Retention of stereochemistry in the microwave assisted synthesis of 1H-tetrazole bioisosteric moiety from chiral phenyl-acetic acid derivatives

Article abstract of DOI:10.1016/j.tetlet.2013.09.022

Chiral substituted phenylethyl-1H-tetrazoles were built-up from the corresponding carboxylic acid derivatives by a useful three-step synthesis. The procedure, that preserves the chiral center from racemization, was successfully applied to a selection of several hit compounds by conversion of the carboxylic acid moiety to the nitrile derivatives and subsequent reaction with trimethylstannyl azide, under microwave conditions. A useful application to the corresponding tetrazole analogue has been found also in the conversion of the aminoacidic moiety like (R)-N-Cbz-phenylglycine showing a wide potential synthetic application.

Full text of DOI:10.1016/j.tetlet.2013.09.022

Accepted Manuscript
Retention of stereochemistry in the microwave assisted synthesis of 1H-tetra‐
zole bioisosteric moiety from chiral phenyl-acetic acid derivatives
Mara Tomassetti, Michela Fanì, Gianluca Bianchini, Sandra Giuli, Andrea
Aramini, Sandro Colagioia, Giuseppe Nano, Samuele Lillini
PII:
S0040-4039(13)01567-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.09.022
TETL 43522
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 July 2013
29 August 2013
6 September 2013
Please cite this article as: Tomassetti, M., Fanì, M., Bianchini, G., Giuli, S., Aramini, A., Colagioia, S., Nano, G.,
Lillini, S., Retention of stereochemistry in the microwave assisted synthesis of 1H-tetrazole bioisosteric moiety
from chiral phenyl-acetic acid derivatives, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.
2013.09.022
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2
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Retention of stereochemistry in the
microwave assisted synthesis of 1H-tetrazole
bioisosteric moiety from chiral phenyl-acetic
acid derivatives
Leave this area blank for abstract info.
Mara Tomassetti, Michela Fanì, Gianluca Bianchini, Sandra Giuli, Andrea Aramini, Sandro Colagioia,
Giuseppe Nano and Samuele Lillini
A
A
R2
R1
H
N
R2
R1
OH
N
O
N
N
A: Me, NHCbz

Tetrahedron Letters
journal homepage: www.elsevier.com
Retention of stereochemistry in the microwave assisted synthesis of 1H-tetrazole
bioisosteric moiety from chiral phenyl-acetic acid derivatives
Mara Tomassettia, Michela Fanìb, Gianluca Bianchinia, Sandra Giulia, Andrea Araminia,b, Sandro
Colagioiaa, Giuseppe Nanoa and Samuele Lillinib,*
a Dompé s.p.a., Via Campo di Pile, 67100 L’Aquila, Italy
b Dompé s.p.a., Via P. Castellino, 80131 Naples, Italy.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Chiral substituted phenylethyl-1H-tetrazoles were build-up from the corresponding carboxylic
acid derivatives by a useful three-step synthesis. The procedure, that preserves the chiral center
from racemization, was successfully applied to a selection of several hit compounds by
conversion of the carboxylic acid moiety to the nitrile derivatives and subsequent reaction with
trimethylstannyl azide, under microwave conditions. A useful application to the corresponding
tetrazole analogue has been found also in the conversion of the aminoacidic moiety like (R)-N-
Cbz-phenylglycine showing a wide potential synthetic application.
Available online
Keywords:
Tetrazole synthesis
Retention of stereochemistry
Trimethylstannyl azide
Bioisostere
2009 Elsevier Ltd. All rights reserved.
C5aR antagonists
Inflammatory and neuropathic pain are the most prevalent types
of pathological pain and represent important health problems.
Whereas inflammatory pain is one of the classical symptoms of
the inflammatory process, neuropathic pain arises from any of
multiple nerve lesions or diseases with symptoms including
hyperalgesia or allodynia.1 Some of the most powerful
painkillers, including opioids and non-steroidal anti-
inflammatory drugs, are generally not selective, only partially
effective and prolonged exposure can cause side effects.2 As a
result, there is a continuous effort to identify novel therapeutics
for pain control with alternative mechanism of action and that
elicit fewer side effects.
selectivity and potency in inhibiting C5a-induced neutrophils
chemotaxis. Lead optimization program has permitted the
investigation of large series of carboxylic acid bioisosteres8
identifying the tetrazole group as the best moiety and DF3966Y
(4a, IC50 = 50nM, Figure 1) as the Lead Compound in terms of
pharmacological profile (ADME and PK), maintaining potency
and selectivity properties.
