2-Oxo-1,2-ethylenedioxy group as a linker for solution-, liquid-, and solid-phase syntheses to discover drug-like small molecules

Article abstract of DOI:10.1016/j.tetlet.2005.04.138

2-Oxo-1,2-ethylenedioxy (2-OED) functionality has been prepared on a soluble MPEG polymer and solid (HM resin and Lanterns) supports as platforms for discovery of drug-like small molecules. The functionality is cleaved either by Yb(OTf)₃ or TMSCHN₂ in MeOH, or by various amines to release small molecules synthesized on the platform in good yield. Application of these platforms aiming for discovery of potent agonists for growth hormone secretagogue is reported by using the Ugi four-component coupling reaction.

Full text of DOI:10.1016/j.tetlet.2005.04.138

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4667–4670
2-Oxo-1,2-ethylenedioxy group as a linker for solution-, liquid-,
and solid-phase syntheses to discover drug-like small molecules
*
Masato Oikawa, Yoshiyuki Takeda and Makoto Sasaki
Laboratory of Biostructural Chemistry, Graduate School of Life Sciences, Tohoku University, Tsutsumidori-Amamiya,
Aoba-ku, Sendai 981-8555, Japan
Received 29 March 2005; revised 18 April 2005; accepted 22 April 2005
Available online 23 May 2005
Abstract—2-Oxo-1,2-ethylenedioxy (2-OED) functionality has been prepared on a soluble MPEG polymer and solid (HM resin and
Lanterns) supports as platforms for discovery of drug-like small molecules. The functionality is cleaved either by Yb(OTf) or
3
2
TMSCHN in MeOH, or by various amines to release small molecules synthesized on the platform in good yield. Application of
these platforms aiming for discovery of potent agonists for growth hormone secretagogue is reported by using the Ugi four-com-
ponent coupling reaction.
Ó 2005 Elsevier Ltd. All rights reserved.
Polymer-supported synthesis is now realized as an effi-
cient strategy to discover drug-like small molecules used
for biological study such as chemical genetics where
liquid-phase synthesis may be used. PEG is soluble in
most solvents, and hence provides rapid reaction rate,
except for ethers such as diethyl ether or tert-butyl
1
4–6
small molecules play a central role. Experimentally,
polymer-supported synthesis is used, (1) to realize a
small molecules library by means of diversity-oriented
methyl ether where PEG easily precipitates.
Once
the library is constructed, it is submitted to biological
screening. Hit compounds found from the screening
are then needed to be synthesized in a larger scale for
quantitative evaluation of the activity. This is carried
out in either liquid-phase or solid-phase using a bigger
2
organic synthesis as a source of biologically interesting
molecules, and (2) to resynthesize hit compounds found
3
from the library by biological screening. Polymers used
TM
in these studies are, in most cases, a soluble polymer
such as poly(ethylene glycol) (PEG) and an insoluble
polystyrene-based cross-linked polymer.
support such as SynPhase Lanterns so that even chemi-
cally unstable intermediates are treated easily.
To develop the chemical studies described above effi-
ciently, a linker of the same chemical structure should
be used to reproduce the reactivity of the small molecules
even on a different support. Here, we report the applica-
tion of the ester-type linker, named 2-oxo-1,2-ethylenedi-
oxy (2-OED, Fig. 1) group, on a soluble PEG polymer as
well as insoluble cross-linked polystyrene-based poly-
mers, HM resin and Lanterns, aiming for discovery of
drug-like small molecules. Though the linker has been
Our strategy for construction of a small molecules li-
brary containing thousands of privileged structure, as
well as for resynthesis of hit compounds is summarized
in Table 1. Process development is at first studied in the
4
solution-phase or in the liquid-phase using PEG to find
the best reaction conditions which provide the desired
products in good yield. One advantage in using PEG
over a solution-phase platform here is that it allows iso-
lation of chemically unstable intermediates such as an
imine compound. The same reactions are also attempted
on solid-phase using small bead support, and after fine
tuning of the reaction conditions, a diverse collection
of small molecules can be constructed on the support.
