Oxidation of H-phosphonates with iodine by intramolecular support of a 2-pyridyl thermolabile protecting group

Article abstract of DOI:10.1021/jo300937k

Acceleration of H-phosphonate diester oxidation with iodine accompanied by a thermolabile protecting group (TPG) is presented. It is shown that the intermediate product of this reaction is an oxazaphospholidine oxide which forms a phosphate diester only w

Full text of DOI:10.1021/jo300937k

Article
pubs.acs.org/joc
Oxidation of H‑Phosphonates with Iodine by Intramolecular Support
of a 2‑Pyridyl Thermolabile Protecting Group
Tomasz Ratajczak and Marcin K. Chmielewski*
́
Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland
S
* Supporting Information
ABSTRACT: Acceleration of H-phosphonate diester oxidation with iodine accompanied by a thermolabile protecting group
(TPG) is presented. It is shown that the intermediate product of this reaction is an oxazaphospholidine oxide which forms a
phosphate diester only when a 2-pyridyl TPG is applied. The intermediate product is formed with exocyclic nitrogen. The
absolute configurations of phosphorothioate diesters, H-phosphonate diesters, and oxazaphospholidine oxides were determined.
31P NMR spectroscopy was used to evaluate the relationship between chemical shift and absolute configuration at the
phosphorus center of H-phosphonate diesters and oxazaphospholidine oxides.
To enhance their stability on a phosphate center, a “click-
clack” approach has been applied.20 The approach increases 2-
pyridyl TPG stability by temporarily forming a five-membered
oxazaphospholidine ring. A linear form of the TPG may be
easily recovered by acid hydrolysis, during which also an H-
phosphonate diester is formed (Figure 1). The removal of a
TPG is now very simple in the thermocyclization process and
takes about 10 min at 90 °C to form an H-phosphonate
monoester.
INTRODUCTION
■
Protecting groups used in organic chemistry more and more
frequently find multifunctional applications. Apart from
protecting reactive sites, they may function as fluorophores,1
as markers for monitoring the reaction progress2,3 and changes
in solubility, or as influencing molecule conformation changes.4
Moreover, the protecting groups can affect the stereospecific
character of chemical reactions. What is significant in nucleic
acid chemistry is the stereospecific reactions on the phosphorus
center to produce phosphorothioates.5 It has been found
recently that five-membered phospholidine rings are useful in
controling this stereospecificity of the phosphorus center.6,7
Another tendency in developing protecting groups is
applying mild and simple physicochemical processes, such as
oxidation,8 protonation,9 reduction,10,11 photosensitivity,12 and
temperature change. The last process has recently led to the
introduction of thermolabile protecting groups (TPGs) in
order to enhance effective protection of a phosphate,13,14
hydroxyl,15 or amine center.16 Removal of these groups is based
on intramolecular cyclization depending on temperature.14
Examples of TPGs presented in the literature show that
introducing a 2-pyridyl moiety in a TPG significantly
accelerates thermocyclization at temperatures over 50 °C15
but at the same time increases its stability at ambient
temperature.17 So far these groups have been applied to
protect a phosphate or hydroxyl center.18
Figure 1. Strategy of the “click-clack” approach: closing and opening
the oxazaphospholidine ring.
However, H-phosphonate diesters also are easily oxidized to
the corresponding phosphates by iodine in the presence of a
base21,22 such as pyridine or lutidine25 or an acid23 but acid-
catalyzed oxidation is complicated by some diester hydrolysis.21
Oxidation of an H-phosphonate diester in the presence of a
base such as pyridine involves a multistep mechanism: first, a
phosphoroiodidate derivative is formed quickly24 and then the
transformation of phosphoroiodidate into a pyridinium
cation25,26 that is easily hydrolyzed is postulated.
2-Pyridyl TPGs are characterized by various thermocycliza-
tion times depending on their structure, and the times are
shorter when the same structure is applied to protect
phosphate, as compared to hydroxyl. However, it has been
found that phosphate triesters with some TPGs are very
unstable.19
Received: May 21, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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a
Scheme 1. Conversion of Cyclic Rings in the Linear Form
a
Legend: (i) benzylthiotetrazole (BTT)/acetonitrile; (ii) I2/acetonirile; (iii) H2O.
applied. Decreasing the nucleophilicity slows down the
oxidation (Table 1).
RESULTS AND DISCUSSION
■
The present paper shows the possibility of oxidizing H-
phosphonate diesters which contain a 2-pyridyl TPG to
phosphate by using only an iodine solution. In this case we
are taking advantage of the nucleophilic and basic properties of
the TPG to perform intramolecular catalysis of the oxidation
reaction. We have also shown that this oxidation mechanism
forms an intermediate product (oxazaphospholidine oxide)
which can be easily opened by means of water when a 2-pyridyl
TPG is used (Scheme 1).
On the basis of these results we have also postulated that the
2-pyridyl moiety from a TPG catalyzes oxidation with the
iodine solution. It has been observed that a cyclic intermediate
product is formed in oxidation only when the exocyclic
nitrogen atom is present (3, 5).28 Our study shows that the 2-
pyridyl TPG, and particularly the exocyclic nitrogen atom, is
involved in a five-membered ring with a phosphate center after
substituting the hydrogen atom by iodine. The oxazaphospho-
lidine oxide 9 formed from the 2-pyridyl TPG opens easily in
the presence of water. The rate of this process is about 1 order
of magnitude faster than intramolecular ring closing. The
formation of the oxazaphospholidine ring in the intermediate
product is proof of the catalytic character of oxidation with
iodine. In order to study the mechanism further, we have used
model compound 5, in which the pyridyl ring is replaced by
phenyl. This mechanism is similar to that for 3, and the formed
oxazaphospholidine oxide 10 does not open as easily as does 9
under the same conditions. The oxazaphospholidine oxide 10 is
stable,29 and its opening requires conditions such as strong
bases or acids.30 Thus, the intermediate product 10 is isolated
and characterized by NMR analysis. Oxidation of the mixture of
2 (oxazaphospholidine ring) and 5 (H-phosphonate diester)
using iodine and pyridine gives the same product: oxazaphos-
pholidine oxide 10 (see Figure 3).
We have observed that adding iodine in acetonitrile to the
present H-phosphonate diesters (see Figure 2) accelerates
Figure 2. Structure of H-phosphonate diesters used in oxidation
without pyridine.
These results confirm that the presence of the exocyclic
nitrogen atom supports the formation of a five-membered ring.
