Triazolbenzo[d]thiazoles: Efficient synthesis and biological evaluation as neuroprotective agents

Article abstract of DOI:10.1016/j.bmcl.2012.07.022

A number of (1H-1,2,3-triazol-1-yl)benzo[d]thiazoles were synthesized utilizing a versatile Cu-catalyzed azide-alkyne click reaction (CuAAC) on tautomeric benzo[4,5]thiazolo[3,2-d]tetrazole (1) and 2-azidobenzo[d]thiazole (2) starting materials. Moreover, one of the resulting products of this investigation, triazolbenzo[d]thiazole 22, was found to possess significant neuroprotective activity in human neuroblastoma (SH-SY5Y) cells.

Full text of DOI:10.1016/j.bmcl.2012.07.022

Bioorganic & Medicinal Chemistry Letters 22 (2012) 5976–5978
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Triazolbenzo[d]thiazoles: Efficient synthesis and biological evaluation
as neuroprotective agents
Belem Avila a, Aaron Roth a, Heather Streets a, Donard S. Dwyer b, Mark J. Kurth a,
⇑
a Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, United States
b Departments of Psychiatry and Pharmacology, LSU Health Sciences Center, 1501 Kings Highway Shreveport, LA 71130, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A number of (1H-1,2,3-triazol-1-yl)benzo[d]thiazoles were synthesized utilizing a versatile Cu-catalyzed
azide–alkyne click reaction (CuAAC) on tautomeric benzo[4,5]thiazolo[3,2-d]tetrazole (1) and 2-azi-
dobenzo[d]thiazole (2) starting materials. Moreover, one of the resulting products of this investigation,
triazolbenzo[d]thiazole 22, was found to possess significant neuroprotective activity in human neuro-
blastoma (SH-SY5Y) cells.
Received 6 June 2012
Revised 3 July 2012
Accepted 6 July 2012
Available online 20 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Click chemistry
Triazolbenzo[d]thiazoles
Neuroprotective agents
Benzothiazole derivatives exhibit a wide array of biological
activities; these include anti-HIV,1a antibacterial,1b antitumor,1c
and neuroprotective.1d Of special interest, thiourea derivatives of
2-amino-(6-trifluoromethoxy)benzothiazole, which are analogs of
riluzole (a neuroprotective drug in animal models of Huntington’s
disease), have been shown to significantly decrease neuronal injur-
y.1d 1,2,3-Triazoles have also been shown to exhibit a variety of
activities, including antimycobacterial,2a antiprotozoal,2b and
anti-inflammatory.2c Given this backdrop, it was envisioned that
benzothiazoles derivatized with 1,2,3-triazoles may afford unique
biological activities. The compounds reported here are also in-
cluded in the NIH Molecular Libraries Small Molecule Repository
(MLSMR) for high-throughput biological screening.
exists between benzo[4,5]thiazolo[3,2-d]tetrazole(1a) and 2-azi-
dobenzo[d]thiazole (2a) – two tautomeric forms where solvent
and substitution patterns govern which tautomer is favored
(Fig. 1).6 A similar tautomeric equilibrium exists with the pyrido-
and quinolinotriazoles.7a The structure of 1a has been confirmed
by X-ray crystallographic analysis8 and, in our hands, the 1H NMR
taken in CDCl3 showed a 1.8:1 ratio of 1a:2a after 15 min, which
evolved to a 1:1 ratio after 1 h. Interestingly, it was recently shown
that analogous pyridotetrazoles could be employed in click reac-
tions with alkynes to form pyrido-1,2,3-triazole derivatives.7a,7b
Thus, we envisaged that a variety of substituted triazolbenzo[d]thi-
azoles could be prepared from 1a ¡ 2a via CuAAC chemistry.9
Scheme 1 illustrates that benzothiazolotetrazole 1a can be ob-
tained from either benzo[d]thiazol-2-amine (3; method A) or
2-chlorobenzo[d]thiazole (4; method B). In method A, direct diazo-
tization using 85% phosphoric acid, nitric acid and aq sodium nitrite
followed by addition of aq sodium azide was employed and led to
the rapid formation (>45 min.) of pure 1a, which precipitated out
of solution in the excellent yield of 84%. In method B, 4 was sub-
jected to sodium azide, easily giving 1a but in only moderate yield
(50%) making this a less useful route.
Previous work reported by one of us3a suggested that benzimid-
azole and benzothiazole scaffolds often afford neuroprotective com-
pounds. Indeed, a pharmacophore hypothesis for neuroprotection
has been developed and suggested to us that triazolbenzo[d]thia-
zoles would nicely conform to this general scheme. Herein, we report
the synthesis of novel triazolbenzo[d]thiazoles as well as the neuro-
protective effects of 2-isopropyl-1-((1-(2-morpholinobenzo[d]thia-
zol-6-yl)-1H-1,2,3-triazol-4-yl)methyl)-1H-indazol-3(2H)-one (22).
Current methods for the preparation of 2-azidobenzo[d]-thia-
zoles include treatment of benzo[d]thiazol-2-amine with tert-butyl
nitrite followed by treatment with aq sodium azide,4 as well as to-
syl azide reaction with heteroaryllithiums (azido transfer).5 How-
ever, reactions involving 2-azidobenzo[d]-thiazole could,
potentially, be problematic since it is known that an equilibrium
⇑
Corresponding author.
E-mail address: mjkurth@ucdavis.edu (M.J. Kurth).
Figure 1. Benzo[4,5]thiazolo[3,2-d]tetrazole (1a) and 2-azidobenzo [d]thiazole
(2a).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.07.022

