Solid-phase synthesis of 4(5),1′,5′-trisubstituted 2,4′-biimidazoles

Article abstract of DOI:10.1021/jo902122e

(Chemical Equation Presented) Amethod for the synthesis of libraries of 4(5),1′,5′-trisubstituted 2,4′-biimidazoles on a solid support was developed.¹ A trivalent scaffold, 2-(5′-amino-4(5)-formyl- 1H,1′H-2,4′-biimidazol-1′-yl)acetic acid, was

Full text of DOI:10.1021/jo902122e

pubs.acs.org/joc
Solid-Phase Synthesis of 4(5),10,50-Trisubstituted 2,40-Biimidazoles
€
Tuomas Karskela* and Harri Lonnberg
Department of Chemistry, University of Turku, Turku, FIN-20014, Finland
tuomas.karskela@utu.fi
Received October 2, 2009
A method for the synthesis of libraries of 4(5),10,50-trisubstituted 2,40-biimidazoles on a solid support
was developed.1 A trivalent scaffold, 2-(50-amino-4(5)-formyl-1H,10H-2,40-biimidazol-10-yl)acetic
acid, was first prepared in solution by a two-step synthesis from ethyl adenin-9-ylacetate and
bromomalonaldehyde. The product was coupled to an amino acid loaded Wang resin and the formyl
group was subsequently derivatized either by reductive amination, oximation, or hydrazone
formation. The 50-amino group of the resin-bound biimidazole was then acylated and the products
were finally released from the resin and purified. 50-Amino-2,40-biimidazole offers a scaffold for lead
compounds of drug discovery, possibly useful in finding leads for protein kinase inhibitors.
Introduction
conformation in the protein-ATP complex.7 Although these
competitive inhibitors of ATP exhibit high affinity and
sometimes additionally interact with binding elements not
involved in ATP binding, they often suffer from lack of
sufficient selectivity.8 To circumvent the selectivity issue, a
growing number of small-molecule inhibitors that do not
directly compete for the ATP-binding site have emerged.9
These inhibitors may target the inactive conformation of the
kinase, allosteric sites, or binding sites for substrate or a
protein partner.2b,6,7 Despite these extensions to the para-
digm, the number of different molecular frameworks sub-
jected to synthesis of combinatorial libraries still is rather
limited. In fact, it appears that a top-heavy distribution of
frameworks occurs in medicinal chemistry, i.e., the more
often a framework has been used for synthesis of a library,
the more likely it will be used for another combinatorial
approach.10 The selectivity issues together with highly con-
gested intellectual property (IP) space resulting from the
limited chemical space covered has increased the IP risk
associated with the pursuit of protein kinase inhibitors.11
Protein kinases (PK) mediate and regulate the majority of
the signal transduction in cells and they, hence, play an
important role in cell growth, metabolism, differentiation,
and apoptosis.2 A number of diseases, including cancer,
diabetes, and inflammation, are linked to perturbation in
these signaling pathways.3 Thereby PKs are an attractive
target for drug development3,4 and they have recently be-
come the most studied class of drug targets of the pharma-
ceutical industry.5 PKs catalyze the transfer of the terminal
phosphate group from ATP to the hydroxyl group of a
serine, threonine, or tyrosine residue of their protein sub-
strate. The binding site for ATP is highly conserved among
PKs and plenty of structural information of the catalytic
domains is available.2b,6 The majority of small-molecule
kinase inhibitors that have been developed so far target the
ATP binding site. The conformation of the kinase engaged in
the protein-inhibitor complex usually closely resembles the
€
(1) Karskela, T.; Lonnberg, H. Chemistry of Nucleic Acid Components.
Collect. Symp. Ser. 2008, 10, 372-373.
(2) (a) Manning, G.; Whyte, D. B.; Martinez, R.; Hunter, T.; Sudarsanam,
S. Science 2002, 298, 1912–1934. (b) Liao, J. J.-L. J. Med. Chem. 2007, 50, 409–
424.
(3) Noble, M. E. M.; Endicott, J. A.; Johnson, L. N. Science 2004, 303,
1800–1805.
(7) Liu, Y.; Gray, N. S. Nat. Chem. Biol. 2006, 2, 358–364.
(8) Fischer, P. M. Curr. Med. Chem 2004, 11, 1563–1583.
(9) Bogoyevitch, M. A.; Fairlie, D. P. Drug Discovery Today 2007, 12,
622–633.
(4) Cohen, P. Nat. Rev. Drug Discovery 2002, 1, 309–315.
(5) Cohen, P. Curr. Opin. Cell Biol. 2009, 21, 317–324.
(6) (a) Cherry, M.; Williams, D. H. Curr. Med. Chem. 2004, 11, 663–673.
(b) Smyth, L.; Collins, I. J. Chem. Biol. 2009, 2, 131–151.