For a small size library (less than a thousand), a
7
used for the synthesis of hydantoin and 1,4-benzodiaze-
8
pine-2,5-dione derivatives on PEG polymer, mild cleav-
age conditions have so far not been explored. The
cleavage procedures have been established here, and
some multi-component coupling products have been pre-
pared on these platforms to discover strong agonists for
growth hormone secretagogue (GHS).
*
Corresponding author. Tel./fax: +81 22 717 8827; e-mail: mao@bios.
tohoku.ac.jp
In the present study, we used a soluble poly(ethylene
glycol) monomethyl ether (MPEG-OH) and an
0
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.138

4668
M. Oikawa et al. / Tetrahedron Letters 46 (2005) 4667–4670
Table 1. Chemical strategy aiming for discovery of drug-like small molecules
Solution-phase
Liquid-phase
(
Solid-phase
(small bead support)
Solid-phase
(big Lantern support)
a
a
PEG support)
Process development
Library realization
Resynthesis of hit compounds
X
X
b
X
X
X
X
a
Density of a functional group is <2 nm for one bead and 16 lm for one unit of Lantern in the present study.
Applied for a synthesis of <1000 compounds.
b
O
erally, yields for the loading reaction were found to be
satisfactorily good (87–100% yield) with (2-naph-
thoyl)oxyacetic acid (2) by the use of 1,3-diisopropylcar-
1
O
2
1
1
O
bodiimide (DIC) and DMAP in CH Cl at rt for 24 h.
2 2
R
Figure 1. 2-Oxo-1,2-ethylenedioxy (2-OED) linker.
Cleavage reaction to release small molecule 4 was next
attempted. As had been expected from our earlier study
6
with the alkoxyacetyl (AAc) linker, the cleavage by
Yb(OTf)3 (procedure A) proceeded quantitatively in
both solution and liquid-phases using CH Cl –MeOH
insoluble cross-linked polymer bearing a hydroxymethyl
(
2
2
1
2
HM) functional group, namely HM resin and hydroxy-
(4:1) as a solvent (runs 1 and 2). For solid-phase cleav-
age from 3C and D (runs 3 and 4), the yields were lower
than the homogeneous reaction probably because these
insoluble supports are not swollen sufficiently in the sol-
vent system employed. In comparison with Yb(OTf)3,
the cleavage was effected in better yield satisfactorily
TM
9
methylated SynPhase Lanterns, since it is one of the
simplest and hence is inexpensive, and in addition, it is
reusable.
Introduction of small molecules onto polymers via the
-OED linker was effected by direct acylation with phen-
2
by the use of TMSCHN (procedure B); it was quantita-
2
oxyacetic acid derivatives. The results for loading 2-
naphthol on various supports via the 2-OED linker are
summarized in Table 2. Supports used were benzyl alco-
hol (solution-phase experiments (run 1)), MPEG-OH
tive for both solution and liquid-phases (runs 1 and 2),
and the yields were around 90% for cleavage from the
solid-phase (runs 3 and 4). The cleavage is also effected
by various amines. Thus, by treatment of the polymer 3
1
3
1
0
(
run 2), HM resin (run 3), and Lanterns (run 4). Gen-
with EtNH in water at rt for 40 h, N-ethyl (2-naphthyl-
2
Table 2. Loading and cleavage yields of 2-naphthol on various support, via the 2-OED linker
O
loading
HO
O
cleavage
NAP
O
O
2
procedure A or B
R* OH
A-1D
R*
O
MeO
1
O
O
DIC, DMAP, CH2Cl2
rt, 24 h
CH2Cl2 / MeOH
NAP
NAP
(
4 : 1)
3A (R* = Bn)
4
rt, 48 h
3
3
3
B (R* = MPEG)
C (R* = HM resin)
D (R* = Lantern)
d
Run
Phase
R*-OH
Product and loading yield (%)
Cleavage yield for 4 (%)
e
f
Procedure A
Procedure B
a
a
a
1
2
3
4
Solution
Liquid
Solid
Benzyl alcohol (1A)
MPEG-OH (1B)
HM resin (1C)
Lantern (1D)
3A (93)
3B (>99)
>99
>99
b
g
g
>99
84
>99
91
c
g
g
3C (87)
3D (>99)
c
g,h
g
Solid
61 (90)
89
a
b
c
Yield for chromatographically pure compound.