However, in 4 (only with the pyridyl ring) the formation of an
intermediate cyclic ring is not observed in oxidation because it
is limited by two factors:
oxidation from 160 min for 5 to 15 min for 3. However,
applying the same reaction conditions in the case of 6 (without
nitrogen atoms) gives a phosphate diester within 5 h.27
Differences in oxidation times (Table 1) depend on the
nucleophility of the nitrogens and their distance from the
phosphate center. The fastest oxidation process (15 min)
occurs in the case of H-phosphonate 3 when a 2-pyridyl TPG is
(i) The only nitrogen atom of the pyridyl ring is too distant
from the phosphate center (a seven-membered ring must
be formed). However, shortening the distance will cause
conformation restrictions in a potential cyclic ring.
(ii) The pKa of pyridine is lower than that of 2-amino-
pyridine.31
Table 1. 31P NMR Analysis of H-Phosphonate Diesters
under Oxidation Conditions without an Additional Base
It has been observed that the oxidation process accelerates
when the distance between pyridine and phosphate is smaller.
Reducing this distance (7, 8) shortens the time of total
oxidation (Table 1), but it is not substantial enough to indicate
that it follows the intramolecular process. What we are probably
observing here is an intramolecular interaction or increasing
electron influence caused by pyridine. Even if the chain is only
one carbon atom 8, the acceleration is not as rapid as in 3.
3
4
5
6
7
8
a
a
time required for complete
conversn to phosphate
diester, min
<15
150
160
300
90
50
a
The reaction stops in the form of phospholidine oxide (10).
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Figure 3. Application of the traditional oxidation method (iodine/pyridine) to produce an oxazaphospholidine oxide ring and the structure
confirmation thereof. Oxidation was performed on the mixture of 2 and 5. The upper 31P NMR profile presents a mixture of 2 and 5, and the lower
31P NMR profile presents the product after oxidation with a traditional iodine/pyridine solution.
Scheme 2. Assignment of Configuration on the Basis of 31P
NMR Spectra
While studying the phosphate cyclic system mechanisms, it is
a
important to assign absolute configuration to the H-
phosphonate center. It is known that transforming H-
phosphonate diesters into phosphorothioate diesters is a
stereospecific reaction and occurs with retention of config-
uration. Thanks to this property as well as the fact that
phosphodiesterase from snake venom stereospecifically digests
only one diester (isomer RP in the case of 5′-O-phosphor-
othioate-O-p-nitrophenyl ester32 or phosphorothioate dinucleo-
sides33), an absolute configuration may be assigned to the
phosphate center. The transformation of H-phosphonate
diesters into phosphorothioate diesters and the result of the
digestion thereof (after removing the 3′-acetyl group) have
been observed by 31P NMR. As shown in Figure 4, one P-
a
Transformations were performed using the 3′-acetylated derivative of
deoxythymidine.
Analyzing the transformation of minor and major thiophos-
phate isomers, the conformation of H-phosphonate diesters 5
can be defined as RP (D ) configuration of the major isomer at
P
11.11 ppm and SP (L ) configuration of the minor isomer at
P
10.51 ppm (Scheme 3).
The correlation method has been also used to assign the
configuration in oxazaphospholidine oxide derivatives 10 (SP
(L ) minor isomer 14.12 ppm and RP (D ) major isomer 13.89
P
P
Figure 4. 31P NMR analysis of thiophosphate 13 digestion by snake
venom phosphodiesterase from Crotalus atrox (EC 3.1.4.1). To
increase the solubility in buffer, the 3′-acetyl group has been removed.
A shift of mutual positions of the major and minor isomers in relation
to 12 is observed.
ppm).
CONCLUSION
■
The paper presents the application of a 2-pyridyl TPG which
intramolecularly catalyzes oxidation of H-phosphonate diesters
with iodine only. We have shown that the nitrogen atom from
the TPG may substitute iodine, forming an oxazaphospholidine
oxide ring. However, forming a phosphate diester in this
reaction is possible only when a 2-pyridyl TPG is applied.
Moreover, the process is very fast and is quantitatively complete
within 15 min. This method is an example of the multifunc-
tional character of TPGs where deprotection occurs simulta-
neously with oxidation. It may be applied in subtle trans-
formations where in one molecule of e.g. synthesized nucleic
acid only one phosphate center is oxidized. Replacing the 2-
pyridyl moiety from a TPG with an aryl-alkyl moiety practically
slows down the oxidation of H-phosphonate diesters using only
iodine. Still, applying only pyridine linked by an alkyl chain
does not accelerate the reaction as much as using a 2-pyridyl
moiety. The absolute configurations assigned to H-phospho-
nate diesters and cyclic oxazaphospholidines will facilitate
further research when this method might be used in
stereospecific transformations of phosphate centers.
diastereoisomer (55.6 ppm, major isomer) has been digested by
enzyme in a buffer solution. However, in this case, the absolute
configuration of the digested diester changes from RP to SP
because of another priority of substitutes in the Cahn−Ingold−
Prelog convention. Because sometimes there are such
discrepancies between absolute configurations in P-nucleotide
analogues and physicochemical or biological properties, the
precise names of phosphorus configurations designated as D
P
and L have been introduced recently.34 Thus, for the sake of
P
clarity, the absolute configuration in phosphorothioate diesters
12 has been assigned: hence, the SP (D ) configuration of the
P
major isomer to the signal at 59.99 ppm and the RP (L )
P
configuration of the minor isomer at 60.24 ppm. As shown in
Scheme 2, the H-phosphonate diesters are disproportional,
which helps to determine the absolute configuration in
subsequent transformations.
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mL/min for 20 min, and then the concentration is increased to 10%
MeCN/min and then held isocratically for 2 min.
Scheme 3. Assignment of the Absolute Configuration of the
Phosphorus Center on the Basis of Enzymatic Digestion
Preparation of 3′-O-Acetyl-5′-[3-pyridyl[1,3,2]oxaza-
phospholidine]thymidine (1). 3′-Acetylthymidine (177 mg, 047
mmol) and 2-N-diisopropyl-3-pyridyl[1,3,2]oxazaphospholidine (0.10
g, 0.47 mmol) were dissolved in 2 mL of dry acetonitrile (MeCN).
Then a solution of benzylthiotetrazole (BTT) in dry MeCN (2.37 mL,
0.2 M, 0.47 mmol) was added dropwise. After 2 h the reaction was
complete and 1 mL of diisoproylamine was added. The mixture was
evaporated, and the residue was dissolved in dichloromethane (DCM)
and extracted three times with a saturated solution of NaHCO3. The
organic layer was dried (anhydrous Na2SO4) and subsequently
evaporated to dryness. From 291 mg of the solid reaction product
145 mg (part A) was used for further purification. Part A was then
placed on a PLC plate and developed in dichloromethane (DCM)/
methanol (MeOH)/triethylamine (89/6/5). Pure product 1 was
obtained (23 mg, 0.051 mmol); yield 21.7%.
