A convenient one-pot synthesis of asymmetric 1,3,5-triazine-2,4,6-triones and its application towards a novel class of gonadotropin-releasing hormone receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2004.11.026

A convenient one-pot synthetic route was developed for the preparation of asymmetric 1,3-dialkyl-1,3,5-triazine-2,4,6-triones from readily available alkyl- or aryl-isocyanates, primary amines and N-chlorocarbonyl isocyanate in excellent yields. Subsequent alkylation with N-protected amino alcohols afforded the desired 1,3,5-triazine-2,4,6-triones in good yields. This methodology was applied to the synthesis of a chemical library acting as antagonists of the hGnRH receptor.

Full text of DOI:10.1016/j.bmcl.2004.11.026

Bioorganic & Medicinal Chemistry Letters 15 (2005) 693–698
A convenient one-pot synthesis of asymmetric
1,3,5-triazine-2,4,6-triones and its application towards a novel
class of gonadotropin-releasing hormone receptor antagonists
Zhiqiang Guo,a,* Dongpei Wu,a Yun-Fei Zhu,a Fabio C. Tucci,a Joseph Pontillo,a
John Saunders,a Qiu Xie,b R. Scott Struthersb and Chen Chena,*
aDepartment of Medicinal Chemistry, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
bDepartment of Endocrinology, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
Received 1 October 2004; revised 8 November 2004; accepted 10 November 2004
Available online 2 December 2004
Abstract—A convenient one-pot synthetic route was developed for the preparation of asymmetric 1,3-dialkyl-1,3,5-triazine-2,4,6-tri-
ones from readily available alkyl- or aryl-isocyanates, primary amines and N-chlorocarbonyl isocyanate in excellent yields. Subse-
quent alkylation with N-protected amino alcohols afforded the desired 1,3,5-triazine-2,4,6-triones in good yields. This methodology
was applied to the synthesis of a chemical library acting as antagonists of the hGnRH receptor.
Ó 2004 Elsevier Ltd. All rights reserved.
1,3,5-Triazine-2,4,6-trione, also known as isocyanurate,
is a class of simple heteroaromatic molecules.1 Symmet-
rical 1,3-disubstituted triazinetriones can be prepared by
condensation of two molecules of isocyanates with a
metal isocyanate (Na, K, Li, as in MNCO) in a dipolar
aprotic solvent (Eq. 1),2 and symmetrical trisubstituted
triazinetriones are synthesized by trimerization of isocya-
nates using a variety of catalysts such as Lewis bases,3
anions,4 and metallic compounds (Eq. 2).5 In addition
to the severe conditions required for these reactions,
these approaches are limited to the 1,3,5-triazine-2,4,6-
triones substituted with two or three identical groups.
However, there are very few literature reports on the
synthesis of asymmetric triazinetriones with two or
three different substituents. Kappe and co-workers syn-
thesized 1-(1H-benzimidazol-2-ylmethyl)-3-phenyl-s-tri-
azine-2,4,6(1H,3H,5H)-trione as an antimicrobial agent
by cyclization of the corresponding urea and N-ethoxy-
carbonyl isocyanate in refluxing bromobenzene in 60%
yield (Eq. 3).6 A more efficient cyclization of N,N0-dialk-
ylureas with N-chlorocarbonyl isocyanate to yield asym-
metric disubstituted triazinetriones is described in an
early patent literature (Eq. 4).7 Recently, this cyclization
has been applied to the solid-support synthesis of asym-
metric 1,3,5-triazine-2,4,6-triones using resin-bound
amino acids for a chemical library (Eq. 5).8
ð1Þ
ð2Þ
We have reported the discovery of a new series of 6-
methyluracils such as 1 (Fig. 1) as potent antagonists
of the human gonadotropin-releasing hormone
(hGnRH) receptor.9 The early lead compound 1a
(Ki = 34nM) has been optimized to much more potent
analogues such as 1b (Ki = 1.1nM); interestingly, the re-
gio-isomer of 1a, 2, did not display measurable binding
affinity (Ki > 20 lM). This data suggest the impor-
tant role of the 4-carbonyl functionality of uracil 1a in
Keywords: GnRH; Antagonist; Triazinetrione.
