthesis of the imide is shown in Scheme 2. Amine 6
was generated from a three component condensation
reaction between 6-amino-1,3-dimethyl uracil, cyano-
malonate, and o-tolualdehyde.3 This amine was very
unreactive toward general electrophiles such as anhy-
drides, isocyanates, and isothiocyanates. In fact, the
only conditions that allowed a reaction to occur at the
exocyclic nitrogen was with excess acid chloride in
refluxing pyridine for prolonged times (12–48 h). With
our recent interest in shortening reaction times using
microwave assisted organic synthesis and in optimizing
chemical reactions using statistical design of experi-
ments,4 we thought this reaction would be an ideal
candidate to apply both principles.
During the course of a library synthesis, we noticed that
N-(6-cyano-1,3-dimethyl-2,4-dioxo-5-substituted-1,3-
dihydropyridino[2,3-d]pyrimidin-7-yl)imides 1 could be
deacylated with a macroporous Tris resin (Scheme 1).2
At this point we realized a potential use of these com-
pounds as acylating reagents and decided to pursue.
We will show the synthesis of this imide using both
conventional organic techniques and microwave tech-
nology (optimization of the microwave parameters will
be discussed in detail) as well as the acylation of several
amines. With the appropriate linker, the imide could be
bound to a solid-phase support. We will show the
To find optimal microwave reaction conditions, we used
design of experiments (DoE). To generate our model,
we chose a quadratic design with three variables—
equivalents of benzoyl chloride, microwave power, and
time of reaction. The worksheet of experiments that
were performed is shown in Table 1. Examination of the
results show that for product formation, there is a
quadratric dependence on equivalents of acid chloride
and also a strong positive interaction between power
and time (i.e., the reaction could be done at low power
for a long time or high power for a short time).5 Max-
imum product formation was predicted and found to
occur with 12.4 equiv acid chloride at 300 W for 2 min.
Scheme 1. Deacylation of imide 1. Reagents and conditions: (a) macro-
porous-Tris amine resin, pyridine/acetonitrile (1:1).
Imide 8 was then reacted with amines in solvents such as
dichloromethane, THF, or acetonitrile to generate ben-
zoyl amides 11 (Scheme 2). Table 2 shows the results
from a variety of amines (see Fig. 1). Note that no
reaction occurred with anilines or sterically hindered
*Corresponding author. Tel.: +1-781-994-0496; fax: +1-781-376-
6019; e-mail: rnicewonger@arqule.com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00272-X

1800
R. B. Nicewonger et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1799–1802
Scheme 2. Synthesis of imide 8 and use as an acylating reagent. Reagents and conditions: (a) isopropanol, reflux, 24 h; (b) benzoyl chloride, pyridine,
reflux, 36 h or microwave irradiation 300 W, 2min; (c) dichloromethane, rt; (d) silica gel purification, then benzoyl chloride, pyridine, reflux, 36 h or
microwave irradiation 300 W, 2min.
amines even at 50 ꢀC for 24 h. Secondary amines reacted
sluggishly, but could be pushed by heating to 50 ꢀC
overnight with selective N-acylation in the presence of
hydroxyl groups (see entries 17–19).
benzaldehyde generated ester 27. Hydrolysis of the ester
proved challenging—even under reflux with conven-
tional heating, no product was observed after 7 days.
We again turned to microwave irradiation conditions—
165 ꢀC for 20 min proved successful. The high tempera-
ture was necessary to get the ester into solution. Stan-
dard peptide coupling conditions afforded the resin-
bound amine 29. The same reaction conditions used to
make the imide in solution were applied to the solid
phase—12.4 equiv acid chloride, 300 W of microwave
power for 2min using pyridine as a solvent. (Note that
if thermal conditions were applied here—excess acid
chloride, 12–36 h in refluxing pyridine—the resin turned
deep brown and could not be thoroughly washed. Even
after extensive Soxhlet extraction, leachables were pre-
sent in high amounts.) The loading of the resin was
0.37 mmol/g as determined by reaction with tryptamine
26.
It should also be noted that the imides were stable to
prolonged treatment in methanol (3 days at 50 ꢀC). Pri-
mary amines and piperazines reacted quickly at room
temperature. The by-product amide 10 of the reaction
could be isolated, reacylated with an acid chloride and
used in further acylations reactions.
The resin-bound version of 1 was constructed as shown
in Scheme 3. The three component condensation of
aminouracil, dicyanomalonate, and 4-carbomethoxy-
Table 1. Experiments generated by CCF design
Experiment
numbera
Benzoyl
chloride (equiv)
Power
(W)
Time
(min)
Relative
reaction yieldb
Table 2. Reaction conversionsa for solution-phase benzoylation of
amines 12–24
1
2
3
4
5
6
7
8
230
20
2300
20
230
2
2300
20
2165
20
11
11
11
11
11
11
11
25
30
253
300
10
0
10
300
6 8
165
30
300
165
165
165
165
165
2
29
100
10
Amine
% Conversion in
2 h at rt
% Conversion
o/n at rt
% Conversion
o/n at 50 ꢀC
2
35
30
9512
0
0
0
0
0
0
0
0
0
0
30
56
65
nr
100
nr
nr
nr
0
0
0
0
0
56
95
72
nr
nr
nr
nr
nr
0
13
14
15
16
17
18
19
20
21
22
23
24
10
10
9
2
10
11
12
13
14
15
16
17
6
6
6
294
10
6
6
6
40
52
32
0
14
35
100
93
100
100
100
32
77
41
62
aReactions were carried out by adding benzoyl chloride to a solution
of 6 in pyridine. All samples were irradiated using a Discover single-
mode microwave instrument from CEM.
bRelative reaction yield was determined by measuring the HPLC area
of the product amide. The largest peak is designated as 100. Values for
each of the other experiments is calculated as a ratio of peak area
divided by the maximum peak area.
aAliquots were removed from reactions at given times and diluted with
DMSO. Conversion was measured by the disappearance of starting
material. Percentages were calculated by measuring the HPLC peak
area of the remaining starting material at the given time divided by the
HPLC peak area of the starting material peak with no acylating
reagent present.

