Alkali metal cations control over nucleophilic substitutions on aromatic fused pyrimidine-2,4-[1H,3H]-diones: Towards new PNA monomers

Article abstract of DOI:10.1016/j.tet.2012.08.028

In this paper we report synthesis of a series of aromatic fused pyrimidine-2,4(3H)-dione-1-yl acetic acid and new PNA monomers containing these polycyclic nucleobase analogues. Introduction of a fused aromatic ring onto the 5,6-positions of the pyrimidine-2,4-[1H,3H]-diones brings about the steric effects and the charge delocalization, both weakening the nucleophilic substitutions on the 1- and 3-positions. We found that alkali metal cations play an important role in this alkylation reaction. LiOH brings out a much more efficient alkylation than NaOH does, while KOH nearly does not work on this reaction. Such influences from the alkali metal cations are probably due to that the charge-pairing interactions between the pyrimidine-2,4-dioxide anions and the alkali metal cations rearrange the charge distribution around the whole aromatic system and increase the negative charge distribution on the 1- and 3-nitrogen atoms, which then strengthens the nucleophilic reactivity on these positions.

Full text of DOI:10.1016/j.tet.2012.08.028

Tetrahedron 68 (2012) 8908e8915
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Alkali metal cations control over nucleophilic substitutions on aromatic fused
pyrimidine-2,4-[1H,3H]-diones: towards new PNA monomers
,
,
a
c
b
,
c
Pengfa Li a b, Chuanlang Zhan a , Shanlin Zhang , Xunlei Ding , Fengqi Guo , Shenggui He ,
*
*
Jiannian Yao a
,
*
a Beijing National Laboratory of Molecular Science, Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Department of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan Province, PR China
c Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100080, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2012
Received in revised form 16 July 2012
Accepted 10 August 2012
Available online 16 August 2012
In this paper we report synthesis of a series of aromatic fused pyrimidine-2,4(3H)-dione-1-yl acetic acid
and new PNA monomers containing these polycyclic nucleobase analogues. Introduction of a fused ar-
omatic ring onto the 5,6-positions of the pyrimidine-2,4-[1H,3H]-diones brings about the steric effects
and the charge delocalization, both weakening the nucleophilic substitutions on the 1- and 3-positions.
We found that alkali metal cations play an important role in this alkylation reaction. LiOH brings out
a much more efficient alkylation than NaOH does, while KOH nearly does not work on this reaction. Such
influences from the alkali metal cations are probably due to that the charge-pairing interactions between
the pyrimidine-2,4-dioxide anions and the alkali metal cations rearrange the charge distribution around
the whole aromatic system and increase the negative charge distribution on the 1- and 3-nitrogen atoms,
which then strengthens the nucleophilic reactivity on these positions.
Keywords:
Nucleophilic substitution
Aromatic fused pyrimidine-2,4(3H,7H)-
diones
Alkali metal cation
PNA monomer
Ó 2012 Elsevier Ltd. All rights reserved.
Solvent effect
1. Introduction
modifications of the PNA backbone (1) by using chiral amino acid to
replace 2-aminoethyl, yielding chiral PNA backbone8 or (2) by using
Peptide nucleic acid (PNA) is a typical class of structural mimics
of nucleic acids,1 in which the natural sugar-phosphate backbone is
replaced by achiral N-(2-aminoethyl) glycine units.2 PNA can bind
to complementary sequence targets with high affinity and se-
quence specificity, forming either duplexes with mixed sequences,3
triplexes with homopyrimidine/homopurine sequences,4 or quad-
ruplexes with G-rich sequences.5 The PNA/DNA (RNA) complexes
generally have a higher thermal stability than the corresponding
DNA/DNA (RNA/RNA) complexes, and this encourages potential
applications for PNA, for example, for antisense and antigene pur-
poses.6 The relative high binding affinity of PNA to natural oligo-
nucleotides is determined by both the neutral backbone and
conformational flexibility. The neutral backbone largely reduces the
electrostatic repulsion formed between naturally DNA (RNA)
strands and the conformational flexibility is compensated by the
formation of internucleobase hydrogen-bonding and stacking as
achieved between nucleic acid strands.1,7 Attempts has been made
to increase the binding affinity and selectivity of PNA. These include
achiral branched or cyclo alkyl units, achieving conformational
constrained PNA backbone.9
Our purpose is to use polycyclic nucleobase analogues to tune
the inter-nucleobase stacking and hydrogen-bonding interactions.
Aromatic fused pyrimidine-2,4(1H,3H)-dione derivatives have
a larger
p-aromatic system and also possess hydrogen-bonding
features similar to that of thymine, showing potential applica-
tions for the medicine or clinic purposes.10,11 Also, these structural
features endow the derivatives with possible utilizations as a typi-
cal class of polycyclic nucleobase analogues.12 Encouraged by this,
we aimed to convert the aromatic fused pyrimidine-2,4(1H,3H)-
diones as a proper substitute for the natural thymine nucleobase of
PNA monomers (Scheme 1a), in which its different aromatic size
and different electron nature will provide different inter-
nucleobase stacking and hydrogen-bonding interactions (Scheme
1b).13
Thymin-1-yl acetic acid,
a key intermediate towards the
thymine-based PNA monomers, is generally synthesized by fol-
lowing these two procedures: one is the alkylation of thymine-1H
with 2-bromoacetate ester in the organic solvents, such as dime-
thylformamide (DMF) with K2CO3 as base and the followed
* Corresponding authors. Tel./fax: þ86 10 82617312; e-mail addresses: clzhan@
iccas.ac.cn (C. Zhan), jnyao@iccas.ac.cn (J. Yao).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.028

P. Li et al. / Tetrahedron 68 (2012) 8908e8915
8909
3-amine-2-carboxylic acid (6, Scheme 3) with urea at 150 ꢀC for
12 h. After workup, the desired product was obtained as a white or
light yellow solid, typically in a yield of 80e95%.
(a)
Scheme 3. Synthesis of the aromatic fused pyrimidine-2,4(3H,7H)-diones. Reaction
conditions: urea, 20 equiv; 150 ꢀC, 12 h; yield: 80e95%.