*
Corresponding authors. Tel.: +1 858 617 7657; fax: +1 858 617
7619; e-mail: zguo@neurocrine.com
URL: http://www.neurocrine.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.11.026

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Z. Guo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 693–698
than 1a. While this data did not provide a conclusive
answer to the question, it prompted us to further inves-
tigate this class of compounds as potential small mole-
cule antagonists of the hGnRH receptor.
ð3Þ
ð4Þ
In order to explore the structure–activity relationship of
this class of compounds, a general synthetic method to
yield the 1,3,5-triazine-2,4,6-triones with three different
substitution groups that would be amenable to the effi-
cient preparation of large numbers of analogues is re-
quired. If we could achieve rapid access to the
asymmetric 1,3-disubstituted 1,3,5-triazine-2,4,6-trione,
this key intermediate could be suitable for Mitsunobu
alkylation with readily available amino alcohols. This
would incorporate a side-chain bearing a basic nitrogen,
an important pharmacophore feature for this class of
GnRH antagonists. In this paper, we report an efficient
one-pot synthesis of 1,3-disubstituted 1,3,5-triazine-
2,4,6-triones bearing two different substituents from
ð5Þ
receptor binding possibly through hydrogen bonding.
Alternatively, the change from the 6-methyl group of
1a to the carbonyl moiety of 2 between the 3-methoxy-
phenyl and the 2,6-difluorobenzyl groups could also
cause a conformational change of molecule 2, which
may account for its loss of potency. To explore the im-
pact of this carbonyl group on GnRH receptor binding
we designed and synthesized the 1,3,5-triazine-2,4,6-tri-
one analogue 3, using the method described in Eq. 3.
Interestingly, compound 3 exhibited a Ki value of
4.2lM, which was superior to 2, but much less potent
readily available material, and subsequent Mitsunobu
reaction with N-protected amino alcohols to prepare
the monocyclic compounds in a screening library for
the human GnRH receptor.
The initial approach to synthesize the targeted 1,3,5-tri-
azine-2,4,6-triones 3 is summarized in Scheme 1. 1-Allyl-
3-(3-methoxy-phenyl)-1,3,5-triazine-2,4,6-trione 5a was
obtained by a cyclization of N-allyl-N0-(3-methoxyphen-
yl)urea 4 with N-ethoxycarbonyl isocyanate in refluxing
bromobenzene in ꢀ30% yield. Compound 5a was then
N
OMe
N
OMe
OMe
F
F
O
N
F
O
N
F
O
N
N
N
N
N
H2N
O
O
O
N
F
F
F
1c
1b
1a
N
N
N
N
OMe
OMe
O
N
N
F
N
O
N
O
O
N
F
O
F
F
3
2
Figure 1. Small molecule GnRH antagonists.

Z. Guo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 693–698
695
OMe
OMe
O
N
F
OMe
O
N
N
F
b
O
a
N
N
O
O
N
H
N
H
O
N
H
5a
O
4
6
OMe
N
N
OMe
O
N
F
O
N
F
O
d
N
N
F
c
N
N
F
O
O
O
O
3
7
Scheme 1. Reagents and conditions: (a) EtOCONCO, bromobenzene, 150°C, 30%; (b) 2,6-difluorobenzyl bromide, TBAF, THF, 62%; (c) NaIO4,
OsO4 (cat), THF/H2O, 92%; (d) 2-(N-methylaminoethyl)pyridine, NaBH(OAc)3, dichloroethane, 85%.
O
O
R3
R1
O
R1
O
N
N
HN
N
b or c
a
R2 NH2
R1 NCO
O
N
+
O
N
R2
R2
8
9
10
5
Scheme 2. Reagents and conditions: (a) 1. dichloromethane, ambient temperature, 2. ClCONCO, dichloromethane; (b) R3OH, triphenylphosphine,
di-t-butyl-azodicarboxylate, THF; (c) R3Br, K2CO3, DMF.
treated with 2,6-difluorobenzyl bromide and n-tetra-
butylammonium fluoride in THF to afford 6, which
was subjected to an oxidative cleavage of the olefin by
NaIO4/OsO4 to yield 7. Finally, the aldehyde 7 was
reacted with 2-(N-methyl-2-aminoethyl)pyridine under
reductive conditions to give the desired product 3.
