Direct Regioselective Synthesis of Tetrazolium Salts by Activation of Secondary Amides under Mild Conditions

Article abstract of DOI:10.1021/acs.orglett.7b01004

Tetrazolium salts are biologically active molecules that have found broad applications in biochemical assays. A regioselective synthesis of tetrazolium salts is described through a formal (3 + 2) cycloaddition. The possibility of employing simple amides and azides as starting material and the mild conditions allow a broad functional group tolerance.

Full text of DOI:10.1021/acs.orglett.7b01004

This is an open access article published under a Creative Commons Attribution (CC-BY)
License, which permits unrestricted use, distribution and reproduction in any medium,
provided the author and source are cited.
Letter
pubs.acs.org/OrgLett
Direct Regioselective Synthesis of Tetrazolium Salts by Activation of
Secondary Amides under Mild Conditions
‡
‡,†
́ ́
Veronica Tona, Boris Maryasin, Aurelien de la Torre, Josefine Sprachmann, Leticia Gonzal
ez,§
‡,†
‡
,
‡
and Nuno Maulide*
‡
University of Vienna, Institute of Organic Chemistry, Wah
̈
ringer Strasse 38, 1090 Vienna, Austria
University of Vienna, Insitute of Theoretical Chemistry, Wahringer Strasse 17, 1090 Vienna, Austria
S Supporting Information
§
̈
*
ABSTRACT: Tetrazolium salts are biologically active mole-
cules that have found broad applications in biochemical assays.
A regioselective synthesis of tetrazolium salts is described
through a formal (3 + 2) cycloaddition. The possibility of
employing simple amides and azides as starting material and
the mild conditions allow a broad functional group tolerance.
he tetrazole core is a recurrent fragment in drugs and
biologically active compounds. In particular, tetrazolium
Scheme 1. Literature Syntheses of Tetrazolium Salts (a−c)
and Method Proposed Herein (d)
1
T
salts occupy a prominent role in clinical biochemistry and have
found wide application in histochemical and cytochemical
assays, in which they are metabolically transformed to their
2
highly colored reduced form, formazans. Furthermore,
applications of tetrazolium salts as energetic ionic liquids with
a low toxicity have progressively gained more space in the field
3
of materials chemistry.
Tetrazolium salts were historically prepared by oxidation of
4
the corresponding formazanes (Scheme 1 a). In order to
increase the level of synthetic flexibility of that method,
different approaches have been developed over the past
decades. The most common, and obvious, of them is alkylation
of preformed aminotetrazoles using suitable alkyl electrophiles
5
(
Scheme 1 b). However, this simple alkylation approach is
plagued by regioselectivity issues, as it is not always possible to
access a single regioisomer in a predictable manner. A different
approach to obtain 1,4,5-substituted tetrazolium salts would be
the synthesis of the tetrazolium core through (3 + 2)
cycloaddition between azides and nitrilium ions. This approach
6
was reported in 1976 by Quast and Bieber and in 1984 by
7
Carboni (Scheme 1c). However, those reactions mandated the
use of isolable nitrilium salts as reagents, which considerably
narrowed the scope of the method.
Secondary amides are a family of diverse and easily accessible
substrates, known to be synthetic precursors of nitrilium ions.
Notably, amide activation using triflic anhydride is now a
powerful chemoselective synthetic tool allowing the generation
In our initial attempt, performed using the amide activation
conditions previously reported by our group, we were pleased
to observe that the amide 8a underwent regioselective
formation of a single cycloaddition product 9a (Scheme 2).
Single-crystal X-ray analysis unambiguously confirmed the
8
−10
of nitrilium ions, among other activated intermediates.
Herein, we report a chemo- and regioselective synthesis of
tetrazolium salts through formal (3 + 2) cycloaddition, using
alkyl azides and secondary amides as starting materials, where
the nitrilium ion is formed in situ through amide activation with
triflic anhydride (Scheme 1d). The versatility of the method is
showcased by a broad array of successful examples.
Received: April 3, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01004
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
proposed connectivity of the 1,4,5-trisubstituted tetrazolium
product.
Scheme 3. Scope of Secondary Amides for the Synthesis of
Tetrazolium Salts
Scheme 2. Initial Attempt and X-ray Analysis
A brief optimization of reaction conditions showed that full
conversion was reached when the reaction time was increased
from 1 to 12 h, along a temperature increase to 40 °C (Table
1). Aqueous workup was found to be unnecessary, and the
Table 1. Optimization of the Direct Tetrazolium Salt
Synthesis
b
entry
time (h)
temp (°C)
workup
yield (%)
1
2
3
4
5
0.5
1
rt
rt
solvent evaporation
solvent evaporation
25
74
64
97
1
rt
NaHCO 1 h
3
12
12
rt
solvent evaporation
solvent evaporation
c
40
100
a
To a mixture of 8a and 2-F-Py was added Tf O at 0 °C. After 15 min,
2
6
a was added, and the reaction was allowed to stir at the reported
b c
temperature. NMR yield. 90% isolated yield.
reaction mixture was simply purified through fast column
chromatography with neutral alumina after evaporation of the
solvent to deliver 9b in an excellent 90% yield.