(b)
2.2. Influences of the solvent on the nucleophilic sub-
stitutions of the 1- and 3-positions of the aromatic fused
pyrimidine-2,4(1H,3H)-diones
As we can see from Scheme 2 that introduction of the fused
aromatic ring, such as naphthalene could produce steric effects,
although slightly, for the nucleophilic reactivity on the 1H position
of naphtha[20,30:5,6]pyrimidine-2,4(1H,3H)-diones. Again, the
charge delocalization from the pyrimidine-2,4(1H,3H)-dioxide an-
ions onto the naphthalene ring may weaken the nucleophilic ability
of the 1- and 3-nitrogen atom. Quantum chemical calculations in-
dicate that the negative charge distributed on the pyrimidine-2,4-
dioxide anions, e.g., on the atoms of N1, C2, N3, C4, C5, C6, O7
and O8, is 96.2% (ꢁ1.923/ꢁ2) for 5-methylpyrimidine-2,4-dioxide
anions (T2ꢁ), whereas it is only 72.4% for naphtha[20,30:5,6]pyrim-
idine-2,4-dioxide anions (A2ꢁ) (Table 1 and Fig. 1). Synthetic ex-
periments further support the weak nucleophilic reactivity on the
N1 and N3 positions. For example, no desired product of 3a, while
only trace of product 4a was obtained by following the traditional
procedures using ethyl 2-bromoacetic ester (BrAcOEt) as the re-
actant, K2CO3 as the base and DMF as the reaction medium (Table 2,
entry 1)14 or using 2-bromoacetic acid (BrAcOH) as the reactant,
KOH as the base and water as the reaction medium (Table 2,
entry 2).8b,15
Scheme 1. (a) The chemical structures of the synthesized PNA monomers containing
polycyclic nucleobase analogues of aromatic fused pyrimidine-2,4(3H)-diones. (b) The
possible WatsoneCrick and Hoogsteen H-bonding between the PNA monomers of 1a
and the natural T-monomer, for example.
hydrolysis of the resultant ester derivative to yield the target
product;14 the other is the alkylation of thymine-1H with 2-
bromoacetic acid with water as the reaction medium and with
KOH as the base.8b,15 Both methods gave a typical yield of w60%.
However, introduction of the fused aromatic ring, for example,
naphtha[20,30:5,6]pyrimidine-2,4-[1H,3H]-diones (2a) (Scheme 2),
dramatically weakened the nucleophilic reactivity on the 1H and
3H positions under the conditions of the above two mentioned
methods and there was only trace of 3-yl acetic acid product
formed, whereas no 1-yl acetic acid product was yielded (Scheme
2). To resolve this problem, we have developed a straightforward
synthetic procedure and report herein the synthesis of the aromatic
fused pyrimidine-2,4(1H,3H)-dione-1-yl acetic acid (Scheme 2) and
further the corresponding PNA monomers (Scheme 1a). In partic-
ular, we have observed that both the organic solvent and the alkali
metal cations play an important role in the nucleophilic sub-
stitutions on the 1H- and 3H-positions.16
Table 1
Charge distributions on 5-methylpyrimidine-2,4-dioxide anions (T2ꢁ), naphtha
[20,30:5,6]pyrimidine-2,4-dioxide anions (A2ꢁ), naphtha[20,30:5,6]pyrimidine-2,4-
dioxide lithium (Li2A), naphtha[20,30:5,6]pyrimidine-2,4-dioxide sodium (Na2A),
and naphtha[20,30:5,6]pyrimidine-2,4-dioxide potassium (K2A), respectively
Compound
S (N1eO8)a
S (C9eC16)b
S (2Mþ)c
N1
N3
T2ꢁ
ꢁ1.923
ꢁ1.449
ꢁ1.762
ꢁ1.759
ꢁ1.772
ꢁ0.077d
ꢁ0.551
0.016
d
ꢁ0.712
ꢁ0.694
ꢁ0.782
ꢁ0.771
ꢁ0.776
ꢁ0.701
ꢁ0.667
ꢁ0.786
ꢁ0.773
ꢁ0.776
A2ꢁ
d
Li2A
Na2A
K2A
1.746
1.811
1.859
ꢁ0.052
ꢁ0.087
a
Total Natural Bond Orbital (NBO) charge density distributed on the atoms of N1,
C2, N3, C4, C5, C6, O7, and O8 (Fig. 1).
b
Total NBO charge density distributed on the atoms of C9H, C10, C11, C12H,
C13H, C14H, C15H, and C16H (Fig. 1).
c
Total NBO charge density distributed on the two alkali metal cations of M17 and
Scheme 2. Synthetic procedures of the naphtha[20,30:5,6]pyrimidine-2,4(3H)-dione-1-
yl acetic acid (3a), naphtha[20,30:5,6]pyrimidine-2,4(1H)-dione-3-yl acetic acid (4a)
and naphtha[20,30:5,6]pyrimidine-2,4-dione-1,3-diyl diacetic acid (5a). Detailed re-
action conditions from 2a to 3a, 4a and 5a are shown in Table 2.
M18 (Fig. 1).
d
For T2ꢁ this represents the total NBO charge density distributed on the 5-
substituted methyl (C9).
We then tried to change the reaction conditions. When we used
2-bromoacetic acid as the reactant and DMF as the reaction me-
dium, while replaced K2CO3 with NaOH, desired products of 3a, 4a,
and 5a were isolated, respectively, in a yield of 3%, 6%, 4% (Table 2,
entry 3). However, the insoluble intermediate of naphtha[20,30:5,6]
pyrimidine-2,4-dioxide sodium and the insoluble sodium salt of
products 3a, 4a, and 5a all mixed in the reaction system and this
may prevent further conversion from the intermediate to the final
2. Results and discussion
2.1. Synthesis of aromatic fused pyrimidine-2,4(1H,3H)-
diones
Aromatic fused pyrimidine-2,4(1H,3H)-diones were successfully
synthesized by heating the mixture of the corresponding aromatic-

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P. Li et al. / Tetrahedron 68 (2012) 8908e8915
2.3. Influences of the alkali metal cations on the nucleophilic
substitutions of the 1- and 3-positions of the aromatic fused
pyrimidine-2,4(1H,3H)-diones
As shown in entries 4, 8 and 9 of Table 2, alkali metal
cations influence the nucleophilic reactivity obviously: LiOH
produced product 3a in a yield of 55% and 4a in a yield of 30%
(entry 8), both are obviously increased as compared to that by
using NaOH as the base (15% for 3a and 10% for 4a, entry 4),
whereas KOH produced products of 3a and 4a both in a low
yield of 10% and 8%, respectively (entry 9).
To get insightful information of the influences of the alkali
metal cations on the nucleophilic reactivity, we carried out the
DFT calculations. Table 1 shows that binding of Liþ (from A2ꢁ to
Li2A) significantly increases (1) the negative NBO charge density
on the N1 and N3, going from ꢁ0.694 and ꢁ0.667 for A2ꢁ to
ꢁ0.782 and ꢁ0.786 for Li2A, respectively, and (2) that on the part
of pyrimidine-2,4-dioxide anions, e.g., on the atoms of N1, C2, N3,
C4, C5, C6, O7, and O8 (Fig. 1), going from ꢁ1.449 for A2ꢁ up to
ꢁ1.762 for Li2A. Both are comparable to those of T2ꢁ. Therefore,
the DFT calculations indicate that the rearrangement of the charge
distributions after binding the alkali metal cations may account
for the increase of the nucleophilic reactivity on the N1 and N3
positions.
Fig. 1. Optimized geometry and molecular labelling numbers in naphtha[20,30:5,6]
pyrimidine-2,4-dioxide salt (M2A).