1H NMR (400 MHz, CDCl3): δ 8.9 (d, J = 4.95 Hz, 1H); 7.62−
7.58 (m, 2H); 7.34 (s, 1H); 6.80−6.77 (m, 1H); 6.20−6.19 (m, 1H);
5.30−5.28 (m, 1H); 4.57−4.48 (m, 2H); 4.14−4.07 (m, 1H); 4.01−
3.97 (m, 1H); 3.59−3.52 (m, 2H); 3.47−3.39 (m, 1H); 2.29−2.26 (m,
1H); 2.20−2.18 (m, 1H); 2.01 (s, 3H); 1.78 (d, J = 1.12 Hz, 3H). 13C
NMR (75 MHz, CD3CN): δ 170.6; 163.0; 156.2; 150.6; 138.3; 128.2;
115.4; 110.4; 107.6; 84.3; 83.6; 74.8; 69.3; 63.8; 45.8; 43.5; 36.9; 20.2;
11.6. 31P NMR: δ (ppm) 129.8 (m, 1P), 125.5 (m, 1P). HRMS (ESI
+): calcd for C19H23N4O7PNa, 473.1304; found, 473.1216. Mp: 86−88
°C.
Preparation of 3′-O-Acetyl-5′-[3-phenyl-[1,3,2]oxaza-
phospholidine]thymidine (2). A 300 mg portion of 2-N-
diisopropylo-3-phenyl[1,3,2]oxazaphospholidine (1.13 mmol) and
500 mg of 3′-O-acetylthymidine (1.34 mmol) were dissolved in 5
mL of MeCN. Then a solution of BTT in MeCN (3 mL, 0.2 M, 0.6
mmol) was added dropwise. After 2 h the reaction was complete; 1 mL
of triethylamine was added and the mixture evaporated to dryness
afterward. The product mixture was dissolved in DCM, moved to a
separating funnel, and extracted three times with saturated NaHCO3
solution. The organic layer was poured into and dried with anhydrous
Na2SO4 and evaporated. The crude product (550 mg) was
chromatographically purified with the eluent DCM/MeOH/triethyl-
amine (89/6/5). Compound 2 (0.45 mmol, 203 mg) was obtained in
80.2% yield.
EXPERIMENTAL SECTION
1H NMR (400 MHz, CDCl3): δ 7.49 (d, J =1.24 Hz, 1H); 7.20−
7.27 (q, J1 = 7.4 Hz, J2 = 16.2 Hz, 2H); 7.00 (t, J = 7.36 Hz, 2H);
6.21−6.18 (m, 1H); 5.18−5.16 (m, 1H); 4.49−4.56 (m, 1H); 4.46−
4.49 (m, 1H); 4.1−4.05 (m, 1H); 3.99−4.10 (m, 1H); 3.56 (m, 1H);
3.48 (t, J1 = 8.8 Hz, 1H); 2.25−2.29 (m, 1H); 2.20−2.23 (m, 1H);
2.13 (m, 1H); 2.01 (s, 3H); 1.84 (d, J = 1.24 Hz, 3H). 13C NMR (75
MHz, DMSO-d6): δ 170.3; 163.4; 150.4; 143.8; 135.5.6; 129.3; 128.3;
120.0; 115.0; 110.5; 84.0; 83.3; 74.4; 69.3; 63.3; 44.8; 36.5; 20.1; 11.6.
31P NMR: δ (ppm) 127.72, 124.47. HRMS (ESI): calcd for
■
General Methods. All reagents (analytical grade) were obtained
from commercial suppliers and used without further purification.
Anhydrous benzene, pyridine, and CH2Cl2 were freshly distilled from
CaH2 and P2O5, respectively, and were kept over activated 4 Å
molecular sieves. All other anhydrous solvents and liquid reagents were
dried through storage over activated 3 Å (2-pyridinepropanol, 2-
pyridineethanol, 2-pyridinemethanol, MeCN) molecular sieves. The
progress of the reactions was monitored by thin-layer chromatography
and 31P NMR spectra, while purification was effected by column
chromatography using silica gel (60−120 mesh).
C21H28N3O8P, 482.1352; found, 482.1714. Mp: 68−70 °C.
Preparation of 3′-O-Acetylthymidine-5-yl N-(2-Pyridyl)-
aminoethyl Phosphonate (3). The thus obtained 1 (23 mg, 0.051
mmol) was dissolved in 500 μL of MeCN with 5 μL of water. A 200
μL portion of BTT in MeCN (0.2 M, 0.04 mmol) was added
dropwise, and the reaction mixture was kept in a refrigerator at 4 °C
for 1 h until complete opening of the oxazaphospholidine ring
(according to 31P NMR). The thus prepared product 3 was ready
without further purification for the oxidation reaction. The residue
(146 mg, part B) of the crude product was dissolved in MeCN. A 200
μL portion of BTT solution (0.2 M, 0.04 mmol) and 50 μL of water
were added. After 1 h the reaction was complete (according to 31P
NMR) and the reaction mixture was dried and purified on a PLC plate
with the eluent DCM/MeOH (9/1) at 4 °C. A 20 mg amount of 3
was obtained (21% yield).
1
1
1H, 13C NMR, H−1H COSY, H−13C HSQC, Mass Spectrom-
eter Parameters. The NMR spectra were recorded at 298 K on a 400
MHz spectrometer operating at the frequencies 400 MHz (1H) and
100 MHz (13C). 31P NMR spectra were recorded on 300 MHz
spectrometer operating at the frequency 121 MHz with 5% H3PO4 in
D2O as an external reference and on a 500 MHz spectrometer
operating at the frequency 202.5 MHz with 100% D2O as an external
reference. The MicroTofQ mass spectrometer was equipped with
electrospray ionization (ESI) sources. Its source parameters are as
follows: ESI source voltage of 3.2 kV, nebulization with nitrogen at 0.4
bar, dry gas flow of 4.0 L/min at temperature 220 °C. The instrument
operated under EsquireControl version 5.1, and data were analyzed
using the Data Analysis version 3.1 package.
