Efficient synthesis of bicyclic oxazolino- and thiazolino[3,2-c]pyrimidine- 5,7-diones and its application to the synthesis of GnRH antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.01.009

Treatment of various 2-methyl oxazolines or thiazolines with chlorocarbonyl isocyanate gives the corresponding bicyclic oxazolino- or thiazolino[3,2-c] pyrimidin-5,7-dione derivatives in very good yield. This reaction has been applied to the rapid syntheses of human gonadotropin-releasing hormone (hGnRH) receptor antagonists for SAR study, resulting in 13e with binding affinity in the low nanomolar range (4.5 nM).

Full text of DOI:10.1016/j.bmcl.2005.01.009

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1407–1411
Efficient synthesis of bicyclic oxazolino- and
thiazolino[3,2-c]pyrimidine-5,7-diones and its application
to the synthesis of GnRH antagonists
Joseph Pontillo* and Chen Chen
Department of Medicinal Chemistry, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
Received 11 November 2004; revised 29 December 2004; accepted 4 January 2005
Available online 25 January 2005
Abstract—Treatment of various 2-methyl oxazolines or thiazolines with chlorocarbonyl isocyanate gives the corresponding bicyclic
oxazolino- or thiazolino[3,2-c]pyrimidin-5,7-dione derivatives in very good yield. This reaction has been applied to the rapid
syntheses of human gonadotropin-releasing hormone (hGnRH) receptor antagonists for SAR study, resulting in 13e with binding
affinity in the low nanomolar range (4.5 nM).
ꢀ 2005 Elsevier Ltd. All rights reserved.
Gonadotropin-releasing hormone (GnRH) is a decapep-
tide released from the hypothalamus.1 It stimulates the
GnRH receptor in the pituitary gland to release folli-
cle-stimulating hormone (FSH) and luteinizing hormone
(LH), which in turn regulate gonadal steroid hormone
production.2 Down-regulation of the hypothalamic–
pituitary–gonadal hormonal axis with peptidic GnRH
agonists has been shown to alleviate disease conditions
associated with endometriosis, uterine fibroids, breast,
and prostate cancer.3 The recent search for orally avail-
able, non-peptide GnRH antagonists has resulted in the
disclosure of several classes of small molecules that
strongly bind to, but do not activate, the human GnRH
(hGnRH) receptor. These include the thieno[2,3-d]pyr-
idin-2,4-diones,4a the 1H-quinolones,4b the 2-arylin-
doles,4c the aminopyrimidines,4d the arylpyrrolo[1,2-
a]pyrimidones,4e the imidazolo[1,2-a]pyrimidones,4f
and very recently the uracils such as 1a4g and 1b4h
(Fig. 1).
Figure 1.
The present work was initiated in an attempt to develop
a bicyclic system that conformationally locks the 1-benz-
yl ring of compounds 1a,b, because in the crystal struc-
ture of a closely related analog, the phenyl ring of this
benzyl group is tilted away from the uracil plane.5 A
bicyclic system such as 12 or 13 may constrain the phen-
yl ring in order to emulate low-energy conformations
of uracils 1, which may result in more potent com-
pounds. The cores of these bicylic systems could in the-
ory be obtained by cyclization of the corresponding
alcohol 2 and thiol 3 under basic conditions, a method
used for furanosyluracil synthesis.6 However, the avail-
ability of the starting material and the strongly basic
*
reaction conditions required are limiting factors in these
syntheses. A Bayer patent described the reaction of
Corresponding author. Tel: +1 858 617 7533; fax: +1 858 617 7998;
e-mail: jpontillo@neurocrine.com
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.01.009

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J. Pontillo, C. Chen / Bioorg. Med. Chem. Lett. 15 (2005) 1407–1411
Table 1 (continued)
benzoxazoles and benzothiazoles with chlorocarbonyl
isocyanate to give benzoxazolopyrimidinediones and
benzothiazolopyrimidinediones.7 We thought that this
reaction would be ideally suited for the synthesis of
the bicyclic oxazolidino- and thiazolidino[3,2-c]pyrim-
idin-5,7-dione systems 6 and 7 from the readily available
oxazolines 4 and thiazolines 5 (Eq. 1). In addition, this
straightforward chemistry would provide rapid access
to analogs for SAR study.
Product
Yield (%)
70
86
82
ð1Þ
The generality of this reaction was explored with a num-
ber of different commercially available8 or synthe-
sized9,10 oxazoline and thiazoline substrates. Thus
treatment of solutions of oxazolines 4 or thiazolines 5
in the presence of dimethylaniline with chlorocarbonyl
isocyanate at 0 ꢁC to room temperature gave the bicyclic
products 6a–e and 7a–e (Table 1).11 The reaction proved
to be successful with a variety of oxazolines and thiazo-
lines, giving simple, non-substituted products (e.g., 6a,
7a) to those incorporating two aromatic rings (6e, 7b–
e). The reactions worked equally well for oxazolines
and thiazolines; the non-optimized yields ranged
from 70% to 89%. Furthermore, the products were
80
76
Table 1. Cyclization reaction results
Product
Yield (%)
86
74
78
89
generally solids, and easily separated from the reaction
mixtures.
The scope of this reaction was further broadened by its
application to the synthesis of the corresponding 6,6-
ring system. Thus reaction of the dihydroxazine 812 with
chlorocarbonyl isocyanate in the presence of dimethyl-
aniline under the same conditions gave the novel, bicy-
clic uracil derivative 9 in 78% yield (Eq. 2).
76
ð2Þ