H
N
F3C
F3C
(three-step
synthesis)
OH
N
N
N
N
N
O
S
S
N
H
N
H
Inflammatory mediators, including cytokines/chemokines, play
critical role in the pathogenesis of inflammatory and
DF2703Y (1a)
DF3966Y (4a)
a
neuropathic pain.3 Emerging evidences suggest that C5a, the
anaphylatoxin produced by the complement activation, has potent
nociceptive activity in several models of inflammatory and
neuropathic pain by interacting with its selective receptor C5aR.4
C5aR belongs to the class A subfamily of the G protein-coupled
receptors (GPCR)5 and is widely expressed in immune cells,
including neutrophils (PMN), monocytes, microglia, and in non-
immune cells, including neurons in the central nervous system
(CNS) and dorsal root ganglia (DRG).6
Figure 1. Conversion of the carboxylic acid moiety to the tetrazole,
the corresponding bioisostere.
The addition of azide ion to nitriles is the most convenient
route to obtain 5-substituted 1H-tetrazoles. Cyano derivatives are
easily available by conversion of the carboxylic acid moieties to
the corresponding amides and subsequent dehydration (Scheme
1). The extreme toxicity, the low boiling point (37°C) and the
explosive nature of the hydrazoic acid (HN3) prevent its use as
free acid.9 Comparatively safe azide’s sources have been
introduced in the last few decades, the most common are, besides
of the sodium azide (NaN3), tin, aluminum and silicon azides10
which are employed in a wide range of reaction conditions.
As for other peptidergic GPCRs, the efforts to identify small
molecular weight C5aR antagonists have led to a limited number
of molecules, mostly lacking adequate potency and selectivity.7
During our discovery programs, we selected a novel class of
(R)-4-(heteroaryl)phenylpropionic derivatives, typified by
compound DF2703Y (1a, Figure 1), which shows strong
Corresponding author. Tel.: +39-081-6132284; fax: +39-081-6132277; e-mail: samuele.lillini@dompe.it

2
It is well-known that azide salts can engage nitriles11 at
elevated temperatures to yield tetrazoles, however, there is
continuous debate on the mechanism of the reaction. Despite of
numerous examples reported in the literature, there are no papers
describing the synthesis of phenylethyl-1H-tetrazole derivatives
from chiral phenylethyl-nitriles with retention of stereochemical
configuration.
HPLC assays. An extensive study of reaction conditions has
been performed in the synthesis of our lead compound 4a
(DF3966Y, Scheme 1) starting from the easily racemizable 3a
(Table 1).
F3C
F3C
O
O
1. EDC, HOBt, 0°C, 1h
2. NH3 (g), 0°C, 2-3h
N
N
Herein, we present a novel method for the synthesis of chiral
phenylethyl 1H-tetrazoles using trialkylstannyl-azides as the
source of azide under microwave conditions.
OH
NH2
S
NH
S
NH
2a
DCM
1a e.e. 96%
The synthesis of our target compound 4a is shown in Scheme
1. Starting from carboxylic acid 1a, amide 2a was obtained via
activation with EDC, HOBt in DCM and subsequent bubbling
with gaseous NH3 at low temperature (0°C).12 Extremely
convenient reaction conditions were found in the use of
trichloroacetyl chloride and TEA in DCM for the conversion of
amide 2a to the corresponding nitrile 3a. Compound 3a was
obtained with 94% of enantiomeric excess in 86% yield (Scheme
1).13 Some other dehydrating reagents such as thionyl chloride,
phosgene and phosphorus oxychloride, used under classical
reaction conditions provided comparable yields but lower optical
purity.