1H NMR (400 MHz, CDCl3): δ 8.76 (s, 1H); 7.50 (d, J = 16.7 Hz,
1H); 7.22−7.33 (m, 2H); 7.19 (t, J = 5.0 Hz, 1H); 6.32−6.36 (q, J1 =
5.3 Hz, J2 = 9.2 Hz, 1H); 5.23−5.28 (dd, J1 = 10.9 Hz, J2 = 6.6 Hz,
1H); 4.31−4.38 (m, 2H); 4.18−4.26 (m, 3H); 3.62−3.70 (m, 1H);
2.40−2.44 (m, 1H); 2.12−2.17 (m, 1H); 2.12 (s, 3H); 1.97 (d, J = 3.9
Procedure for Chromatographic Analysis. HPLC analyses
were performed using a UFLC system with LC-20AD pump and 3 μm
C(18)2 100 Å column (15 cm × 4.6 mm) according to the following
conditions: starting from 0.01 M triethylammonium acetate (pH 7.0),
a linear gradient of 2.5% MeCN/min is pumped at a flow rate of 0.9
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Hz, 3H); 1.36−1.4 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 170.7;
163.4; 150.2; 134.9; 129.8; 119.9; 111.9; 84.2; 82.6; 74.2; 64.9; 62.6;
36.9; 34.4; 29.6; 23.4; 20.9; 16.4; 12.5. 31P NMR (121 MHz,
CH3CN): δ (ppm) (RP,SP) 10.49; 9.79, (1JPH = 706.1 Hz; 3JPH = 7.55
Preparation of 3′-O-Acetylthymidine-5-yl (2-Pyridyl)ethyl
Phosphonate (7). An 85 mg amount of 3′-acetylthymidine 5′-H-
phosphonate monoester (0.19 mmol) was dissolved in 1.8 mL of
DCM and 0.2 mL of pyridine. A 24 μL portion of 2-pyridineethanol
(25.6 mg, 0.21 mmol) and 55 μL of diphenyl chlorophosphate (70.8
mg, 0.3 mmol, ρ = 1.296 g/L) were added. After 1 h the reaction was
over (according to 31P NMR). Subsequent purification was carried out
as in 4, which gave 101 mg of 7 with 68.5% yield.
3
Hz), (1JPH = 711.4 Hz; JPH = 7.98 Hz).
Preparation of 3′-O-Acetylthymidine-5-yl (2-Pyridyl)propyl
Phosphonate (4). A 93 mg amount of 3′-acetylthymidine 5′-H-
phosphonate monoester (0.21 mmol) was dissolved in 1.8 mL of
DCM and 0.2 mL of pyridine. 2-Pyridinepropanol (28.3 mg, 0.21
mmol) and 60 μL of diphenyl chlorophosphate (78 mg, 0.33 mmol, ρ
= 1.296 g/L) were added. After 1 h the reaction was over (according
to 31P NMR). The reaction mixture was moved to a separating funnel
and extracted three times with water. The organic layer was poured
and dried with anhydrous Na2SO4. The DCM was evaporated. The
thus obtained oily product was purified on preparative thin-layer
chromatography with the eluent MeCN/DCM (7/3). The solution
was evaporated and lyophilized. A 42 mg amount of 4 was obtained
(42.8% yield).
1H NMR (400 MHz, CDCl3): δ 8.53−8.52 (m, 1H); 7.61 (t, J1 =
7.58 Hz, 1H); 7.61−7.56 (m, 1H); 7.40−7.38 (dd, J =1.03 Hz, 1H);
7.13−7.18 (m, 2H); 6.32−6.29 (q, J1 = 5.46 Hz, J2 = 9.01 Hz, 1H);
5.26 (s, 1H); 5.18 (t, J1 = 6.87 Hz, 1H); 4.57−4.48 (m, 2H); 4.26−
4.19 (m, 1H); 3.17 (t, J = 6.31 Hz, 2H); 2.37−2.32 (m, 1H); 2.16−
2.08 (m, 1H); 2.06 (s, 3H); 1.84 (d, J = 8.5 Hz, 3H). 13C NMR (75
MHz, CDCl3): δ 170.9; 164.3; 157.2; 151.1; 149.7; 137.5; 135.3;
124.2; 122.4; 112.2; 84.9; 83.1; 74.6; 65.7; 65.4; 38.9; 37.4; 21.2; 12.8.
31P NMR (121 MHz, CH3CN): δ (ppm) (RP,SP) 8.99; 7.98 ppm (1JPH
3
3
= 703.0 Hz, JPH = 8.32 Hz) (1JPH = 707.29 Hz, JPH = 8.61 Hz).
HRMS (ESI): calcd for C19H24N3O8PNa, 476.1301; found, m/z
476.1196.
1H NMR (400 MHz, CDCl3): δ 8.53 (d, J1 = 4.60 Hz, 1H); 7.61 (t,
J1 = 7.58 Hz, 1H); 7.46 (d, J = 6.78 Hz, 1H); 7.35 (s, 2H); 7.12−7.17
(q, 2H); 6.33−6.40 (m, 1H); 5.22−5.27 (dd, J1 = 12.6 Hz, J2 = 6.6 Hz
1H); 4.34 (d, J = 6.8 Hz, 1H); 4.30−4.38 (m, 2H); 4.16−4.2 1(m,
3H); 2.90 (t, J = 7.57 Hz, 2H); 2.38−2.43 (m, 1H); 2.13−2.17 (m,
1H); 2.11 (s, 3H); 1.92 (d, J = 3.9 Hz, 3H). 13C NMR (75 MHz,
CDCl3): δ 170.6; 163.8; 150.6; 149.2; 136.6; 135.2; 128.2; 122.9;
121.4; 111.9; 84.4; 82.5; 74.2; 65.7; 65.0; 36.8; 33.7; 30.1; 20.9; 12.6.
31P NMR (121 MHz, CH3CN): δ (ppm) (RP,SP) 9.02; 7.99 ppm, 1JPH
Preparation of 3′-O-Acetylthymidine-5-yl (2-Pyridyl)methyl
Phosphonate (8). An 85 mg amount of 3′-acetylthymidine 5′-H-
phosphonate monoester (0.19 mmol) was dissolved in 1.8 mL of
DCM and 0.2 mL of pyridine. A 21 μL portion of 2-pyridinemethanol
(23.7 mg, 0.21 mmol) and 50 μL of diphenyl chlorophosphate (63.7
mg, 0.27 mmol, ρ = 1.296 g/L) were added. After 1 h the reaction was
complete (according to 31P NMR). Subsequent purification was
carried out as for compound 4. A 37 mg amount of compound 8 was
obtained (44.2% yield).
= 702.98 Hz; 3JPH = 8.37 Hz, 1JPH = 702.88 Hz; 3JPH = 7.77 Hz; HRMS
(ESI): calcd for C20H27N3O8P+, 468.1458; found, 468.1550.