J. Pontillo, C. Chen / Bioorg. Med. Chem. Lett. 15 (2005) 1407–1411
1409
Scheme 1. Reagents and conditions: (a) ClC(O)NCO, PhNMe2, DCM
(81%); (b) NBS, THF (86%); (c) BnN(Boc)CH2CH2OH, Ph3P, DEAD,
THF (80%); (d) [(2-F,3-OMe)Ph]B(OH)2, Pd(Ph3)4 (cat.), Ba(OH)2
(aq), dioxane/C6H6/EtOH (10:9:1), D; (e) TFA, DCM (67%, two steps).
Scheme 2. Reagents and conditions: (a) HOBt, EDC, DCM (91%); (b)
P2S5, DCM (91%); (c) ClC(O)NCO, PhNMe2, DCM (79%); (d) N-Boc-
(R)-phenylglycinol, Ph3P, DEAD, THF; (e) TFA, DCM (70%, two
steps).
thiazoline 5f in 91% yield.9b The key cyclization reaction
in the presence of chlorocarbonyl isocyanate gave the
thiazolinopyrimidinedione 7f in 79% yield. Subsequent
Mitsunobu reaction with N-Boc-(R)-phenylglycinol un-
der standard conditions, followed by Boc deprotection,
provided the amine 13a in 70% yield. Since this synthesis
ended with a Mitsunobu coupling (or alternatively, sim-
ple alkylation), it was ideally suited to SAR study on the
left side of the system.
The success of this reaction led to the development of
efficient syntheses of small molecule GnRH antagonists.
The first representative synthesis (Scheme 1) involves
reaction of the known oxazoline 4f, prepared by conden-
sation of R-phenylglycinol with triethylorthoacetate,12
with chlorocarbonyl isocyanate to give the oxazolino-
pyrimidinedione 6f (81%). Subsequent bromination with
NBS to give 10, followed by Mitsunobu reaction with
(N-Boc, N-benzyl)ethanolamine, yielded the 6-bromo-
oxazolinopyrimidinedione derivative 11. Finally, incor-
poration of the right (2-fluoro, 3-methoxy)phenyl ring
by Suzuki coupling, and subsequent Boc deprotection
gave the oxazolinopyrimidinedione derivative 14 in
67% yield (two steps). Because this synthesis ended with
a Suzuki coupling, it was well suited to SAR study on
the right side of the system.
Binding affinities of an initial set of compounds were
determined13 and are summarized in Table 2. The set
was modeled on analogs 1a,b in hopes of achieving good
binding affinities to the hGnRH receptor. Indeed, the
first compound 14, which has a benzylaminoethyl side
chain similar to 1a, displayed modest binding activity
at 460 nM. In this series, however, the side chain derived
from (R)-phenylglycinol (as in 1b) gave better binding
activity and was thus utilized in subsequent compounds.
In addition, compounds with ÔSÕ chirality at C3 of
the bicyclic core were much more active; (R)-14 and
(R)-13a (not shown) had binding affinities in the high
The second representative synthesis (Scheme 2) begins
with the coupling of (R)-phenylglycinol with 2-fluoro-
phenylacetic acid under standard coupling conditions.
The amide 15 thus obtained was then converted to the
Table 2. Binding affinities of compounds 12–14 to the hGnRH receptor13
Compound
C3 chirality
Ar1
Ar2
Ki (nM)
14
S
Phenyl
Phenyl
(2-Fluoro, 3-methoxy)phenyl
2-Fluorophenyl
2-Fluorophenyl
460
700
230
870
125
110
220
40
12a
12b
12c
12d
13a
13b
13c
13d
13e
RS
RS
RS
RS
S
2-Fluorophenyl
Phenyl
Phenyl
3-Methoxyphenyl
(2-Fluoro, 3-methoxy)phenyl
2-Fluorophenyl
2-Fluorophenyl
Phenyl
RS
RS
RS
RS
2-Fluorophenyl
2-Fluorophenyl
2-Fluorophenyl
2-Fluorophenyl
(2-Fluoro, 3-methoxy)phenyl
2-Chlorophenyl
(2-Chloro, 3-methoxy)phenyl
93
4.5