Cl3CCOCl, TEA,
RT, 1h
DCM
H
N
F3C
F3C
Table 1
N
N
N
N
N
N
S
S
NH
NH
3a e.e. 94%, yield 86%
4a (DF3966Y)
e.e. 90%, yield 89%
The enantiomeric excess values were evaluated comparing the
chiral compound with its corresponding racemate by chiral
Scheme 1. Synthesis of the target compound 4a (DF 3966Y)
Table 1. Study of reaction conditions in the tetrazole synthesis
Entry
Azide source
TMSN3(9eq), TBAF(9eq)
TMSN3(1eq), TBAF(1eq)
NaN3(1eq)
Solvent
Conditions
18h, 85°C
Conversion (%) Yield (%)
% e.e.
1
2
3
4
5
6
7
8
THF
THF
80
100
100
82
60
76
92
73
18
35
89
45
0
0
MW 1h, 130°C
MW 1h, 150°C
MW 1h, 150°C
MW 1h, 150°C
NMP/AcOH/H2O; 7:2:1 (Vol)
NMP/AcOH; 7:3 (Vol)
DMF
0
NaN3(4eq)
0
NaN3(1.05eq) , NH4Cl(1.05eq)
NaN3(1.1eq), ZnBr2(1eq)
Me3SnN3(1.5eq)
44
0
IPA/ H2O/NMP; 9:0.5:0.5 (Vol) MW 1h, 120°C
45
0
toluene
toluene
MW 1.5h, 130°C
24 h, reflux
100
50
90
80
Me3SnN3(1.5eq)
It was clearly evident from the literature that a high yielding and
fast tetrazole formation would have to involve a high temperature
regime and pressure control, in an environmentally safe reactor.
In order to improve these preparation protocols, we performed
our study under microwave irradiation, using this powerful
technique to accelerate the thermally-driven chemical reactions.
We began our investigations by evaluating several reported
methods of tetrazole synthesis, with the aim of finding the one
that would be appropriate to obtain the target compound in good
yield and most of all, with high enantiomeric excess (Table 1).
Under our reaction conditions screening, three different azide’s
sources were used, moving from TMSN3/TBAF in THF (Entries
1 and 2), in which complete conversion and good yield were
obtained (Entry 2) to NaN3 in a mixture of polar solvents (Entries
3-6). It is noteworthy that the reaction carried out in a
The complete conversion and excellent yield were performed
using NaN3 in NMP/AcOH/H2O heated at 150°C under
microwave irradiation (Entry 3). All reaction conditions in Table
1 (Entries 1-6) lead to complete racemization. As an alternative
to inorganic azide salts, the trimethylstannyl azide (Me3SnN3)
was adopted (Entry 7). This reagent offers several advantages
over the other reagents including better solubility in organic
solvents and it is comparatively safer to use. The reaction was
carried out in toluene using 1.5 equivalents of tin reagent, heating
by microwave irradiation at 130°C for 1.5h. The reaction
proceeds in a high yield (89%) and with an excellent retention of
configuration (90% of enantiomeric excess, see Table 1 and
Table 2), proving that the reaction conditions have negligible
effect on the enantiomeric purity of compound 4a.13 As
comparison with the normal heating (Table 1, Entry 8), it should
be pointed out that microwave irradiation improves yield,
reaction time and markedly the optical purity.
conventional heating required longer times and an excess of
reactants to obtain a good conversion (Entry 1).

3
To explore the scope and limitations of this method a
significant selection of diversely substituted chiral phenylethyl-
nitrile derivatives was synthesized starting from the
corresponding carboxylic acid derivatives (Scheme 2 and Table
2). Para and meta EWGs and EDGs were chosen in order to
evaluate the direct effect to the acidity of the proton at the
benzylic position.
1. EDC, HOBt, 0°C, 1h (1a-g
)
Me3SnN3
MW, 130°C, 1.5h
H
N
R2
R1
or CDI, 0°C, 1h (1h
)
R2
O
R2
R1
R2
R1
O
2. NH3 (g), 0°C, 2-3h
Cl3CCOCl, TEA, RT, 1h
DCM
N
N
NH2
R1
N
OH
Toluene
DCM
N
1a-h
3a-h
2a-h
4a-h
1. Me3SnN3, MW, 130°C, 1.5h, toluene
2. Crystallizzation from reaction
medium
SnMe3
R2
R1
N
acidic work-up
N
N
N
5f,h
Scheme 2. Synthesis of chiral phenylethyl 1H-tetrazoles from chiral benzyl-nitriles.