1H NMR (400 MHz, CDCl3): δ 9.89 (s, 1H); 8.53−8.57 (m, 2H);
7.39−7.36 (dd, J = 8.2 Hz, 1H); 7.22−7.19 (m, 2H); 6.30−6.25 (m,
1H); 5.11−5.21 (q, J1 = 13.2 Hz, J2 = 24.3 Hz, 1H); 4.67 (s, 1H);
4.25−4.33 (m, 2H); 4.06−4.10 (m, 1H); 3.82 (t, J1 = 2.2 Hz, 1H);
3.72 (t, J1 = 10.9 Hz, 1H) 2.26−2.34 (m, 1H); 2.06−2.14 (m, 1H); 2.0
(d, J = 3.9 Hz, 3H); 1.78 (s, 3H). 13C NMR (75 MHz, DMSO-d6): δ
171.1; 164.5; 151.1; 150.0; 136.6; 128.7; 124.2; 121.9; 111.7; 85.9;
75.3; 64.7; 62.7; 37.7; 37.4; 21.4; 21.2; 12.9. 31P NMR (121 MHz,
Preparation of 3′-O-Acetylthymidine-5-yl (2-Phenyl)-
aminoethyl Phosphonate (5). Half of the previously obtained
crude 2 (part B; 550 mg) was dissolved in DCM with addition of
water (100 μL) and 2.9 mL of dichloroacetic acid (0.2 M, 0.5 mmol).
After 30 min the reaction was complete. The aqueous layer was poured
off while the organic layer was concentrated and purified on a
chromatographic column. The eluent was DCM/MeOH. Product 5
ran out at 4% of methanol, giving 245 mg (80.7% yield).
1H NMR (400 MHz, CDCl3): δ 8.88−8.86 (m, 1H); 7.42 (d, J =
12.4 Hz, 1H); 7.15−7.19 (m, 2H); 6.60−6.63 (q, 2H); 6.32−6.36 (q,
1H); 5.19−5.24 (dd, J1 = 12.6 Hz, J2 = 6.8 Hz, 1H); 4.34 (d, J = 6.8
Hz, 1H); 4.33 (d, J = 8.1 Hz, 1H); 4.29−4.32 (m, 2H); 4.12−4.14 (m,
1H); 3.44−3.47 (q, J1 = 5.04 Hz, J2 = 9.09 Hz, 2H) 2.33−2.40 (m,
1H); 2.13−2.18 (m, 1H); 2.10 (s, 3H); 1.93 (d, J = 3.9 Hz, 3H). 13C
NMR (75 MHz, DMSO-d6): δ 170.5; 163.4.; 150.3; 147.1; 134.9;
129.2 (d, J = 3 Hz); 118.2; 112.9; 111.8; 84.5; 82.6; 73.9; 65.2; 44.1;
37.0; 20.8; 12.5. 31P NMR (121 MHz, CH3CN): δ (ppm) (RP,SP)
11.11; 10.51 ppm, 1JPH = 719.22 Hz; 3JPH = 8.23 Hz, 1JPH = 722.0 Hz;
3JPH = 7.38 Hz. MS (ESI): calcd for C20H26N3O8PNa, 490.1458;
3
CH3CN): δ (ppm) (RP,SP) 9.41; 8.38 (1JPH = 713.68 Hz; JPH = 8.64
3
Hz) (1JPH = 716.36 Hz; JPH = 9.12 Hz). HRMS (ESI): calcd for
C18H22N3O8PNa, 462.1145; found, m/z 462.1053.
Preparation of 3′-O-Acetyl-5′-[3-pyridyl[1,3,2]oxaza-
phospholidine oxide]thymidine-5-yl (10), 3′-O-Acetyl-5′-[3-
phenyl[1,3,2]oxazaphospholidine oxide]thymidine-5-yl (11),
and 3′-O-Acetylthymidine-5-yl 2-N-(2-Pyridyl)ethyl Phospho-
nate (9). Portions of both compound 1 (12 mg, 0.026 mmol) and 2
(12 mg, 0.026 mmol) were taken and dissolved in dry MeCN. BTT
(0.2 M, 65 μL, 0.013 mmol) in dry MeCN and 2 drops of water were
added to each substrate. After 1 h the reactions were complete
(according to 31P NMR and TLC), giving products 3 and 5,
respectively. Then molecular sieves 4 Å were added to dry solutions.
After 2.5 h 7 mg of iodine (0.026 mmol) was added to each solution.
The course of reaction was followed with the use of 31P NMR. The
reaction was assumed as complete when peaks corresponding to H-
phosphonate diesters totally disappeared. This way compounds 9
(from 3) and 10 (from 5) were obtained (100% yield according to 31P
NMR). Addition of BTT dissolved in acetonirile (0.2 M, 130 μL, 0.026
mmol) to each product 9 and 10 resulted in hydrolysis only in the first
case, giving compound 11. It was obtained after chromatographic
purification (isocratic phase acetonirile/dichloromethane 1/1) as a
white powder with 86% yield (11 mg).
found, 490.1370.
Preparation of 3′-O-Acetylthymidine-5-yl (2-Phenyl)propyl
Phosphonate (6). A 200 mg amount of 3′-acetylthymidine 5′-H-
phosphonate monoester (0.44 mmol) was dissolved in 4.5 mL of
DCM and 0.5 mL of pyridine. 3-Phenylpropanol (60 mg, 0.44 mmol)
and 101 μL of diphenyl chlorophosphate (129.8 mg, 0.55 mmol, ρ =
1.296 g/L) were added. After 1 h the reaction was complete
(according to 31P NMR). Subsequent purification was carried out as in
compound 4. A 101 mg amount of 6 was obtained (49.2% general
yield).
1H NMR (400 MHz, CDCl3): δ 9.31 (s, 1H); 7.45−7.48 (dd, J1 =
1.05 Hz, J2 = 11.12 Hz, 1H); 7.25−7.27 (m, 1H); 7.15−7.19 (m, 2H);
6.60−6.63 (q, 2H); 6.35−6.39 (m, 1H); 5.22−5.27 (dd, J1 = 12.6 Hz,
J2 = 6.8 Hz 1H); 4.34 (d, J = 6.8 Hz, 1H); 4.33 (d, J = 8.1 Hz, 1H);
4.13−4.17 (m, 2H); 4.12−4.14 (m, 1H); 2.68−2.73 (m, 2H); 2.38−
2.44 (m, 1H); 2.13−2.20 (m, 1H); 2.10 (s, 3H); 1.99−2.06 (m, 2H);
1.93 (d, J = 1.9 Hz, 3H). 13C NMR (100.6 MHz MHz, CDCl3): δ
170.5; 163.6.; 150.5; 134.7; 128.5; 128.3; 126.2; 111.9; 111.8; 84.5;
82.6; 74.1; 73.9; 65.2; 65.0; 37.0; 31.8; 31.7; 31.5; 20.8; 12.4. 31P NMR
Data for 9 are as follows. 31P NMR (121 MHz, CH3CN): δ (ppm)