The resulting mixture was stirred at ambient tempera-
ture for 1h. N-Chlorocarbonyl isocyanate (1equiv,
0.6mL, 7.5mmol) was then added slowly (caution: the
reaction was usually exothermic), and the reaction mix-
ture was stirred for one additional hour. An additional
equivalent of N-chlorocarbonyl isocyanate (0.6mL)
was slowly introduced and the resulting mixture was
stirred overnight. If precipitate formed, it was filtered,
and washed with cold dichloromethane to yield the
desired triazinetrione product. Otherwise, the reaction
was diluted with dichloromethane, washed with satu-
rated aqueous NaHCO3 and then brine, and the organic
layer dried over Na2SO4, and evaporated to give the
final product as white solid.11
Since this route was inefficient, we focused on the devel-
opment of a new synthetic method for 1,3,5-triazine-
2,4,6-triones with two different groups at the 1- and 3-
positions. Typically, an N,N0-dialkylurea is prepared
by the treatment of a primary amine with an isocyanate,
then the isolated urea is cyclized into the triazinetrione.
We found that by combining these two steps in dichlo-
romethane, first by mixing an equimolar amount of
amine and isocyanate, followed by addition of N-chlo-
rocarbonyl isocyanate (1.5–2equiv, excess amount was
required for the reaction to completion), the desired tri-
azinetrione usually precipitated out from the reaction
mixture to afford the pure product. Thus, a convenient
method to synthesize the asymmetric disubstituted
1,3,5-triazine-2,4,6-trione 5 in an one-pot procedure
was developed (Scheme 2).
In general, this efficient, two-step, one-pot reaction
sequence afforded 1,3-dialkyl-1,3,5-triazine-2,4,6-triones
in very good to excellent yields, while minimal workup
and purification were required. Over 100 compounds
with a wide variety of substitutions were prepared using
this method. A selection of compounds and their yields
are listed in Table 1.
These disubstituted 1,3,5-triazine-2,4,6-triones 5 were
further modified by introduction of the third substitu-
ent, by either a conventional alkylation with an alkyl
halide in the presence of a base such as potassium
carbonate (Scheme 2) or by Mitsunobu reaction with
A general procedure for this synthesis of 1,3-dialkyl-
1,3,5-triazine-2,4,6-triones is described as follows.10 To
a solution of a primary amine (7.5mmol) in CH2Cl2
(10mL), an isocyanate (7.5mmol) was added slowly.

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Z. Guo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 693–698
Table 1. Synthesis of disubstituted 1,3,5-triazine-2,4,6-diones 5b–t by
one-pot procedure
O
O
R1
O
R1
O
R
H2N
a, b
OH
N
N
HN
N
Compound R1 (isocyanates)
R2 (amines)
Yielda (%)
87
NH
+
R
Boc
O
N
O
N
R2
R2
5b
12
11
5
5c
91
Scheme 3. Reagents and conditions: (a) PPh3, di-t-butyl-azodicarb-
oxylate, THF; (b) TFA/CH2Cl2 (1:1).
F
5d
76
82
83
alcohols to afford the asymmetric triazinetriones 12
(Scheme 3). The successful Mitsunobu reaction of the
disubstituted triazinetriones allowed us to introduce,
by utilizing readily available N-protected amino
alcohols, an aminoalkyl side-chain, which is important
for high affinity GnRH receptor binding. Examples of
this Mitsunobu alkylation followed by deprotection of
Boc group are listed in Table 2.12
F
5e
5f
F
F
5g
95
81
We have also successfully adopted solid-supported
triphenylphosphine (Aldrich, 1mmol/g loading) in the
synthesis of library compounds, which overcame the dif-
ficulty of separating the triphosphine oxide by-product
from the reaction mixture. Despite the large excess of
solid-supported triphenylphosphine (ꢀ10equiv) that
was required (instead of 1.5equiv of triphenylphosphine
in solution) and the longer reaction times (10–16 vs
1–4h), the solid-supported reaction offered an easy puri-
fication of the desired product by filtration. This method
was especially useful when the product had similar
polarity to triphenylphosphine oxide.