Table 2. Reaction scope
Acid (1)*
Amide (2)
Nitrile (3)
Tetrazole (4)
Entry
Compound
R1 (para)
R2 (meta)
% e.e.
% yield
% e.e. % yield
% e.e. %yield
% e.e.
F3C
N
1
a
H
H
97
68
95
86
94
89
90
S
N
H
O
F
3C
S
2
3
b
c
96
96
56
83
85
87
95
93
88
90
O
O
O
O
F3C
O
S
H
4
5
d
e
H
98
97
80
85
80
82
60
67
92
93
H
H
O
6
f
98
90
83
96
90
93
7
8
g
h
F
98
97
97
97
84
96
70
64
96
94
0
4-NO2
H
86**
92**
*
Optical purity of the starting materials.
The acid 1h was activated by CDI (1.5eq) in DCM and treated with gaseous NH3 at low temperature (0°C).
**

4
Moderate loss of enantiomeric purity was observed for para
and ortho-triflate derivatives, respectively 4b and 4c (Entries 2
and 3).
nitrile followed by ring closure, and a concerted [2 + 3]
cycloaddition.18 In our case both mechanisms do not necessarily
involve the chiral center.
The method proved to be very efficient in the synthesis of a
single tetrazole’s enantiomer structurally related to some active
pharmaceutical ingredients, such as: Ibuprofen 4d (Table 2, entry
4), Ketoprofen 4f (Entry 6) and Flurbiprofen 4g (Entry 7). A
comparable result was achieved for the meta-alkyl compound 4e.
To further examine the range of applicability of this reaction,
the 4-nitrophenyl derivative 4h was synthesized. The common
method used for the preparation of the amides 2a-g, via
activation by EDC/HOBt gave a complete racemization when
applied to synthesis of intermediate 2h. The mentioned amide 2h
was differently obtained with 92% e.e. by activation with CDI
and subsequent bubbling with gaseous ammonia (Table 2, entry
8). A loss of optical purity occurred in the dehydration step to
obtain nitrile derivatives 3h, (70%, Table 2, Entry 8). We then
applied our standard protocol with Me3SnN3 but tetrazole was
obtained as a racemic mixture. All attempts done for the
synthesis of chiral tetrazole 4h, unfortunately failed.
In order to assess the (2R)-2-(4-nitrophenyl)propanenitrile (3h)
stability without any source of azide, the compound was
subjected to a set of different conditions mimicking the standard
ones or testing mechanism hypotheses. Mild anhydrous acidic
conditions (cat. AcOH), mild anhydrous basic (an. sodium
bicarbonate), mild aqueous basic (sodium bicarbonate) and
simple heating in toluene at 130°C, all led to optical purity loss.
Negligible loss of enantiomeric purity was observed for the
nitrile derivatives 3a-g by heating at 130°C for 30 minutes.
All these tests taken together point toward tautomeric
mechanisms assisting the proton abstraction that causes
racemization in the 4-nitro derivative. Furthermore, the absence
of a solvent capable of proton coordination indicates that
intermolecular coordination between reacting nitrile molecules
may play a significant role in the supposed mechanism.
Differently from the two-step mechanism, which is generally
reported for azide ion in polar solvent, the concerted one is
described for covalent species such as alkyl azides. In our
hypothesis a greater degree of covalency in the nitrogen-metal
bond of the stannylazide as compared to sodium azide,19
presumably drives the reaction towards a 1,3-dipolar concerted
cyclization, as demonstrated by the stability of the isolated
intermediates 5f,h and the use of an apolar solvent like toluene.
In summary, we have demonstrated an exceedingly simple
protocol for the transformation of a wide variety of chiral
phenylacetonitrile into the corresponding 1H-tetrazoles; the
method has been successfully applied to an easily racemizable
alpha amino acid. By using Me3SnN3 we obtained a good to high
retention of configuration in high yields. The use of
trimethylstannyl azide under neutral condition prevents the
formation of toxic and explosive HN3. All the reactions were
carried out in a safe microwave reactor environment improving
time reaction, yield and considerably the optical purity in
comparison with the conventional thermal methods. Further
investigation is currently in progress in order to explore the
application to other chiral nitriles.
