3-Hydroxy-4-oxo-4H-pyrido[1,2-a]pyrimidine-2-carboxylates-fast access to a heterocyclic scaffold for HIV-1 integrase inhibitors

Article abstract of DOI:10.1016/j.tetlet.2008.09.010

An efficient and reliable synthesis of the heterocyclic scaffold methyl-3-hydroxy-4-oxo-4H-pyrido[1,2-a]pyrimidine-2-carboxylate is described. The scope of the synthesis regarding the introduction of substituents on the pyrido-fused ring is explored.

Full text of DOI:10.1016/j.tetlet.2008.09.010

Tetrahedron Letters 49 (2008) 6556–6558
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
3-Hydroxy-4-oxo-4H-pyrido[1,2-a]pyrimidine-2-carboxylates—fast
access to a heterocyclic scaffold for HIV-1 integrase inhibitors
b
a
b
Olaf D. Kinzel a, , Richard G. Ball , Monica Donghi , Courtney K. Maguire ,
*
Ester Muraglia a, Silvia Pesci a, Michael Rowley a, Vincenzo Summa a
a Department of Medicinal Chemistry, IRBM-MRL Rome, Via Pontina, Km 30.600, 00040 Pomezia, Rome, Italy
b Center for Physico-Chemical Characterization, Merck Research Laboratories, 126 E. Lincoln Ave, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2008
Accepted 3 September 2008
Available online 7 September 2008
An efficient and reliable synthesis of the heterocyclic scaffold methyl-3-hydroxy-4-oxo-4H-pyrido-
[1,2-a]pyrimidine-2-carboxylate is described. The scope of the synthesis regarding the introduction of
substituents on the pyrido-fused ring is explored.
Ó 2008 Elsevier Ltd. All rights reserved.
In the context of our studies on tetrahydro-pyridopyrimidinon-
es of type A as HIV-1 integrase inhibitors,1 we became interested in
the related unsaturated heterocyclic compounds of type B (Fig. 1).
The corresponding core scaffold methyl-3-hydroxy-4-oxo-4H-pyr-
ido[1,2-a]pyrimidine-2-carboxylate C (R = H, R0 = OMe, Fig. 2) has
been reported in the literature.2 The synthesis is described as a
low yielding (17%) condensation reaction between 2-aminopyr-
idine and dimethyl diacetoxyfumarate used in large excess. Unfor-
tunately, we were unable to obtain the desired product following
the described procedure. Thus, a new and reliable synthetic route
was needed in order to conduct medicinal chemistry studies.
Our retrosynthetic analysis of methyl-3-hydroxy-4-oxo-4H-
pyrido[1,2-a]pyrimidine-2-carboxylate or the corresponding car-
boxamides C is summarized in Figure 2. Our first attempt to obtain
the pyridopyrimidinones by a dehydrogenation of the related tet-
rahydropyridopyrimidinones gave unsatisfactory results (option I,
Fig. 2). When heated in high boiling solvents over palladium on
carbon the dehydrogenated products were observed along with
extensive degradation, and the yields were variable and very low.
The best results were obtained when a methylamino-substituent
was present on the tetrahydropyrido-ring (scheme 1).
In the light of these results, our attention turned to options II
and III, that is, the formation of C via a condensation reaction
between dimethylacetylene dicarboxylate (DMAD) and 2-amino-
pyridine-N-oxides. We reasoned that this transformation could
be possible because 2-aminopyridine-N-oxide can be regarded as
a tautomeric form of cyclic amidoxime D (Fig. 2). Amidoximes,
either acyclic or cyclic ones, are known to react with DMAD to give
the desired hydroxypyrimidinone derivatives.1,3 The introduction
of the substituent in the 9-position on the pyrido-fused ring would
be ideally achieved after formation of the core heterocycle in order
to have a flexible late stage derivatization (option II, Fig. 2). Alter-
natively, already functionalized 2-aminopyridine-N-oxides could
be used in the cyclization reaction (option III, Fig. 2).