With these optimized conditions, we studied the scope of the
reaction (Scheme 3). The reactions were carried out on a 0.2
mmol scale; however, it was possible to perform the reaction on
a gram scale without significantly affecting the yield (9b). As
depicted, tetrazolium salts were formed in uniformly good to
high yields starting from secondary amides carrying various
aliphatic chains (9d,g,i,j). Aromatic substituents or unsatura-
tions (9h,k,l) did not disrupt the reaction. Interestingly, other
carbonyl functionalities, such as esters (9m), were tolerated,
with selective activation at the amide being observed. The use
of a large-ring secondary amide as substrate leads to a fused
bicyclic tetrazolium as product (9n). Different substituents at
nitrogen of the amide partner were also tolerated, as shown in
examples 9o and 9p. Unfortunately, N-arylamides were found
to give products in low yields.
a
Reaction performed on a 7 mmol scale.
10
With these results in hand, we decided to examine the
possibility of varying the azide partner (Scheme 4). Pleasingly,
different functional groups were tolerated, and the reaction
proceeded with high yields in the presence of a cyano moiety
On the basis of prior literature, we postulated that amide 8,
via formation of an iminium triflate and ensuing 2-F-Py-
mediated deprotonation, would lead to a nitrilium ion 10. That
intermediate could then react with the alkyl azide in a formal (3
+ 2) cycloaddition (Scheme 5).
(9t) or substituted aromatic rings (9r).
The reaction was also performed with a (naphthylmethyl)
A concerted cycloaddition mechanism would conceivably
allow the formation of two different regioisomers. Exper-
imentally, however, a single regioisomer 9, where the
azide (9s) with more modest yield; however, again the same
regioisomer as in previous examples was observed exclusively,
in spite of the considerable steric congestion of product 9s.
3
1
substituents R and R (see Scheme 5) are located on the
B
DOI: 10.1021/acs.orglett.7b01004
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Scope of Azides in the Direct Tetrazolium
Synthesis
−
1
Figure 1. Computed reaction profile (ΔG2
, kcal mol ) for the
98,DCM
direct synthesis of tetrazoliums from nitrilium ions and azides.
Scheme 5. Proposed Mechanism
same side of the molecule, was consistently observed (cf.
Schemes 2−4). Furthermore, the regioselectivity of the process
is undisturbed even in cases where steric hindrance would
appear to favor an alternative outcome (cf. Scheme 4, 9s). Why
is 9 the only observed regioisomer? Is the mechanism of this
cycloaddition step concerted? To address these (and other)
questions, a quantum chemical study was performed (see the SI
for computational details). Indeed, we found that the reaction
proceeds via a stepwise and not a concerted mechanism. Figure
Figure 2. Computed structures of reactant complex A and
intermediate B with NBO charges (au).
cycloaddition. Our methodology possesses the advantage of
forming the highly reactive nitrilium intermediate in situ
through amide activation with triflic anhydride and a base under
mild conditions, allowing a broad functional group tolerance.
Quantum chemical calculations explain the observed regiose-
lectivity and elucidate the stepwise, charge-controlled nature of
the reaction mechanism.
1
shows the calculated reaction profile for the model system
1
2
3
with R = Et, R = R = Me. The first step is the formation of
intermediate B with one newly built C−N bond. The second
step is an annulation to the final product C. This mechanism
explains the experimentally observed regioselectivity. It must be
noted that several attempts to locate a concerted transition
state did not lead to any alternative mechanisms.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01004.
Figure 2 shows the computed structures of the reactant
complex A (left) and the intermediate B (right). Calculated
NBO (Natural Bond Orbital) charges of several atoms are also
presented. The attractive electrostatic force between the N4
Experimental procedures, computational details, and full
characterization data for new compounds; X-ray data for
compound 9a and 9g (PDF)
(the most negatively charged) and C5 (the most positively
charged) atoms of the complex A allows the formation of
intermediate B and predetermines the regioselectivity. In an
analogous manner to the attractive interaction ultimately
leading to the N4−C5 bond, there is an attraction between
the atoms N2 and N1 in intermediate B which is conducive to
the final annulation step.
In conclusion, we report a practical regio- and chemo-
selective synthesis of tetrazolium salts using simple secondary
amides as starting materials through a formal 1,3-dipolar
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: nuno.maulide@univie.ac.at.
ORCID
Leticia Gonzal
Nuno Maulide: 0000-0003-3643-0718
́
ez: 0000-0001-5112-794X
C
DOI: 10.1021/acs.orglett.7b01004
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Author Contributions
†
A.d.l.T. and B.M. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Generous support of this research by the DFG, FWF, and ERC
StG FLATOUT and CoG VINCAT) is acknowledged. We are
■
(
grateful to the University of Vienna for continued support of
our research program. Calculations were partially performed at
the Vienna Scientific Cluster (VSC). N.M. is the recipient of
the Elisabeth-Lutz Preis of the Austrian Academy of Sciences.
Generous support of this research by the DFG, FWF (P
27194), and ERC 153 (StG FLATOUT and CoG VINCAT) is
acknowledged.
