An Improved Protocol for the Synthesis of 1-(Mesitylenesulfonyl)-3-nitro-1, 2,4-triazole (MSNT)

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Organic Preparations and Procedures
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An Improved Protocol for the Synthesis
of 1-(Mesitylenesulfonyl)-3-nitro-1,2,4-
triazole (MSNT)
Rico Petersena, Jakob F. Jensena & Thomas E. Nielsena
a Department of Chemistry, Technical University of Denmark,
Kemitorvet, Building 201, DK-2800 Kgs. Lyngby, Denmark
Published online: 27 May 2014.
To cite this article: Rico Petersen, Jakob F. Jensen & Thomas E. Nielsen (2014) An Improved Protocol
for the Synthesis of 1-(Mesitylenesulfonyl)-3-nitro-1,2,4-triazole (MSNT), Organic Preparations
and Procedures International: The New Journal for Organic Synthesis, 46:3, 267-271, DOI:
10.1080/00304948.2014.903145
To link to this article: http://dx.doi.org/10.1080/00304948.2014.903145
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Organic Preparations and Procedures International, 46:267–271, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2014.903145
OPPI BRIEF
An Improved Protocol for the Synthesis
of 1-(Mesitylenesulfonyl)-3-nitro-1,2,4-triazole
(MSNT)
Rico Petersen, Jakob F. Jensen, and Thomas E. Nielsen
Department of Chemistry, Technical University of Denmark, Kemitorvet,
Building 201, DK-2800 Kgs. Lyngby, Denmark
Since the pioneering work of Merrifield,1–5 solid-phase synthesis has become a key tech-
nology for the rapid parallel synthesis of polypeptides and other oligomeric compounds for
drug discovery and chemical biology research. With an increasing number of peptide and
peptide mimetic drug candidates in the pipelines of large pharmaceutical companies, solid-
phase synthesis will likely continue to have a major impact on pharmaceutical chemistry.
For solid-phase synthesis, it is essential that all transformations on the solid support
proceed in a quantitative fashion. Until the final step of a synthetic sequence, reaction
products remain covalently bound to the support and can only be purified by simple
washings and filtrations, rendering any incomplete reaction step along the way a source of
decreased yields of products.
Solid-phase synthesis typically involves the creation of amide- or ester bonds,
either as part of the linkage strategy, or to build the final product. In this context, a
multitude of coupling reagents has been developed. For example, reagents for esterifi-
cation on solid support include N,Nꢀ-dicyclohexylcarbodiimide (DCC) in conjunction
with 4-(N,N-dimethylamino)pyridine (DMAP)6 or 1-hydroxybenzotriazole (HOBt),7
2,6-dichlorobenzoyl chloride,8 and diethyl azodicarboxylate–triphenylphosphine.9
However, these activators generally result in inferior conversions and product yields.
1-(Mesitylenesulfonyl)-3-nitro-1,2,4-triazole (MSNT) has most often been used for the
formation of phosphate- and phosphorothiolate esters in oligonucleotide synthesis,10–18
and, to a limited extent for amide-bond formation in peptide synthesis.19,20 Another
important application of MSNT is its usefulness to anchor the C-terminal of amino acids to
hydroxyl-functionalized supports.21–23 MSNT-based esterification protocols generally pro-
ceed with minimal racemization in high yields, and have therefore frequently been
Submitted by July1, 2013.
Address correspondence to Thomas E. Nielsen, Department of Chemistry, Technical University
of Denmark, Kemitorvet, Building 201, DK-2800 Kgs. Lyngby, Denmark. E-mail: ten@kemi.dtu.dk
267

268
Nielsen et al.
employed for the solid-phase synthesis of peptides, glycopeptides, and
peptidomimetics.24–36 It is therefore desirable that the reagent be obtainable in a scalable,
efficient, and reliable manner. The only method known37 describes the preparation of MSNT
in 63% yield in two steps from 3-amino-1,2,4-triazole, through the intermediacy of 3-nitro-
1,2,4-triazole (2). This intermediate can be prepared from 3-amino-1,2,4-triazole (1) using
sodium nitrite/acetic acid,38 sodium nitrite/acetic acid/sulfuric acid,39 sodium nitrite/sulfuric
acid40 or hydrogen peroxide/trifluoroacetic acid,41 sodium tungstate/hydrogen peroxide,42
sodium perborate/acetic acid.43 However, the yields of these procedures (23%–77%) are
either lower than or similar to that reported by Jones et al.37 The only other method using
sodium nitrite/hydrochloric acid44 reported to give a higher yield (87%) was not repro-
ducible in our hands. Starting with commercial 3-amino-1,2,4-triazole (Sigma-Aldrich),
we now describe a reproducible, scalable, and highly practical protocol for the synthesis of
MSNT.
In the first step, 3-amino-1,2,4-triazole (∼25 g) is diazotized by slow addition of reagent
in a 2 l round-bottomed flask under mechanical stirring. The slow rate of addition, the use
of a large volume flask and mechanical stirring are experimental parameters that were
carefully chosen to overcome the occurrence of extensive foaming and precipitation. The
product (3-nitro-1,2,4-triazole) was subsequently collected and purified by recrystallization
in 62%–68% yields. The generation of highly toxic NOx gases requires the use of a well-
ventilated fume hood. In the second step, 3-nitro-1,2,4-triazole (∼20 g) is sulfonylated using
mesitylenesulfonyl chloride to give MSNT (Scheme 1), which is isolated in 46%–48% yield
after extraction and purification by recrystallization. The purity of both the intermediate
and the product can be determined by routine NMR and RP-HPLC techniques. In summary
we have developed a practical synthesis of MSNT in 29%–33% overall yield, which is safer
and more reproducible than previously reported methods.
Scheme 1
Experimental Section
All reagents and materials used were purchased commercially and were used without
purification. The solvents used were of standard HPLC grade. Thin-layer chromatography
was performed on aluminum plates, pre-coated with silica gel (Merck 25, 20 × 20 cm,
60 F254) using the indicated eluent systems. Visualization was carried out by illumination
with a UV-lamp (254 nm). Flash column chromatography was performed using silica gel
(FLUKA 60758 silica gel 60, particle size 0.035–0.070 mm, 220–440 mesh ASTM) in