(10) Lipkus, A. H.; Yuan, Q.; Lucas, K. A.; Funk, S. A.; Bartelt, W. F.;
Schenck, R. J.; Trippe, A. J. J. Org. Chem. 2008, 73, 4443–4451.
(11) Akritopoulou-Zanze, I.; Hajduk, P. J. Drug Discovery Today 2009,
14, 291–297.
9446 J. Org. Chem. 2009, 74, 9446–9451
Published on Web 11/06/2009
DOI: 10.1021/jo902122e
r
2009 American Chemical Society

€
Karskela and Lonnberg
JOCArticle
As a result, there is a permanent need for new scaffolds.
50-Amino-2,40-biimidazoles constitute a rarely studied group
of compounds that may afford an interesting new scaffold
for lead search. They obey well the 2-0 rule of kinase-
likeness proposed by Aronov et al.,12 they are easy to
synthesize, and they contain six points of diversification.
The structure of 50-amino-2,40-biimidazole is a rigid rod, but
rotation around the 2,40-bond connecting the imidazoles is
possible, although hindered. Depending on substituents and
pH, there is a possibility for intramolecular hydrogen bond-
ing between the 50-amino proton and the pyridine-like
nitrogen of the second imidazole ring.13 This hydrogen bond
results in the formation of a pseudo-six-membered ring
between the imidazoles thus making the ring system planar.
Furet has successfully applied a similar pseudoring design
approach to the kinase inhibitor scaffold search.14 Different
twisted conformations between the imidazole rings can be
evoked by the choice of substituents. The multifaceted core
structure makes it possible to design a spatially diverse
set of compounds with a single scaffold. To exemplify
the 50-amino-2,40-biimidazole scaffold synthesis, we have
devised a method to produce 5,10,50-trisubstituted 2,40-
biimidazoles on a solid support.
SCHEME 1a
aReagents and conditions: (i) HCBr(CHO)2, HCO2H, 2,6-lutidine,
DMF; (ii) HCl (aq), (iii) TFA/DCM.
(55 °C) in a 1:1 mixture (v/v) of THF and aq NaOH, the
overall hydroxide ion concentration being 0.08 mol L-1. The
acid-catalyzed hydrolysis was carried out as an overnight
reaction at room temperature in 3.6 mol L-1 HCl in dioxane
containing 10% water. However, part of the support-bound
material was observed to be cleaved off during the hydrolytic
ring-opening. Neither of these solid-supported reactions was,
after all, applied to the library synthesis. Since 3 was a common
precursor of all the library members, its preparation in solution
phase followed by postsynthetic immobilization to commer-
cially available supports loaded with various amino acids
offered a simple way for parallel synthesis of several subli-
braries.
Wang polystyrene resins loaded with Fmoc-protected
amino acids (Leu, Asp, Trp, Ser) were used for the solid-
supported library synthesis. Since 3 and its 1-hydroxybenzo-
triazole ester were sparingly soluble in DMF or DMF/
diisopropylethylamine, compound 3 and 2-(1H-Benzotria-
zol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate
(HBTU), used as an activator, were dissolved in warm dry
pyridine and in DMF, respectively. The combined solutions
were added onto the deprotected solid support (Scheme 2)
and the reaction was allowed to proceed several hours to
ensure completion of the coupling. Extensive washing was
carried out to remove the surpluses of reactants. The sup-
ports obtained, 4a-d, were divided into three portions. The
first portion was oximated with methoxylamine. The second
portion was allowed to react with either 2,6-dinitrophenyl-
hydrazine (DNPH) or phenylhydrazine. Unfortunately, the
applicability of hydrazines as diversifying reagents turned
out to be rather limited. Among the two products obtained,
2,6-dinitrophenylhydrazone nicely gave a single product, 6a,
upon subsequent acylation of the 50-amino function, whereas
the phenylhydrazone gave a product mixture. Evidently, the
central nitrogen atom of the phenylhydrazone still was
sufficiently nucleophilic to undergo acylation, in contrast
to the dinitrophenyl (DNP) derivative that was deactivated
by the strongly electron-withdrawing 2,6-dinitro groups.
The resulting N-acylated phenylhydrazones were relatively
labile and they were partially decomposed to formylbiimi-
dazoles and 1-phenylhydrazides. Changing the order of these
two reactions, hydrazone formation and acylation, did not
Results and Discussion
The common core structure of the library, viz. 2-(50-
amino-5-formyl-1H,10H-2,40-biimidazol-10-yl)acetic acid (3,
Scheme 1), was prepared as a hydrochloride from ethyl
2-(adenin-9-yl)acetate 1 and bromomalonaldehyde along a
two-step route that did not contain chromatographic puri-
fications. In the first step, an additional imidazole ring was
formed by bridging N1 and N6 of the adenine moiety with a
formyletheno group under the conditions originally devel-
oped for the solid supported synthesis of similar com-
pounds.15 The pyrimidine ring of the resulting 3H-
imidazo[1,2-i]purine derivative (2a) could be opened by
either a base-16 or acid-catalyzed17 hydrolysis. Attempted
base-catalyzed hydrolysis led to a dark brown product
mixture, while the acid-catalyzed hydrolysis gave a white
or yellowish product that precipitated from the solution. The
product obtained by the latter method was sufficiently pure
to be used as such for further derivatization.