Acknowledgments
We thank Lucio De Simone for Chiral HPLC support.
References Note
1. (a) Kidd, B.L.; Urban, L.A. Br J Anaesth 2011, 87, 3-11; (b) Baron, R.;
Binder, A.; Wasner, G. Lancet Neurol 2010, 9, 807–819.
2. (a) Schaffer, D.; Florin, T.; Eagle, C.; Marschner, I.; Singh, G.; Grobler,
M.; Fenn, C.; Schou, M.; Curnow, K. M. Med J Aust 2006, 185, 501–506.
(b) Devulder, J.; Jacobs, A.; Richarz, U.; Wiggett, H. Br J Anaesth 2009,
103, 576–585.
3. (a) White, F. A.; Jung, H.; Miller, R. J. Proc Natl Acad Sci USA 2007, 104,
20151–20158. (b) Cunha, M.; Barsante, M. M.; Guerrero, A. T.; Verri Jr,
W.A.; Ferreira, S. H.; Coelho, F. M.; Bertini, R.; Di Giacinto, C.;
Allegretti, M.; Cunha, F. Q.; Teixeira, M.M. Br J Pharmacol 2008, 154,
460–470.
Tin-tetrazole adducts 5f,h were isolated directly from the
reaction medium: after 1.5 hours of microwave heating, the
mixture was cooled to room temperature and concentrated
without any workup and the products crystallized.15,16 Subsequent
acidic hydrolysis of 5h,f yielded the corresponding tetrazoles,
respectively 4f and 4h.
Continuing the investigations in the field of the carboxylic acid
replacement, the method has been successfully applied in the
synthesis of the tetrazole derivative of (R)-N-Cbz-phenylglycine.
Starting from chiral α-amino nitrile 6, obtained with an e.e. of
86% (Scheme 3), tetrazole 7 was synthesized obtaining a good
yield (79%) and a good enantiomeric purity (e.e. 80%). To the
best of our knowledge, derivative 7 has never been obtained with
an e.e. higher than 39%.17
4. (a) Ting, E.; Guerrero, A.T.; Cunha, T.M.; Verri Jr, W.A.; Taylor, S.M.;
Woodruff, T.M.; Cunha, F. Q.; Ferreira, S.H. Br J Pharmacol 2008, 153,
1043–1053. (b) Griffin, R.S.; Costigan, M.; Brenner, G.J.; Ma, C.H.;
Scholz, J.; Moss, A.; Allchorne, A.J.; Stahl, G.L. ; Woolf, C.J. J Neurosci
2007, 27, 8699–8708.
5. Gerard, C.; Gerard, N.P. Annu Rev Immunol 1994, 12, 775–808.
6. (a) Zwirner, J.; Fayyazi, A.; Gotze, O. Mol Immunol 1999, 36, 877–884.
(b) Jang, J. H.; Liang, D.; Kido, K.; Sun, Y.; Clark, D.J.; Brennan, T.J. J
Neuroinflammation 2011, 8, 80–97. (b) Jang, J.H.; Clark, J. D.; Li, X.;
Yorek, M.S.; Usachev, Y. M.; Brennan, T.J. Pain 2010, 148, 343–352.
7. (a) Himo, F.; Demko, Z.P.; Noodleman, L.; Sharpless, K.B. J. Am. Chem.
Soc. 2002, 124, 12210 and references cited therein.
N
H
N
H
N
N
N
N
Cbz
N
H
Cbz
Me3SnN3
8. Nicholas A. Meanwell J. Med. Chem. 2011, 54(8), 2529–2591.
9. Gutmann, B.; Roduit, J.P.; Roberge, D.; Kappe, C.O. Angew. Chem. Int.
Ed. 2010, 49, 7101–7105.
Toluene
MW, 130°C, 1h
10. (a) Carlier, P.R.; Taylor, P.; Venkateshwarlu, M.G.; Premalatha, G.;
Rajanna, A.; Saiprakash, K.C. Synth. Commun. 2009, 39, 4479–4485; (c)
Demko, Z.P.; Sharpless, K.B. J. Org. Chem. 2001, 66, 7945–7950; (d)
Amantini, D.; Fringuelli, F.; Pizzo, F.; Vaccaro, L. J. Org. Chem. 2001, 66,
6734; (e) Corriu, R.J.; Perez, R.; Reye, C. Tetrahedron 1983, 39, 999–
1009; (f) Witteberger, S.J.; Donner, B.G. J. Org. Chem. 1993, 58, 4139–
4141; (g) Breitonmoser, R.A.; Heimgartner, H. Helv. Chim. Acta 2002, 85,
885–912.
