New ways to derivatize at position 6 of 7,7-dimethyl-7,8-dihydropterin

Article abstract of DOI:10.1016/j.tetlet.2011.09.047

Reported are the synthesis of two intermediates for derivatization at position 6 of 7,7-dimethyl-7,8-dihydropterin: 6-carboxylic acid ethyl ester-7,7-dimethyl-7,8-dihydropterin, which is a novel compound, and 6-aldehyde-7,7-dimethyl-7,8-dihydropterin, which is synthesized by a new method with a yield of 90%.

Full text of DOI:10.1016/j.tetlet.2011.09.047

Tetrahedron Letters 52 (2011) 6174–6176
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New ways to derivatize at position 6 of 7,7-dimethyl-7,8-dihydropterin
⇑
Genbin Shi, Xinhua Ji
Biomolecular Structure Section, Macromolecular Crystallography Laboratory, National Cancer Institute, 1050 Boyles Street, Frederick, MD 21702, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2011
Revised 8 September 2011
Accepted 11 September 2011
Available online 16 September 2011
Reported are the synthesis of two intermediates for derivatization at position 6 of 7,7-dimethyl-7,8-dihy-
dropterin: 6-carboxylic acid ethyl ester-7,7-dimethyl-7,8-dihydropterin, which is a novel compound, and
6-aldehyde-7,7-dimethyl-7,8-dihydropterin, which is synthesized by a new method with a yield of 90%.
Published by Elsevier Ltd.
Keywords:
Pterin
Carboxylation
Bromination
Ester
Aldehyde
The folate biosynthetic pathway is essential for bacterial
growth. Targeting key enzymes in the pathway, sulfonamides
and diaminopyrimidines were developed as antibacterial drugs.1–
4 As bacteria have developed resistance to those drugs, new agents
are needed for the treatment of bacterial infection. 6-Hydroxy-
methyl-7,8-dihydropterin pyrophosphokinase (HPPK) is another
key enzyme in the folate pathway. It catalyzes the transfer of pyro-
phosphate from ATP to 6-hydroxymethyl-7,8-dihydropterin (HP)
to form AMP and 6-hydroxymethyl-7,8-dihydropterin pyrophos-
phate (HPPP).5–8
at position 8, which is important for binding to HPPK by forming a
hydrogen bond with the carbonyl oxygen of a leucine residue of
the protein.10,11
Stuart, Suckling, Wood, and colleagues12–15 developed methods
for the synthesis of both 6,7,7-trimethyl-7,8-dihydropterin (2b,
Scheme 1) and 6-hydroxymethyl-7,7-dimethyl-7,8-dihydropterin
(2c, Scheme 1), but 2c was found to be degraded to 6-hydroxy-
7,7-dimethyl-7,8-dihydropterin (2d, Scheme 1) upon prolonged
incubation with HPPK.11 In addition, the synthesis of 2c was trou-
blesome and the yield was low. A number of modifications were
O
O
N
N
HPPK
HN
OH
HN
OPP
3
6
+
ATP
+
AMP
1
N
8
N
H
N
N
H
H2N
H2N
HP
HPPP
Targeting HPPK, we previously synthesized a series of bisubstrate
analogs by linking 6-hydroxymethylpterin (1c, Scheme 1) to adeno-
sine through 2, 3, or 4 phosphate groups, and submicromolar affinity
totheenzymewasobtainedforthetetraphosphateinhibitor.9 To im-
prove the affinity and linker properties of such bisubstrate analogs,
we have designed new compounds by replacing the pterin moiety
(1a, Scheme 1) with 7,7-dimethyl-7,8-dihydropterin (2a, Scheme
1). Mimicking HP, 2a has one more hydrogen bond donor than 1a
O
N
O
R
N
R
N
N
HN
H2N
HN
6
8
N
N
H2N
H
1
2
⇑
Corresponding author. Tel.: +1 301 846 5035; fax: +1 301 846 6073.
E-mail address: jix@mail.nih.gov (X. Ji).
Scheme 1. Reagents: Pterin compounds: (a) R is absent, (b) R@CH3, (c) R@CH2OH,
(d) R@OH, and (e) R@CH2NHR0.
0040-4039/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2011.09.047