1410
J. Pontillo, C. Chen / Bioorg. Med. Chem. Lett. 15 (2005) 1407–1411
4. (a) Sasaki, S.; Cho, N.; Nara, Y.; Harada, M.; Endo, S.;
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Jiang, J.; Ujjainwalla, F.; Toupence, R. B.; Parikh, M.;
Huang, S. X.; Fair, J. A.; Goulet, M. T.; Wyvratt, M. J.;
Lo, J. L.; Ren, N.; Yudkovitz, J. B.; Yang, Y. T.; Cheng,
K.; Cui, J.; Mount, G.; Rohrer, S. P.; Schaeffer, J. M.;
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E.; Vincent, S.; Carlin, J. R.; Cameron, J.; Smith, R. G. J.
Med. Chem. 2001, 44, 917, and references cited therein; (c)
Young, J. R.; Huang, S. X.; Walsh, T. F.; Wyvratt, M. J.;
Yang, Y. T.; Yudkovitz, J. B.; Cui, J.; Mount, G. R.; Ren,
R. N.; Wu, T. J.; Shen, X.; Lyons, K. A.; Mao, A. H.;
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Gregory, M.; Vazir, H.; May, J. M.; Anderson, M. B.
Bioorg. Med. Chem. Lett. 2002, 12, 827; (e) Tucci, F. C.;
Zhu, Y.-F.; Guo, Z.; Gross, T. D.; Conners, P. J.;
Struthers, R. S.; Reinhart, G. J.; Wang, X.; Saunders, J.;
Chen, C. Bioorg. Med. Chem. Lett. 2002, 12, 3491, and
references cited therein; (f) Zhu, Y.-F.; Guo, Z.; Gross, T.
D.; Gao, Y.; Conners, P. J.; Struthers, R. S.; Xie, Q.;
Tucci, F. C. J. Med. Chem. 2003, 46, 1769, and references
cited therein; (g) Guo, Z.; Zhu, Y.-F.; Gross, T. D.; Tucci,
F. C.; Gao, Y.; Moorjani, M.; Connors, P. J., Jr.;
Rowbottom, M. W.; Chen, Y.; Struthers, R. S.; Xie, Q.;
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5. Neurocrine, unpublished result.
micromolar range. Other compounds 12a–d and 13b–e
were tested as mixtures of diastereomers.
In comparing the oxazolino[3,2-c]pyrimidin-5,7-dione
cases 12a and 12b, incorporation of a 2-fluoro substitu-
ent on the bottom phenyl ring slightly increased activity
to 230 nM. The 3-methoxyphenyl derivative 12c showed
modest activity (870 nM). However, incorporation of
the 2-fluoro, 3-methoxyphenyl substitution pattern in
12d resulted in good binding activity (125 nM).
The thiazolino[3,2-c]pyrimidin-5,7-diones 13 showed
somewhat similar SAR patterns. The sulfur derivative
13b (220 nM) was equipotent to oxo-derivative 12b.
Incorporation of the (2-fluoro, 3-methoxy)phenyl moi-
ety in 13c increased binding activity to 40 nM, an almost
6-fold increase. Interestingly, incorporation of the 2-
chlorophenyl substituent (13d) also slightly improved
binding affinity over the 2-fluorophenyl case. Moreover,
the best substitution pattern in this series is the (2-
chloro, 3-methoxy)phenyl of 13e. This potent compound
displayed a hGnRH binding affinity of 4.5 nM.
In summary, we have shown that reaction of substituted
2-methyl oxazolines or thiazolines with chlorocarbonyl
isocyanate gives the oxazolino- or thiazolino[3,2-c]pyr-
imidin-5,7-dione derivatives in very good yield. This
reaction has been applied to the rapid syntheses of sub-
strates active against the hGnRH receptor. The first syn-
thesis ends with a Suzuki coupling, making the synthesis
ideally suited to the development of SAR on the right
side of the molecule, while the second synthesis ends
with a Mitsunobu coupling, rendering the synthesis well
suited to development of SAR on the left side of the sys-
tem. The best compound in the series displayed binding
in the low nanomolar range with 13e having a Ki of
4.5 nM. Further SAR studies of these systems will be