3
(RP,SP) 8.99; 7.98 ppm (1JPH = 703.0 Hz, JPH = 8.32 Hz) (1JPH
707.29 Hz, JPH = 8.61 Hz).
=
3
Data for 10 are as follows. 1H NMR (400 MHz, CDCl3): δ 7.46 (d,
J =11.8 Hz, 1H); 7.29−7.34 (m, 2H); 7.12−7.14 (q, J1 = 2.31 Hz, J2 =
8.25 Hz, 2H); 7.05 (t, J = 7.35 Hz, 1H); 6.30−6.36 (m, 1H); 4.55−
4.61 (m, 1H); 4.45−4.51 (m, 1H); 4.35−4.37 (dd, J = 2.6 Hz, 1H);
4.29−4.32 (m, 1H); 4.12−4.15 (m, 1H); 3.84−3.90 (m, 2H); 3.70−
3.74 (q, J1 = 7.01 Hz, J2 = 14.02 Hz, 4H); 2.33−2.40 (m, 1H); 2.13−
2.18 (m, 1H); 2.10 (s, 3H); 1.94 (s, 1H); 1.85 (s, 1H). 13C NMR (75
1
(121 MHz, CH3CN): δ (ppm) (RP,SP) 9.60, 8.62 ppm; JPH = 711.96
3
1
3
Hz; JPH = 8.23 Hz, JPH = 713.78 Hz; JPH = 9.20 Hz. HRMS (ESI):
calcd for C21H27N2O8PNa, 489.1505; found, m/z 489.1420.
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Article
MHz, CDCl3): δ 170.5; 163.3; 150.2; 134.9; 129.6; 128.3; 122.7;
116.2; 115.9; 111.8; 111.6; 84.5; 82.6; 82.7; 74.4; 63.7; 46.1; 45.9;
37.1; 20.8; 12.4. 31P NMR δ (ppm) (SP,RP) 14.12, 13.89. HRMS
(ESI): calcd for C20H23N3O8P, 464.1301; found, m/z 464.1200.
Data for 11 are as follows. 1H NMR (400 MHz, D2O): δ 7.73 (t, J =
7.56 Hz, 1H); 7.66 (d, J = 5.76 Hz, 1H); 7.56 (s, 1H); 6.89 (d, J = 9.12
Hz, 1H); 6.75 (t, J = 6.56 Hz, 1H); 6.23−6.20 (q, J1 = 6.32 Hz, J2 =
8.24 Hz, 1H); 5.24 (d, J = 5.08 Hz, 1H); 4.01 (m, 2H); 3.99 (m, 1H);
3.58−3.54 (q, J1 = 5.24 Hz, J2 = 9.80 Hz, 2H); 2.38−2.33 (m, 1H);
2.27−2.20 (m, 1H); 2.12 (s, 3H); 1.79 (s, 3H); 1.76 (s, 3H). 13C
NMR (75 MHz, D2O): δ 173.5; 166.1; 153.1; 151.4; 143.4; 137.0;
133.4; 112.6; 111.6; 84.7; 83.1; 75.1; 67.8; 65.3; 64.0; 42.1; 36.4; 24.1;
dissolved in an acetonitrile/triethylamine mixture (2/1) and left until
full deprotection occurred (confirmed by 31P NMR), giving
quantitatively the oily product 16. Compound 16 was chromato-
graphically purified using a DCM/methanol mixture as eluent. A 340
mg amount (0.75 mmol) of an oily product was obtained (98.8%
yield).
Data for 16 are as follows. 31P NMR (121 MHz, CH3CN): δ (ppm)
1
3
6.25; JPH = 546.68 Hz; JPH = 16.3 Hz.
Oxidation with Iodine Solution. Each of substrates 4 (0.027
mmol, 13 mg), 6 (0.025 mmol, 12 mg), 7 (0.024, 11 mg), and 8 (0.03
mmol, 13 mg) was dissolved in 500 μL of dry acetonitrile separately. A
5 mg portion of iodine and a few drops (5 to 6) of water were added
to each solution. The course of the oxidation reactions was followed
with the use of 31P NMR. The reaction was assumed as complete when
peaks referred to as the substrate totally disappeared.
3
11.5. 31P NMR (121 MHz, CH3CN): δ (ppm) 0.66; JPH = 4.86 Hz.
HRMS (ESI): calcd for C19H25N4O9PNa, 507.1359; found, 507.1274
[M + Na]+.
Preparation of 3′-O-Acetylthymidine-5-yl (2-phenyl)-
aminoethyl thiophosphonate (12). An 8.7 mg amount (0.27
mmol) of sublimed sulfur was added to 32 mg of 5 (0.0685 mmol) in a
MeCN/pyridine (1/9) solvent mixture. The reaction mixture was
stirred and kept at room temperature for 1 h. After that time the
reaction proceeded quantitatively according to 31P NMR. The mixture
was evaporated and kept under reduced pressure until complete
removal of pyridine. The product was purified on a PLC plate with the
eluent toluene/MeCN/triethylamine (45/45/10). The product was
washed out from silica gel with MeOH/DCM (7/3). A 32 mg amount
of compound 12 was obtained (94.1% yield).
In the case of H-phosphonates 3 and 5 these compounds were
prepared for oxidation in a different way. Compounds 1 (12 mg, 0.026
mmol) and 2 (12 mg, 0.026 mmol) were dissolved in dry MeCN. BTT
(0.2 M, 65 μL, 0.013 mmol) in dry MeCN and 2 drops of water were
added to each substrate. After 1 h the reactions were complete
(according to 31P NMR and TLC), giving products 3 and 5,
respectively. Then molecular sieves 4 Å were added to dry solutions.
After 2.5 h 4 mg of iodine was added to each solution. The course of
reaction was followed with the use of 31P NMR. The reaction was
assumed as complete when peaks referred to as H-phosphonates
diesters totally disappeared. This way compound 9 (from 3) and 10
(from 5) were obtained (100% yield according to 31P NMR).
The addition of 20 μL of water to both products 9 and 10 resulted
in hydrolysis only in the first case (compound 11 was obtained), while
product 10 remained stable. The hydrolysis time of 9 was shorter than
1 min when the first 31P NMR spectrum was recorded. Product 10 was
further purified on a PLC plate in the eluent DCM/MeOH/
triethylamine (89/5/6) and washed out from the gel with DCM/
MeCN (3/7). A 10 mg amount of pure compound 10 was obtained
(82.7% yield).