F
MeO
5h
F
F
5i
87
98
F
5j
F3CO
5k
100
90
All final compounds were evaluated for their ability to
inhibit des-Gly10[125I-Tyr5, DLeu6, NMeLeu7, Pro9-
NEt]-GnRH radioligand binding to the cloned hGnRH
receptor stably expressed in HEK293 cells using a 96-
well filtration assay format.13 The binding affinities of
a few selected compounds 12 are summarized in Table
3. Several compounds from this collection were found
to exhibit moderate potency with the 3-methoxyphenyl
compound derived from (R)-phenylglycinol (12c) having
the highest binding affinity (Ki = 37nM). The 2-fluoro-
phenyl analogue 12g displayed a Ki value of 89nM.
Interestingly, the 2,6-difluorophenyl analogue (12h,
220nM) was slightly less potent than 12g.
5l
H3CO
5m
100
94
H3CO
F3CO
MeO
5n
H3CO
5o
5p
95
In comparison with the corresponding uracil derivative
1c (Ki = 2.3nM), 12c displayed lower affinity, but only
by about 16-fold. This data indicates that the change
from a methyl group to a carbonyl moiety between the
3-methoxyphenyl and 2,6-difluorobenzyl groups in 1c
has some impact, but is not a key factor in hGnRH
receptor binding. Thus the loss of potency in 2 is most
likely caused by the loss of the carbonyl group between
the 3-methoxyphenyl and the aminoethyl groups for
possible hydrogen bonding.
94
5q
99
96
93
92
O
O
O
5r
O
O
5s
O
In summary, we have developed a convenient two-step,
one-pot cyclization procedure for assembly of the 1,3,5-
triazine-2,4,6-triones with disubstitutions in excellent
yields. Mitsunobu reaction of these disubstituted triazine-
triones was also developed and applied to intro-
duce, from readily available amino alcohols, a third
F3CO
MeO
O
5t
O
a Isolated yield.

Z. Guo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 693–698
Table 2. Synthesis of 1,3,5-triazine-2,4,6-diones 12a–e by Mitusnobu reaction12
697
O
R3
R1
O
N
N
12a-e
O
N
R2
Compound
R1 (isocyanates)
R2 (amines)
R3
Yielda (%)
12a
78
92
N
H
F
12b
H2N
F
F
MeO
12c
12d
89
90
86
NH2
F
NH
H3CO
H3CO
F3CO
12e
NH
a Isolated yield.
Table 3. Binding affinities of compounds 3, 12 on the hGnRH receptor14
O
R3
R1
O
N
N
F
O
N
F
3, 12c, 12f-j
Compound
R3
Chirality
R1
Ki (nM)
OMe
OMe
N
N
3
4200
12c
12f
12g
12h
12i
R
37
120
89
NH2
F
OMe
RS
R
NH2
NH2
F
F
R
220
410
NH2
NH2
F
N
S
N
R
F
12j
RS
380
NH2
HN
F

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Z. Guo et al. / Bioorg. Med. Chem. Lett. 15 (2005) 693–698
10. For detailed experimental conditions, see: Guo, Z.; Wu,
D.; Chen, C.; Tucci, F. C.; Regan, C. WO 0311839, Feb
13, 2003; U.S. Patent 6,677,340, Jan 13, 2004.
substitution group bearing a basic nitrogen. This conve-
nient synthetic route was utilized to synthesize a screen-
ing library for the human gonadotropin-releasing
hormone receptor. The initial work on this series pro-
vided us with compounds of moderate affinity. The best
compound (12c) from the initial SAR study had a Ki
value of 37nM. Further structure–activity relationships
of this series of compounds as hGnRH antagonists will
be presented elsewhere.
11. (S)-1-((S)-2-Phenyl-cyclopropyl)-3-(4-trifluoromethoxy-
benzyl)-[1,3,5]triazinane-2,4,6-trione (5j) was obtained as a
white solid (98% yield): mp 192.8–193.0°C; 1H NMR
(TMS/CDCl3): d 1.43–1.48 (m, 1H), 1.62–1.67 (m, 2H),
2.36–2.37 (m, 1H), 2.74–2.78 (m, 1H), 5.023 (s, 1H), 7.17–
7.56 (m, 9H), 8.48 (br s, 1H). HRMS (ESI) m/z calcd for
C20H16F3N3O4 (MNa+) 442.0985, found 442.0988. Anal.