6
e.e. 86%
7
e.e. 80%, yield 79%
Scheme 3. Tetrazole analogous of (R)-N-Cbz-phenylglycine.
As described by Sharpless and coworkers in a relevant
computational study, two plausible mechanisms can be involved
at the same time in the the addition of the azide ion to the nitrile
leading to the tetrazole. The two considered are: a two-step
mechanism, wherein the azide first nucleophilically attacks the
11. Bräse, S.; Gil, C.; Knepper, K.; Zimmermann V. Angew. Chem. Int. Ed.
2005, 44, 5188–5240.

5
characterized by NMR. 1H NMR (400 MHz, DMSO-d6) δ ppm 0.56 (br.
12. General procedure for the synthesis of amide (2a-g). A solution of the (R)-
acid 1a-g (1.0 eq) in dry DCM (0,05 mmol/mL) was cooled to 0°C under
nitrogen atmosphere. To the mixture 1.3 eq of both 1-
Hydroxybenzotriazole (HOBt) and N-(3-Dimethylaminopropyl)-N′-
ethylcarbodiimide hydrochloride (EDC) were added to the solution and the
reaction was stirred for 1h and allowed to reach R.T.. The solution was
cooled to 0°C again and gaseous ammonia was bubbled in the mixture until
the reaction was completed. After solvent removal under reduced pressure,
the residue was dissolved in ethyl acetate and the organic layer was washed
with NaHCO3 s.s. then with a 2M (pH = 2) sodium phosphate buffer. The
organic layers were dried over anhydrous Na2SO4 and the solvent removed
to afford amides 2a-g. The products were directly used for the next step
without further purification unless otherwise specified (see Supplementary
s., 9 H) 1.62 (d, J=7.2 Hz, 2 H) 4.49 (q, J=7.2 Hz, 1 H) 7.34 - 7.85 (m, 8
H).
16. Langry, K.C. J. Org. Chem. 1991, 56, 2400–2404.
17. Sharpless, B.K.; Deemko, Z.P. Org. Lett. 2002, 4, 2525–2527.
18. (a) Himo, F.; Deemko, Z.P.; Noodleman, L.; Sharpless, B.K. J. Am. Chem.
Soc. 2002, 124, 12210–216; (b) Deemko, Z.P.; Sharpless, B.K. J. Org.
Chem. 2001, 66, 7945–7950.
19. (a) Bräse, S.; Barnet, K. Organic Azide: Syntheses and Applications 2010,
34; (b) Cheng, H.S.; Herber, R.H. Inorg. Chem. 1970, 9, 1686–1690.
Data).
(2R)-2-(4-{[4-(trifluoromethyl)-1,3-thiazol-2-yl]amino}phenyl)
propanamide (2a). The enantiomeric excess was found by Chiral HPLC
Column: Chiracel OF, elution: HEX/IPA 80:20, flow rate: 1mL/min,
injected volume: 10 µL, λ: 254 nm, sample concentration 2 mg/mL
(MeOH): t1= 10.31; t2= 13.86 (S/R 2.5:97.5) e.e. 95%. 1H NMR (400
MHz, DMSO-d6) δ (ppm) 1.30 (d, J=7.03 Hz, 3 H), 3.53 (q, J=7.03 Hz, 1
H), 6.80 (br. s., NH), 7.22 - 7.32 (m, 2 H), 7.34 (br. s., NH), 7.42 - 7.56 (m,
2 H), 7.63 (br s, 1 H), 10.52 (s, NH). LC-MS(ESI): [M+H]+ m/z 316.01.
[α]D20 (c = 1.0 in MeOH) = -17.9°.