Table 2
Collections of the yields of products 3a, 4a, and 5a under different reaction
conditionsa
Entries
Reactant
Base
Solvent
T (ꢀC)
Yield of products (%)
Smaller alkali metal cation from Kþ to Naþ and Liþ can pro-
duces much stronger charge-pairing interactions between the
metal cation and the oxide anion. As revealed by the DFT cal-
culations, the binding energy between the alkali metal cations
and the naphtha[20,30:5,6]pyrimidine-2,4-dioxide anions in-
creases from ꢁ13.40 eV for Kþ to ꢁ15.06 eV for Naþ and then
ꢁ17.25 eV for Liþ, respectively (Table 3). This means that smaller
alkali metal cation possesses stronger tendency to bind with the
oxide anion in the DMSO solution and this contributes to the
countering alkali metal cation effects on the nucleophilic re-
activity, which decreases from Liþ to Naþ and then Kþ, as ob-
served from the experiments. Noted that Table 1 shows that the
calculated NBO charge density on the pyrimidine-2,4-dioxide
anions and on the 1- and 3-nitrogen atoms are both compara-
ble to each other when binding with Liþ, Naþ, and Kþ, re-
spectively. This is because (1) delocalization of the partial
negative charge from A2ꢁ onto the 2Mþ yields a decreases of
total positive charge of the alkali metal cations, going from 1.746
for Liþ to 1.811 for Naþ and 1.859 for Kþ, respectively (Table 1 and
2) the calculations are based on the gas models, and therefore,
the solvated effects are excluded.
3a
4a
5a
1
2
3
4
5
6
7
8
9
BrAcOEt
BrAcOH
BrAcOH
BrAcOH
BrAcOH
BrAcOH
BrAcOH
BrAcOH
BrAcOH
K2CO3
KOH
DMF
H2O
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
40
40
40
40
25
30
50
40
40
d
d
3
30
8
16
15
55
10
Trace
Trace
6
20
10
15
10
30
8
d
d
4
10
3
10
8
5
NaOH
NaOH
NaOH
NaOH
NaOH
LiOH
KOH
3
a
The reaction procedure and conditions are as follows: 1 equiv of naphtha
[20,30:5,6]pyrimidine-2,4(1H,3H)-diones was dissolved or dispersed into a proper
solvent (10 mL) and then 5 equiv of inorganic base was added. After the mixture was
stirred at the selected temperature for 1 h, to which 3 equiv of reactant in 2 mL of
this same solvent was dropwise added.
products, decreasing the yield of the desired products. We tried to
solve this problem by adding water (in a volume ratio of 1e20%)
and we observed that the addition of water could really dissolve the
insoluble solids, but unfortunately, the yield of the desired product
was also decreased obviously, only trace of desired product was
detected by TLC. This may be due to that the strong hydrogen-
bonding from water hinders naphtha[20,30:5,6]pyrimidine-2,4-
dianions from attacking BrAcOH.
We then selected dimethyl sulfoxide (DMSO) as the reaction
medium because it is aprotic and its dielectric constant (46.7) is
larger than that of DMF (38.0). As expected, DMSO can dissolve the
solids completely. As a result, the yield of products 3a, 4a, and 5a
was then significantly improved, up to 30%, 20%, and 10%, re-
spectively (Table 2, entry 4). The solvent influences from water to
DMF and DMSO imply that (1) a SN2-type reaction may account for
the alkylation on the 1- and 3-nitrogen atoms with 2-bromoacetic
acid as the reactant and DMSO as the reaction medium17 and (2) the
balance between solubility and nucleophilicity is a key factor for
selection of a reaction solvent.
Table 3
ꢀ
Binding energy (E/eV) and distances (r1, r2, r3 and r4/A) between the naphtha
[20,30:5,6]pyrimidine-2,4-dioxide anions (A2ꢁ) and alkali metal cations: E¼E(AM2)ꢁ
E(A2ꢁ)ꢁ2*E(Mþ
)
ꢀ
ꢀ
ꢀ
ꢀ
E (eV)
r1 (A)
r2 (A)
r3 (A)
r4 (A)
Liþ
Naþ
Kþ
ꢁ17.25
ꢁ15.06
ꢁ13.40
1.87
2.23
2.54
1.90
2.23
2.56
1.83
2.19
2.48
1.90
2.24
2.58
The optimized geometry and distance of r1, r2, r3 and r4 are shown in Fig. 1.
Various reaction conditions, such as temperature, reaction time,
and the relative equivalence of reactant and inorganic base as well
as the solvent volume were checked out to optimize the reaction
conditions. It was observed that the reaction temperature showed
obvious influences on the yield (Table 2, entries 5e7). 40 ꢀC was
better for this reaction. Low or high reaction temperature, for ex-
ample, 25 ꢀC, 30 ꢀC and 50 ꢀC, all produced a low yield of the de-
sired product.
By following this optimized reaction conditions, a series of
aromatic fused pyrimidine-2,4(3H)-dione-1-yl acetic acid were
successfully synthesized. As shown in Table 4, the yield of the
desired product with the 1H position substituted by acetic
acid was dependent on the utilization of the base, decreasing
in the order of LiOH, NaOH and KOH for all these seven
products.

P. Li et al. / Tetrahedron 68 (2012) 8908e8915
8911
Table 4
Synthesis of product 3 under the optimum reaction conditionsa
Entries
Reactant
Product
Yield (%)
LiOH
NaOH
KOH
1
60e80
60e75
60e80
20e30
8e15
2
3
16e30
20e30
6e13
10e15
4
50e65
50e60
10e22
5e10
8e16
<5
Fig. 2. Molecular structures (a), 1H NMR (b) and 13C NMR (c) spectra of products 3c and
4c, both in DMSO-d6.
5
6
40e50
40e50
10e20
12e21
10e15
8e14
7
a
The reaction procedure and conditions are as follows: 1 equiv of aromatic fused
pyrimidine-2,4(1H,3H)-diones was dissolved or dispersed into DMSO (10 mL) and
then 5 equiv of base was added. After the mixture was stirred at 40 ꢀC for 1 h, to this
solution 3 equiv of reactant in 2 mL of DMSO was dropwise added.
2.4. Discriminations of the aromatic fused pyrimidine-
2,4(1H,3H)-dione-1-yl acetic acid (3) and aromatic fused py-
rimidine-2,4(1H,3H)-dione-3-yl acetic acid (4)
In the 1H NMR spectra of products 3c and 4c (Fig. 2b), seven sets
of signals with the same integrals were observed in the region of
2e14 ppm. These signals include two singlets (protons a and k) in
the region of 2e5 ppm, two doublets (protons c and d) and one
singlet (proton g) in the region of 7e8 ppm, and two singlets
(protons i and l) in the region of 11e14 ppm. These signals are in
agreements well with the structure of the products of 3c and 4c
(Fig. 2a). In the region of 4e5 ppm, the singlet was assigned to the
protons of the methylene groups. For the 1-acetic acid derivatives
(3c), they appear at the 4.78 ppm, whereas they occur at 4.55 ppm
for the 3-acetic acid derivatives. This is the most useful feature in
the chemical shifts to discriminate products 3c and 4c.