G. Shi, X. Ji / Tetrahedron Letters 52 (2011) 6174–6176
6175
O
N
HN
N
N
H
H2N
2b
i
ii
iv
Br
O
N
O
Br
O
N
Br
Br
N
N
N
HN
H2N
Br
HN
Br
HN
H2N
N
H
N
N
H
N
H
H2N
3a
3b
3c
vi
v
iii
x
O
N
O
O
N
CHO
N
COOH
N
HN
H2N
NHR'
HN
HN
vii
N
H
viii
N
N
H
N
N
H
H2N
H2N
2e
4b
4c
ix
x
Cl
O
O
N
N
N
HN
OH
HN
H2N
O
N
N
H
N
H
H2N
2c
5a
Scheme 2. Derivatization of compound 2b: (i, ii) as reported,13,14 (iii–viii) this work, (ix, x) as reported.23,12
made, but the yield was still low.16 In contrast, 2b was easier to
synthesize and stable, and the yield was high.12 In this study, we
focused on the derivatization of 2b at position 6.
LC–MS, but the products decomposed to a yellow fluorescent com-
pound that was identified as 4b (Scheme 2vii). To test whether we
could derive 2e from 4b, we needed a better method for the syn-
thesis of 4b. Using direct oxidation of methyl group by SeO2 as
O’Neal and Jacobi reported previously,24 we derived 4b from 2b
and the yield of 4b was 90% (Scheme 3).
The first reaction of 2b derivatization was bromination.
Previously, Stuart13,14 prepared 6-bromomethyl-7,7-dimethyl-
7,8-dihydropterin (3a, Scheme 2) and 6-dibromomethyl-7,7-di-
methyl-7,8-dihydropterin (3b) with bromine in glacial acetic acid
(Scheme 2, i and ii, respectively). Compound 3b could be used to
derive 6-aldehyde-7,7-dimethyl-7,8-dihydropterin (4b) at high
temperature,17–21 but we found that the yield of the reaction was
poor (Scheme 2iii). In general, bromination of methyl aromatic
compounds could give tribromomethyl aromatic compounds that
could be hydrolyzed to carboxylic acid aromatic compounds with
silver nitrate in aqueous ethanol or sulfuric acid,22 but we found
that although 2b gave 6-tribromomethyl-7,7-dimethyl-7,8-dihy-
dropterin (3c, Scheme 2iv), compound 3c did not yield the sug-
gested 6-carboxy-7,7-dimethyl-7,8-dihydropterin (4c, Scheme
2v). Instead, multiple products were found in the reaction mixture.
We attempted to make compound 2e by reacting 3a with
amines (Scheme 2vi). The desired products could be detected by
Our method for the synthesis of 4b: A solution of 2b (207 mg,
1.0 mmol) in DMF (10 mL) and pyridine (105 lL, 1.30 mmol) was
treated with SeO2 (145 mg, 1.30 mmol) and stirred at room tem-
perature for 5 h. The reaction was then heated to 80 °C for
15 min. The solvent was evaporated under high vacuum and the
residue purified by flash chromatography (silica gel, methanol/
dichloromerhane = 2:8) to give 4b (199 mg, 0.9 mmol, 90%) as a
yellowish powder. HRMS (ESI) calculated for C9H11N5O2 ([M+H]+)
222.0981, found 222.0991; 1H NMR (400 MHz, CD3OD) d 1.53 (6
H, s), 9.33 (1 H, s); UV (0.1%AcOH), 400 nm.
Indeed, with 4b, compound 2e could be made by reductive ami-
nation reacting with an amine (Scheme 2viii). Again, the desired
products, which could be detected by LC–MS, decomposed to the
starting material (Scheme 2vii). Taken together, compound 2e
could be made with either 3a or 4b, but the products would still
hydrolyze to 4b (Scheme 2). Previously, Suckling and co-work-
ers23,12 reported that 4b was obtained by gently oxidizing 2c or a
pterin ether analog (5a) (Scheme 2, ix and x, respectively).
Compound 2e was not stable. We thought that the methylene
group conjugating C@N could be a problem, and that an amide
could improve the stability. Therefore, our second derivative of
2b contains a carboxyl group linked to the pterin moiety. For the
synthesis of carboxylic acid substituted aromatic compounds from
methyl or aldehyde substituted aromatic compounds, various
O
N
O
N
CHO
N
N
SeO2/DMF
HN
H2N
HN
H2N
N
H
N
H
2b
4b
Scheme 3. New method of deriving 6-aldehyde-7,7-dimethyl-7,8-dihydropterin
(4b) from 6,7,7-trimethyl-7,8-dihydropterin (2b).