The pyrimidine ring could also be opened on a solid
support. For that purpose, 2-(7-formyl-3H-imidazo[1,2-i]-
purin-3-yl)acetic acid18 (2c, Scheme 1) was coupled to an amino
acid loaded Rink amide-PEG-PS resin (base-catalyzed
hydrolysis) or HMBA-PS resin (acid-catalyzed hydrolysis).
The base-catalyzed hydrolysis required overnight heating
(12) Aronov, A. M.; McClain, B.; Moody, C. S.; Murcko, M. A. J. Med.
Chem. 2008, 51, 1214–1222.
€
€
(13) Maki, J.; Klika, K.; Sjoholm, R.; Kronberg, L. J. Chem. Soc., Perkin
Trans. 1 2001, 1216–1219.
(14) (a) Furet, P.; Bold, G.; Hofmann, F.; Manley, P.; Meyer, T.;
Altmann, K.-H. Bioorg. Med. Chem. Lett. 2003, 13, 2967–2971. (b) Furet,
P.; Caravatti, G.; Guagnano, V.; Lang, M.; Meyer, T.; Schoepfer, J. Bioorg.
Med. Chem. Lett. 2008, 18, 897–900.
€
(15) Karskela, T.; Lonnberg, H. Org. Biomol. Chem. 2006, 4, 4506–4513.
(16) (a) Yip, K. F.; Tsou, K. C. Tetrahedron Lett. 1973, 33, 3087–3090. (b)
€
€
Maki, J.; Sjoholm, R.; Kronberg, L. J. Chem. Soc., Perkin Trans. 1 2000,
4445–4450.
(17) (a) Shaw, G.; Smallwood, B. M. J. Chem. Soc. C 1970, 2206–2208. (b)
Sattsangi, P. D.; Barrio, J. R.; Leonard, N. J. J. Am. Chem. Soc. 1980, 102,
770–774.
(18) See Supporting Information for synthesis and characterization.
J. Org. Chem. Vol. 74, No. 24, 2009 9447

€
Karskela and Lonnberg
JOCArticle
SCHEME 2a
aReagents and conditions: (i) 20% piperidine in DMF; (ii) 3, HBTU, pyridine, DMF; (iii) R1ONH2 HCl, pyridine; (iv) R2NHNH2, AcOH, DMF,
3
heating; (v) morpholine, HCO2H, NaCNBH3, MeOH, DMF; (vi) BzCl, pyridine; (vii) Ac2O, pyridine; (viii) isobutyryl chloride, pyridine; (ix) 2.5% H2O,
2.5% triisopropylsilane in TFA. bFor annotation of variable groups see Table 1.
improve the situation, as the conditions needed for the
hydrazone formation resulted in cleavage of the acyl group
from the 50-amino function. Accordingly, the synthesis was
continued with 6a and 6b, but not with 6c. Half of the
remaining 4c was instead oximated with phenoxyamine that
circumvented the acylation problem. With 4d, the hydrazone
formation was replaced with a similar oximation. The third
portion of 4a-d was aminated with morpholine by using a
method developed earlier.15 The 50-amino group of biimida-
zole is relatively unreactive which, taken together with the
presence of other reactive groups in the molecules, limits the
selection of chemistries applicable to the solid-supported
library synthesis. Active ester approach and anhydride
activation both failed to give good yields in preliminary
experiments (data not included). For this reason, acyl halides
and acetic anhydride were used in the library synthesis.
Products 5-7 were divided into two portions (except 5e,
5f, and 7c) which were acylated with benzoyl chloride, acetic
anhydride, or isobutyryl chloride. Acyl halides reacted par-
tially twice, but the second substituent was easily removed by
a short ammonia treatment. Initially, the unsubstituted
50-amino group was intended to be used as one of the
diversity groups. For this purpose, compound 8a was synthe-
sized. Because the product remained brown, although homo-
geneous according to HPLC, recrystallization was
attempted. Unfortunately the product decomposed upon
recrystallization and therefore the free 50-amino was aban-
doned as a diversity group.
9448 J. Org. Chem. Vol. 74, No. 24, 2009

€
Karskela and Lonnberg
JOCArticle
TABLE 1. Members of the Synthesized 4(5),10,50-Trisubstituted
2,40-Biimidazole Test Library and Isolated Overall Yields
compd
V1
V2
isolated yield (%)a
8a
8b
9a
9b
9c
9d
9e
9f
9g
9h
9i
10a
10b
10c
10d
10e
11a
11b
11c
11d
11e
11f
Leu
Trp
Leu
Leu
Leu
Asp
Asp
Asp
Trp
Ser
CHNOMe
CHNNHPh
CHNOMe
2,6-DNPH
morpholine
CHNOMe
CHNNHPh
morpholine
CHNOMe
CHNOMe
morpholine
morpholine
morpholine
CHNOMe
CHNOMe
CHNOPh
52
n.p.
