Combinatorial Synthesis of 2-Thioxo-4-dihydropyrimidinones

Full text of DOI:10.1021/jo971302v

9358
J . Org. Chem. 1997, 62, 9358-9360
Sch em e 1. Com bin a tor ia l Syn th esis of
Com bin a tor ia l Syn th esis of
2-Th ioxo-4-d ih yd r op yr im id in on es
Th ioh yd a n toin s (n ) 1) a n d Th ioxop yr im id in on es
(n ) 2)
Mui Mui Sim, Cheng Leng Lee, and Arasu Ganesan*
Institute of Molecular and Cell Biology, National University
of Singapore, 30 Medical Drive, Singapore 117609
Received J uly 15, 1997
Combinatorial methods of organic synthesis1 are now
established as an important source of compound diversity
for drug discovery. These libraries are usually prepared
by solid-phase techniques, although solution-phase2 routes
are attracting increasing attention. Recently, we re-
ported3 the solution-phase synthesis of a thiohydantoin
library from R-amino acids, aldehydes, and isothiocyan-
ates. We were also interested in the homologous syn-
thesis of 2-thioxo-4-dihydropyrimidinones4 (dihydro-2-
thiouracils), a class of compounds known5 to possess
biological activity, and disclosed some preliminary re-
sults. Here, we present further details of this work and
its application to the synthesis of a library comprising
125 discrete compounds.
have been reported7 using strong acid or base; however,
these conditions are less suitable for compounds contain-
ing sensitive functionality. We have found that the
thioureas undergo the desired cyclization in refluxing
triethylamine.8 Typically, the reaction was complete
after 1 day, while further heating (for example, 3 days)
did not lead to product decomposition. When the alde-
hyde component (2) was 4-acetoxybenzaldehyde, partial
hydrolysis of the acetate ester was observed in the
thioxopyrimidinone products.
Resu lts a n d Discu ssion
In our thiohydantoin synthesis, the cyclization (Scheme
1, n ) 1, 5 to 6) was considerably faster for N-alkylamino
acid esters compared to their unsubstituted counterparts.
Indeed, with secondary amines (3), the cyclization oc-
curred during the conditions of isothiocyanate (4) addi-
tion (rt, 1 mol equiv triethylamine6), and we were unable
to isolate intermediate 5. The related six-membered ring
(Scheme 1, n ) 2) cyclization to thioxopyrimidinones was
less facile and did not occur likewise. Related cyclizations
We have investigated a variety of â-amino acid esters,
aromatic aldehydes, and isothiocyanates as reaction
inputs. Over 80 thioxopyrimidinones were successfully
prepared during this validation phase. In general, the
thioxopyrimidinone cyclization is more susceptible to
steric hindrance than the thiohydantoin synthesis. Thus,
thioureas (7, Figure 1) derived from cyclohexyl isothio-
cyanate did not undergo cyclization, whereas thiohydan-
toins were successfully formed3 with tert-butyl and
1-adamantyl isothiocyanates. In the case of methyl
nipecotate, the thiourea intermediate (8) also did not
cyclize, and decomposed after one week of heating.
Finally, anthranilate esters (9) gave variable yields in
the reductive alkylation and isothiocyanate addition.
We next prepared a library of 125 discrete thioxopy-
rimidinones from a set of 5 permutations each for the
â-amino acid esters, aldehydes, and isothiocyanates
(Table 1). Reductive alkylations of the â-amino acid
esters were carried out on a large scale, and the crude
products divided into portions followed by reaction with
individual isothiocyanates. These reactions proceeded
uneventfully, except for the reductive alkylation of â-ala-
nine methyl ester (the least hindered â-amino acid),
which was plagued by dialkylation. With 3-methoxy-
benzaldehyde and 4-bromobenzaldehyde, chromatographic
purification was necessary to remove the dialkylated
product. For benzaldehyde and furfural, only dialkylated
products were formed, and the reaction was instead
carried out with limiting aldehyde (0.5 mol equiv) to
obtain the monoalkylated amino acid after chromatog-
(1) For recent reviews of broad scope, see: (a) Balkenhohl, F.; von
dem Bussche-Hu¨nnefeld, C.; Lansky, A.; Zechel, C. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2289. (b) Thompson, L. A.; Ellman, J . A. Chem.