Assignments of the protons on 3c and 4c are confirmed by the
HeH NOESY, HSQC and HMBC spectra (Fig. 3aed and 4). In HeH
NOESY spectrum of 3c two obvious signals were observed between
the protons of k and d (Fig. 3a), whereas no signals appeared be-
tween the protons of i and d. In contrast, two signals occurred
between the protons of i and d and no signals appeared between
the protons of k and d in the HeH NOESY spectrum of 4c (Fig. 3b).
Similarly, in the 13C NMR spectrum, the signals appearing in the
region of 40e45 ppm are assigned to the carbons of the methylene
groups (Fig. 1c). The methylene carbons in the 1-yl acetic acid
Fig. 3. Partial HeH NOESY (a and b) and HMBC (c and d) spectra of products 3c
(a and c) and 4c (b and d), all in DMSO-d6.
derivative appear at 44.2 ppm, while those in the 3-yl acetic acid
derivative occur at 41.7 ppm. The aromatic carbons were fully
assigned further from the HSQC (Fig. 4) and HMBC spectra (Fig. 3c
and d). In HMBC spectra, product 3c shows three sets of strong
signals between the protons k and the carbons e, h and l, re-
spectively (Fig. 3c), whereas 4c gives out three sets of signals be-
tween the protons k and the carbons h, j and l, respectively (Fig. 3d).
2.5. Synthesis of the PNA monomers with aromatic fused
pyrimidine-2,4(1H,3H)-diones as the base analogues
The Boc-protected PNA monomers were synthesized by fol-
lowing the procedure shown in Scheme 4, similar to that reported
in the references.14 In the condensation of
3 and 7 DCC

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P. Li et al. / Tetrahedron 68 (2012) 8908e8915
Chemical shifts were referenced to Si(CH4)4 signals. ESI mass
spectrometric were obtained on LC-MS 2010 and BRUKER Apex IV
FTMS and MALDI-TOF mass spectrometric measurements were
performed on Bruker Biflex III MALDI-TOF spectrometer.
4.2. Quantum chemical calculations
Density functional theory (DFT) calculations were performed
using the Gaussian 03 program18 with the B3LYP exchange-
correlation functional.19 All-electron triple-
x valence basis sets
with diffusion and polarization functions (6-311þþG**)20 are used
for all atoms. Geometry optimizations were performed with full
relaxation of all atoms. Calculations were performed in gas phase
without solvent effects. Vibrational frequency calculations were
performed to check that the stable structures had no imaginary
frequency. Charge distribution of the molecules was calculated by
natural population analysis.21
Fig. 4. Partial HSQC spectra of products 3c (a) and 4c (b), both in DMSO-d6.
4.3. Synthetic procedures
4.3.1. Synthesis of aromatic fused pyrimidine-2,4(1H,3H)-dio-
nes. The typical procedure to aromatic fused pyrimidine-
2,4(1H,3H)-diones (2) is as follows. Aromatic-3-amine-2-carboxylic
acid (0.1 mol) and urea (2 mol) were both mixed in a flask and then
heated at 150 ꢀC for 12 h. When the mixture was cooled down to
100 ꢀC, 40 mL of water was added and the resultant mixture was
stirred for another 10 min to dissolve the unreacted urea. After the
reaction mixture was cooled down to room temperature, it was
filtered and 400 mL, 1 mol/L NaOH aqueous solution was then
added. Another 1 h later 55 mL of acetic acid was dropwise added
for acidification and the resultant light yellow or white precipitates
were filtered and dried to give the desired product 2 in a yield of
80e95%.
Scheme 4. Synthesis of the aromatic fused pyrimidine-2,4(3H,7H)-diones. Reaction
conditions: (1), compound 7, 1 mol; DhbtOH 1.1 mol; compound 3, 1.1 mol; 1:1 DMF
and DCM, 7.5 mL; 0 ꢀC, 3 h; yield: 60e80%; (2), compound 8, 1.0 mmol; THF, 5 mL; 1 M
LiOH, 5 mL; rt, 45 min.
(dicyclohexylcarbodiimide) and DhbtOH (3,4-dihydro-3-hydroxy-
4-oxo-1,2,3-benzotriazine) were both utilized to get a high yield of
product 8. In this procedure, trace of DCU (dicyclohexylurea) may
mix with product 8 and they may be appeared as white precipitate
during the hydrolysis of product 8. The white precipitate should be
removed by filtering the reaction mixture before the pH value of
the filtration was adjusted to 2 by using 1 M hydrochloric acid
aqueous solution.
For 2a: 1H NMR (400 MHz, DMSO-d6)
d: 11.31 (s, 1H, eNH), 11.20
(s, 1H, eNH), 8.64 (s, 1H, enapheH), 8.10 (d, 1H,
J¼8.4 Hz, enapheH), 7.90 (d, 1H, J¼8.4 Hz, enapheH), 7.61 (t, 1H,
J¼8.0 Hz, enapheH), 7.52 (s, 1H, enapheH), 7.47 (t, 1H,
J¼8.0 Hz, enapheH); 13C NMR (DMSO-d6, 100 MHz)
d: 162.9, 150.3,
136.5, 136.3, 129.5, 129.2, 129.0, 128.5, 126.7, 124.9, 115.2, 110.2;
Anal. Calcd: C, 67.92; H, 3.80; N, 13.20; found: C, 68.07; H, 3.74; N,
13.17; GCT-MS m/z (%): 212.0 (Mþ, 100%).
3. Conclusions
For 2b: 1H NMR (400 MHz, DMSO-d6)
d
: 11.36 (s, 1H, eNH), 11.21
(s, 1H, eNH), 7.29 (s, 1H, epheH), 6.71 (s, 1H, epheH), 3.85 (s,
3H, eCH3), 3.80 (s, 3H, eCH3); 13C NMR (DMSO-d6, 100 MHz)
We have developed a straightforward approach to the synthesis
of aromatic fused pyrimidine-2,4(3H)-dione-1-yl acetic acid. We
found that both a proper solvent of DMSO and a proper alkali metal
hydroxide, such as LiOH yielded efficient alkylation of 2-
bromoacetic acid on the 1- and 3-positions of the aromatic fused
pyrimidine-2,4(3H)-dione derivatives. By using this approach we
successfully synthesized a series of new PNA monomers containing
these polycyclic base analogues. Currently, incorporation of these
PNA monomers into PNA oligomers is undergoing.
d
:
161.6, 149.8, 137.2, 136.4, 132.1, 126.8, 115.3, 113.2, 56.2, 54.3; Anal.
Calcd: C, 54.05; H, 4.54; N, 12.61; found: C, 53.97; H, 4.70; N, 12.57;
GCT-MS m/z (%): 222.2 (Mþ, 100%).