Calcd for C20H16F3N3O4: C, 57.28; H, 3.85; N, 10.02.
Found: C, 57.26; H, 3.69; N, 10.19. 1-(4-t-Butyl-benzyl)-3-
(4-methoxybenzyl)-[1,3,5]triazinane-2,4,6-trione (5l) was
References and notes
obtained as
a white solid (90% yield): mp 131.1–
1
1. Bartholomew, D. In Chapter 6.12: 1,3,5-Triazines in
Comprehensive Heterocyclic Chemistry II; Boulton, A. J.,
Ed.; Pergamon, 1996; pp 575–636.
2. Argabright, P. A.; Philips, B. L.; DePuy, C. H. J. Org.
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7. Hagmann H. DOS 1927921, 1970. For a review article on
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131.8°C; H NMR (TMS/CDCl3): d 1.29 (s, 9H), 3.78 (s,
3H), 4.94 (s, 2H), 4.97 (s, 2H), 6.83–7.44 (m, 8H), 8.69 (br
s, 1H). HRMS (ESI) m/z calcd for C22H25N3O4 (MꢁHꢁ)
394.1772, found 394.1765. Anal. Calcd for C22H25N3O4:
C, 66.82; H, 6.37; N, 10.63. Found: C, 67.05; H, 6.16; N,
10.68.
12. Typical procedure: a solution of 5l (32mg, 0.08mmol) in
anhydrous THF (1.0mL) was treated with N-Boc-(R)-
1-(1,2,3,4-tetrahydro-isoquinolin-3-yl)-methanol (22mg,
0.08mmol), Ph3P (42mg, 0.12mmol) and di-t-butyl-azo-
dicarboxylate (28mg, 0.12mmol) at ambient temperature.
The reaction mixture was stirred at ambient temperature
for 3h. Volatiles were evaporated, and the residue was
treated with 1:1 TFA/DCM (2mL) for 1h. The reaction
mixture was then evaporated and purified by reverse phase
HPLC (C-18 column, 15–75% ACN/water) to give 1-(4-t-
butyl-benzyl)-3-(4-methoxybenzyl)-5-[(R)-1-(1,2,3,4-tetra-
hydro-isoquinolin-3-yl)methyl]-[1,3,5]triazinane-2,4,6-tri-
8. Yu, Y.; Ostresh, J. M.; Houghten, R. A. J. Comb. Chem.
2002, 4, 484, and references cited therein.
1
9. (a) Zhu, Y.-F.; Gross, T.; Guo, Z.; Gao, Y.; Connors, P.
J., Jr.; Tucci, F. C.; Struthers, R. S.; Reinhart, G. J.;
Saunders, J.; Chen, C. J. Med. Chem. 2003, 46, 2023; (b)
Guo, Z.; Zhu, Y.-F.; Tucci, F. C.; Gao, Y.; Struthers, R.
S.; Saunders, J.; Gross, T. D.; Xie, Q.; Reinhart, G. J.;
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one (12e, 45mg, 86% yield) was obtained as TFA salt: H
NMR (TMS/CDCl3): d 1.28 (s, 9H), 3.10 (d, J = 7.5Hz,
2H), 3.77 (s, 3H), 3.91–4.13 (m, 2H), 4.48–4.93 (m, 7H),
6.82 (d, J = 9.0Hz, 2H), 7.09–7.37 (m, 10H); MS (CI) m/z
541.2. Anal. Calcd for C32H36N4O4ÆTFA: C, 62.38; H,
5.70; N, 8.56. Found: C, 62.19; H, 5.62; N, 8.45.
13. Perrin, M. H.; Haas, Y.; Rivier, J. E.; Vale, W. W. Mol.
Pharmacol. 1983, 23, 44.
14. On each assay plate a standard antagonist of comparable
affinity to those being tested was included as a control for
plate-to-plate variability. Overall, Ki values were highly
reproducible with an average standard deviation of 45%
for replicate Ki determinations. Key compounds were
assayed in 3–8 independent experiments.