Multi-gram synthesis of enantiopure 1,5-disubstituted tetrazoles via Ugi-azide 3-component reaction

Article abstract of DOI:10.3390/molecules23112758

Tetrazoles have been widely studied for their biological properties. An efficient route for large-scale synthesis of 1,5-disubstituted tetrazoles (1,5-DTs) is presented. The strategy exploits a reductive approach to synthetize a cyclic chiral imine substrate which is then converted into the target product through an Ugi-azide three-component reaction (UA-3CR). The final products are equipped with additional functionalities which can be further elaborated for the generation of combinatorial libraries of enantiopure heterocycles.

Full text of DOI:10.3390/molecules23112758

molecules
Article
Multi-Gram Synthesis of Enantiopure 1,5-Disubstituted
Tetrazoles Via Ugi-Azide 3-Component Reaction
Pietro Capurro , Lisa Moni 1 , Andrea Galatini , Christian Mang 2 and Andrea Basso 1,*
1
1
1
Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31,
1
6146 Genoa, Italy; pietro.capurro@outlook.it (P.C.); lisa.moni@unige.it (L.M.);
andrea@chimica.unige.it (A.G.)
AnalytiCon Discovery GmbH, Hermannswerder Haus 17, 14473 Potsdam, Germany;
c.mang@ac-discovery.com
2
*
Correspondence: andrea.basso@unige.it; Tel.: +39-010-353-6117
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Cimarelli Cristina
Received: 5 October 2018; Accepted: 22 October 2018; Published: 25 October 2018
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: Tetrazoles have been widely studied for their biological properties. An efficient route for
large-scale synthesis of 1,5-disubstituted tetrazoles (1,5-DTs) is presented. The strategy exploits a
reductive approach to synthetize a cyclic chiral imine substrate which is then converted into the target
product through an Ugi-azide three-component reaction (UA-3CR). The final products are equipped
with additional functionalities which can be further elaborated for the generation of combinatorial
libraries of enantiopure heterocycles.
Keywords: multicomponent reactions; Ugi-azide; chiral imines; tetrazoles; diversity-oriented synthesis
1
. Introduction
Since their discovery in 1885, tetrazoles have been widely studied, and their chemistry and
biological properties have been deeply explored. In particular, syntheses of 1,5-disubstituted tetrazoles
1,5-DTs) have been thoroughly investigated as these compounds, being bioisosteres of cis-amide
bond in peptides, play a relevant role in modern bioresearch (Figure 1a) [ ]. 1,5-DTs have shown
(
1
biological activity as HIV-protease inhibitors (e.g., analogues of Saquinavir are currently being studied),
but they have also been proved to be good candidates for new antiseptic, anti-inflammatory, and
analgesic drugs. Highly active nucleoside analogues and peptidomimetics (Figure 1b) containing the
tetrazole ring have also been developed [2]. However, drug research is nowadays facing a slowdown,
and the approval of new active compounds is stalling because of the low availability of adequate
starting points for library generation. In fact, classical synthetic methods hardly permit differently
functionalized products featuring a common scaffold to be obtained, and therefore a method to
straightforwardly access complex structures with different decorations is highly desirable. Within
this field, multicomponent reactions (MCRs) are a powerful tool that can be exploited to synthetize
a plethora of potentially active structures with few steps and relatively simple building blocks [3–5].
A convenient synthesis of 1,5-DTs for combinatorial libraries and diversity-oriented synthesis (DOS)
employs the Ugi-azide three- or four-component reaction (UA-3CR/UA-4CR), a modification of the
classical and well-known Ugi reaction. Despite these considerations, very few examples of UA-3CRs
on cyclic or chiral imines are reported [
6,
7] On the contrary, the Ugi-Jullié reaction with such substrates
has been more widely exploited by us and others [
8
–10]. In order to fill this gap, we wish to report a
novel synthetic route able to afford large amounts of enantiopure 1,5-DTs through a simple two-step
protocol under very mild conditions starting from a chiral lactam formally derived from glutamic
acid. These products can then be further processed for the generation of combinatorial libraries in
accordance to synthetic approaches already applied by us to other substrates [11].
Molecules 2018, 23, 2758; doi:10.3390/molecules23112758
www.mdpi.com/journal/molecules