B. Avila et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5976–5978
5977
Table 1
Synthesis of 2-(1H-1,2,3-triazol-1-yl)benzo[d]thiazolesabc
Scheme 1. Synthesis of 1a ¡ 2a. Method A: H2PO4, HNO3, aq NaNO2, aq NaN3, 85%.
Method B: aq NaN3, DMSO, 100 °C, 50%.
Scheme 2. Synthesis of 2-(4-phenyl-1H-1,2,3-triazol-1-yl)benzo[d]thiazole.
With an effective route to 1a ¡ 2a in hand, we proceeded by
reacting 1a ¡ 2a with phenyl acetylene and CuSO4/ascorbate in
t-BuOH/H2O. The 1a ¡ 2a equilibrium mixture proved to be inert
to these conditions and triazolobenzothiazole 5 was not obtained.
Fortunately, utilizing stoichiometric CuI in THF at room temper-
ature for 1 h delivered triazolobenzothiazole 5 in excellent yield
(84%; Scheme 2). Given this success, we set out to synthesize sev-
eral triazolobenzothiazoles using 1a¡2a and a number of diverse
alkynes. The yields for these cycloaddition reactions ranged from
64–84% (5-13; Table 1) and, as expected, tolerated phenyl, ether,
nitrile, ester, and alkene functionality in the alkyne moiety.
We further explored substitution on the benzothiazole ring by
employing tetrazole/azide starting materials 1b¡2b (6-methyl
substituted) and 1c¡2c (6-phenyl substituted) in reaction with a
number of alkynes (? 10–13; Table 1). We found that the tetra-
zole/azide starting materials need not be purified (that is, could
be used crude) without significant loss in yield or complication
in product isolation/purification (? 12–13; 64–69% yield; Table 1).
Next, 6-nitrobenzothiazolotetrazole 14 was employed using the
same CuI reaction conditions and triazolobenzothiazole 15 was ob-
tained in 50% yield (Scheme 3). It is interesting to note that this
reaction proceeded moderately well despite the fact that starting
6-nitrobenzo[4,5]thiazolo[3,2-d]tetrazole (14) was the near exclu-
sive tautomer as confirmed by 1H NMR in CDCl3.
We next set out to prepare triazolobenzothiazoles where the
triazole moiety was placed at C6 and an amine placed at C2. The
key intermediate for this work, morpholinobenzobenzo[d]thiazol-
6-amine (19), was prepared in three steps (50% overall yield) from
6-nitrobenzo[d]thiazol-2-amine (16; Scheme 4). Diazotization of
16 followed by addition of aq CuSO4 and NaCl yielded 17 (75%)
and subsequent ipso displacement of chloride by morpholine
yielded aminated product 18 (67%). Zinc-mediated reduction of
the nitro group in 18 (Zn powder/AcOH in EtOH) delivered pure
19 in nearly quantitative yield. With 19 in hand, method A condi-
tions (H2PO4, HNO3, aq NaNO2, aq NaN3; see Scheme 1) were used
for the in situ ÀNH2 ? ÀN3 conversion and subsequent one-pot
CuI + alkyne cycloaddition in THF gave 20–22 in yields ranging
from 51–83%.
a
Products were characterized by 1H, 13C NMR, IR, and ESI MS.
b
Isolated yields after purification by chromatography on silica gel.
c
Starting tetrazole/azide ratios (1:2) were determined by 1H NMR in CDCl3:
1a:2a::1:1; 1b:2b::1.5:1.
Scheme 3. Synthesis of 3-(1-(6-nitrobenzo[d]thiazol-2-yl)-1H-1,2,3-triazol-4-
yl)propanenitrile.
SH-SY5Y against serum deprivation. Under these conditions, the
cells normally slow their rate of cell division and undergo modest
shrinkage. Analog 22 afforded significant neuroprotection as de-
picted in Fig. 2. In addition, 22 caused a small but significant
(P <0.05) increase in cell size at both 20 and 40 lM when compared
to control. While these concentrations are higher than desired for a
drug candidate, they are typical for hits obtained during screening
of compound libraries.3b The positive effects of 22 on the SH-SY5Y
cell line could be mediated by several mechanisms, but are most
consistent with activation of the serine-threonine kinase Akt. Akt
has previously been implicated in neuroprotection, and regulates
cell and organismal size.3b Additional studies will be required to
confirm a role for this signaling pathway. Compounds 20 and 21
did not protect against serum deprivation; however, they have
limited solubility in cell culture medium, which complicates
interpretation of these negative results. By contrast, analog 22
was readily soluble beyond 40
bles a previous neuroprotective hit from screening (Hoechst
lM. It is noteworthy that 22 resem-
6-Triazolo-2-morpholinobenzothiazoles 20–22 were evaluated
for their ability to protect the human neuroblastoma cell line