REFERENCES
■
(
5
1) Wei, C.-X.; Bian, M.; Gong, G.-H. Molecules 2015, 20, 5528−
553.
2) (a) S
M. V.; Herst, M. P. Biotech. Ann. Rev. 2005, Elsevier B. V. 11, 127.
c) Berridge, M. V.; Tan, A. S.; McCoy, R. D.; Wang, R. Biochemica
996, 4, 14.
3) Jones, C. B.; Haiges, R.; Schroer, T.; Christe, K. O. Angew. Chem.,
Int. Ed. 2006, 45, 4981.
4) (a) Nieham, A. W. The Chemistry of Formazans and Tetrazolium
Salts, Research Laboratories; May & Baker Ltd.: Essex, 1954.
b) Buzykin, B. I. Chem. Heterocycl. Compd. 2010, 46, 379−408.
5) Klapotke, T. M.; Mayer, P.; Schulz, A.; Weigand, J. J. J. Am. Chem.
Soc. 2005, 127, 2032−2033. (b) Klapotke, T. M.; Sabate, C. M.;
Rusan, M.; Welch, J. M. Eur. J. Inorg. Chem. 2009, 2009, 880−896.
(
̈
̧enoz, H. Hacettepe J. Biol. Chem. 2012, 40, 293. (b) Berridge,
(
1
(
(
(
(
̈
̈
́
(
c) Aridoss, G.; Laali, K. K. Eur. J. Org. Chem. 2011, 2011, 6343−6355.
(
(
(
6) Quast, H.; Bieber, L. Tetrahedron Lett. 1976, 17, 1465.
7) Carboni, B.; Carrie, R. Tetrahedron 1984, 40, 4115.
8) For general reviews about amide activation, see: Charette, A. B.;
́
Grenon, M. Can. J. Chem. 2001, 79, 1694. (b) Kaiser, D.; Maulide, N.
J. Org. Chem. 2016, 81, 4421.
(
9) For recent examples of amide activation, see: (a) Movassaghi, M.;
Hill, M. D. J. Am. Chem. Soc. 2006, 128, 4592−4593. (b) Movassaghi,
M.; Hill, M. D.; Ahmad, O. K. J. Am. Chem. Soc. 2007, 129, 10096−
10097. (c) Barbe, G.; Charette, A. B. J. Am. Chem. Soc. 2008, 130, 18−
19. (d) Pelletier, G.; Bechara, W. S.; Charette, A. B. J. Am. Chem. Soc.
2010, 132, 12817−12819. (e) Bechara, W. S.; Pelletier, G.; Charette,
A. B. Nat. Chem. 2012, 4, 228−234. (f) Xiao, K.-J.; Wang, A.-E.;
Huang, P.-Q. Angew. Chem., Int. Ed. 2012, 51, 8314−8317. (g) Peng,
B.; Geerdink, D.; Maulide, N. J. Am. Chem. Soc. 2013, 135, 14968−
1
4971. (h) Huang, P.-Q.; Huang, Y.-H.; Xiao, K.-J.; Wang, Y.; Xia, Y.-
E. J. Org. Chem. 2015, 80, 2861−2868. (i) Lumbroso, A.; Behra, J.;
Kolleth, A.; Dakas, P.-Y.; Karadeniz, U.; Catak, S.; Sulzer-Mosse, S.; De
Mesmaeker, A. Tetrahedron Lett. 2015, 56, 6541−6545. (j) Kolleth, A.;
Lumbroso, A.; Tanriver, G.; Catak, S.; Sulzer-Mosse, S.; De
́
́
Mesmaeker, A. Tetrahedron Lett. 2016, 57, 2697−2702. (k) Huang,
P.-Q.; Huang, Y.-H.; Xiao, K.-J. J. Org. Chem. 2016, 81, 9020−9027.
(
(
l) Kaiser, D.; Maulide, N. J. Org. Chem. 2016, 81, 4421−4428.
́
m) Tona, V.; de la Torre, A.; Padmanaban, M.; Ruider, S.; Gonzalez,
L.; Maulide, N. J. Am. Chem. Soc. 2016, 138, 8348−8351.
10) For examples of nitrilium ion generation from amides using
(
triflic anhydride, see: (a) Wang, A.-E.; Chang, Z.; Sun, W.-T.; Huang,
P.-Q. Org. Lett. 2015, 17, 732−735. (b) Zheng, J.-F.; Qian, X.-Y.;
Huang, P.-Q. Org. Chem. Front. 2015, 2, 927−935. (c) Huang, P.-Q.;
Ou, W.; Ye, J.-L. Org. Chem. Front. 2015, 2, 1094. (d) Geng, H.;
Huang, P.-Q. Tetrahedron 2015, 71, 3795−3801. (e) Huang, P.-Q.;
Lang, Q.-W.; Wang, A. E.; Zheng, J.-F. Chem. Commun. 2015, 51,
1
096−1099. (f) Huang, P.-Q.; Huang, Y.-H.; Geng, H.; Ye, J.-L. Sci.
Rep. 2016, 6, 28801.
D
DOI: 10.1021/acs.orglett.7b01004
Org. Lett. XXXX, XXX, XXX−XXX