For 2c: 1H NMR (400 MHz, DMSO-d6)
d: 11.21 (s, 1H, eNH), 11.05
(s, 1H, eNH), 7.69 (s, 1H, ephH), 7.47 (d, 1H, J¼7.2 Hz, ephH), 7.08
(d, 1H, J¼7.2 Hz, ephH), 2.32 (s, 3H, eCH3); 13C NMR (DMSO-d6,
100 MHz) d: 162.8, 150.3, 138.7, 135.9, 131.5, 126.5, 115.3, 114.2, 20.2;
Anal. Calcd: C, 61.36; H, 4.58; N, 15.90; found: C, 61.11; H, 4.74; N,
4. Experimental part
16.07; GCT-MS m/z (%): 176.0 (Mþ, 100%).
For 2d: 1H NMR (400 MHz, DMSO-d6)
d: 11.21 (s, 1H, eNH), 11.10
4.1. Materials and methods
(s, 1H, eNH), 8.00 (d, 1H, J¼7.6 Hz, ephH), 7.71 (t, 1H, J¼7.6 and
7.6 Hz, ephH), 7.32 (d, 1H, J¼8.4 Hz, ephH), 7.30 (t, 1H, J¼7.6 and
Starting materials are all commercially available reagents and
solvents used as received except for statements. Tetrahydrofuran
(THF) was distilled on sodium. All solvents were purified using the
standard procedures. Reactions were monitored by thin-layer
chromatography (TLC) on pre-coated silica gel plates (Yantai Shi
Huagxue Gongye Yanjiusuo) and visualized using UV irradiation
(254 or 365 nm). Flash chromatography was performed on silica gel
H60 (Qingdao Haiyang Huagongchang).
7.5 Hz, ephH); 13C NMR (DMSO-d6, 100 MHz)
d: 162.6, 150.1, 138.6,
135.8, 131.4, 126.5, 115.2, 114.1; Anal. Calcd: C, 59.26; H, 3.73; N,
17.28; Found: C, 59.17; H, 3.74; N, 17.07; GCT-MS m/z (%): 162.0 (Mþ,
100%).
For 2e: 1H NMR (400 MHz, DMSO-d6)
d: 11.44 (s, 1H, eNH), 11.27
(s, 1H, eNH), 7.82 (s, 1H, ephH), 7.70 (d, 1H, J¼8.4 Hz, ephH), 7.19
(d, 1H, J¼8.4 Hz, ephH); 13C NMR (DMSO-d6, 100 MHz)
d: 161.8,
150.1, 139.7, 134.8, 126.3, 115.9, 117.5, 115.8; Anal. Calcd: C, 48.88; H,
2.56; N, 14.25; Found: C, 48.97; H, 2.70; N, 14.27; GCT-MS m/z (%):
196.0 (Mþ, 100%).
1H and 13C NMR spectra were recorded on a Bruker AVANCE 400
or AVANCE 600 spectrometer and referenced to solvent signals.

P. Li et al. / Tetrahedron 68 (2012) 8908e8915
8913
For 2f: 1H NMR (400 MHz, DMSO-d6)
2H, eNH), 7.60 (m, 2H, ephH), 7.21 (s, 1H, ephH); 13C NMR
(DMSO-d6, 100 MHz) : 162.1, 158.5, 156.1, 150.1, 137.5, 123.0,
d
: 11.29 (s, broad,
114.9, 112.3, 44.1; Anal. Calcd: C, 50.43; H, 2.96; N, 11.76; found: C,
50.25; H, 3.09; N, 11.58; ESI-MS m/z (%): 237.0 (Mꢁ, eHþ, 100%).
d
For 3g: 1H NMR (400 MHz, DMSO-d6)
d: 13.27 (s, 1H, eCOOH),
122.8, 117.6, 117.5, 115.4, 115.3, 112.1, 111.8; Anal. Calcd: C, 53.34;
H, 2.80; N, 15.55; found: C, 53.17; H, 2.74; N, 15.37; GCT-MS m/z
(%): 180.0 (Mþ, 100%).
11.98 (s, 1H, eNH), 8.60 (d, 1H, J¼8.0 Hz, epyH), 8.27 (d, 1H,
J¼8.0 Hz, epyH), 7.23 (q, 1H, J¼6.4, 6.4 and 6.8 Hz, epyH), 4.68 (s,
2H, eCH2); 13C NMR (DMSO-d6, 100 MHz)
d: 170.5, 161.6, 154.4,
151.9,151.3,136.7,118.4,109.5, 44.3; Anal. Calcd: C, 48.87; H, 3.19; N,
19.00; found: C, 48.68; H, 3.07; N, 19.14; ESI-MS m/z (%): 220.0
(Mꢁ, eHþ, 100%).
For 2g: 1H NMR (400 MHz, DMSO-d6)
d
: 11.64 (s, 1H, eNH), 11.48
(s, 1H, eNH), 8.60 (d, 1H, J¼8.0 Hz, epyH), 8.26 (d, 1H,
J¼8.0 Hz, epyH), 7.26 (q, 1H, J¼4.8 and 6.8 Hz, epyH); 13C NMR
(DMSO-d6, 100 MHz) d: 162.2, 154.8, 152.4, 150.4, 136.7, 118.9, 109.9;
Anal. Calcd: C, 51.54; H, 3.09; N, 25.76; found: C, 51.37; H, 2.99; N,
4.3.3. Synthesis of the PNA monomers containing aromatic fused
pyrimidine-2,4(3H)-diones. Briefly, ethyl N-[2-Boc-aminoethyl]gly-
cinate (7, 1.0 mmol), DhbtOH (3,4-dihydro-3-hydroxy-4-oxo-1,2,3-
benzotriazine, 1.1 mmol), and aromatic fused pyrimidine-
2,4(13H)-dione-1-yl acetic acid (3, 1.1 mmol) were dissolved in 1:1
mixed DMF and DCM (7.5 mL). After the solution was cooled to 0 ꢀC,
DCC (6.6 mmol) was added and the reaction was allowed to react
for 3 h at this temperature. After removal of DCU by filtration, the
organic layer was washed with dilute aqueous NaHCO3 and brine
aqueous solution, respectively. After the solvent was removed un-
der vacuum, the resultant oil residue was dissolved in DCM (3 mL)
and the precipitate was filtered out. The product was precipitated
by the addition of petroleum ether (3 mL) at 0 ꢀC. The precipitate
was re-dissolved in DCM and precipitated once more with petro-
leum ether, affording 8. The yield is 60e80%.
25.67; GCT-MS m/z (%): 163.0 (Mþ, 100%).