13. General procedure for the synthesis of nitriles (3a-h). To a solution of 2 in
dry DCM (0.05 mmol/mL) TEA (2.0 eq) was added and the mixture was
cooled to 0°C and stirred for 15 min under nitrogen atmosphere. A solution
of trichloroacetyl chloride (2.0 eq) in dry DCM (1.0 mmol/0.6 mL) was
added dropwise and the reaction was allowed to reach room temperature
and stirred for 1h. The reaction was quenched with water and the organic
layer was separated. The aqueous layer was extracted again with ethyl
acetate (2×10 mL) and the organic layers were collected, washed with brine
(1×10 mL) and dried over anhydrous Na2SO4. After solvent removal, nitrile
3 was isolated by purification on silica gel.
(2R)-2-(4-{[4-(trifluoromethyl)-1,3-thiazol-2-yl]amino}phenyl)
propanenitrile (3a). The enantiomeric excess was determined by Chiral
HPLC Coloumn: PHENOMENEX LUX AMYLOSE-2, gradient: n-
hexane/isopropyl alcohol 80:20, flow rate: 0.5 mL/min, Volume of
injection: 2 µL, λ: 300 nm, sample concentration 1.7 mg/mL: t1= 8.62 min;
t2= 9.15 min (S/R 3:97) e.e. 94%. 1H NMR (400 MHz, CDCl3) δ (ppm)
1.66 (d, J=7.28 Hz, 3 H) 3.91 (q, J=7.28 Hz, 1 H) 7.09 (q, J=1.00 Hz, 1 H)
7.34 - 7.43 (m, 4 H) 7.51 - 7.67 (br s, 1 H); LC-MS(ESI): [M+H]+ m/z
297.97, [M-H]- m/z 295.94. [α]D20 (c = 2.0, CH3OH) = +7.5°.
14. General procedure for the synthesis of 1H-tetrazol-derivatives (4a-h and 7).
In a MW vial, the nitrile 3 was dissolved in toluene (0.20 mmol/ mL) and
the trimethylstannyl azide (1.5 eq.) was added to the solution. The mixture
was irradiated in a microwave apparatus at 130°C for 1.5h. The solvent was
removed under vacuum and an aqueous solution of 1N HCl (1mL/ 0.10
mmol of substrate) was added to the crude. The suspension was stirred for
1h at r.t. and the product extracted with ethyl acetate (2 x 25 mL). The
collected organic layers were washed with water (2 x 10 mL) and brine (1 x
10 mL), dried over anhydrous Na2SO4, and the solvent removed under
reduced pressure. Tetrazoles 4 were purified on silica gel as specified in
each of the following examples.
N-{4-[(1R)-1-(1H-tetrazol-5-yl)ethyl]phenyl}-4-(trifluoromethyl)-1,3-
thiazol-2-amine 4a. The enantiomeric excess was determined by Chiral
HPLC Coloumn: CHIRACEL OJ, gradient: n-hexane/isopropyl alcohol
90:10 + 0.5% AcOH, flow rate: 1 mL/min, injection volum: 2 µL, λ: 300
nm, sample concentration 2 mg/1mL: t1= 65.5 min; t2= 85.3 min (R/S
92:8) e.e. 84%. 1H-NMR (DMSO-d6) δ (ppm): 1.65 (d, J = 7.2 Hz, 3H),
4.51 (q, J = 7.3 Hz, 1H), 7.24 (d, J = 8.6 Hz, 2H), 7.53 (d, J = 8.6 Hz, 2H),
7.64 (s, 1H), 10.6 (s, 1H). LC-MS(ESI): [M+H]+ m/z 340.93, [M-H]- m/z
338.99. HRMS (ESI) calc’d for C13H12F3N6S [M+H]+ m/z 341.0791, found
341.0793. [α]D20 (c = 2.0, MeOH) = -12.9°.
15. Synthesis of 5f (1R)-phenyl(3-(1-(2-trimethylstannyl)-2H-tetrazol-5-
yl)ethyl)phenyl)phenyl) methanone.
The nitrile 3f (0.150 g, 0.64 mmol, 1.0 eq) was dissolved in toluene (3.2
mL, 0.20 mmol/ mL) in a MW vial. Trimethylstannyl azide (0.197 g, 0.957
mmol, 1.5 eq) was added to the solution and the reaction was irradiated in a
microwave apparatus at 130°C for 1.5h. The solvent was evaporated and
the white precipitate was triturated in n-hexane stirring 1h. This solid was