1
Determined by H NMR (500 MHz).
Determined by weight gain of the polymer after the reaction.
d
e
In all cases, R*-OH (1A–D) were recovered after the cleavage quantitatively.
0 mol% of Yb(OTf) was used.
(hexane solution) was used.
3
3
f
10 equiv of TMSCHN
2
g
h
1
Cleaved 4 from the polymer was pure (>95%) as judged from H NMR (500 MHz).
In parentheses is a combined yield after second cleavage.

M. Oikawa et al. / Tetrahedron Letters 46 (2005) 4667–4670
4669
oxy)acetamide was obtained in 80% and >99% yields for
1
yield (81%) for HM resin and in a lower but an accept-
able yield (62%) for Lantern. The reaction temperature
was specific to the polymer supports; the reaction pro-
ceeded smoothly at rt in the liquid-phase (5B), whereas
heating up to 40–50 °C was required for HM resin 5C
and Lantern 5D, respectively, to lead the reaction to
completion. As for the reaction time, the reaction was
completed within 24–48 h for MPEG 5B and HM resin
5C, respectively. In contrast to this, the Ugi reaction
with Lantern-immobilized aldehyde 5D stopped after
24 h, and prolonged reaction time did not improve the
yield.
4
3
C and D, respectively (data not shown).
With these results in hand, we carried out multi-compo-
nent coupling reactions to show the usefulness of these
platforms in construction of a small molecules library.
The reaction demonstrated here is a Ugi reaction which
is a four-component coupling reaction between alde-
hyde, amine, isonitrile, and carboxylic acid to synthesize
1
5
a-(acylamino)carboxylic acid amide. N-Boc-glycine,
-fluorobenzylamine, and benzyl isocyanide were inde-
4
pendently treated with 4-substituted benzaldehyde
immobilized onto MPEG (0.18 mmol/g), HM resin
(
1.32 mmol/g), and Lanterns (16 lm/unit), prepared by
Release of small molecule 7 from the polymer supports
was effected by the same two procedures, A and B,
shown in Table 2. In both cases, the cleavage proceeded
in the range of good (Lanterns) to excellent (MPEG)
yields. In view of the operational efficiency, the proce-
dure using TMSCHN is advantageous over Yb(OTf) ,
acylation of 1B–D with 2-(4-formylphenoxy)acetic acid
in >94% yield. The optimized conditions and the result
for each platform are shown in Scheme 1. A standard
solvent, MeOH, gave a reasonable result (82% yield)
for MPEG-immobilized aldehyde 5B. For HM resin
2
3
5
C and Lantern 5D, CH Cl , and THF were needed as
2
since the reagent can be easily removed under reduced
pressure after the reaction.
2
co-solvents to swell the polymer, respectively. In these
solvent systems, the reaction proceeded in a comparable
Our attention then moved to a practical application of
these strategies to discover biologically active small mole-
cules. The privileged scaffold shown in Figure 2 has
been recently discovered as a potent agonist for growth
hormone secretagogue (GHS) which controls the release
of growth hormone from somatotrophs in the pituitary
gland at the low-micromolar level. With this signifi-
cantly different structure from known GHS agonists,
we have determined to construct an appendage diverted
library to discover more potent GHS agonists. Though
the structure was shown to be readily accessible by the
Ugi reaction, details for synthetic work are not reported.