4.3.2. Synthesis of the aromatic fused pyrimidine-2,4(3H)-dione-1-yl
acetic acid. The typical procedure is as follows: 2 (10 mmol) and
LiOH (25 mmol) was dissolved into DMSO (60 mL) and stirred at
40 ꢀC for 2 h, to which a 5 mL DMSO solution of BrCH2COOH
(10 mmol) was dropwise added inside within 5 min. The reaction
was continued at 40 ꢀC for 12 h. Then it was poured into 500 mL
ethyl acetate. The resultant white or light yellow precipitates were
collected and re-dissolved into 100 mL water. The water solution
was adjusted to pH¼6 using 4 M hydrochloric acid and then was
put at 3e6 ꢀC for 2 h. The precipitates (2) were then filtered out and
the solution was again adjusted to pH¼2 and put at 3e6 ꢀC for
another 2 h. The product 3 was filtered out in a typical yield shown
in Tables 1 and 2, respectively. The yield for each compound of 3aeg
was listed in Table 2.
Compound 8 (1.0 mmol) was suspended in THF (5 mL) and 1 M
LiOH (aqueous, 5 mL) was added. The mixture was stirred for
45 min at room temperature and the white precipitate of DCU was
filtered out. The filtrate was then extracted with DCM and water for
several times until no white precipitate appeared. The water layer
was collected and cooled to 0 ꢀC and its pH was adjusted to 2 by the
dropwise addition of 1 M HCl. The water layer was extracted with
EtOAc. After removal of EtOAc, the residue was re-dissolved in
methanol and the desired product 1 was re-precipitated by using
cyclohexane as a colourless solid. The yield for each PNA monomer
is in the range of 70e85%.
For 3a: 1H NMR (400 MHz, DMSO-d6)
d: 13.21 (s, 1H, eCOOH),
11.76 (s, 1H, eNH), 8.76 (s, 1H, enapheH), 8.15 (d, 1H,
J¼8.4 Hz, enapheH), 7.99 (d, 1H, J¼8.4 Hz, enapheH), 7.81 (s,
1H, enapheH), 7.64 (t, 1H, J¼8.0 and 8.0 Hz, enapheH), 7.52 (t, 1H,
J¼8.0 and 8.0 Hz, enapheH), 4.90 (s, 2H, eCH2); 13C NMR (DMSO-
d6, 100 MHz) d: 169.6, 161.8, 150.3, 136.7, 136.5, 129.5, 129.2, 128.2,
127.3, 125.6, 115.7, 110.5, 56.0; Anal. Calcd: C, 62.22; H, 3.73; N,
10.37; found: C, 62.42; H, 3.60; N, 10.17; ESI-MS m/z (%): 269.0
(Mꢁ, eHþ, 100%).
For 3b: 1H NMR (400 MHz, DMSO-d6)
d: 13.08 (s, 1H, eCOOH),
For 1a: 1H NMR (400 MHz, DMSO-d6)
d: 13.09 (s, 0.2H, eCOOH)
11.51 (s, 1H, eNH), 7.40 (s, 1H, epheH), 6.82 (s, 1H, epheH), 4.85 (s,
and12.61 (s, 0.8H, eCOOH),11.67 (s,1H, eNH), 8.71 (s,1H, enapheH),
8.15 (d, 1H, J¼8.4 Hz, enapheH), 7.95 (d, 1H, J¼8.4 Hz, enapheH),
7.63 (t, 1H, J¼8.0 Hz, enapheH), 7.59 (s, 1H, enapheH), 7.48 (t, 1H,
J¼8.0 Hz, enapheH), 6.93 (s, 0.8H, eCOOH) and 6.74 (s, 0.2H,
eCOOH), 4.90 (s, 1.2H, eCH2) and 4.70 (s, 0.8H, eCH2), 4.28 (s, 0.8H,
eCH2) and 3.97 (s, 1.2H, eCH2), 3.47 (t,1.2H, J¼6.0 and 6.4 Hz, eCH2)
and 3.32 (d, 0.8H, J¼6.4 and 6.4 Hz, eCH2), 3.20 (d, 1.2H, J¼6.4 Hz,
2H, eCH2), 3.88 (s, 3H, eCH3), 3.81 (s, 3H, eCH3); 13C NMR (DMSO-
d6, 100 MHz) d: 169.8, 161.2, 155.0, 150.5, 145.0, 136.9, 108.0, 107.4,
98.0, 56.4, 55.8, 44.0. Anal. Calcd: C, 51.43; H, 4.32; N, 10.00; found:
C, 51.23; H, 4.42; N, 10.20; ESI-MS m/z (%): 279.1 (Mꢁ, eHþ, 100%).
For 3c: 1H NMR (400 MHz, DMSO-d6)
d: 13.16 (s, 1H, eCOOH),
11.64 (s, 1H, eNH), 7.83 (s, 1H, ephH), 7.56 (d, 1H, J¼8.2 Hz, ephH),
7.25 (d, 1H, J¼8.4 Hz, ephH), 4.79 (s, 2H, eCH2), 2.36 (s, 3H, eCH3);
eCH2) and 3.02 (d, 0.8H, J¼6.4 Hz, eCH2), 1.37 (s, 9H, eBoceH); 13
C
13C NMR (DMSO-d6, 100 MHz)
d
: 170.2, 162.4, 150.7, 139.5, 136.7,
NMR (DMSO-d6, 100 MHz) d: 170.9, 170.6, 166.8, 166.5, 161.7, 155.7,
132.5, 127.5, 115.9, 115.0, 44.4, 20.4; Anal. Calcd: C, 56.41; H, 4.30; N,
11.96; found: C, 56.21; H, 4.35; N, 12.06; ESI-MS m/z (%): 233.1
(Mꢁ, eHþ, 100%).
149.8,141.3,136.4,135.0,129.6,129.4,128.5,126.8,125.0,114.3,110.2,
78.1, 77.1, 49.0, 47.4, 44.8, 44.4, 38.1, 37.7, 28.2; Anal. Calcd:C, 58.72; H,
5.57; N, 11.91; found: C, 58.48; H, 5.49; N, 12.16; TOF-MS m/z (%):
493.2 (MþNaþ, 100%).
For 3d: 1H NMR (400 MHz, DMSO-d6)
d: 13.17 (s, 1H, eCOOH),
11.71 (s, 1H, eNH), 8.03 (d, 1H, J¼7.6 Hz, ephH), 7.75 (t, 1H, J¼7.6
and 7.6 Hz, ephH), 7.35 (d, 1H, J¼8.4 Hz, ephH), 7.29 (t, 1H, J¼7.6
and 7.5 Hz, ephH), 4.81 (s, 2H, eCH2); 13C NMR (DMSO-d6,
For 1b: 1H NMR (400 MHz, DMSO-d6)
d: 13.05 (s, 0.2H, eCOOH)
and 12.62 (s, 0.8H, eCOOH), 11.50 (s, 1H, eNH), 7.29 (s, 1H, epheH),
6.94 (s, 0.8H, eCOOH) and 6.73 (s, 0.2H, eCOOH), 6.73 (s,
1H, epheH), 4.81 (s, 1.2H, eCH2) and 4.70 (s, 0.8H, eCH2), 4.28 (s,
0.8H, eCH2) and 3.97 (s, 1.2H, eCH2), 3.88 (s, 3H, eCH3), 3.81 (s,
3H, eCH3), 3.47 (t, 1.2H, J¼6.0 and 6.4 Hz, eCH2) and 3.32 (d, 0.8H,
J¼6.4 and 6.4 Hz, eCH2), 3.20 (d, 1.2H, J¼6.4 Hz, eCH2) and 3.02 (d,
0.8H, J¼6.4 Hz, eCH2), 1.37 (s, 9H, eBoceH); 13C NMR (DMSO-d6,
100 MHz) d: 169.6, 161.7, 150.3, 141.1, 135.4, 127.6, 122.8, 115.4, 114.6,
43.7; Anal. Calcd: C, 54.55; H, 3.66; N, 12.72; found: C, 54.35; H,
3.78; N, 12.85; ESI-MS m/z (%): 219.0 (Mꢁ, eHþ, 100%).