2-Aminopyridine-N-oxide reacted smoothly with one equiva-
lent DMAD in chloroform at 0 °C, forming an adduct as a mixture
of E/Z-isomers (ca. 1:10, Scheme 2). NMR-analysis (via 1H–13C-
HMBC experiment) established this adduct to be the enamine 1
instead of the desired O-adduct 1a. The diagnostic proton-carbon
correlations detected for the NH-proton of 1 are reported in
Scheme 2.4 Upon heating to 150–165 °C in o-xylene, a rearrange-
ment/cyclization to the desired methyl-3-hydroxy-4-oxo-4H-
pyrido[1,2-a]pyrimidine-2-carboxylate
2 occurred. Mechanistic
studies of this reaction have not been performed, but one possible
mechanism is the rearrangement of 1 to the O-adduct (1a) which
then undergoes a similar rearrangement/cyclization reaction as de-
scribed for the saturated alkyl amidoximes,1,3 involving the cleav-
age of the N–O-bond. For alkyl-amidoximes, a detailed mechanistic
study of this rearrangement has been published recently.5 After O-
protection of 2 as a pivalate in order to facilitate its purification 2a
was isolated in 33% yield.6,7
O
N
O
N
OH
F
OH
F
N
N
H
H
N
N
R
O
R
O
A
B
With protected bicycle 2a in hand, we then proceeded to seek a
way to introduce a substituent R, ideally an amino group which
then could be further modified for SAR studies. Selective nitration
under mild conditions was unsuccessful, and our attention
turned to the introduction of a bromine atom which then could
Figure 1. Tetrahydropyridopyrimidinone HIV-1 integrase inhibitors (A) and the
targeted pyridopyrimidinone structure (B).
* Corresponding author. Tel.: +39 06 91093334; fax: +39 06 91093654.
E-mail address: olaf_kinzel@merk.com (O. D. Kinzel).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.010

O. D. Kinzel et al. / Tetrahedron Letters 49 (2008) 6556–6558
6557
O
OH
N
I
N
COR'
R
O
N
O
N
CO2Me
CO2Me
O-
6
II
OP
N+
OH
N
7
8
+
N
NH2
CO2Me
COR'
9
R
C
III
OH
NH
N
D
CO2Me
O-
N+
+
NH2
R = OP or NHP
P = protecting group
R'=NH-p-F-benzyl, OMe
CO2Me
R
Figure 2. Retrosynthetic analysis of pyridopyrimidinone core scaffold C.
Pd(C),
O
O
N
DIPEA (4 eq.)
o-xylene,
OH
F
OH
F
N
N
155 ˚C, 7 h
H
H
N
N
N
13%
NHMe
O
NHMe
O
Scheme 1. Dehydrogenation of tetrahydropyridopyrimidinones.
conceivably be transformed by Pd-catalyzed amination. After a
thorough screening of bromination conditions, compound 2a was
successfully brominated with NBS in a mixture of acetonitrile
and acetic acid at 5 °C, leading to addition product 3. The formation
of an addition product indicates the partially olefin-like character
of the pyrido-fused ring of 2a. On the other hand, 2a did not react
with elementary bromine as one would expect for simple alkenes.
Treatment of 3 with triethylamine in dichloromethane at room
temperature afforded brominated elimination product 4 in moder-
ate yield. The position of the bromine atom in 4 was confirmed by
X-ray crystallography (Fig. 3).8
Figure 3. X-ray crystal structure of 4.
N-oxides already bearing a suitable functionality at the desired po-
sition (option III, Fig. 2).
Although bromo-intermediate 4 proved to be a suitable sub-
strate for further derivatization (e.g., Pd-catalyzed cross coupling
reactions)9 the 9-position was our preferred site of modification.