reported in due course.
6. For examples, see: (a) Otter, B. A.; Falco, E. A.; Fox, J. J.
J. Org. Chem. 1968, 33, 3593; (b) Tanai, K.; Maruyama,
O. J. Carbohydr. Nucleos. Nucleot. 1976, 3, 25; (c)
Maruyama, T.; Sato, S.; Honjo, M. Chem. Pharm. Bull.
1982, 30, 2688; (d) Urata, H.; Miyagoshi, H.; Kakuya, H.;
Tokumoto, H.; Kawahata, T.; Otake, T.; Akagi, M.
Chem. Pharm. Bull. 1998, 46, 458.
7. Bayer Patent (Germany) DE 2126148; Chem. Abstr. 1972,
78, 11360.
8. The oxazolines 4a, 4c–d and thiazoline 5a were purchased
from Sigma-Aldrich. The oxazoline 4b was purchased
from Lancaster Synthesis.
Acknowledgements
9. (a) The oxazoline 4e was synthesized according to known
literature procedures, see: Shafer, C. M.; Molinski, T. F. J.
Org. Chem. 1996, 61, 2044; (b) The thiazolines 5b–f were
synthesized according to known literature procedures, see:
Aitken, R. A.; Armstrong, D. P.; Galt, R. H. B.; Mesher,
S. T. E. J. Chem. Soc., Perkin Trans. 1 1997, 2139.
We thank Qiu Xie (Neurocrine Biosciences) for per-
forming the hGnRH binding assays.
References and notes
1
10. All new compounds were characterized by H NMR, 13C
1. Schally, A. V.; Arimura, A.; Kastin, A. J.; Matsuo, H.;
Baba, Y.; Redding, T. W.; Nair, R. M. G.; Debeljuk, L.;
White, W. F. Science 1971, 173, 1036.
NMR, LCMS, and elemental analysis or HRMS.
11. Representative procedure (see Scheme 1): To (S)-2-
methyl-4-phenyl-4,5-dihydrooxazole (4f) (1.82 g, 11.3
mmol) and dry N,N-dimethylaniline (1.49 g, 12.3 mmol)
in dry dichloromethane (8 mL) under N2 at 0 ꢁC was
added dropwise chlorocarbonyl isocyanate (1.24 g,
11.8 mmol). The mixture was allowed to warm to room
temperature and stirred for 14 h, during which time a thick
white precipitate formed. The suspension was filtered, and
the solid triturated with cold dichloromethane (3 · 2 mL)
to give 3-phenyl-2,3-dihydroxazolo[3,2-c]pyrimidine-5,7-
(6H)-dione (6f) as a white amorphous powder (2.11 g,
2. (a) Kutscher, B.; Bernd, M.; Beckers, T.; Polymeropoulos,
E. E.; Engel, J. Angew. Chem., Int. Ed. 1997, 36, 2149, and
references cited therein; (b) Cui, J.; Smith, R. G.; Mount,
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R. J.; Goulet, M. T.; Schaeffer, J. M.; Cheng, K. Mol.
Endocrinol. 2000, 14, 671; (c) Zhou, W.; Rodic, V.;
Kitanovic, S.; Flanagan, C. A.; Chi, L.; Weinstein, H.;
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kos, T., Eds.; IOS: Amsterdam, 1999; Vol. 22, pp 275–285.
1
81%). H NMR (DMSO-D6, 500 MHz): d 10.95 (s, 1H);
7.42–7.39 (m, 2H); 7.37–7.33 (m, 1H); 7.28–7.26 (m, 2H);
5.57 (dd, J = 3.7, 8.3 Hz, 1H), 5.11 (s, 1H); 5.00 (dd,
J = 8.8, 8.8 Hz, 1H); 4.54 (dd, J = 3.4, 8.8 Hz, 1H). 13C

J. Pontillo, C. Chen / Bioorg. Med. Chem. Lett. 15 (2005) 1407–1411
1411
NMR (DMSO-D6, 500 MHz): d 164.9, 162.8, 147.4, 138.3,
128.9, 128.4, 126.0, 77.2, 75.5, 57.4. HRMS (EI) m/e (M)+
calcd for C12H10N2O3: 230.0691, found 230.0686.
NEt]-GnRH radioligand binding to the cloned hGnRH
receptor stably expressed in HEK293 cells, using a 96-well
filtration assay format; see: Perrin, M. H.; Haas, Y.;
Rivier, J. E.; Vale, W. W. Mol Pharmacol. 1983, 23, 44,
Data are the average of two or more independent
experiments.
12. Kamata, K.; Agata, I. J. Org. Chem. 1998, 63, 3113.
13. All final compounds were evaluated for their ability to
inhibit des-Gly10[125I-Tyr5, DLeu6, NMeLeu7, Pro9-