S. Kalhor-Monfared et al. / European Journal of Medicinal Chemistry 122 (2016) 436e441
437
Table 2
MTT (3-(4,5-dimethylthiazol -2-yl)-2,5-diphenyltetrazolium bro-
mide) assay. From these results, we could find a few triazolic de-
rivatives with cytotoxicity effect in micromolar range.
Synthesis of arylhydroxymethyl alkynes a7-12 by reaction of aldehydes with ethynyl
Grignard reagent.
Nꢀ
R
Yielda
Nꢀ
R
Yielda
a7
a8
a9
4-NO2Ph
4-CF3Ph
4-CH3Ph
40
80
97
a10
a11
a12
2-naphthyl
4-MeOPh
2,3,4-(MeO)3Ph
74
78
57
2. Chemistry
In order to prepare a library of triazolic derivatives, an approach
using common aldehyde precursors has been selected to prepare
aryl alkynes and arylhydroxylmethyl alkynes.
a
Isolated yield.
Aryl alkynes were prepared from aldehydes by Seyferth-Gilbert
homologation using the Bestmann-Ohira reagent as shown in
Table 1 [18,19]. Aldehydes were treated with 1.5 equivalents of this
reagent in the presence of 2 equivalents of potassium carbonate in
methanol at room temperature. Three alkynes a1, a4 and a6 were
prepared using this reaction, and alkynes a2-3 and a5 were com-
mercial. Good yields were obtained except for a6 (45% yield).
In order to synthesize hydroxymethyl alkyne analogues, alkynes
a7-12 were prepared through a reaction with 1.3 equivalents of
ethynyl magnesium bromide (0.5 M in THF) in THF under argon
atmosphere at 0 ꢀC (Table 2). The final products obtained in more or
less good yields (97-40%).
Table 3
Synthesis of azides b1-7.
Nꢀ
R
Yielda
Nꢀ
R
Yielda
b1
b2
PhCH2
EtO2CCH2
91
99
b4
b5
4-MeOPhCH2
81
99
b3
HO(CH2)2
99
b6
99
a
Isolated yield.
All of these terminal alkynes bear either an electron donating
(EDG) or electron-withdrawing group (EWG) on their aromatic ring
to study their effect on biological activity of the final compounds.
Finally, the azides required for the CuAAC preparation of tri-
azoles were prepared by azide anion SN2 displacement of a bro-
mide, chloride or mesylate, as a function of the starting material
(Table 3). All azide derivatives b1-6 were obtained in very good
yields.
Triazolic libraries were then synthesized by CuAAC using A-
21 CuI in methylene chloride overnight (Schemes 1 and 2). For the
C-4 aryl compounds a1b1 to a6b6, the library was obtained with an
average yield of 90%. The majority of the triazoles were isolated
with a yield between 71 and 99%. Only compounds a1b3 (38%) and
a3b6 (57%) were obtained in a low yield. It is interesting to note
that very few compounds needed a purification after the CuAAC
reaction.
For the synthesis of arylhydroxymethyl derivatives a7b1 to
a12b6, the average yield was 86%. In this case, the obtained yields
were lower, but still between 74 and 99% for most of the triazoles.
Yields were often lower in these series for triazoles with the “(S)- or
(R,S) dioxolane” substituent [(4S)-2,2-dimethyl-1,3-dioxolan-4-yl)
methyl] due to the use of the azide b6. Thus yields for derivatives
a8b6 (65%), a9b6 (64%), a11b5 (42%), a11b6 (46%) and a12b6 (59%)
were much lower. Furthermore, the reaction was incomplete under
the same condition used for the other derivatives. We observed
formation of precipitates that prevented the completion of the re-
action even with incubation for a longer time. Our hypothesis is
that the dioxolane system can form stable complexes with reacting
species, and that these complexes prevent further reaction under
these conditions.
IC50 in Tables 4 and 5. Unfortunately, some compounds could not be
evaluated in the standard assay due to solubility problems. Thus
results only show a partial view of the activity of these two libraries.
For the first library of triazoles a1b1 to a6b6 (Table 4), com-
pound a1b1 (R1
IC50 ¼ 5.12 0.39
decreased the activity to 19.05
¼
4-NO2Ph, R2
¼
PhCH2) gave an
m
M. Changing R2 to 4-MeOPhCH2 in a1b4,
0.73
mM. Other compounds
m
resulted from a1 gave cytotoxcities >100
M. When R1 ¼ 4-CF3Ph,
compound with R2 ¼ PhCH2 (a2b1) was not active. However, where
R2 is 4-MeOPhCH2 for compound a2b4, an IC50 similar to that one of
a1b4 was observed (17.37 0.35
m
M). For a2b5, with R2 ¼ “ (R/S)-
dioxolane”, a slightly better activity was obtained (13.48 1.26
Other 4-trifluoromethylphenyl derivatives were beyond 100
mM).
mM.
Changing the 4-CF3 to a methyl group in the a3 series gave again
two compounds with R2 ¼ 4-MeOPhCH2 (a3b4) and R2 ¼ “ (R/S)-
dioxolane” (a3b5) with IC50 of 19.49 1.53 and 41.68 6.89 mM
respectively, being at the same level as a2b4 for a3b4, and less
active for a3b5 when compared to a2b5. The best activity was
obtained for a4b1 (R1 ¼ 2-naphthyl, R2 ¼ PhCH2) with an
IC50 ¼ 3.89 0.31
m
M. Replacing R2 by “(R/S)-dioxolane” (a4b5) and
(S)-dioxola-ne (a4b6) gave a lower IC50 of 6.60
5.61 and
12.02 1.5
m
M respectively. Finally, when R1 ¼ 2,3,4-(MeO)3Ph and
R2 ¼ PhCH2 (a6b1), an IC50 of 27.54 2.06
mM was measured.
For the second series of triazoles presented in Scheme 2 having a
hydroxymethyl between the triazole and the R1 group, solubility
problems were an issue. Most of the compounds cannot be tested
or showed an IC50 > 100
m
M. Only the compounds a10b1 (R1 ¼ 2-
naphthyl, R2 ¼ PhCH2) and a10b1 (R1 ¼ 2-naphthyl, R2 ¼ (R/S)-
dioxolane) gave moderate values of 28.84
1.33 and
26.91 7.00
mM respectively.
3. Biological activity
4. Discussion
The triazole libraries were then tested for their biological ac-
tivity against B16 melanoma cells, and cytotoxicities are reported as
For the active compounds, while looking at the various sub-
stituents, some similarities could be found (Fig. 1).
When the triazole had an N1-benzyl resulted from b1 in the
presence of a 2-naphthyl on C-4 (a4b1) gave the most active
Table 1
Synthesis of aryl alkynes a1, a4 and a6 using Bestmann-Ohira reagent.
Nꢀ
R
Yielda
Nꢀ
R
Yielda
compound (3.89
nitrophenyl (a1b1) or a 2,3,4-trimethoxyphenyl (a6b1) progres-
sively reduced the activity to 5.12 and 27.54 M.
The C-4 2-naphthyl is also found in derivatives in the series of
N1-[2,2-dimethyl-1,3-dioxolan-4-yl)methyl] (“dioxolane”)
mM). Replacing the C-4 substituent by a 4-
a1
a2
a3
4-NO2Ph
4-CF3Ph
4-CH3Ph
99
eb
eb
a4
a5
a6
2-naphthyl
4-MeOPh
2,3,4-(MeO)3Ph
83
eb
45
m
a
Isolated yield.
b
Commercial alkyne.