Efficient solid-phase synthesis of 4,5-dihydro-1,2,4-triazin-6(1H)-ones

Article abstract of DOI:10.1016/S0040-4039(02)02591-1

A new and easy protocol for the formation of substituted 4,5-dihydro-1,2,4-triazin-6(1H)-ones was developed on solid support. The heterocyclic compounds were formed by nucleophilic reaction of hydrazine on thioamide esters. As cyclization was concomitant with cleavage from the support, substituted 4,5-dihydrotriazinones were obtained in high purity.

Full text of DOI:10.1016/S0040-4039(02)02591-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 459–462
Efficient solid-phase synthesis of
4,5-dihydro-1,2,4-triazin-6(1H)-ones
Damien Boeglin, Sonia Cantel, Jean Martinez and Jean-Alain Fehrentz*
Laboratoire des Aminoacides, Peptides et Prote´ines (LAPP), UMR 5810 CNRS Universite´s Montpellier I et II,
Faculte´ de Pharmacie, 15 Avenue Charles Flahault, BP 14491, 34093 Montpellier Ce´dex 5, France
Received 21 October 2002; revised 14 November 2002; accepted 15 November 2002
Abstract—A new and easy protocol for the formation of substituted 4,5-dihydro-1,2,4-triazin-6(1H)-ones was developed on solid
support. The heterocyclic compounds were formed by nucleophilic reaction of hydrazine on thioamide esters. As cyclization was
concomitant with cleavage from the support, substituted 4,5-dihydrotriazinones were obtained in high purity. © 2002 Elsevier
Science Ltd. All rights reserved.
Heterocyclic compounds are of great interest in modern
medicinal chemistry. 4,5-Dihydro-1,2,4-triazin-6(1H)-
ones are heterocycles which can be considered as pepti-
domimetic structures of dipeptides or as potential
templates for anchoring various pharmacophore groups
in a privileged structure approach. Three different
strategies have been formally applied for the prepara-
tion of 4,5-dihydrotriazinone derivatives. The first route
was achieved by condensation of hydrazine, considered
as a 1,2-dinucleophile on a 1,4-dielectrophile,1–5 the
second one concerned a cyclocondensation of an a-
aminohydrazide (1,5-dinucleophile) onto an orthoester
as 1,1-dielectrophile.6 Recently, Saniere et al. reported
the use of N-thioacylphtalimides as 1,1-dielectrophiles
in a reaction with a-amino hydrazides to yield dihydro-
triazinones.7 As far as we know, no publication
describes the solid-phase synthesis (SPS) of 4,5-dihy-
drotriazin-6-ones while the SPS of 3-amino-1,2,4-tri-
azin-5(4H)-ones was reported once.8 As part of our
search into heterocyclic compounds9 we decided to use
a more practical approach to synthesize these molecules
with the aim to generate quickly and easily libraries of
compounds for lead compound research. To obtain a
wide diversity and to apply this strategy to combinato-
rial chemistry, we needed to transfer this methodology
to SPS. This was performed by hydrazine condensation
on an a-(thioacyl)aminoester5 linked to the solid sup-
port via its C-terminal carboxylic group and a hydra-
zine cyclization/cleavage step as shown in Scheme 1.
Hydroxymethyl polystyrene resin 1 (1.1 mmol/g) was
converted into the corresponding N-protected a-
aminoester 2 by activation of the carboxylic acid func-
tion with diisopropylcarbodiimide (DIC) in the
presence of 4-dimethylaminopyridine (DMAP) as cata-
lyst.10 The unreacted hydroxymethyl groups were
capped with Ac2O in DCM, and after drying, an
aliquot of the resin was used for loading determination
by UV dosage of the Fmoc group release. The loading
of the resin was in the range of 0.4 to 0.6 mmol/g
depending of the N-protected a-aminoacid. After Fmoc
deprotection of 2, the amine was acylated with a
selected carboxylic acid R3-COOH (5 equiv.) using
BOP11 as coupling reagent in the presence of diiso-
propylethylamine (DIEA). The amide bond of 3 was
then converted into thioamide on the solid support
using Lawesson’s reagent (LR) in refluxing THF as
already described.12,13 Three experiments were per-
formed on a model dipeptidyl-resin with different quan-
tities of LR (0.5, 1 and 3 equiv.) to optimize this
reaction. At this stage, completion of the thionation
reaction was necessary to avoid undesired formation of
dipeptide hydrazide 6 during the hydrazine cleavage
final step. Complete conversion of 3 into 4 was checked
by cleavage of resin aliquots with TFA for these three
experiments. Mass spectrometry and HPLC analysis of
4% revealed that 3 equiv. of LR were needed. The
reaction can also be conveniently monitored by IR
analysis of the resin beads which show a disappearance
of the amide carbonyl stretch at 1669 cm−1. This thion-
ation reaction on solid support was found very practi-
cal compared to solution synthesis. LR used in excess
was eliminated by simple filtration in the case of SPS
whereas the same reaction in solution needed a tedious
Keywords: Lawesson’s reagent; 4,5-dihydrotriazin-6-one; thioamide;
cyclocleavage.
* Corresponding author. Fax: (33)467548654; e-mail: fehrentz@
colombes.pharma.univ-montp1.fr
0040-4039/03/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02591-1