R. B. Nicewonger et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1799–1802
1801
Scheme 3. Synthesis of polymer bound imide and use as an acylating reagent. Reagents and conditions: (a) 4-carbomethoxybenzaldehyde, cyano-
malonate, isopropanol, reflux, 24 h; (b) microwave irradiation, 165 ꢀC, 20 min 1 M HCl/MeOH (3:1); (c) nova aminomethyl polystyrene, HATU,
NMM, DMF; (d) 12.4 equiv benzoyl chloride, pyridine, microwave irradiation 300 W, 2 min; (e) RNH2, dichloromethane, 2–24 h, 25–50 ꢀC.
Table 3. Reaction conversionsa for solid-phase benzoylation of
selected amines
Amine
Conversion o/n at rt
15
16
21
22
23
24
25
26
0
0
89
98
93
96
88
96
aConversion was measured by the disappearance of starting material.
Percentages were calculated by measuring the HPLC peak area of the
remaining starting material at a given time divided by the HPLC peak
area of the starting material peak with no acylating resin present.
and found to be 0.34 mmol/g, only slightly lower than
the original 0.37 mmol/g.
In conclusion, we have demonstrated a synthesis of N-
(6-cyano-1,3-dimethyl-2,4-dioxo-5-substituted-1,3-dihy-
dropyridino[2,3-d]pyrimidin-7-yl)imides 1 using both
conventional and microwave heating. These imides
function as acylating reagents in solution. Primary
amines and piperazines reacted quickly at room tem-
perature, while more hindered secondary amines
required more time and heat. Anilines were unreactive.
A solid-phase imide was also prepared and shown to be
equally effective as an acylation reagent. This resin
bound acylating reagent was shown to be recyclable
after washing with DMF and reactivation using novel
microwave conditions.
Figure 1. Amines used in the acylation study.
Compound 30 was used to acylate the same amines as in
the previous solution-phase examples. Table 3 shows
the results. Resin bound imide 30 was equally effective
at acylating primary amines and piperazines. Just as its
solution phase counterpart, 30 did not acylate anilines.
The by-product of the acylation is resin bound amide
32, which could be washed and reactivated for acyl
donation by repeating the microwave acylation condi-
tions. After one such cycle, the loading was measured
References and Notes
1. (a) For a more complete list of coupling reagents, refer to:
Larock, R. C. Comprehensive Organic Transformations: A
Guide to Functional Group Preparations; VCH: New York,
1989; p. 972. (b) March, J. Advanced Organic Chemistry:

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R. B. Nicewonger et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1799–1802
Reactions, Mechanisms, and Structure; John Wiley & Sons:
New York, 1985; p. 371, and references cited therein.
2. Nicewonger, R. B.; Ditto, L.; Varady, L. Tetrahedron Lett.
2000, 41, 2323.
3. For a two-step sequence to make the amine, refer to: Bhuyan,
P.; Boruah, R. C.; Sandhu, J. S. J. Org. Chem. 1990, 55, 568.
4. (a) Nicewonger, R. B.; Kerr, D.; Varady, L. Book of
Abstracts, Part 2, 222nd National Meeting of the American
Chemical Society, Chicago, IL, Aug 26–30, 2001; American
Chemical Society: Washington, DC, 2001; ORGN 528. (b)
Kerr, D.; Nicewonger, R.B.; Varady, L. Book of Abstracts,
Part 2, 222nd National Meeting of the American Chemical
Society, Chicago, IL, Aug 26–30, 2001; American Chemical
Society: Washington, DC, 2001; ORGN 291.
5. For all DoE experiments, we used Modde 6.0 software
package from Umetrics, Inc.