For 3e: 1H NMR (400 MHz, DMSO-d6)
d: 13.23 (s, 1H, eCOOH),
11.89 (s, 1H, eNH), 7.95 (s, 1H, ephH), 7.79 (d, 1H, J¼8.4 Hz, ephH),
7.42 (d, 1H, J¼8.8 Hz, ephH), 4.81 (s, 2H, eCH2); 13C NMR (DMSO-
100 MHz) d: 170.9, 170.6, 167.3, 167.3, 167.0, 161.1, 155.7, 155.1, 150.0,
d6, 100 MHz)
d
: 169.4, 160.7, 150.1, 140.1, 135.0, 127.2, 126.5, 117.1,
145.2, 135.2, 107.5, 105.5, 97.6, 78.0, 77.7, 55.9, 55.7, 49.1, 47.0, 44.8,
44.4, 38.1, 37.7, 28.2; Anal. Calcd: C, 52.50; H, 5.87; N, 11.66; found:
C, 52.38; H, 5.73; N, 11.74; TOF-MS m/z (%): 503.2 (MþNaþ, 100%).
114.9, 44.0; Anal. Calcd: C, 47.17; H, 2.77; N, 11.00; found: C, 47.08;
H, 2.85; N, 11.15; ESI-MS m/z (%): 253.0 (Mꢁ, eHþ, 100%).
For 3f: 1H NMR (400 MHz, DMSO-d6)
d
: 13.22 (s, 1H, eCOOH),
For 1c: 1H NMR (400 MHz, DMSO-d6)
d: 13.07 (s, 0.2H, eCOOH)
11.91 (s, 1H, eNH), 7.85 (s, 1H, ephH), 7.85 (d, 1H, J¼8.4 Hz, ephH),
and 12.64 (s, 0.8H, eCOOH), 11.64 (s, 1H, eNH), 7.82 (s, 1H, ephH),
7.54 (d, 1H, J¼8.0 Hz, ephH), 7.24 (d, 1H, J¼8.0 Hz, ephH), 6.93 (s,
0.8H, eCOOH) and 6.74 (s, 0.2H, eCOOH), 4.80 (s, 1.2H, eCH2) and
7.43 (d, 1H, J¼8.8 Hz, ephH), 4.80 (s, 2H, eCH2); 13C NMR (DMSO-
d6, 100 MHz) d: 169.5, 160.8, 150.0, 140.2, 135.1, 127.3, 126.6, 117.2,

8914
P. Li et al. / Tetrahedron 68 (2012) 8908e8915
4.71 (s, 0.8H, eCH2), 4.29 (s, 0.8H, eCH2) and 3.98 (s, 1.2H, eCH2),
3.47 (t, 1.2H, J¼6.0 and 6.4 Hz, eCH2) and 3.33 (d, 0.8H, J¼6.4 and
6.4 Hz, eCH2), 3.21 (d, 1.2H, J¼6.4 Hz, eCH2) and 3.02 (d, 0.8H,
J¼6.4 Hz, eCH2), 2.38 (s, 3H, eCH3), 1.37 (s, 9H, eBoceH); 13C NMR
Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.tet.2012.08.028. These data include MOL
files and InChIKeys of the most important compounds described in
this article.
(DMSO-d6, 100 MHz) d: 170.9, 170.7, 167.2, 166.8, 161.7, 155.8, 149.9,
137.4, 136.2, 131.9, 126.8, 115.2, 113.4, 78.1, 77.1, 49.1, 46.7, 44.8, 44.4,
38.1, 37.7, 28.2, 20.3; Anal. Calcd: C, 55.29; H, 6.03; N, 12.90; found:
C, 55.36; H, 6.07; N, 12.80; TOF-MS m/z (%): 457.3 (MþNaþ, 100%).
References and notes
For 1d: 1H NMR (400 MHz, DMSO-d6)
d: 13.06 (s, 0.2H, eCOOH)
and 12.63 (s, 0.8H, eCOOH), 11.64 (s, 1H, eNH), 8.03 (d, 1H,
J¼7.6 Hz, ephH), 7.75 (t, 1H, J¼7.6 and 7.6 Hz, ephH), 7.30 (m, 2H,
J¼8.4, 7.6 and 7.5 Hz, ephH), 6.93 (s, 0.8H, eCOOH) and 6.74 (s,
0.2H, eCOOH), 4.81 (s, 1.2H, eCH2) and 4.71 (s, 0.8H, eCH2), 4.29 (s,
0.8H, eCH2) and 3.98 (s, 1.2H, eCH2), 3.47 (t, 1.2H, J¼6.0 and
6.4 Hz, eCH2) and 3.34 (d, 0.8H, J¼6.4 and 6.4 Hz, eCH2), 3.20 (d,
1.2H, J¼6.4 Hz, eCH2) and 3.02 (d, 0.8H, J¼6.4 Hz, eCH2), 1.37 (s,
1. Nielsen, P. E. Peptide Nucleic Acids, Protocols and Applications, 2nd ed.; Horizon
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3. (a) Egholm, M.; Buchardt, O.; Christensen, L.; Behrens, C.; Freier, S. M.; Driver,
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9H, eBoceH); 13C NMR (DMSO-d6, 100 MHz)
d: 170.9, 170.6, 167.1,
166.8, 161.6, 155.7, 149.9, 139.4, 135.2, 127.4, 122.6, 115.2, 113.5, 78.0,
77.7, 49.0, 46.9, 44.8, 44.4, 38.1, 37.7, 28.2; Anal. Calcd: C, 54.28; H,
5.75; N, 13.33;found: C, 54.43; H, 5.70; N, 13.43; TOF-MS m/z (%):
443.2 (MþNaþ, 100%).
4. (a) Bentin, T.; Nielsen, P. E. J. Am. Chem. Soc. 2003, 125, 6378e6379; (b) Griffith,
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Zengeya, T.; Rozners, E. J. Am. Chem. Soc. 2010, 132, 8676e8681.