1-(Mesitylenesulfonyl)-3-nitro-1,2,4-triazole
269
varying sizes of glass columns equipped with a sintered glass filter, a joint pressure inlet,
and a stopcock.
Melting points were measured using a Buch & Holm A/S melting point apparatus
and are uncorrected. 1D NMR spectra were recorded using a Varian Mercury-300 MHz
instrument in DMSO-d6 or CDCl3 using the residual CHCl3 or DMSO solvent peaks,
respectively, as the internal standards. All 13C NMR spectra were proton decoupled. IR
analyses were carried out on a Bruker Alpha FT-IR spectrometer. Products were analyzed
on a Waters Alliance reverse-phase HPLC system consisting of a Waters 2695 Separations
Module equipped with a Symmetry C18 column (3.5 μm, 4.6 × 75 mm, column temp 25◦C,
flow rate 1 ml/min) and a Waters Photodiode Array Detector (detecting at 215 nm). Elution
was carried out in a linear reversed phase gradient fashion combining water and acetonitrile
(buffered with 0.1% trifluoroacetic acid).
3-Nitro-1,2,4-triazole (2). A 2.0 l, three-necked, round-bottomed flask equipped with
an overhead mechanical stirrer, a 100 ml pressure-equalizing addition funnel, and a glass-
stopper was charged with 3-amino-1,2,4-triazole (26.3 g, 0.297 mol) and an aqueous
sodium nitrite solution (100.0 g, 1.45 mol in 150 ml water) in an efficient fumehood.
The suspension was cooled in an ice-water bath and the mechanical stirrer was started.
After 10 min of stirring, the light suspension was treated dropwise with conc. nitric acid
(85 ml) via the addition funnel over a period of 2.5–3.0 h (foaming).45 After complete
addition of nitric acid, the ice-water bath was removed and the yellow suspension was
stirred for additional 1.0 h at room temperature until foaming stopped.46 The crude product
was collected and the yellow solid filter cake was dried overnight under oil pump vac-
uum, to give a yellow solid (47 g). The crude product was dissolved in boiling methanol
(300 ml) for 30 min, and then filtered hot using house vacuum. The methanol filtrate
was allowed to cool to rt. and then placed overnight in a freezer at −15◦C whereupon
3-nitro-1,2,4-triazole crystallized as a light yellow solid, which was collected, washed with
ice-cold methanol, and dried overnight at 1.9 mmHg to afford 21–23 g (62%–68%) of
the title compound, mp. 208◦C–211◦C (lit.37 208–210).1H NMR (300 MHz, DMSO-d6):
δ 8.86 (s, 1H); 13C NMR (75 MHz, DMSO-d6): δ 163.1, 146.3; IR (solid): 3162, 2861,
2776, 2730. The purity of the product (97%) established by RP-HPLC, tR = 2.01 min
(254 nm).
1-(Mesitylenesulfonyl)-3-nitro-1,2,4-triazole (3). An oven dried 1.0 l round-bottomed
flask containing a magnetic stir bar was equipped with a Claisen adapter. 3-Nitro-1,2,4-
triazole (19.5 g, 0.171 mol), dry dioxane (200 ml) and triethylamine (1.0 equiv., 17.3 g,
0.171 mol) was transferred to the reaction flask and the solution was cooled in an ice-bath
with magnetic stirring.47 The Claisen adapter was fitted with a 200 ml pressure-equalizing
addition funnel, containing a dioxane solution (150 ml) of mesitylenesulfonyl chloride
(37.4 g, 0.171 mol). The solution of mesitylenesulfonyl chloride was added dropwise over
a period of approx. 0.5 h and the final suspension was stirred for an additional 1 h and
then warmed to rt. After removal of the precipitated Et3N·HCl by filtration, the filtrate
was concentrated in vacuo to give a yellow solid, which was dissolved in dichloromethane
(150 ml) and the solution was washed with water (150 ml).48 The organic layer was dried
over anhydrous Na2SO4 and evaporated in vacuo to give a yellow solid. Recrystallization
from boiling toluene (20–30 ml) followed by washing with ice-cold toluene and drying
overnight at 1.9 mmHg, provided 24–25 g (46%–48%) of the title compound as light yellow

270
Nielsen et al.
solid,49 mp. 131◦C–133◦C (lit.37 130◦C–132◦C). Rf = 0.23 (20% hexane in CH2Cl2, UV);
1H NMR (300 MHz, CDCl3): δ 8.84 (s, 1H), 7.07 (s, 2H), 2.69 (s, 6H), 2.34 (s, 3H);
13C NMR (75 MHz, CDCl3): δ 162.9, 147.5, 145.2, 142.5, 133.0, 127.9, 23.2, 21.3; IR
(solid): 3126, 2983, 2947, 1597.
Anal. Calcd. for C11H12N4O4S: C, 44.59; H, 4.08; N, 18.91. Found: C, 44.69; H,
3.88; N, 18.72. The purity of the product (96%) established by RP-HPLC, tR = 8.41 min
(254 nm).
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