Therefore, we decided to start the synthesis with aminopyridine-
As precursors for further derivatives a benzyl ether as well as a
benzyl carbamate was chosen. To this end, 3-(benzyloxy)pyridin-
2-amine 1-oxide (5) was prepared by N-oxidation of the commer-
cially available 3-(benzyloxy)pyridin-2-amine, while benzyl
O
150-165 ˚C,
o-xylene
O-
CO2Me
OH
O-
CHCl3, 0 ˚C
N+ CO2Me
N
N+
2
+
CO2Me
N
CO2Me
N
87%
NH2
H
CO2Me
1
O-
N+ CO2Me
PivCl, pyr
O
CO2Me
N
33%
2 steps
1
CO2Me
N
NH CO2Me
H
N1H-13C-couplings
1a
detected via HMBC
O
N
O
NBS, MeCN/
HOAc, 5 ˚C
OAc O
N
OPiv
TEA/DCM
Br
OPiv
Br
OPiv
N
N
27%
2 steps
CO2Me
N
CO2Me
N
CO2Me
2a
4
3
Scheme 2. Synthesis of pyridopyrimidinones.

6558
O. D. Kinzel et al. / Tetrahedron Letters 49 (2008) 6556–6558
1. DMAD, CHCl3, 25 ˚C
O
N
2. 160 ˚C, o-xylene
O-
OBz
N+
3. BzCl, pyr
N
m-CPBA
76%
N
CO2Me
NH2
22%
NH2
OBn
OBn
OBn
6
5
1. BnOCOCl,
THF/pyr, 0 ˚C
2. m-CPBA
O
N
O-
DMAD, CHCl3,
OH
N+
N
pTSA, reflux
N
NH2
CO2Me
NH2
35%
14%
NHCbz
NHCbz
NH2
7
8
Scheme 3. Synthesis of substituted pyridopyrimidinones 6 and 8.
combined organic phases were concentrated to dryness to afford the product as
a light brown oil (7.5 g, 87%). The product was a mixture of E/Z-isomers (ca.
1:10) and was used without separation of the isomers. Main isomer: 1H NMR
(300 MHz, DMSO-d6) d: 10.49 (s, 1H), 8,26 (m, 1H), 7.31 (m, 1H), 7.03 (m, 2H),
5.70 (s, 1H), 3.78 (s, 3H), 3.71 (s, 3H). 13C NMR (125 MHz, DMSO-d6) d: 167.4,
162.7, 144.3, 142.5, 137.9, 126.9, 118.6, 113.9, 98.8. MS m/z: 253 (M+H)+.
5. Pye, P. J.; Zhong, Y.-L.; Jones, G. O.; Reamer, R. A.; Houk, K. N.; Askin, D. Angew
Chem., Int. Ed. 2008, 4722, 4134–4136.
(2-amino-1-oxidopyridin-3-yl)carbamate (7) was prepared by N-
oxidation of benzyl (2-aminopyridin-3-yl)carbamate, derived from
a 3-N-protection of 2,3-diaminopyridine (Scheme 3). With these
starting materials in hand, we proceeded to investigate the forma-
tion of DMAD adducts and subsequent rearrangement/cyclization
reactions. In the case of the benzyl ether 5, the adduct formation
with DMAD proceeded to completion at room temperature within
8 h. Without isolation the formed adduct was treated in the same
way as for 1, leading to the expected methyl 9-(benzyloxy)-3-
hydroxy-4-oxo-4H-pyrido[1,2-a]pyrimidine-2-carboxylate which
was benzoylated to give compound 6 in 22% overall yield. The ad-
duct formation of pyridine-N-oxide 7 with DMAD was very slow at
room temperature. At reflux in chloroform in the presence of cata-
lytic p-TSA, the direct formation of the desired product 8 was
observed which was isolated in low yield (14%).10
In summary, a new and reliable procedure for fast access
to substituted methyl-3-hydroxy-4-oxo-4H-pyrido[1,2-a]pyrimi-
dine-2-carboxylates, a new scaffold for HIV-1 integrase inhibitors,
is described. The synthesis starts from commercially available or
readily prepared starting materials and furnishes a complex het-
erocyclic scaffold in essentially one step. Attempts to introduce a
substituent on the naked scaffold indicated position 7 of the pyr-
ido-fused ring as the most reactive one toward electrophilic attack.