44
38
33
57
6
17
8
53
44
29
21
8
71
47
36
45
n.p.
7c
Ser
Leu
Asp
Trp
Ser
Ser
Leu
Asp
Asp
Trp
Trp
Ser
2,6-DNPH
CHNOMe
CHNNHPh
CHNOPh
morpholine
morpholine
12
25
aAccording to 13C NMR all as TFA salts except 9b,9e,10c, and 11e.
bn.p. = not purified. cCombined yield of E and Z isomers.
Products 8-11 (Table 1) were cleaved from the support
with TFA, using water and triisopropylsilane as scavengers.
Regardless of the scavengers, the tryptophan derived com-
pounds (9g, 10c, 11d-e) gave low yields. Although the
majority of the products (see Figure 1 for illustrative HPLC
traces) were relatively pure after cleavage and, in some cases,
ether precipitation, they were still purified by HPLC for
NMR analysis. Methoxyaminated products existed as E/Z
isomers that could not be separated by the RP-HPLC
method employed. The isomeric ratio was approximately
1
1:2, according to the H NMR integrals. To identify the
isomers, a nondecoupled 13C spectrum of 10d was recorded
(13C satellite signals were not visible in 1HNMR). The 1JCH
coupling constants of the oxyimino carbons of the isomers
were 173 (δ 137.8 ppm) and 182 Hz (δ 133.9 ppm). Since a
larger 1JCH coupling constant is expected for a hydrogen syn
to the imine nitrogen lone pair,19 it was concluded that the
carbon with 182 Hz coupling constant referred to the Z
isomer. Hence, it could be seen from the 2D spectra that Z
was the major isomer of the methoxyaminated compounds.
The isomeric ratio of the phenoxyamine analogues 11d and
10e was approximately 1:1. Isomers of 11d were separated by
HPLC. Because of the prototropic tautomerism and hin-
dered rotation about the C40-C2 bond adjoining the imida-
zole rings,17b,13 the 13C and 1H NMR signals of the imidazole
rings were broadened in most of the products. With several
samples, some imidazole ring carbon signals were missing or
weak. In those cases, the chemical shift assignment relied on
the 2D spectra. In addition, signals referring to the morpho-
lino group and methyl and methine groups connecting the
outer imidazole ring to the second diversity group were
broadened. No acid was added to the NMR samples to
speed the exchange processes, but probe temperature was
FIGURE 1. Illustrative examples of HPLC traces of crude products
cleaved from the resin (RP HPLC 0-100% MeCN, 0.1% TFA, λ =
220 nm).
(19) (a) Yonezawa, T.; Morishima, I. J. Mol. Spectrosc. 1968, 27, 210–
217. (b) Gil, V. M. S.; Philipsborn, W. V. Magn. Reson. Chem. 1989, 27, 409–
430.
raised or lowered in many experiments to improve the
spectral quality.
J. Org. Chem. Vol. 74, No. 24, 2009 9449

€
Karskela and Lonnberg
JOCArticle
Conclusion
deprotected resin. The reaction suspension was shaken in an
orbital shaker for 7 h, filtered, and washed with pyridine, 30%
DMF/pyridine, 10% water/pyridine, water, pyridine, 30%
DMF/pyridine, DMF, DCM, 10% MeOH/DCM, and MeOH.
Resin 4 was dried in suction and in a vacuum desiccator.
General Procedure for Oximation on a Solid Support (5).
Methoxylamine hydrochloride (83.5 mg, 1.00 mmol) in pyridine
(2 mL) was added onto resin 4 (0.1 mmol). The suspension was
shaken for 1 h and then filtered and washed with pyridine, 10%
water/pyridine, pyridine, DCM, 10% MeOH/DCM, and
MeOH and dried as before.
Alternatively, phenoxyamine hydrochloride (36.4 mg, 0.25
mmol) in pyridine (1 mL) was added onto resin 4 (0.05 mmol).
The suspension was shaken for 2 h and then filtered and washed
subsequently with pyridine, 10% water/pyridine, pyridine,
DCM, 10% MeOH/DCM, and MeOH and dried as before.
General Procedure for Hydrazone Formation on a Solid Sup-
port (6). 2,6-Dinitrophenylhydrazine (67% , 295.7 mg, 1.0
mmol) and acetic acid (250 μL) were dissolved in DMF (2.25
mL) and added onto resin 4 (0.1 mmol). The suspension was
stirred 22 h with a magnetic stirrer at 88 °C and then filtered and
washed with DMF, 10% AcOH/DMF, DMF, 10% MeOH/
DMF, water, MeOH, 5% TEA/DCM, DCM, 10% MeOH/
DCM, and MeOH and dried as before.