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D. Boeglin et al. / Tetrahedron Letters 44 (2003) 459–462
Scheme 1. Solid-phase synthesis of 4,5-dihydro-1,2,4-triazin-6(1H)-ones. Reagents and conditions: (i) 10 equiv. FmocAA1OH, 5
equiv. DIC, DMAP cat.; (ii) Ac2O/DCM 50/50 (v/v); (iii) DMF/Pip 80/20 (v/v); (iv) 5 equiv. R3COOH, 5 equiv. BOP, 10 equiv.
DIEA, DMF; (v) 3 equiv. LR, THF, 65°C, 3 h; (vi) TFA/TIS/H2O 95/2.5/2.5 (v/v/v); (vii) 30 equiv. NH2NH2, H2O, dioxane,
90°C, 16 h.
then 15 min. After classical washings, Z-L-tryptophan
purification step. As the thionation reaction was per-
formed at 65°C, it was specific to the amide bond and
neither the ester nor the urethane reacted with LR even
used in large excess.14 The linked compound 4 was then
placed in dioxane at 90°C in the presence of 30 equiv.
of hydrazine hydrate for 16 h and the solutions contain-
ing the desired compounds 5 were then recovered after
simple filtration. These crude samples were directly
examined by HPLC to check their purity.
(269 mg, 5 equiv., 0.79 mmol) and BOP (352 mg, 0.79
mmol) were added to the resin in 5 ml DMF, followed
by 270 ml of DIEA (1.59 mmol). After 45 min of
mechanical stirring, the reaction mixture was drained
and the beads were washed twice with DMF and THF.
LR (193 mg, 3 equiv., 0.48 mmol), then 5 ml of THF
were added to the resin and placed under stirring at
65°C for 3 h. The reaction mixture was drained and the
resin was sequentially washed with THF (2×) and diox-
ane (2×). 5 ml of dioxane and 222 ml of hydrazine
hydrate (30 equiv., 4.77 mmol) were added to the
reactor which was placed at 90°C under stirring for 16
h. The reactor was allowed to warm up to room
temperature, the filtrate was recovered and the resin
was washed with 5 ml dioxane which were combined
with the previous ones. The crude mixture was dried
under vacuum, dissolved in CH3CN/H2O (50/50, v/v)
and lyophilized to yield a white foam which was ana-
lyzed. This procedure was applied concomitantly to all
obtained compounds on the Quest apparatus. All com-
pounds were characterized by HPLC and LC/MS (Fig.
1 and Table 1). As an example, physico-chemical data
from compounds f and g are given.15
The following experimental procedure is representative
of all synthesized compounds. 8.52 g of Fmoc-L-phenyl-
alanine (22 mmol) was placed at 0°C in an ice bath and
0.5 equiv. of DIC (1.7 ml, 11 mmol) was added. After
30 min at this temperature, this mixture was added to 2
g of Wang resin (1.1 mmol/g) in a fritted SPS reactor.
DMAP in a catalytic amount (0.14 g, 1.1 mmol) was
added and the reaction was left overnight under
mechanical stirring. After draining, the resin was
sequentially washed twice with DMF, MeOH and
DCM. The remaining hydroxyl groups were then
capped with DCM/Ac2O (50/50, v/v) for 30 min. After
washings, the resin was dried in vacuo till constant
weight. An aliquot of the resin was used for determina-
tion of the resin substitution by Fmoc release measure-
ment followed by UV. A loading of 0.53 mmol/g was
found. 300 mg of this resin (0.159 mmol) was placed in
a reaction vessel on a Quest 210 organic synthesizer
(Argonaut Technologies, San Carlos, CA). The tempo-
rary Fmoc group of the N-terminal function was
removed by a 80/20 solution of piperidine/DMF for 5
As this methodology was devoted to lead compound
discovery, no special effort was done to avoid epimer-
ization of the second introduced amino residue.
Epimerization was detected by HPLC and LC/MS in
the case of diastereomers (compounds a, b, c, d and f).
Saniere et al.7 reported that the use of mercury diac-

D. Boeglin et al. / Tetrahedron Letters 44 (2003) 459–462
461
Figure 1. Synthesized molecules.
Table 1. Isolated yield, purity and physico-chemical characteristics of synthesized compounds
Compd
HPLC purity of crude product (%) at 214 nm/254 nma
Isolated yield (%)b
Retention time tR (min)a
a
b
c
d
e
f
g
h
i
83/86c
98/91c
95/100c
89/84c
68/64
75/58c
99/96
92/94
81/81
73/93
79
75
61
80
60
68
92
90
71
79
8.47
6.88
7.07
7.20
4.30
7.75 (maj.)/8.03 (min.)d
5.57
7.30
6.52
6.55
j
a Symmetry Shield Waters Column, C18, 5 mm, 150×3.9 mm; gradient from A to B in 15 min (A: H2O, 0.1% TFA; B: CH3CN, 0.1% TFA).
b Yields were calculated from the measured resin substitution and after flash purification.
c Both diastereoisomers were combined.
d Diastereoisomers were separated.
etate as a catalyst during the cyclization reaction under
mild conditions resulted in practically no epimerization.
acids can be used as building blocks and as N1 and N4
sites are potential alkylation positions, generation of
large libraries of compounds is possible. The products
were obtained with good to excellent yields and with
high purities.
4,5-Dihydro-1,2,4-triazin-6(1H)-ones are interesting
scaffolds as it is theoretically possible to introduce four
different groups on the heterocycle as shown in Figure
2. Attempts for the cyclization/cleavage step with sub-
stituted hydrazines are under investigation in our labo-
ratory (R4"H) and the alkylation of the amidine
moiety is also under study (R2"H).
Acknowledgements
We are grateful to Mr. Pierre Sanchez for his indispens-
able help in obtaining mass spectrometry data.
In conclusion, we described in this paper an easy,
general and efficient solid-phase access to 4,5-dihydro-
1,2,4-triazin-6(1H)-ones. As aminoacids and carboxylic
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462
D. Boeglin et al. / Tetrahedron Letters 44 (2003) 459–462
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