Our strongest concern was to realize the Ugi reaction
between such heavily functionalized components on
the polymer support. Here, two aldehydes immobilized
on HM resin (5C and its meta-isomer) were used for
the aldehyde component in combination with two car-
O
5B (R* = MPEG)
5
5
C (R* = HM resin)
D (R* = Lantern)
R*
O
O
1
6
CHO
CO2H
H2N
O
BocN
H
F
CN
O
O
R*
O
a
N
H
boxylic acids (5-phenyl-2-pyrrolecarboxylic acid and 5-
17
O
N
Ugi reaction
(2-methoxyphenyl)-2-pyrrolecarboxylic acid) and two
6
6
6
B
C
D
amines (3-(diethylamino)propylamine and N-(3-amino-
propyl)diethanolamine). In all reactions, cyclohexyl iso-
cyanide was used. Efficient reaction conditions, which
provide the scaffold in Figure 2 by the Ugi reaction were
at first explored using the aldehyde 5C immobilized on
HM resin, since our first goal is construction of a
GHS agonist library comprising thousands of small
molecules. Though the same reaction conditions used
for the synthesis of 6C (Scheme 1) gave the product in
only a trace amount (<20% yield), we found that addi-
tion of a catalytic amount of perchloric acid was effec-
tive for improvement of the yield after several
experiments. With the optimized reaction conditions,
BocN
H
F
O
MeO
O
O
b
N
H
O
N
cleavage
BocN
H
7
F
H
O
N
N
R
Scheme 1. Reagents and conditions: (a) (for 5B) MeOH, rt, 24 h, 82%;
(
for 5C) MeOH–CH
2
Cl
2
(4:1), 40 °C, 48 h, 81%; (for 5D) MeOH–THF
(30 mol%), MeOH–CH Cl (1:4),
HN
(2:1), 50 °C, 24 h, 62%, (b) Yb(OTf)
3
2
2
O
N
rt, 48 h, 99% from 6B, 80% from 6C, and 62% from 6D; TMSCHN
10 equiv), MeOH–CH Cl (1:4), rt, 48 h, 99% from 6B, 90% from 6C,
and 52% from 6D.
2
(
2
2
16
Figure 2. A GHS agonist scaffold (R = alkyl, aryl).

4670
M. Oikawa et al. / Tetrahedron Letters 46 (2005) 4667–4670
a,b
Table 3. Preparation of GHS agonist candidates on the HM resin
3. Clemons, P. A.; Koehler, A. N.; Wagner, B. K.; Sprigings,
T. G.; Spring, D. R.; King, R. W.; Schreiber, S. L.; Foley,
M. A. Chem. Biol. 2001, 8, 1183–1195; Blackwell, H. E.;
Perez, L.; Stavenger, R. A.; Tallarico, J. A.; Eatough, E.
C.; Foley, M. A.; Schreiber, S. L. Chem. Biol. 2001, 8,
L
M
X
H
N
O
N
1
167–1182.
. Bayer, E.; Mutter, M. Nature 1972, 237, 512–513.
4
5
HN
. Oikawa, M.; Ikoma, M.; Sasaki, M. Tetrahedron Lett.
2
O
N
Y
Y
005, 46, 415–418; Oikawa, M.; Tanaka, T.; Kusumoto,
S.; Sasaki, M. Tetrahedron Lett. 2004, 45, 787–790;
Oikawa, M.; Tanaka, T.; Fukuda, N.; Kusumoto, S.
Tetrahedron Lett. 2004, 45, 4039–4042.
Y
6
. Oikawa, M.; Ikoma, M.; Sasaki, M. Tetrahedron Lett.
2
H
OH
004, 45, 2371–2375.
c
c
d
d
OMe
H
90%
60%
>99%
>99%
7. Lee, M. J.; Sun, C. M. Tetrahedron Lett. 2004, 45, 437–
440.
8. Cheng, M. F.; Fang, J. M. J. Comb. Chem. 2004, 6, 99–
X
a
For experimental procedures, see Scheme 1. In all cases, a catalytic
amount of perchloric acid was added to the reaction mixture.
1
04.
TM
9
. MPEG-OH, HM resin, and SynPhase Lanterns were
purchased from Aldrich Co. (catalog no. 20251-7),
Advanced ChemTech, Inc. (catalog no. SA5020), and
Mimotopes Pty Ltd (catalog no. SP-PS-L-HOM-015),
respectively.