A small 22 members library of 5,10,50-trisubstituted 2,40-
biimidazoles was synthesized on a solid support. With the
developed method, larger libraries can easily be produced by
increasing the selection of the diversity reagents used, namely
the amino acid resin, O-substituted hydroxylamine, amine,
and acid halide. By contrast, the use of hydrazines has
limitations because of the side reactions. Nevertheless hydra-
zines having electron-withdrawing groups can be used as the
second diversity reagent. Products isolated after cleavage
reaction and ether precipitation are relatively pure, though in
some cases additional purification is desirable. Nevertheless,
the method is applicable for production of 2,40-biimidazoles
potentially useful for lead search of kinases.
Experimental Section
Ethyl 2-(7-Formyl-3H-imidazo[1,2-i]purin-3-yl)acetate (2a).
Ethyl 2-(9H-adenin-9-yl)acetate (2.45 g, 11.5 mmol), bromoma-
˚
lonaldehyde (3.48 g, 23.1 mmol), and 3 A molecular sieves were
suspended into a mixture of formic acid (4.33 mL, 115 mmol),
2,6-lutidine (2.80 mL, 24.2 mmol), and DMF (20 mL). The
reaction mixture was stirred 3 h with a magnetic stirrer at 60 °C
in a closed bottle. The mixture turned to dark orange and a white
precipitate formed during the reaction. The reaction was fol-
lowed with TLC (10% H2O/MeCN). After completion, the
mixture was kept in a freezer overnight. The solidified reaction
Alternatively, phenylhydrazine (98.5 μL, 1.0 mmol) and
acetic acid (250 μL) were dissolved in DMF (2.25 mL) and
added onto resin 4 (0.1 mmol). The suspension was stirred 22 h
with a magnetic stirrer at 77 °C and then filtered and washed
with DMF, 10% AcOH/DMF, DMF, 10% MeOH/DMF,
water, MeOH, 5% TEA/DCM, DCM, 10% MeOH/DCM,
and MeOH and dried as before.
€
mixture was transferred to a Buchner funnel with water. The
white crude product was washed additionally with water. The
mother liquor was evaporated to dryness and the remaining
deposit was suspended in water, suction filtered, and washed
with water. The air-dried product was further dried over phos-
phorus pentaoxide in a vacuum desiccator. The product was
recrystallized from 25% EtOH/EtOAc yielding 2.43 g (77%) of
white fibrous product. 1H NMR (500 MHz, (CD3)2SO) δ 1.22
(t, J = 7.1 Hz, 3 H, CH3,Et), 4.20 (q, J = 7.1 Hz, 2 H, CH2,Et),
5.34 (s, 2H, CH2Im), 8.53 (s, 1H, H2), 8.59 (s, 1H, H8), 9.91 (s,
1H, H5), 10.00 (s, 1H, CHO) ppm; 13C NMR (125 MHz,
(CD3)2SO) δ 14.0 (CH3,Et), 44.9 (CH2Im), 61.6 (CH2,Et), 122.1
(C9b), 124.7 (C7), 136.8 (C5), 141.9 (C3a), 144.3 (C2), 144.8
(C9a), 147.9 (C8), 167.6 (CO2), 179.1 (CHO) ppm. HRMS (ESI-
TOF) calcd for [MHþ] C12H12N5O3þ 274.0935, found 274.0936.
2-(50-Amino-4(5)-formyl-1H,10H-2,40-biimidazol-10-yl)acetic
Acid Hydrochloride (3). 2a (1.00 g, 3.66 mmol) was stirred in 6 M
hydrochloric acid (20 mL) until the reaction was completed
according to ESI-MS. The white product precipitated during the
reaction. The solution was cooled and suction filtered and washed
with water. The mother liquor was concentrated and acetone was
added until the mixture turned cloudy. After cooling, the precipi-
tate was collected by suction filtration and washed with water.
Product phases were combined and dried over phosphorus penta-
General Procedure for Reductive Amination on a Solid Support
(7). Morpholine (87 μL, 1.0 mmol) and formic acid (92 μL)
dissolved in 1.00 mL of DMF were pipetted on resin 4 (0.1
mmol). After half an hour 85% NaCNHB3 (37 mg, 0.4 mmol)
and MeOH (138 μL) dissolved in 1.00 mL of DMF were added.
The suspension was shaken for 18 h. The suspension was then
filtered and washed with DMF, 10% MeOH/DMF, water,
MeOH, DCM, 10% MeOH/DCM, and MeOH. The resin was
dried as before.
General Procedure for Benzoylation on a Solid Support (9).