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Figure 1. (a) 1,5-DTs as isosteres of cis-amide bonds. (b) Example of an opiate-like peptidomimetic
containing the 1,5-DT structural moiety.
2
. Results and Discussion
We have recently reported that cyclic chiral imines, which are easily synthetized via reduction
of the corresponding lactam with Schwartz’s reagent, can be employed in the Betti reaction with
phenol derivatives [12]. In order to expand this approach, we reasoned that the same imines could
be ideal substrates for the UA-3CR. Starting from commercially available chiral lactam
imine was synthetized in two steps via protection of the primary alcoholic group and subsequent
Zr-mediated reduction of lactam (Scheme 1). Protection of the hydroxyl functionality was essential
for the reduction to occur smoothly, and the silyl protecting group was preferred to other derivatives
i.e., acetyl) that could interact with Schwartz’s reagent. Despite the fact that this reducing agent
is widely used in organic synthesis, it is often used in small-scale reactions [13 16], and thus we
developed a robust methodology that allowed us to react up to 6.0 g of substrate in a one pot process
1, chiral
3
2
(
–
with a simple purification step. This straightforward procedure to prepare chiral imine
one previously reported by us [8].
3 exceeds the
With imine
3
in hand, we moved to test the UA-3CR with different isocyanides. We found that
the reaction of
3 with TMS-N and isocyanides occurred fast (30 min) under mild reaction conditions
3
◦
(
MeOH, 0 C). With this approach, the necessity of high reaction temperatures or prolonged times to
force the imine formation (as reported by Dömling and co-workers) was avoided [17].
Scheme 1. Synthetic route. Conditions: (i) TBS-Cl, imidazole, DCM, N atm., r.t., overnight; (ii) Cp ZrHCl,
2
2
◦
◦
THF, −20 C to r.t, 1 h 30 min; (iii) R-NC, TMS-N , MeOH, 0 C, 30 min.
3
Under these conditions, the diastereoselectivity of the process proved to be moderate, ranging
from a 54:46 ratio up to 72:28, and usually resulting in around 68:32 (trans:cis). These results are
consistent with previous studies by our group where compound ent-3 (enantiomer of chiral imine
3),
when employed in Ugi-Joullié three-component reactions, gave similar results in terms of overall yield

Molecules 2018, 23, 2758
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and asymmetric induction (Scheme 2) [
8]. According to the commonly accepted mechanisms of these
reactions, the stereoselection in both cases was determined by the facial selectivity of the incoming
isocyanide and only marginally influenced by the substituent on the pyrroline ring.
However, as the two diastereoisomers were separable by column chromatography, the moderate
induction did not represent a problem. In fact, both isomers are valid entry points for generating
libraries for diversity-oriented synthesis applications.
H
U-3CR
TBSO
TBSO
N
R1
N
trans:cis 60:40 up to 68:32
yield: 46-85%
N
O
R2
O
ent-3
our previous work
R
N
UA-3CR
TBSO
TBSO
N
N
trans:cis 54:46 up to 72:28
yield: 56-75%
N
H
N N
3
this work
Scheme 2. Analogies between yields and asymmetric induction for products of Ugi-Joullié and Ugi-
azide reactions using cyclic chiral imine 3 and its enantiomer ent-3.
This synthetic protocol allowed us to synthetize multi-gram quantities (up to 5.0 g) of the targeted
1,5-DTs in just two steps from lactam 2. Imine formation was fast (1 h 30 min) and its purification
by filtration was trivial while the UA-3CR was even faster (30 min) and its products easily separated
upon column chromatography. It is worth noting that in some cases (depending on the nature
of the isocyanide employed), small quantities of 1-substituted tetrazole
5 was formed (Scheme 3).
This product was derived from the cycloaddition reaction between the HN formed in situ and the
3
isocyanide. However, with stoichiometric amounts of the azide source at low temperatures and in
absence of an acid catalyst, the reaction is known to be rather slow, and thus the formation of the side
product can be easily kept under 5 mol% [
are reported in Table 1. During the scope, the synthetic route proved to be robust and very reliable,
apart from isocyanides bearing acidic protons on the position which reacted poorly (entries l and m).
1]. The results of our scope with commercial isocyanides
α
On the other hand, isocyanides bearing a basic nitrogen, although affording the desired products in
comparable yield, reacted more slowly (entry k).
Scheme 3. Mechanism of formation of 1-substituted tetrazole
5 via cycloaddition between the HN3
formed in situ and the isocyanide.
The configuration of each diastereoisomer was determined via NMR NOESY-1D and 2D experiments.
In particular, products with cis configuration showed a marked nOe effect between the protons directly
bonded to the chirality centers, while products in trans configuration did not. The nOe effect was
accurately measured on both isomers of product 4g with NOESY-1D experiments (Figure 2). Upon
irradiation of proton
In the same molecule, irradiation of proton
same experiment on molecule 4g-trans showed no visible nOe between the same irradiated protons.
a
in the cis isomer (molecule 4g-cis), proton
b
gave a smooth and clear nOe (2%).
b
induced a 1.5% nOe on proton
a. On the other hand, the