Rev. (Washington, D.C.) 1996, 96, 555.
(2) We are aware of the following examples published between J an-
Aug 1997: (a) Maehr, H.; Yang, R. Bioorg. Med. Chem. 1997, 5, 493.
(b) Boger, D. L.; Chai, W.; Ozer, R. S.; Andersson, C.-M. Bioorg. Med.
Chem. Lett. 1997, 7, 463. (c) Adamczyk, M.; Gebler, J . C.; Grote, J .
Bioorg. Med. Chem. Lett. 1997, 7, 1027. (d) Chng, B. L.; Ganesan, A.
Bioorg. Med. Chem. Lett. 1997, 7, 1511. (e) Boger, D. L.; Ozer, R. S.;
Andersson, C.-M. Bioorg. Med. Chem. Lett. 1997, 7, 1903. (f) An, H.;
Cummins, L. L.; Griffey, R. H.; Bharadwaj, R.; Haly, B. D.; Fraser, A.
S.; Wilson-Lingardo, L.; Risen, L. M.; Wyatt, J . R.; Cook, P. D. J . Am.
Chem. Soc. 1997, 119, 3696. (g) Flynn, D. L.; Crich, J . Z.; Devraj, R.
V.; Hockerman, S. L.; Parlow, J . J .; South, M. S.; Woodard, S. J . Am.
Chem. Soc. 1997, 119, 4874. (h) Booth, R. J .; Hodges, J . C. J . Am.
Chem. Soc. 1997, 119, 4882. (i) Pop, I. E.; De´prez, B. P.; Tartar, A. L.
J . Org. Chem. 1997, 62, 2594. (j) Studer, A.; J eger, P.; Wipf, P.; Curran,
D. P. J . Org. Chem. 1997, 62, 2917. (k) Parlow, J . J .; Mischke, D. A.;
Woodard, S. S. J . Org. Chem. 1997, 62, 5908. (l) Baldino, C. M.;
Casebier, D. S.; Caserta, J .; Slobodkin, G.; Tu, C.; Coffen, D. L. Synlett
1997, 488. (m) Lawrence, R. M.; Biller, S. A.; Fryszman, O. M.; Poss,
M. A. Synthesis 1997, 553. (n) Gayo, L. M.; Suto, M. J . Tetrahedron
Lett. 1997, 38, 513. (o) Mukhopadhyay, M.; Bhatia, B.; Iqbal, J .
Tetrahedron Lett. 1997, 38, 1083. (p) Siegel, M. G.; Hahn, P. J .;
Dressman, B. A.; Fritz, J . E.; Grunwell, J . R.; Kaldor, S. W. Tetrahe-
dron Lett. 1997, 38, 3357. (q) Neuville, L.; Zhu, J . Tetrahedron Lett.
1997, 38, 4091. (r) Falorni, M.; Giacomelli, G.; Nieddu, F.; Taddei, M.
Tetrahedron Lett. 1997, 38, 4663.
(3) Sim, M. M.; Ganesan, A. J . Org. Chem. 1997, 62, 3230.
(4) For
a solid-phase synthesis of dihydropyrimidine-2,4-diones,
see: Kolodziej, S. A.; Hamper, B. C. Tetrahedron Lett. 1996, 37, 5277.
(5) For example, see: Soliman, R. J . Med. Chem. 1979, 22, 321.
(6) Two other groups have recently independently reported the use
of secondary and tertiary amines to effect hydantoin and thiohydantoin
cyclization on solid-phase: (a) Kim, S. W.; Ahn, S. Y.; Koh, J . S.; Lee,
J . H.; Ro, S.; Cho, H. Y. Tetrahedron Lett. 1997, 38, 4603. (b) Matthews,
J .; Rivero, R. A. J . Org. Chem. 1997, 62, 6090.
