1980
H. Chen et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1979–1983
pharmacophores and aromatic bridges (Fig. 4), and 22
commercially available alkynes as the side chain
(Fig. 4). When devising a suitable azide building block,
two key criteria were considered: (1) that the pharmaco-
phore and aromatic bridge were retained without major
variations, and that (2) the attachment of the azido
group to the aromatic bridge was facile. Both criteria
were fulfilled by the method shown in Scheme 2. The
bromides 46 and 67 were synthesized based on published
methods. The desired azides 5 and 7 were prepared in
high yield by nucleophilic substitution of bromides 4
and 6 with sodium azide in DMSO at room tempera-
ture8 (Scheme 2). Therefore, a total of five different
azides were synthesized. Additionally, 22 alkynes with
a variety of substituted aromatic rings and cycloalkenes
were obtained commercially.
1
N
R
N1
N
4
Cu(I)
1
2
R
N
+
3
R
2
R
Scheme 1. Cu(I)-catalyzed Huisgen [2 + 3] cycloaddition.
for chromatographic purification; (2) inoffensive by-
products; (3) mild reaction conditions, operating in a
water–alcohol system; and (4) the azido group and al-
kyne are tolerant to most chemical manipulations.
Therefore, this transformation is especially useful for
drug discovery since it is a reliable linking reaction.
Moreover, the triazole moiety usually has favorable
physicochemical properties. It often interacts with the
biological target through hydrogen bonding and dipole
interactions. Approaches based on ‘click chemistry’ were
shown to be a highly versatile and effective strategy for
the synthesis and identification of biologically active
leads with a number of biological activities,5 such as
antitumor,5a antibacterial,5b and antiviral activities;5c
metalloprotease,5d HIV protease,5e,f sulfotransferase,5g
fucosyltransferase,5h protein tyrosine phosphatase,5i
and acetylcholinesterase inhibitors.5j However, the
application of this approach to the modification of anti-
fungal b-methoxyacrylates has not been explored.
Next, the 110-member b-methoxyacrylate library was
assembled using ‘click chemistry’ (Fig. 5). Each of the
five azides was mixed with each of the 22 alkynes (in
slight excess; see Supporting information for details) in
a t-BuOH/H2O solution, followed by the addition of
catalytic sodium ascorbate and CuSO4Æ5H2O. The ‘click
reaction’ proceeded very well at room temperature. The
resulting products were extracted with ethyl acetate. All
the products were submitted to MS analysis to verify
their authenticity. Indeed, all the mass spectra were con-
sistent with the anticipated product structure (see Sup-
porting information for details). The purity of each
member was then assessed by HPLC. The results
showed that all the products were over 70% pure (see
Supporting information for details).
Herein, we describe, for the first time, a ‘click chemistry’
protocol for the rapid synthesis of a small library of
b-methoxyacrylate antifungal antibiotic analogues. Our
library design was based on the general structure of
b-methoxyacrylates (Fig. 3). The pharmacophores and
aromatic bridges were preserved and the side chain
was attached to the aromatic bridges via a triazole ring.
Presumably, our strategy has the following advantages:
(1) the linkage by a triazole ring allows a parallel synthe-
sis strategy for the analogues, thus facilitating the
library synthesis; (2) the [1,2,3]-triazoles are stable to
acidic and basic hydrolysis as well as oxidation and
reduction, thus providing ideal stability for the ana-
logues; (3) the triazole moiety is capable of participation
in hydrogen bonding, dipole–dipole, and p-stacking
interactions, thus improving both the potency and spec-
ificity of the resulting analogues.
The 110-member library was screened for antifungal
activity in a bioassay consisting of 3-mm diameter
mycelial plugs of Verticillium lamellicola incubated in
50 lg/mL crude products in Tinline minimal medium
agar. In this screening, fifteen products displayed
growth inhibition of the fungus comparable to that seen
in 50 lg/mL azoxystrobin (a commercial fungicide) (see
Supporting information for details). All the 15 crude
products were then purified by flash column chromatog-
raphy. The purified compounds and, as controls, the 10
alkyne building blocks of these compounds were then
retested for their effect on the mycelia growth of V. lam-
ellicola (see Table 1 for details).
In order to test the hypothesis that click chemistry could
be feasibly used for the rapid parallel synthesis of
b-methoxyacrylate analogues, a library of 110 triazole-
modified analogues was designed. The library’s diversity
was insured through the use of five azides, with varied
The resulting bioassay revealed that four compounds
from the library with a chlorine atom on the aromatic
bridge displayed the greatest activity (see Fig. 6 for
structures and Table 1 for bioassay results). Of the al-
kyne starting material, only one, B18, ethynyl p-tolyl
sulfone (B18) showed very strong fungicidal activity at
the concentration of 50 lg/mL (see Table 1), resulting
in complete inhibition.
Side Chain
R
Aromatic bridge
Side Chain Aromatic bridge
N
N
R
N
The minimum inhibitory concentration (MIC) of the
four compounds (A3B1, A3B13, A3B18, and A3B20)
and ethynyl p-tolyl sulfone (B18) was analyzed in 96-
well plates with 1-mm diameter mycelia pieces incubated
in varying concentrations of compounds in Czapek Dox
minimal medium. The MIC was defined as the lowest
concentration at which no mycelial growth was evident.
MeO
OMe
MeO
OMe
O
O
Pharmacophore
Pharmacophore
Figure 3. Design of triazole-modified b-methoxyacrylates.
Chen, Hao