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We attributed the configurations of the two diastereoisomers of molecule 4g accordingly to these results.
In all other couples of diastereoisomers, NOESY-2D experiments were performed. These experiments
a
b
showed clear nOe effects between the respective H and H protons on the less abundant diastereoisomer,
while the most abundant one showed no nOe at all between the very same protons. These results proved
that, with each isocyanide employed, stereoselectivity was always in favor of the trans isomer.
Table 1. Results for UA-3CR depicted in Scheme 1 under condition iii.
Entry
R 1
Yield [%] 2
d.r. (trans:cis) 3
a
b
c
d
e
f
g
h
i
Me
cPr
cBu
tBu
nBu
iBu
cHex
Bn
62
75
56
74
74
59
65
73
76
46
66
62:38
64:36
62:38
54:46
67:33
69:31
67:33
62:38
62:38
69:31
72:28
(2,6-diMe)Ph
(4-MeO)Ph
2-MorphEt
EtOOC-CH2-
Tos-CH2-
j
4
k
l
m
no reaction
no reaction
1
R group of products 4-trans and 4-cis; 2 isolated yield; 3 d.r. on crude products, determined via H-NMR; reaction
1
4
time 48 h.
nOe
TBSO Ha
N
Hb
N
Ha
N
TBSO
N
HN
N
HN
N
Hb
N
N
4g-cis
4g-trans
a
b
Figure 2. Irradiated protons on the cis and trans isomer of molecule 4g. Protons H and H gave a
smooth nOe only in the first case, allowing us to attribute the correct configuration to each of the
two molecules.
3
. Materials and Methods
3
.1. General
1
13
Mono- and bidimensional NMR spectra ( H, C, gCOSY, gHSQC, gHMBC, 1D and 2D-NOESY)
1
13
were obtained on a Varian (Palo Alto, CA, USA) Mercury 300 (300 MHz for H, 75 MHz for C) at
◦
2
7 C in CDCl + 0.03% TMS. Chemical shift values are reported in ppm (parts per million) using TMS
3
signal (0.00 ppm) as reference.
HPLC-UV were performed on a Varian (Palo Alto, CA, USA) Agilent 1100 series using a Phenomenex
(
1
Torrance, CA, USA) Gemini C -Phenyl 150
×
3 mm column, VWD = 210 nm. Sample concentration
6
mg/mL (MeOH + 0.1% HCOOH:H O + 0.1% HCOOH 1:1), injection volume 5 L, flow 0.34 mL/min,
µ
2
◦
T = 30 C, gradient 0 min = 90% A, 20 min = 0% A (A: H O + 0.1% HCOOH, B: MeOH + 0.1% HCOOH).
2
ESI-MS analyses were performed on a Microsaic (Woking, Surrey, UK) 4000 MiD (full scan
1
00-800 m/z, positive ions, tip voltage 750 V).
HR-MS analyses were carried out on a Waters (Milford, MA, USA) Synapt G2 QToF mass
spectrometer. MS signals were acquired from 50 to 1200 m/z in ESI positive ionization mode.