2
Cleavage was effected by TMSCHN .
b
c
Yields are for two steps including Ugi and cleavage reactions, and
were determined by LC–MS.
HM resin-immobilized aldehyde 5C was used for an aldehyde com-
ponent. L = –O–CH
2
COOMe, M = H.
The meta-substituted aldehyde corresponding to 5C was used for an
aldehyde component. L = H, M = –O–CH COOMe.
d
10. Polymers were recovered after the reactions by precipita-
tion using diethyl ether for the liquid-phase synthesis, and
by a simple washing in a stopcock-connected column for
the solid-phase synthesis.
2
1
1. A stepwise reaction comprising acylation with bromoace-
tic acid followed by nucleophilic addition of phenol-type
nucleophile is also effective. For the synthesis of 3B
we have successfully synthesized the appendage-diverse
GHS agonist candidates in good yield on the platform
comprising HM resin and 2-OED linker (Table 3). To
the best of our knowledge, this is the first example for
(
MPEG) and 3C (HM resin), the loading proceeded in
good yield (89%) and in poor yield (25%), respectively.
The low yields are due to low reactivity of phenols toward
alkyl bromide on the cross-linked polymer. Since various
derivatives can be commercially available for phenol, we
are now investigating high yield loading procedure via
nucleophilic addition of phenol derivatives.
1
8
perchloric acid-assisted solid-phase Ugi reaction.
We have thus shown that the 2-OED linker on the HM
resin is an efficient platform to synthesize the GHS ago-
nist candidates rapidly. We are now applying the meth-
od to construct a library comprising thousands of the
drug candidates on the platform. We have also shown
the usefulness of the linker in combination with MPEG
1
2. As can be seen in run 1, (2-naphthyloxy)acetyl group is
potentially useful for an efficient protecting group of
hydroxy functionality. For phenoxyacetyl group for pro-
tection of secondary alcohol, see; Shimada, K.; Kaburagi,
Y.; Fukuyama, T. J. Am. Chem. Soc. 2003, 125, 4048–
TM
and Synphase Lanterns for resynthesizing hit com-
pounds to be found from biological evaluation of the li-
brary. The 2-OED linker will be widely used for
discovery of drug-like small molecules through both li-
quid-phase and solid-phase organic syntheses because
of its (1) ready availability on various polymer supports,
4
049.
13. Since the reaction takes place by the nucleophilic attack of
MeOH used as a solvent in the presence of basic
TMSCHN
investigation.
4. Other amines such as MeNH , PrNH , BuNH , allyla-
mine, ethylenediamine, and piperidine were also effective
for the cleavage.
5. Ugi, I. Angew. Chem., Int. Ed. Engl. 1959, 71, 386–386.
6. Shoda, M.; Harada, T.; Kogami, Y.; Tsujita, R.; Akashi,
H.; Kouji, H.; Stahura, F. L.; Xue, L.; Bajorath, J. J. Med.
Chem. 2004, 47, 4286–4290.
2
, the cleavage in various solvents is under
n
n
1
2
2
2
(
2) chemical stability, (3) chemospecific reactivity under
cleavage conditions, and (4) ability to provide moderate
to excellent reactivity to small molecules loaded on both
soluble and insoluble cross-linked polymers.
1
1
References and notes
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Murgioni, C.; La Colla, P. Chem. Pharm. Bull. 2001, 49,
1406–1411.
18. A solution-phase Ugi-type three-component coupling
reaction using perchloric acid has been reported, see;
Bienayme, H.; Bouzid, K. Angew. Chem., Int. Ed. 1998,
37, 2234–2237.
1
2
. Schreiber, S. L. Chem. Eng. News 2003, 81, 51–61;
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Stockwell, B. R. Trends Biotechnol. 2000, 18, 449–455;
Schreiber, S. L. Science 2000, 287, 1964–1969; Schreiber,
S. L. Bioorg. Med. Chem. 1998, 6, 1127–1152.
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2
004, 43, 46–58.