Enzymatic Determination of the Correlation between
Absolute Configuration at the Phosphorus Atom and
Chemical Shifts in 13. A 10 mL amount of buffer solution
containing 50 mM of Tris-HCl (78.8 mg, 0.5 mmol), 72 mM of NaCl
(42.1 mg, 0.72 mmol), and 14 mM of MgCl2 (13.3 mg, 0.14 mmol)
and the proper quantity of Milli-Q water were prepared.36 All salts
were dissolved at room temperature. The buffer solution pH was
adjusted to 8.5 with 0.1 M HCl solution and sterilized by membrane
filtration (pore diameter 0.22 μm) at room temperature, giving buffer
solution A. A 26 mg amount of compound 13 obtained as a white
powder by the method previously mentioned was dissolved in 1 mL of
solution A. The optical density of such a mixture was 80 OD. A 25 μL
portion of the obtained mixture was taken (2 OD) and added to 500
μL of solution A. The whole mixture was added to 20 mg of lysate
from Crotalus atrox containing phosphodiesterase I (0.026 unit/mg,
EC 3.1.4.1) afterward. The thus obtained solution was incubated at 27
°C for 7 days. Every 24 h a one-dimensional 31P NMR spectrum was
recorded at 298.1 K with 100% D2O as an external standard. The 31P
NMR parameters were as follows: sweep width 4045 Hz, pulse width
7.3 μs, acquisition time 8.1 s collected in 65.5 k, number of transients
2048, line broadening 1 Hz, and temperature of recorded spectra 298.1
K.
The thus obtained 32 mg (0.064 mmol) of compound 12 was
treated with a mixture of methylamine and 32% ammonia (2 mL, 1/1)
to remove the 3′-acetyl protecting group. The reaction mixture was
kept for 10 min at 65 °C. The thus obtained compound 13 was
chromatographically purified using the isocratic phase DCM/MeOH
(9/1), evaporated to dryness, and lyophilized afterward. Full
deprotection of 12 was confirmed with HPLC. A 26 mg amount of
compound 13 was obtained (0.057 mmol, 89.1% yield).
31P NMR (121 MHz, buffer A): δ (ppm) (RP,SP) 60.24, 59.99 ppm;
3
3JPH = 7.38 Hz, JPH = 6.41 Hz. Mp: 166−170 °C.
Data for 13 are as follows. 31P NMR (121 MHz, buffer A): δ (ppm)
(SP,RP) 55.65; 55.33 ppm. HRMS (ESI): calcd for C18H25N3O7PS,
458.1073; found, m/z 458.1134 [M + H]+.
The preparation of ammonium fluorenylomethyl-H-phosphonate
monoester 14 was conducted according to the method presented in
the literature.35
Preparation of Fluorenylomethyl-3′-acetylthymidine 5′-H-
Phosphonate Diester (15). An 816.3 mg amount (2.90 mmol) of
oily ammonium fluorenylomethyl-H-phosphonate monoester 14,
prepared according to the method presented in the literature,35 was
dissolved in 10 mL of a pyridine/triethylamine mixture (4/1 v/v) and
evaporated to dryness. Then 7.2 mL of DCM, 0.8 mL of pyridine, and
997 mg (2.67 mmol) of 3′-acetylthymidine were added. The mixture
was stirred for 10 min, and then 141 μL of pivaloyl chloride was added
dropwise. The reaction mixture was kept under inert gas, and after 30
min the reaction was complete (according to 31P NMR). Subsequent
extraction was carried out as in 4, while further chromatographic
purification was conducted with the eluent MeCN/DCM, giving solid
product 15 (65.5% yield).
Data for 15 are as follows. 1H NMR (400 MHz, CDCl3): δ 9.31 (s,
1H); 7.74 (d, J = 7.52 Hz, 2H); 7.58 (t, J = 7.48 Hz, 2H); 7.39 (t, J =
7.44 Hz, 2H); 7.32−7.30 (m, 2H); 7.28 (d, J = 1.16 Hz, 1H); 6.32−
6.27 (m, 1H); 5.11 (d, J = 6.56 Hz, 1H); 4.58−4.53 (dd, J1 = 6.04 Hz,
J2 = 7.72 Hz, 2H); 4.23−4.19 (q, J1 = 5.52 Hz, J2 = 9.12 Hz, 1H);
4.14−4.13 (dt, J1 = 3.2 Hz, J2 = 7.76 Hz, 1H); 4.05 (m, 1H); 2.37−
2.29 (m, 1H); 2.08 (s, 3H); 2.07−1.96 (m, 1H); 1.80 (d, J = 2.04 Hz,
3H). 13C NMR (75 MHz, CDCl3): δ 170.4; 163.6; 150.4; 142.6;
141.4; 134.7; 128.1; 127.2; 124.7; 120.1; 111.8; 84.4; 82.5; 75.0; 67.4;
64.8; 53.4; 48.1; 37.0; 27.1; 20.8; 12.3. 31P NMR (121 MHz,
Determination of Intermolecular versus Intramolecular
Oxidation Mechanism with the Use of Iodine Solution in the
Case of Substances 4 and 6−8. A 10 mg portion of 4 (0.021
mmol) and 10 mg of 6 (0.021 mmol) were dissolved together in dry
MeCN. Then 5 mg of iodine and 3 drops of water were added. The
course of the reaction was followed with the use of 31P NMR. The
reaction was assumed as complete when peaks corresponding to H-
phosphonate diesters totally disappeared.
Confirmation of the Structure of Oxazaphospholidine Oxide
(10) by Oxidation with Iodine in the Presence of Pyridine and
a Mixture of Oxazaphospholidine (2) and Diester H-
phosphonate (5). A 10 mg amount of compound 2 and 10 mg of
compound 5 were dissolved together in dry MeCN. Then 5 mg of
CH3CN): δ (ppm) 9.15, 8.09; 1JPH = 711.12 Hz; 3JPH = 6.4 Hz, 1JPH
=
3
712.81 Hz; JPH = 7.38 Hz. HRMS (ESI): calcd for C26H27N2O8PNa,
549.1505; found, 549.1419. Mp: 63−66 °C.
Preparation of 3′-Acetylthymidine 5′-H-Phosphonate
Monoester (16). A 200 mg amount (0.38 mmol) of 15 was
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Article
iodine was added. The reaction progress was followed by 31P NMR
(see Figure 3).
́
(25) Garegg, P. J.; Regberg, T.; Stawinski, J.; Stromberg, R. J. Chem.
Soc., Perkin Trans. 1 1987, 6, 1269−1274.