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TLC analyses were performed on Merck (Darmstadt, Germany) 60 F254 0,25 mm silica plates
and visualized with ninidrine (900 mg ninidrine, 300 mL n-BuOH, 9 mL AcOH) or Hanessian stain
(
21 g (NH )MoO4 4H O, 1 g Ce(SO ) , 31 mL H SO 98%, 369 mL H O) development (dipping in the
·
4
2 4 2 2 4 2
corresponding solution and warming).
Flash chromatographies were performed using SiO 230-400 mesh (Merck Ge Duran SI60) and
2
solvent bought from Sigma-Aldrich, Tokyo Chemical Industry, Carlo Erba, Honeywell/Riedel-de-Haen,
Alfa Aesar, VWR International, Acros Organics, Merck (Darmstadt, Germany). Celite employed for
filtration was Acros Organic Celite 545 (Thermo Fischer Scientific, Geel, Belgium).
3
.2. Chemistry
3
.2.1. Protection of Chiral Lactam 1
(
S)-HMP (
04 mmol, 2 eq.) were added to a flask under nitrogen atmosphere, dissolved in 120 mL of dry DCM
0.4 M), and left reacting at room temperature overnight. The crude was diluted to 150 mL and was
1) (6.00 g, 52.1 mmol, 1 eq.), TBS-Cl (9.42 g, 62.5 mmol, 1.2 eq.) and imidazole (7.08 g,
1
(
washed three times with deionized water and once with brine. Eventually, the crude was purified by
flash chromatography (PE:EtOAc 1:9, EtOAc:MeOH 9:1). Yield: 11.65 g (97%).
3
.2.2. Reduction with Schwartz’s Reagent of Chiral Lactam 2 to Imine 3
A solution of TBS-(S)-HMP ( ) (5.35 g, 23.3 mmol, 1 eq.) in dry THF (50 mL, 0.5 M) chilled at 0 C
◦
2
was gently dripped under vigorous stirring and an argon atmosphere into a suspension of Cp ZrHCl
2
◦
(
1
i.e., Schwartz’s reagent, 12.6 g, 48.9 mmol, 2.1 eq.) in dry THF (100 mL, 0.5 M) at
h 30 min, the solvent was removed under reduced pressure and the crude was suspended in iced
−
20 C. After
pentane. The precipitated Cp Zr=O waste was filtered off under vacuum over celite. The eluted
2
solution was dried at the rotary evaporator. Yield: 3.73 g (75%).
3
.2.3. UA-3CR of Imine 3 with TMS-N and R-NC
3
Imine
3
(4.50 g, 21.1 mmol, 1 eq.) was dissolved in dry MeOH (80 mL, 0.25 M) under nitrogen
◦
atmosphere. The solution was chilled at 0 C in ice bath, then R-NC (23.2 mmol, 1.1 eq.) and TMS-N
3
(
3.06 mL, 2.67 g, 1.1 eq.) were added in sequence. After 30 min, the solution was checked for complete
consumption of the imine. The solvent was removed under reduced pressure and the crude was
eventually purified by flash chromatography. See Supplementary Materials for characterization of
the compounds.
4
. Conclusions
In conclusion, the synthetic route that we optimized and reported allows easy access to bulk
amounts of diverse 1,5-DTs of general formula as mixtures of diastereoisomers. Such mixtures can be
4
easily separated upon column chromatography and the resulting enantiopure diastereoisomers can
both be used as valid entry points for the creation of combinatorial libraries. This synthetic path is
particularly advantageous for its mild conditions and operational simplicity as well as its robustness
and reliability. Moreover, the synthetic pathway we propose proved to be strongly time efficient, as
the whole procedure from protection of the substrate to the purification of the final product could be
performed in less than two days, yielding usually more than 4 g of final products. Similar libraries have
been further diversified by our exploitation of the additional functionalities on the pyrrolidine ring and
generating large collections of enantiopure heterocycles to be employed in drug discovery platforms.
Supplementary Materials: Supplementary Material associated with this article can be found, in the online version.
Analytical data for tetrazoles 4a–k and copies of NMR spectra.
Author Contributions: P.C. synthesized the compounds and performed the NMR experiments, L.M. interpreted
the results and wrote the paper, A.G. performed NMR experiments and HPLC-MS analyses, C.M. set the topic,
A.B. set the topic, designed the study, interpreted the results, and wrote the paper.

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Funding: This research was partially funded by AnalytiCon Discovery GmbH.
Conflicts of Interest: The funders had no role in the collection, analyses, and interpretation of data, and in the
writing of the manuscript.
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©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(