(7) (a) Okawara, T.; Nakayama, K.; Furukawa, M. Chem. Pharm.
Bull. 1983, 31, 507. (b) Lorente, A.; Aurrecoechea, L. M. Heterocycles
1994, 38, 1077.
(8) These conditions were used to cyclize ureas derived from
penicillamine: Hatam, M.; Ko¨pper, S.; Martens, J . Heterocycles 1996,
43, 1653.
S0022-3263(97)01302-9 CCC: $14.00 © 1997 American Chemical Society

Notes
J . Org. Chem., Vol. 62, No. 26, 1997 9359
Ta ble 2. Ma ss Recover y a n d HP LC P u r ity of
Rep r esen ta tive Th ioxop r im id in on es
mass
HPLC
mass
HPLC
thioxoprimi- recoverya purity thioxoprimi- recoverya purity
dinone
(%)
(%)
dinone
(%)
(%)
1Ae
1Bc
1Cd
1Db
1Ec
2Aa
2Bd
2Cc
2De
2Eb
3Ac
3Ba
3Cd
86
87
84
79
101
71
92
62
99
64
67
66
63
75
85
73
87
81
61
84
78
81
60
80
79
59
3De
3Eb
4Ae
4Bd
4Cb
4Dd
4Eb
5Ac
5Ad
5Cd
5Db
5Ea
70
63
96
115
116
116
112
95
108
92
101
89
70
60
83
91
80
91
70
87
85
83
85
72
F igu r e 1.
Ta ble 1. Bu ild in g Block s for th e Th ioxop yr im id in on e
Libr a r y
a
Calculated as the ratio of isolated mass over theoretical yield,
expressed as a percentage.
less abundant, but they can be prepared11 by homologa-
tion of the R-amino acid.
Exp er im en ta l Section
Gen er a l. For general methods and instrumentation, see ref
3. IR, 1H, and 13C NMR spectra were recorded in CDCl3; 1H
and 13C NMR were taken at 300 and 75 MHz, respectively.
Methyl esters 2-5 (Table 1) were prepared from the correspond-
ing â-amino acids by reaction with SOCl2 and MeOH, followed
by recrystallization from EtOH and Et2O. Methyl â-aminobu-
tyrate (3, Table 1) was obtained as a yellowish syrup which was
insoluble in CH2Cl2. Reductive alkylations with this compound
were carried out in MeOH.
Libr a r y Syn th esis. A solution of â-amino acid methyl ester
hydrochloride salt (0.5 mmol), triethylamine (1.1 mol equiv),
aldehyde (1.1 mol equiv), and sodium triacetoxyborohydride (1.5
mol equiv) in CH2Cl2 was stirred overnight at rt and then
quenched with water. The organic phase was washed with
water (2×) and dried over MgSO4, and aliquots were evenly
distributed into 1.5 mL vials. Triethylamine (14 µL, 0.1 mmol)
and isothiocyanate (0.11 mmol) were added into each vial, and
the solution shaken for 3 h. Aminomethylated polystyrene
resin12 (0.2-0.5 mol equiv) was added, and the vials were shaken
overnight. The resin was filtered off, and the solution was
heated at 80 °C for 1 day. The remaining solvent and triethyl-
amine were finally removed by evaporation in a SpeedVac to
yield the crude thioxopyrimidinones. Compounds 10-14 were
chromatographed on silica (hexanes-ethyl acetate 75:25 eluent).