Benzoyl chloride (29 μL 0.25 mmol) dissolved in 1.00 mL of
pyridine was pipetted onto resins 5-7 (0.05 mmol). The suspen-
sion was stirred for 3 h, filtered, and washed with pyridine,
10% water/pyridine, and pyridine. The resin was then washed
for 2 min with pyridine containing 10% water and 20% con-
centrated aqueous ammonia and finally with pyridine, 10%
water/pyridine, water, pyridine, DCM, 10% AcOH/DCM,
DCM, 10% MeOH/DCM, and MeOH. The resin was dried as
before.
General Procedure for Acetylation on a Solid Support (10).
Acetic anhydride (23.5 μL 0.25 mmol) dissolved in 1.00 mL of
pyridine was pipetted onto resins 5-7 (0.05 mmol). The suspen-
sion was stirred for 3 h and then filtered and washed with
pyridine, 10% water/pyridine, water, pyridine, DCM, 10%
AcOH/DCM, DCM, 10% MeOH/DCM, and MeOH. The resin
was dried as before. The reaction was repeated with 1 h reaction
time if necessary.
General Procedure for the Isobutyrylation on the Solid Support
(11). Isobutyryl chloride (26 μL 0.25 mmol) dissolved in 1.00 mL
of pyridine was pipetted onto resins 5-7 (0.05 mmol). The
suspension was stirred for 3 h, filtered, and washed with
pyridine, 10% water/pyridine, and pyridine. The resin was then
washed for 1 min with pyridine containing 10% water and 20%
concentrated aqueous ammonia and finally with pyridine, 10%
water/pyridine, water, pyridine, DCM, 10% AcOH/DCM,
DCM, 10% MeOH/DCM, and MeOH. The resin was dried as
before.
1
oxide in a vacuum desiccator. Quantitative yield. H NMR (500
MHz, (CD3)2SO) δ 4.96 (s, 2H, CH2Im), 8.25 (s, 1H, H20), 8.26 (s,
1H, H4), 9.73 (s, 1H, CHO) ppm; 13C NMR (125 MHz,
(CD3)2SO) δ 45.6 (CH2Im), 101.2 (C40), 131.2 (C4), 133.2 (C20),
133.8 (C5), 141.3 (C50), 142.1 (C2), 168.3 (CON), 180.7 (CHO)
ppm. HRMS (ESI-TOF) calcd for [MHþ] C9H10N5O3þ 236.0778,
found 236.0782.
General Procedure for Coupling of 3 to a Solid Support (4). The
Fmoc-protected amino acid Wang polystyrene resin (0.3 mmol)
was deprotected with 20% piperidine in DMF for 20 min. Resin
was successively washed with DMF, DCM, and MeOH and
dried in vacuum. Compound 3 (421 mg, 1.5 mmol) was dissolved
in warm pyridine (6 mL). The solution turned to orange and
some undissolved 3 remained. HBTU (569 mg, 1.5 mmol) in
DMF (3 mL) was added and the mixture was pipetted onto the
9450 J. Org. Chem. Vol. 74, No. 24, 2009

€
Karskela and Lonnberg
JOCArticle
(S,E/Z)-2-{2-[50-Benzamido-4(5)-(methoxyiminomethyl)-1H,10-
H-2,40-biimidazol-10-yl]acetamido}-4-methylpentanoic acid (9a):
Yield as TFA salt 13.1 mg (44%). 1H NMR (500 MHz, CD3OD,
298 K) δ 0.82 (d, J = 6.1 Hz, 3 H, δLeu), 0.85 (d, J = 6.0 Hz, 3 H,
HδLeu), 1.51-1.61 (m, 3 H, HβLeu, HγLeu), 3.97 (s, 0.9 H, OMeE),
4.05 (s, 2.1 H, OMeZ), 4.41 (m, 1 H, HRLeu), 4.90 (s, CH2,Ac), 7.51
(s, 0.7 H, CHNOZ), 7.56 (m, 2 H, H3Bz), 7.66 (m, 1 H, H4Bz), 7.70