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B. Avila et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5976–5978
Figure 2. Neuroprotection by compound 22. (a) Human neuroblastoma cells were
transferred to serum-free medium in the absence (control) or presence of
compounds 20–22 at the concentrations indicated. After 3 d, cell viability was
measured by conversion of tetrazolium dye (CellTiter 96) as described previously.3
Cells were assayed in triplicate in 96-well culture plates. Results are expressed as
the percentage of control values averaged from 6 experiments. The bars represent
the standard deviations, and asterisks indicate significant differences from control,
P <0.01. (b) Automated pharmacophore alignment of previous hit, Hoechst 33258
(cyan, middle panel) with 22 (dark blue). This alignment was generated with
MarvinSketch 5.3.2 from ChemAxon. For comparison, the structures of the active
compounds are shown in similar orientations.
Scheme 4. Synthesis of 6-triazolo-2-morpholinobenzothiazoles 20-22.
33258, LSU-31)3a as judged by automated pharmacophore align-
ment (Fig. 2).
References and notes
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In conclusion, a number of triazolobenzothiazole derivatives
were synthesized via the CuAAC reaction of mixed tetrazole¡azide
starting materials. One compound (22) was found to be neuropro-
tective and to also increase cell size similar to previously described
screening hits that are structurally related. The novel chemistry
and scaffold reported here may provide the basis for deriving
highly potent neuroprotective compounds and additional efforts
in this regard are currently underway.
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Financial support was provided by the National Science Foun-
dation (CHE-0910870, CHE-0449845) and the National Institutes
of Health (GM089153). B.A. thanks the Bristol-Myers Squibb Grad-
uate Chemist Fellowship and Alfred P. Sloan Minority Ph.D. Pro-
gram for fellowship support. We also thank Mr. Michael Donald
and Mr. Wayne Conrad (UC Davis Department of Chemistry) for
synthetic assistance, and Mr. Parrin Patterson (LSU Health Sciences
Center) for assistance with bioassays.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.07.022.