1-(P h en ylm eth yl)-3-(2-ph en yleth yl)-tetr ah ydr o-2-th ioxo-
4(3H)-p yr im id in on e (10, thioxopyrimidinone 1Ac): 22 mg,
67%; pale yellow foam; IR νmax 1705 cm-1; 1H NMR δ 2.61 (2H,
t, J ) 6.8 Hz), 3.01-3.06 (2H, m), 3.37 (2H, t, J ) 6.8 Hz), 4.54-
4.59 (2H, m), 5.29 (2H, s), 7.20-7.38 (10H, m); 13C NMR δ 31.4,
33.9, 43.3, 47.7, 58.2, 126.3, 127.8, 128.1, 128.3, 128.9, 129.1,
135.4, 138.7, 165.8, 181.7. HRMS calcd for C19H20N2OS 324.12964
(M+), found 324.12929. HPLC purity 91%.
raphy. With the more hindered 2-chlorobenzaldehyde,
no dialkylation was observed, and the crude product was
used directly.
In some cases, the thiourea intermediates (5) in our
library underwent cyclization to the thioxopyrimidinone
without the need for heating. This was particularly so
for compounds containing ethyl isothiocyanatoacetate as
a building block. In our earlier thiohydantoin synthesis,
the final purification consisted of an aqueous wash with
glycine as a scavenger. We have since found that
aminomethylated polystyrene9 is a convenient means of
removing excess aldehyde and isothiocyanate. This
avoids the need for an aqueous workup10 and was the
protocol used for the thioxopyrimidinone library. The
1
purity of the compounds was assessed by H NMR, MS,
and HPLC analysis (for 25 representative examples, see
Table 2). Five of these compounds (10-14, Figure 2)
were chromatographed and characterized fully (see Ex-
perimental Section).
In summary, we have demonstrated the utility of the
three-step sequence (reductive alkylation, isothiocyanate
addition, cyclization) for the combinatorial synthesis of
thioxopyrimidinones. Two of the three building blocks,
aldehydes and isothiocyanates, are readily available with
high diversity. Commercially available â-amino acids are
1-[(4-Br om op h en yl)m eth yl]-3-(4-m eth ylp h en yl)-5-m eth -
yl-tetr a h yd r o-2-th ioxo-4(3H)-p yr im id in on e (11, thioxopy-
rimidinone 2De): 38 mg, 94%; white solid, mp 144-146 °C; IR
ν
max 1719 cm-1; 1H NMR δ 1.22 (3H, d, J ) 7.1 Hz), 2.40 (3H, s),
2.82-2.90 (1H, m), 3.40 (1H, dd, J ) 10.9, 13.2 Hz), 3.60 (1H,
dd, J ) 5.8, 13.2 Hz), 5.23 (1H, d, J ) 14.8 Hz), 5.34 (1H, d, J
) 14.9 Hz), 7.02-7.53 (8H, m); 13C NMR δ 12.8, 21.3, 35.6, 50.0,
57.5, 122.2, 128.8, 129.7, 129.8, 132.0, 134.4, 137.4, 138.2, 169.4,
182.4. HRMS calcd for C19H19BrN2OS 402.04016 (M+), found
402.03894. HPLC purity 86%.
(9) For the first report of polymer-bound reagents as scavengers in
solution-phase library synthesis, see: Kaldor, S. W.; Siegel, M. G.;
Fritz, J . E.; Dressman, B. A.; Hahn, P. J . Tetrahedron Lett. 1996, 37,
7193.
(10) We also tried using the resin to effect thioxopyrimidinone
cyclization as well as scavenging excess reagents. However, the results
were less satisfactory than with triethylamine. It is possible that
stronger polymer-bound bases would accomplish this transformation.
(11) For example: (a) Pelleti, J .; Seebach, D. Liebig’s Ann. 1995,
1217. (b) Enantioselective Synthesis of â-Amino Acids; J uaristi, E., Ed.;
VCH: Weinheim, 1997.
