3
700
T. Wang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 3699–3703
Scheme 1. Reagents and conditions: General syntheses of 4-amino-purido[2,3-d]pyrimidin-5(8H)-ones. (a) 160 °C, 2 h; (b) Ac
NaOH, EtOH, 130 °C, 2 h, followed by acidification with HCI, >90% from 3–8; (e) Ph O, 260 °C, 10 h; (f) for alkyl halides, K CO , DMF, 80 °C, 10 h; for epoxides, K
DMF, 80–100 °C, 10 h, 50–60%; (g) mCPBA, 0 °C, chloroform or DMF; (h) for ROH, tBuOK, DMF, 30–50% overall yields from 9; for RSH, K CO , DMF, 40–50% overall yields from
; for X = NH, 5 equiv RNH , DMF, 56% from 9; (i) mCPBA, DMF.
2
O, 150 °C, 1 h; (c) Ph
2
O, 260 °C, 40 min; (d) 2 N
2
2
3
2
CO or NaOH,
3
2
3
9
2
nitrogen with a ring-closing ethylene linker would reduce the polar
surface area while replacing the third key hydrogen bond with a
similar interaction between a polarized C–H in alpha to the ketone
and the same backbone carbonyl oxygen.
Another key element of this design was the synthetic opportu-
nity (Scheme 1) to simultaneously allow facile exploration of the
hydrophobic and sugar subsites. The C2 and N8 positions of the
pyridopyrimidinone scaffold appeared to enable this design strat-
egy as suggested by docking calculations.
or secondary alcohols activated by potassium t-butoxide, was con-
verted to the 2-alkyloxy derivatives, 12. Similarly, C2 thio, 13 and
amino, 14 analogs could be obtained by reaction of the sulfoxide
with thiols or primary amines, respectively. Alternatively, the 2-
methylthiol in 9 could be oxidized first with mCPBA in DMF. The
resulting crude sulfoxide could then be substituted by either pri-
mary alcohols or thiols to generate intermediate 16 (X = O and S),
which upon reaction with an electrophile, could be converted to
desired analogs 12–14.
The synthesis of 4-amino-pyrido[2,3-d]pyrimidin-5(8H)-one
analogs started with diamino-methylthio-pyrimidine, 3. Conden-
sation of neat 3 with diethyl 2-(ethoxymethylene)malonate 4 at
A brief investigation was conducted for the heteroatom linker
attached at C2 using the hydroxyethyl N8 analogs 12a–14a. As
seen in Table 1, both compounds 12a and 13a with oxygen and sul-
fur linkers give similar IC50 values for enzyme inhibition, while the
amine linkage (analog 14a) resulted in loss of activity. We then
focused our effort only on oxygen and sulfur ether linkages at C2.
Next, a variety of C2 alkoxy groups were examined (Table 2).
12
1
60 °C gave diethyl 2-((6-amino-2-(methylthio)pyrimidin-4-yla-
mino)methylene)malonate 5, which was acetylated with acetic
anhydride (150 °C, 1 h) to afford the N-acetylated intermediate 6.
Upon refluxing in diphenyl ether, the ring-closure pyridone prod-
uct 7 was obtained as a solid after cooling and simple hexane wash.
The crude solid 7 was directly taken into 2 N sodium hydroxide
and ethanol. Heating at 100 °C for 2 h followed by quenching with
HCl gave the acid 8 as a pure solid. Finally, decarboxylation of 8
was conducted in refluxing diphenyl ether for 10 h to afford the de-
sired key intermediate, 4-amino-2-(methylthio)pyrido[2,3-d]pyr-
imidin-5(8H)-one, 9. The key feature of this sequence was that
these five steps could be telescoped to produce 9 in multigram
quantities in an expedited manner.
Table 1
SAR of C2 heteroatom linkage
NH
2
O
N
N
X
N
Doubly reactive centers at the C2 and N8 positions allowed
elaboration at these two positions: C2 substitutions targeted the
bacterial-specific hydrophobic site while N8 substitutions targeted
the NAD ribose-binding region. The nucleophilic pyridone NH in 9
was used to react with electrophiles such as alkyl halides and
epoxides resulting in the N-alkylated derivatives, 10. The C2
methylthiol group was then oxidized with mCPBA to generate
the 2-methylsulfoxide 11, which, upon treatment with primary
OH
12a to 14a
Compound
X
E. coli LigA IC50 (lM)
12a
O
S
NH
0.51
0.35
>10
1
1
3a
4a