6176
G. Shi, X. Ji / Tetrahedron Letters 52 (2011) 6174–6176
O
N
In summary, we synthesized a novel intermediate, 6-carboxylic
O
N
acid ethyl ester-7,7-dimethyl-7,8-dihydropterin (6a, Scheme 4),
and found a new way to derive 6-aldehyde-7,7-dimethyl-7,8-dihy-
dropterin (4b, Scheme 3). With these two compounds, it is easy to
extend the side chain at position 6 of 7,7-dimethyl-7,8-dihydrop-
terin (2a, Scheme 1). Such derivatives of these two compounds
can be used as antifolate agents (antibacterials, antimalarials, and
anticancer drugs),3 as nitric oxide synthase activators for the treat-
ment of cardiovascular diseases,26,27 and as pteridine reductase
inhibitors targeting African sleeping sickness and the Leishmanias-
es.28 The method can also be applied to other systems from methyl
heteroaromatic and aromatic compounds to carboxylic ester het-
eroaromatic and aromatic compounds.
COOEt
N
N
HN
H2N
HN
H2N
Br2/EtOH
or NBS/EtOH
N
H
N
H
2b
6a
Scheme 4. Preparation of 6-carboxylic ethyl ester-7,7-dimethyl-7,8-dihydropterin
(6a).
methods with the use of oxidation reagents can be found in the lit-
erature.22 However, we found that these methods were not appli-
cable for the synthesis of 2b derivatives. When 2b was mixed with
the oxidation reagents, the characterized fluorescence of the com-
pound disappeared.
For the synthesis of 3a, a variety of bromination reagents were
tested, including bromine, triphenylphosphine dibromide, N-bro-
mosuccinimide, and phosphorus tribromide. We also tested a vari-
ety of solvents such as chloroform, carbon tetrachloride, ethanol,
N,N-dimethylformamide, and tetrahydrofuran. Usually, this kind
of reaction gives a mixture of mono, bi, and tribromomethyl com-
pounds. However, when ethanol was used as the solvent and the
reaction mixture was separated by silica gel chromatography,
light-yellow plate-shaped crystals were found in the collecting
tubes. These crystals had a blue fluorescence, different from 4b
(yellow fluorescent) and 3a (green fluorescent). The crystals were
collected and identified as a new compound, 6-carboxylic ethyl es-
ter-7,7-dimethyl-7,8-dihydropterin (6a, Scheme 4). We report here
that 6a was produced by heating 2b with bromine in an ethanol
solution (Scheme 4).
Acknowledgments
We thank Dr. Larry Keefer for critical reading of the manuscript.
This research was supported by the Intramural Research Program
of the NIH, National Cancer Institute, Center for Cancer Research
and the Trans NIH/FDA Intramural Biodenfense Program Y3-RC-
8007-01 from the NIAID. Mass spectrometry experiments were
conducted with an Agilent 1100 series LC/Mass Selective Detector
maintained by the Biophysics Resource in the Structural Biophysics
Laboratory, an Agilent 1200 LC/MSD-SL system in the Chemical
Biology Laboratory, and a Thermoquest Surveyor Finnigan LCQ
deca maintained by the Comparative Carcinogenesis Laboratory
of National Cancer Institute at Frederick.
To generate 6a, 2b (0.5 g, 2.4 mmol) was dissolved in 80 mL eth-
anol in a heavy wall pressure vessel, bromine (0.43 mL 8.4 mmol)
was added dropwise to the solution, and the pressure vessel was
sealed with a Teflon bushing. The solution was heated overnight
at 120 °C. The ethanol was evaporated and the residue was purified
by a Teledyne column chromatography system. The desired com-
pound 6a (35%) was obtained as a yellow solid. N-bromosuccini-
mide (NBS) is one of the best bromination reagents. Using NBS
instead of bromine in this procedure, however, the yield of 6a
was similar. HRMS (ESI) calculated for C11H15N5O3 ([M+H]+)
266.1248, found 266.1270; 1H NMR (400 MHz, CD3OD) d 1.52 (6
H, s), 1.32 (3 H, t), 4.23 (2 H, q); UV (0.1% AcOH), 380 nm.
Goswami and coworkers25 reported the side chain bromination
of methyl heteroaromatic and aromatic compounds by solid phase
NBS reaction under microwave to produce dibromo and carbalde-
hyde heterocyclic compounds. We tested the reaction with micro-
wave but in solution. Compound 2b (0.5 g, 2.4 mmol) and NBS
(1.28 g, 7.2 mmol) were dissolved in 20 mL ethanol and the reac-
tion mixture was heated in a Biotage microwave initiator for 5–
25 min. The ethanol was evaporated and the residue was purified
by the Teledyne column chromatography system. The desired com-
pound 6a (52%) was obtained as a yellow solid. We also added
azobisisobutyronitrile (AIBN) in the reaction, but the yield (44%)
was not improved. Bromine was also tested in the microwave reac-
tion system. Due to a pressure-related issue, we had to decrease
the reaction temperature and extend the reaction time; however,
the yield (14%) was poor.
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Overall, the microwave-assisted reaction had higher yield than
conventionally heated reaction, but the microwave gave one
byproduct which was identified to be 2d: HRMS (ESI) calculated
for C8H11N5O2 ([M+H]+) 210.0986, found 210.0988; 1H NMR
(400 MHz, CD3OD) d 1.39 (s). The removal of the byproduct, how-
ever, was difficult in the reaction workup. In this reaction, NBS and
bromine played the same role. Without the microwave, we prefer
the use of bromine because the reaction was cleaner.
27. McInnes, C. R.; Suckling, C. J.; Gibson, C. L.; Morhtala, R. R.; Daff, S.; Gazur, B.
Pterieines 2009, 20, 27–35.
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