(s, 0.3 H, H4E), 7.94 (s, 0.3 H, H20E), 7.96 (s, 0.7 H, H20Z), 8.03 (m,
2 H, H2Bz), 8.03 (s, 0.7 H, H4Z), 8.12 (s, 0.3 H, CHNOE), 8.67 (m,
NH) ppm; 13C NMR (125 MHz, CD3OD) δ 21.7 (CδLeu), 23.3
(CδLeu), 25.9 (CγLeu), 41.5 (CβLeu), 48.1 (CH2,Ac), 52.4 (CRLeu),
63.1 (OMeE), 63.6 (OMeZ), 120.0 (C40), 121.0 (C4E), 124.6 (C4Z),
124.8 (C5Z), 128.2 (C5E), 129.3 (C2Bz), 129.9 (C3Bz), 133.7
(C1Bz,Z), 133.8 (C1Bz,E), 134.0 (ImCHNZ), 134.1 (C4Bz), 137.7
(ImCHNE), 140.4 (C20), 140.9 (C2Z), 142.3 (C2E), 168.4 (CH2CO),
169.1 (COBz,E), 169.2 (COBz,Z), 175.5 (COLeu) ppm. HRMS
(S,E/Z)-2-{2-[50-Acetamido-4(5)-(phenoxyiminomethyl)-1H,10-
H-2,40-biimidazol-10-yl]acetamido}-3-hydroxypropanoic acid (10e):
Yield as TFA salt 13.3 mg (47%). 1H NMR (500 MHz, CD3OD,
273 K) δ 2.26 (s, 1.5 H, CH3,Ac), 2.27 (s, 1.5 H, CH3,Ac), 3.89 (dd,
J = 3.6 Hz, J = 11.2 Hz,1 H, HβSer,E,Z), 3.96 (dd, J = 5.0 Hz, J =
11.2 Hz, 1 H, HβSer,E,Z), 4.56 (m, 1 H, HR Ser,E,Z), 4.90 (m, 2 H,
CH2,Ac,E,Z), 7.08 (m, 1 H, H4Ph,E), 7.12 (m, 1 H, H4Ph,Z), 7.29 (m, 1
H, H3Ph,E), 7.35 (m, 1 H, H3Ph,Z), 7.37 (m, 2 H, H2Ph), 7.88 (s, 0.5
H, CHNOZ), 7.88 (s, 0.5 H, H4E), 7.93 (s, 0.5 H, H20E), 7.95 (s, 0.5
H, H20Z), 8.33 (s, 0.5 H, H4Z), 8.55 (s, 0.5 H, CHNOE) ppm; 13C
NMR (125 MHz, CD3OD 273 K) δ 23.2 (MeAc), 48.0 (CH2,Ac),
56.4 (CRSer), 62.6 (CβSer), 115.4 (C3Ph,E), 115.8 (C3Ph,Z), 119.2
(C40E), 119.6 (C40Z), 122.8 (C4E), 124.1 (C4Ph,E), 124.4 (C5Z), 124.6
(C4Ph,Z), 126.0 (C4Z), 127.3 (C5E), 130.5 (C2Ph,E), 130.6 (C2Ph,Z),
137.2 (ImCHNZ), 140.1 (C20), 141.2 (C2Z), 141.3 (ImCHNE), 142.8
(C2E), 160.2 (C1Ph,Z), 160.4 (C1Ph,E), 168.3 (CH2COZ), 168.4
(CH2COE), 173.1 (COSer), 173.4 (COAc,E), 173.7 (COAc,Z) ppm.
Calcd for [MHþ] C20H22N7O6þ 456.1626, found 456.1624.
(ESI-TOF) calcd for [MHþ] C23H28N7O5 482.2146, found
þ
482.2126.
(S)-2-(2-{50-Benzamido-4(5)-[2-(2,4-dinitrophenyl)hydrazono-
methyl]-1H,10H-2,40-biimidazol-10-yl}acetamido)-4-methylpen-
tanoic acid (9b): Yield 12.1 mg (38%). H NMR (400 MHz,
(S)-3-(1H-Indol-3-yl)-2-{2-[50-isobutyramido-4(5)-(morpholino-
methyl)-1H,10H-2,40-biimidazol-10-yl]acetamido}propanoic acid
(11e): Yield 3.5 mg (12%). H NMR (500 MHz, (CD3)2CO,
1
1
(CD3)2SO, 323 K) δ 0.79 (d, J = 6.1 Hz, 3 H, HδLeu), 0.81 (d,
J = 6.2 Hz, 3 H, HδLeu), 1.49 (m, 2 H, HβLeu), 1.54 (m, 1 H,
HγLeu), 4.27 (m, 1 H, HRLeu), 4.77 (d, 16.8 Hz, CH2,Ac), 4.82 (d,
J = 16.8 Hz, CH2,Ac), 7.54 (m, 2 H, H3Bz), 7.64 (m, 1 H, H4Bz),
7.76 (s, 1 H, H4), 7.93 (s, 1 H, H20), 8.04 (m, 2 H, H2Bz), 8.18 (d,
9.6 Hz, 1 H, 6DNP), 8.25 (dd, 2.5 Hz, 9.6 Hz, 1 H, 5DNP), 8.42 (d,
7.9 Hz, 1 H, NH), 8.59 (s, 1 H, CHNN), 8.87 (d, 2.5 Hz, 3 DNP),
10.25 (b, 1 H, NH), 11.63 (s, 1 H, NH) ppm; 13C NMR (100
MHz, (CD3)2SO, 323 K) δ 21.1 (CδLeu), 22.4 (CδLeu), 24.0
(CγLeu), 39.9 (CβLeu), 46.7 (CH2,Ac), 50.3 (CRLeu), 117.0