The cyclization reaction was found to be sensitive to the presence
of additional substituents on the aminopyridine-N-oxide. Addi-
tionally, the desired products with substitution at the 9 position
could be synthesized from the appropriate amino pyridines, but
in low yields. Biological results regarding the newly obtained class
of inhibitors will be reported elsewhere.
6. Methyl-3-[(pivaloyl)oxy]-4-oxo-4H-pyrido[1,2-a]pyrimidine-2-carboxylate 2a:
1
(7.56 g, 29.7 mmol) was suspended in dry o-xylene (400 mL), and the
suspension was stirred and heated to reflux for 3 h. The mixture was cooled
to room temperature, the solvent was removed under reduced pressure, and
the residue was dissolved in pyridine (154 mL). Pivaloyl chloride (3.75 mL,
29.7 mmol) was added and the mixture was stirred for 2 h at rt. The solvent
was removed under reduced pressure and the residue was partitioned between
EtOAc (500 mL) and 0.6 M aq HCl (500 mL). The aq phase was extracted with
EtOAc (3 ꢀ 500 mL). The combined org. phases were washed with water, brine,
dried over Na2SO4, and concentrated to dryness. The product was purified by
silica gel chromatography (petrol ether/EtOAc). The pooled product fractions
were concentrated to dryness to afford the title compound as a light brown
solid (2.99 g, 33%). 1H NMR (300 MHz, DMSO-d6) d: 8.92 (d, J = 7.8 Hz, 1H), 8.05
(m, 1H), 7.84 (d, J = 7.5 Hz, 1H), 7.48 (m, 1H), 3.88 (s, 3H), 1.32 (s, 9H). 13C NMR
(75 MHz, DMSO-d6) d: 174.8, 163.4, 153.3, 148.1, 144.1, 137.5, 127.8, 127.2,
126.4, 117.5, 52.8, 26.7. MS m/z: 305 (M+H)+. Mp (recrystallized from
methanol): 149.2 °C.
7. For characterization of unprotected 2 a sample of the reaction crude was
purified by reverse phase MPLC with subsequent crystallization from
methanol. 1H NMR (400 MHz, CDCl3) d: 10.43 (s, 1H), 8.87 (d, J = 7.2 Hz, 1H),
7.66 (d, J = 9.2 Hz, 1H), 7.51 (m, 1H), 7.03 (m, 1H), 4.11 (s, 3H). 13C NMR
(75 MHz, DMSO-d6) d: 169.6, 154.2, 143.3, 143.1, 132.5, 127.7, 127.0, 126.2,
115.2, 53.7. MS m/z: 221 (M+H)+.
8. Crystals were grown from 1:1 EtOAc:heptane. Compound C15H15Br N2O5,
Mr = 383.200, triclinic, P1, a = 12.089(3), b = 12.687(3), c = 12.812(3) Å,
63.406(3)°, b = 88.062(4)°,
= 64.406(3)°, V = 1553.4(6) Å3, Z = 4, Dx
1.638 g cmꢁ3, monochromatized radiation k(Mo) = 0.71073 Å,
a
=
=
,
c
l
= 2.67 mmꢁ1
F(000) = 776, T = 100 K. Data were collected on a Bruker CCD diffractometer to
a h limit of 26.32°, which yielded 18,499 reflections. There are 6303 unique
reflections with 4626 observed at the 2
direct methods (SHELXS-97, Sheldrick, G. M. Acta Crystallogr., Sect. A, 1990, 46,
467–473) and refined using full-matrix least-squares on F2
r level. The structure was solved by
(SHELXL-97,
References and notes
Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal Structures.
Univ. of Göttingen, Germany). The final model was refined using 423
parameters and all 6303 data. All non-hydrogen atoms were refined with
anisotropic thermal displacements. The final agreement statistics are: R = 0.040
1. (a) Muraglia, E.; Kinzel, O.; Gardelli, C.; Crescenzi, B.; Donghi, M.; Ferrara, M.;
Nizi, E.; Orvieto, F.; Pescatore, G.; Laufer, R.; Gonzalez-Paz, O.; Di Marco, A.;
Fiore, F.; Monteagudo, E.; Fonsi, M.; Felock, P. J.; Rowley, M.; Summa, V. J. Med.