For 1e: 1H NMR (400 MHz, DMSO-d6)
d
:
13.04 (s,
0.2H, eCOOH) and 12.62 (s, 0.8H, eCOOH), 11.67 (s, 1H, eNH), 7.93
(s, 1H, ephH), 7.82 (d, 1H, J¼8.4 Hz, ephH), 7.46 (d, 1H,
J¼8.8 Hz, ephH), 6.93 (s, 0.8H, eCOOH) and 6.74 (s,
0.2H, eCOOH), 4.81 (s, 1.2H, eCH2) and 4.70 (s, 0.8H, eCH2), 4.28
(s, 0.8H, eCH2) and 3.97 (s, 1.2H, eCH2), 3.47 (t, 1.2H, J¼6.0 and
6.4 Hz, eCH2) and 3.32 (d, 0.8H, J¼6.4 and 6.4 Hz, eCH2), 3.20 (d,
1.2H, J¼6.4 Hz, eCH2) and 3.02 (d, 0.8H, J¼6.4 Hz, eCH2), 1.37 (s,
5. (a) Marin, V. L.; Armitage, B. A. J. Am. Chem. Soc. 2005, 127, 8032e8033; (b)
Petraccone, L.; Pagano, B.; Esposito, V.; Randazzo, A.; Piccialli, G.; Barone, G.;
Mattia, C. A.; Giancola, C. J. Am. Chem. Soc. 2005, 127, 16215e16223.
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Nature 1996, 379, 214; (b) Shiraishi, T.; Pankratova, S.; Nielsen, P. E. Chem. Biol.
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cellor, M.; Huang, L. Mol. Pharmacol. 2006, 3, 398e406; (d) Janowski, B. A.;
Kaihatsu, K.; Huffman, K. E.; Schwartz, J. C.; Ram, R.; Hardy, D.; Mendelson, C.
R.; Corey, D. R. Nat. Chem. Biol. 2005, 1, 210e215; (e) Knudsen, H.; Nielsen, P. E.
Nucleic Acids Res. 1996, 24, 494e500; (f) Pieck, J. C.; Kuch, D.; Grolle, F.; Linne,
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Koppelhus, U. Bioconjugate Chem. 2006, 17, 750e758; (c) Dragulescu-Andrasi, A.;
Rapireddy, S.; He, G.; Bhattacharya, B.; Hyldig-Nielsen, J. J.; Zon, G.; Ly, D. H. J. Am.
Chem. Soc. 2006, 128, 16104e16112.
9H, eBoceH); 13C NMR (DMSO-d6, 100 MHz)
d: 170.8, 170.5, 166.9,
166.5, 160.6, 155.7, 149.6, 138.4, 135.2, 126.6, 126.3, 117.5, 114.9,
78.0, 77.7, 48.9, 46.9, 44.7, 44.2, 38.1, 37.7, 28.2; Anal. Calcd: C,
50.17; H, 5.10; N, 12.32; found: C, 50.23; H, 5.00; N, 12.32; TOF-MS
m/z (%): 467.3 (MþNaþ, 100%).
For 1f: 1H NMR (400 MHz, DMSO-d6)
d
:
13.07 (s,
8. (a) Lusvarghi, S.; Murphy, C. T.; Roy, S.; Tanious, F. A.; Sacui, I.; Wilson, W. D.; Ly,
D. H.; Armitage, B. A. J. Am. Chem. Soc. 2009, 131, 18415e18424; (b) Katritzky, A.
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0.2H, eCOOH) and 12.64 (s, 0.8H, eCOOH), 11.66 (d, 1H, eNH),
7.64 (m, 2H, J¼8.8 and 8.4 Hz, epheH), 7.27 (q, 1H, J¼4.0, 4.0,
4.4 Hz, epheH), 6.94 (s, 0.8H, eCOOH) and 6.74 (s,
0.2H, eCOOH), 4.81 (s, 1.0H, eCH2) and 4.70 (s, 1.0H, eCH2), 4.28
(s, 0.8H, eCH2) and 3.97 (s, 1.2H, eCH2), 3.47 (t, 1.2H, J¼6.0 and
6.4 Hz, eCH2) and 3.32 (d, 0.8H, J¼6.4 and 6.4 Hz, eCH2), 3.20 (d,
1.2H, J¼6.4 Hz, eCH2) and 3.02 (d, 0.8H, J¼6.4 Hz, eCH2), 1.37 (s,
9H, eBoceH); 13C NMR (DMSO-d6, 100 MHz)
d: 170.7, 170.6,
166.9, 166.6, 160.5, 155.7, 149.6, 138.5, 135.3, 126.6, 126.4, 117.6,
115.0, 78.1, 77.7, 49.0, 46.8, 44.7, 44.2, 38.1, 37.7, 28.2; Anal. Calcd:
C, 52.05; H, 5.29; N, 12.78; found: C, 52.16; H, 5.22; N, 12.89; TOF-
MS m/z (%): 461.3 (MþNaþ, 100%).
For 1g: 1H NMR (400 MHz, DMSO-d6)
d
:
13.06 (s,
11. (a) Abdel-Razik, H. H. J. Chin. Chem. Soc. 2005, 52, 141e148; (b) Nikpour, F.;
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0.2H, eCOOH) and 12.63 (s, 0.8H, eCOOH), 12.07 (d, 1H, eNH),
8.66 (d, 1H, J¼4.8 Hz, epyeH), 8.31 (d, 1H, J¼8.0 Hz, epyeH),
7.32 (q, 1H, J¼5.6, 7.2, 6.8 Hz, epyeH), 6.96e6.70 (s, broad,
1H, eCOOH), 4.81 (s, 1.0H, eCH2) and 4.68 (s, 1.0H, eCH2), 4.27
(s, 0.8H, eCH2) and 3.96 (s, 1.2H, eCH2), 3.49 (t, 1.2H, J¼6.4 and
6.4 Hz, eCH2), 3.32 (t, 0.8H, J¼6.4 and 6.4 Hz, eCH2), 3.20 (d,
1.2H, J¼6.4 Hz, eCH2), 3.02 (d, 0.8H, J¼6.4 Hz, eCH2), 1.37 (s,
ꢁ
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17. (a) Zhang, X.; Pang, S. F.; Zhang, Z. G.; Ding, X. L.; Zhang, S. L.; He, S. G.; Zhan, C.
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9H, eBoceH); 13C NMR (DMSO-d6, 100 MHz)
d: 171.7, 171.5,
166.9, 166.3, 161.3, 155.7, 155.0, 151.0, 150.1, 136.9, 119.3, 109.0,
78.0, 77.6, 51.0, 48.1, 44.8, 44.3, 38.1, 37.7, 28.6; Anal. Calcd: C,
51.30; H, 5.50; N, 16.62; found: C, 51.13; H, 5.58; N, 16.42; TOF-
MS m/z (%): 444.2 (MþNaþ, 100%).
Acknowledgements
This work was financially supported by NSFC (Nos. 20973182
and 21173233), the Chinese Academy of Sciences, Projects 973
(2011CB808400) and 863 (2009AA03Z323).

P. Li et al. / Tetrahedron 68 (2012) 8908e8915
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