(C6DNP), 121.7 (C40), 122.6 (C3DNP), 123.9 (C4), 127.8
(C2Bz), 128.1 (C3Bz), 128.9 (C5DNP), 129.2 (C2DNP), 130.3
(C5), 131.8 (C4Bz), 133.1 (C1Bz), 136.9 (C4DNP), 137.5 (C20),
139.4 (ImCHN), 142.6 (C2), 144.2 (C1DNP), 165.7 (CH2CO),
298 K) δ 1.05 (d, J = 6.6 Hz, 3 H, CH3,i-Bu), 1.12 (d, J = 6.6 Hz,
3 H, CH3,i-Bu), 2.73 (sep, J = 6.6 Hz, 1 H, CHi-Bu), 3.22 (dd, J =
8.3 Hz, J = 15.0 Hz, 1 H, HβTrp), 3.25 (br, 4 H, NCH2), 2 H, 3.36
(dd, J = 4.6 Hz, J = 15.0 Hz, 1 H, HβTrp), 3.89 (br, 4 H, OCH2),
4.44 (br, 2 H, ImCH2N), 4.71 (d, J = 16.7 Hz, 1 H, CH2,Ac), 4.77
(d, J = 16.7 Hz, 1 H, CH2,Ac), 4.82 (m, 1 H, HR Trp), 7.01 (m, 1
H, H7Trp), 7.09 (m, 1 H, H6Trp), 7.18 (s, 1 H, H2Trp), 7.37 (m, 1
H, H5Trp), 7.58 (m, 1 H, H8Trp), 7.62 (s, 1 H, H4), 7.72 (s, 1 H,
NH), 7.74 (s, 1 H, H20), 9.97 (s, 1 H, NH), 10.26 (s, 1 H, NH)
ppm; 13C NMR (125 MHz, (CD3)2CO, 298 K) δ 19.6 (C3i-Bu),
28.1 (CβTrp), 35.3 (C2i-Bu), 48.3 (CH2,Ac), 51.6 (ImCH2N), 52.4
(NCH2,morpholine), 53.8 (CRTrp), 65.0 (OCH2,morpholine), 110.6
(C1Trp), 112.2 (C5Trp), 119.1 (C8Trp), 119.6 (C7Trp), 121.1 (C40),
121.9 (C4), 122.2 (C6Trp), 124.3 (C2Trp), 126.3 (C5), 128.3
(C9Trp), 137.4 (C4Trp), 138.8 (C20), 142.2 (C2), 166.7
(CH2CO), 173.1 (COTrp), 178.6 (COi-Bu) ppm. Calcd for
[MHþ] C28H35N8O5þ 563.2725, found 563.2722.
166.4 (COBz), 173.2 (COLeu
)
ppm. Calcd for [MHþ]
C28H29N10O8þ 633.2164, found 633.2154.
(S)-2-{2-[50-Benzamido-4(5)-(2-phenylhydrazonomethyl)-1H,10H-
2,40-biimidazol-10-yl]acetamido}succinic acid (9e): Yield 1.7 mg
(6%). 1H NMR (500 MHz, CD3OD, 278 K) δ2.83 (m, 2 H, HβAsp),
4.78 (m, 1 H, HRAsp),4.89(d,J=16.8Hz,1H,CH2,Ac),4.93(d,J=
16.8 Hz, 1 H, CH2,Ac), 6.83 (m, 1 H, H4Ph), 7.10 (m, 2 H, H3Ph), 7.19
(m, 2 H, H2Ph), 7.49 (s, 1 H, H4), 7.56 (m, 2 H, H3Bz), 7.66 (m, 1 H,
H4Bz), 7.72 (s, 1 H, CHNN), 7.94 (s, 1 H, H20), 8.07 (m, 2 H,
H2Bz) ppm; 13C NMR (125 MHz, CD3OD, 278 K) δ 36.7 (CβAsp),
48.3 (CH2,Ac), 50.6 (CRAsp), 113.7 (C2Ph), 117.8 (C4), 120.8 (C40),
121.2 (C4Ph), 123.6 (ImCHN), 129.4 (C2Bz), 130.0 (C3Bz), 130.1
(C3Ph), 132.8 (C5), 133.8 (C1Bz), 134.1 (C4Bz), 140.0 (C20), 141.4
(C2), 145.9 (C1Ph), 168.5 (CH2CO), 169.7 (COBz), 173.7 (COAsp),
Acknowledgment. We thank the National Graduate
School of Organic Chemistry and Chemical Biology for
€
financial support. We also thank Dr. H. Kivela for helpful
discussions about NMR experiments.
Supporting Information Available: General experimental
methods, experimental procedures, and characterization of
compounds 2b,c, characterization of products not reported
above, tables of 13C NMR data of products 9-11, and 1H and
13C NMR spectra of products. This material is available free of
charge via the Internet at http://pubs.acs.org.
þ
173.8 (COAsp) ppm. Calcd for [MHþ] C26H25N8O6 545.1892,
found 545.1897.
J. Org. Chem. Vol. 74, No. 24, 2009 9451