Chem. 2008, 514, 861–874; (b) Kinzel, O. D.; Monteagudo, E.; Muraglia, E.;
Orvieto, F.; Pescatore, G.; Rico Ferreira, M.; Rowley, M.; Summa, V. Tetrahedron
Lett. 2007, 48, 6552–6555; (c) Crescenzi, B.; Kinzel, O.; Muraglia, E.; Orvieto, F.;
(based on 4626 reflections with I > 2r(I)), wR = 0.097, S = 1.00 with (D/
r)max < 0.01. The maximum peak height in a final difference Fourier map is
1.268 e Åꢁ3. CCDC 692685 contains the supplementary crystallographic data
for this Letter. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
9. Donghi, M.; Kinzel, O.; Summa, V. HIV Integrase Inhibitors. WO 2007039218
A1.
Pescatore, G.; Rowley, M.; Summa, V.
A Preparation of Tetrahydro-4H-
pyrido[1,2-a]pyrimidine Derivatives, Useful as HIV Integrase Inhibitors. WO
2004058757 A1.
2. Tisler, M.; Zupet, R. Org. Prep. Proced. Int. 1990, 224, 532–534.
3. (a) Koch, U.; Attenni, B.; Malancona, S.; Colarusso, S.; Conte, I.; Di Filippo, M.;
Harper, S.; Pacini, B.; Giomini, C.; Thomas, S.; Incitti, I.; Tomei, L.; De Francesco,
R.; Altamura, S.; Matassa, V. G.; Narjes, F. J. Med. Chem. 2006, 49, 1693–1707;
(b) Zhong, Y.-L.; Zhou, H.; Gauthier, D. R., Jr.; Askin, D. Tetrahedron Lett. 2006,
47, 1315–1317.
4. Dimethyl-2-[(1-oxidopyridin-2-yl)amino]but-2-enedioate 1: To a stirred solution
of 2-aminopyridine-N-oxide (3.76 g, 34 mmol) in CHCl3 (150 mL) at 0 °C was
added dropwise a solution of DMAD (4.27 mL, 34 mmol) in CHCl3 (20 mL). After
addition the cooling bath was removed and stirring was continued for 1 h.
Further, DMAD (1 mL) was added and stirring was continued for 2 h. The
solution was filtered over a silica gel plug (20 g), and after washing with EtOAc/
petrol ether (4:6, 1 L) the product was eluted with MeOH/EtOAc (1:4, 1 L). The
10. Methyl-9-{[(benzyloxy)carbonyl]amino}-3-hydroxy-4-oxo-4-H-pyrido-[1,2a]-pyr-
imidine-2-carboxylate 8: To a solution of benzyl (2-amino-1-oxidopyridin-3-y1)
carbamate 7 (2.0 g, 7.69 mmol) in CHCl3 (250 mL, filtered over alumina) was
added DMAD (1.04 mL, 8.46 mmol) and pTSA (50 mg). The suspension was
stirred at 70 °C for 12 h. The solvent was removed under reduced pressure, and
to the residue was added MeOH. The formed solid was filtered off, washed with
MeOH, and dried under vacuum to afford the title compound as a light brown
solid (400 mg, 14%). 1H NMR (400 MHz, CD3CN) d: 9.95 (s, br, 1H), 8.68 (s, br,
1H), 8.43 (d, J = 7.3 Hz, 1H), 8.19 (d, J = 7.3 Hz, 1H), 7.51–7.43 (m, 4H), 7.12 (m,
1H), 5.3 (s, 2H), 4.04 (s, 3H). 13C NMR (125 MHz, DMSO-d6) d: 165.4, 154.6,
152.5, 138.3, 135.9, 135.6, 131.5, 130.9, 128.5, 128.2, 119.0, 115.6, 114.9, 66.8,
52.5.MS m/z: 370 (M+H)+.