1-[(2-Ch lor op h en yl)m eth yl]-3-p r op yl-6-m eth yl-tetr a h y-
d r o-2-th ioxo-4(3H)-p yr im id in on e (12, thioxopyrimidinone
3Ba ): 19 mg, 60%; pale yellow foam; IR νmax 1703 cm-1; 1H NMR
δ 0.95 (3H, t, J ) 7.4 Hz), 1.24 (3H, d, J ) 6.7 Hz), 1.64-1.78
(2H, m), 2.56 (1H, dd, J ) 1.9, 16.2 Hz), 2.84 (1H, dd, J ) 6.3,
16.2 Hz), 3.70 (1H, dp, J ) 1.8, 6.7 Hz), 4.23 (1H, ddd, J ) 6.3,
(12) From Nova Biochem, loading 1.62 mmol/g.

9360 J . Org. Chem., Vol. 62, No. 26, 1997
Notes
F igu r e 2.
9.4, 12.8 Hz), 4.43 (1H, ddd, J ) 6.0, 9.3, 13.1 Hz), 4.80 (1H, d,
3.25 (1H, dd, J ) 6.8, 16.4 Hz), 3.81 (3H, s), 4.16 (1H, d, J )
15.0 Hz), 4.94 (1H, d, J ) 6.4 Hz), 6.48 (1H, d, J ) 15.0 Hz),
6.88-7.49 (13H, m); 13C NMR δ 39.1, 55.3, 56.7, 57.0, 113.4,
113.9, 115.5, 115.7, 120.2, 125.5, 128.9, 129.6, 130.0, 130.2, 136.1,
137.1, 140.6, 140.8, 160.1, 164.5, 182.0. HRMS calcd for
J ) 15.8 Hz), 6.02 (1H, d, J ) 15.8 Hz), 7.24-7.42 (4H, m); 13
C
NMR δ 11.2, 16.8, 21.2, 37.8, 48.0, 49.9, 54.1, 127.3, 128.8, 129.2,
129.8, 133.3, 133.4, 165.2, 180.9. HRMS calcd for C15H19N2OS
275.12180 ([M - Cl]+), found 275.12195. HPLC purity 82%.
1-(2-F u r ylm eth yl)-3-(ca r beth oxym eth yl)-cis-octa h yd r o-
2-t h ioxo-4(3H )-q u in a zolin on e (13, thioxopyrimidinone
C
91%.
24H21N2O2SF 420.13077 (M+), found 420.12820. HPLC purity
4Eb): 28 mg, 80%; pale yellow syrup, IR νmax 1709, 1743 cm-1
;
1H NMR δ 1.28 (3H, t, J ) 7.0 Hz), 1.22-1.82 (7H, m), 2.48 (1H,
br d, J ) 13.2 Hz), 2.92 (1H, br s), 3.64-3.71 (1H, m), 4.21 (2H,
q, J ) 7.2 Hz), 4.74 (1H, d, J ) 15.3 Hz), 4.98 (1H, d, J ) 16.9
Hz), 5.35 (1H, d, J ) 17.0 Hz), 5.69 (1H, d, J ) 15.4 Hz), 6.37
(1H, t, J ) 1.7, 3.4 Hz), 6.44 (1H, d, J ) 2.9 Hz), 7.39 (1H, d, J
) 2.4 Hz); 13C NMR δ 14.1, 21.0, 24.4, 24.7, 25.8, 40.2, 47.4,
50.0, 57.4, 61.3, 109.8, 110.7, 142.6, 149.1, 167.8, 168.7, 179.4.
HRMS calcd for C17H22N2O4S 350.13004 (M+), found 350.12851.
HPLC purity 82%.
Ack n ow led gm en t. This work was supported by
grants from the National Science and Technology Board
of Singapore.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
thioxopyrimidinones 10-14 (5 pages). This material is con-
tained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
1-[(3-Meth oxyph en yl)m eth yl]-3-(3-flu or oph en yl)-6-ph en -
yl-tetr a h yd r o-2-th ioxo-4(3H)-p yr im id in on e (14, thioxopy-
rimidinone 5Cd ): 29 mg, 68%; pale yellow foam, mp 54-60 °C;
1
IR νmax 1715 cm-1; H NMR δ 3.08 (1H, dd, J ) 1.8, 16.4 Hz),
J O971302V