(26) (a) The pyridinium cation is postulated to be formed but it is
not improved. The pyridinium adduct is described by: Stromberg.;
et al. J. Chem. Soc. Perkin Trans. 1 1987, 6, 1269−1274. In this report,
it is formed by attacking the silylated iodo monoester by further
hydrolysis. They also oxidized H-phosphorodiesters where iodo
derivatives were formed but the authors mentioned only hydrolysis
in a pyridine/water mixture. Moreover, in ref 15 of this paper the
authors talk about the addition of pyridine to a phosphoroiodidate
solution, which results in immediate replacement of the original signal
(−41 ppm; iodo derivative) by a new one at ca. −13 ppm. (b)
Epimerization during oxidation in the presence of pyridine may occur,
but according to Seela and Kretschmer, the reaction with iodine/
pyridine/[18O]H2O preferentially leads to one of the diastereoisomers:
Seela, F.; Kretschmer, U. J. Org. Chem. 1991, 56, 3861−3869.
(27) Oxidation with iodine may follow the iodine disproportionation
mechanism.
(28) H-phosphonate diesters 3 and 5 have been obtained from a
cyclic oxazaphospholidine by hydrolysis and oxidized only with the
iodine solution without further isolation and purification. The reaction
has occurred in the presence of a small amount of water (Scheme 1).
(29) Moerat, A.; Modro, T. A. Phosphorous Sulfur Relat. Elem. 1983,
14, 179−184.
(30) Brown, Ch.; Boudreau, J. A.; Hewitson, B.; Hudson, R. F. J.
Chem. Soc., Perkin Trans. 2 1976, 8, 888−895.
(31) Brown, H. C. et al. In Determination of Organic Structures by
Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic Press:
New York, 1955.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C NMR, 1H−1H COSY, 1H−13C HSQC, and MS spectra
for 1−15. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: maro@ibch.poznan.pl.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the European Regional
Development Fund within the Innovative Economy Pro-
gramme (Grant No. POIG.01.03.01-30-045).
REFERENCES
■
(1) Jana, A.; Atta, S.; Sarkar, S. K.; Singh, N. D .P. Tetrahedron 2010,
66, 9798−9807.
(2) Leikauf, E.; Koster, H. Tetrahedron 1995, 51, 5557−5562.
(3) Sehaller, H.; Weimann, G.; Lerch, B.; Khorana, H. G. J. Am.
Chem. Soc. 1963, 85, 3821−3827.
(32) Eckstein, F.; Burgers, P. J. M.; Hunneman, D. H. J. Biol. Chem.
1979, 254, 7476−7478.
(4) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Chem. Rev. 2009, 109,
2455−2504.
́
(33) Almer, H.; Stawinski, J.; Stromberg, R.; Thelin, M. J. Org. Chem.
(5) Guga, P.; Koziołkiewicz, M. Chem. Biodivers. 2011, 9, 1642−81.
(6) Stec, W. J.; Karwowski, B.; Boczkowska, M.; Guga, P.;
Koziołkiewicz, M.; Sochacki, M.; Wieczorek, M. W.; Blaszczyk, J. J.
Am. Chem. Soc,. 1998, 120, 7156−7167.
1992, 57, 6163−6169.
́
(34) (a) Sobkowski, M.; Stawinski, J.; Kraszewski, A. Nucleosides,
Nucleotides, and Nucleic Acids 2005, 24, 1301−1307. (b) Sobkowski,
M.; Stawinski, J.; Kraszewski, A. Nucleosides, Nucleotides, and Nucleic
ski, J.;
́
(7) Wilk, A.; Grajkowski, A.; Philips, L. R.; Beaucage, S. L. J. Am.
Chem. Soc,. 2000, 122, 2149−2156.
Acids 2006, 25, 1377−1389. (c) Sobkowski, M.; Stawin
́
Kraszewski, A. Nucleosides, Nucleotides, and Nucleic Acids 2006, 25,
1363−1375.
(8) Tlais, S. F.; Lam, H.; House, S. E.; Dudley, G. B. J. Org. Chem.
2009, 74, 1876−85.
́
(35) Romanowska, J.; Szymanska-Michalak, A.; Pietkiewicz, M.;
(9) Reese, C. B.; Thompson, A. J. Chem. Soc., Perkin Trans. 1 1988,
2881−2885.
́
Sobkowski, M.; Boryski, J.; Stawinski, J.; Kraszewski, A. Lett. Org.
Chem. 2009, 6, 496−499.
(10) Mairanovsky, V. G. Angew. Chem., Int. Ed. Engl. 1976, 15, 281.
(11) Yus, M.; Martínez, P.; Guijarro, D. Tetrahedron 2001, 57, 10119.
(12) Givens, R. S.; Conrad, P. G. II; Yousef, A. L.; Lee, J.-I.
Photoremovable Protecting Groups. In CRC Handbook of Organic
Photochemistry and Photobiology, 2nd ed.; Horspool, W. M., Ed.; CRC
Press: Boca Raton, FL, 2003; Chapter 69, pp 69.1−69.46.
(13) Wilk, A.; Chmielewski, M. K.; Grajkowski, A.; Phillips, L. R.;
Beaucage, S. L. J. Org. Chem. 2002, 67, 6430−6438.
(14) Grajkowski, A.; Wilk, A.; Chmielewski, M. K.; Phillips, L. R.;
Beaucage, S. L. Org. Lett. 2001, 3, 1287−1290.
(36) Cummins, L. L.; Owens, S. R.; Risen, L. M.; Lesnik, E. A.;
Freier, S. M.; McGee, D.; Guinosso, C. J.; Cook, P. D. Nucleic Acids
Res. 1995, 23, 2019−2024.
́
(15) Chmielewski, M. K.; Marchan, V.; Cieslak, J.; Grajkowski, A.;
Livengood, V.; Munch, U.; Wilk, A.; Beaucage, S. L. J. Org. Chem.
2003, 68, 10003−10012.
(16) Ohkubo, A.; Kasuya, R.; Miyata, K.; Tsunoda, H.; Seio, K.;
Sekine, M. Org. Biomol. Chem. 2009, 7, 687−694.
(17) Chmielewski, M. K.; Tykarska, E.; Markiewicz, W. T.;
Rypniewski, W. R. New J. Chem. 2012, 36, 603−612.
(18) Chmielewski, M. K. Tetrahedron Lett. 2012, 6, 666−669.
(19) Ciesl
10129.
́
ak, J.; Beaucage, S. L. J. Org. Chem. 2003, 68, 10123−
(20) Chmielewski, M. K. Org. Lett. 2009, 11, 3742−3745.
(21) Lewis, E. S.; Spears, L. G. J. Am. Chem. Soc. 1985, 107, 3918−
3921.
(22) Nylen, P. Z. Anorg. Allg. Chem. 1938, 235, 161−182.
(23) Silver, B.; Luz, Z. J. Am. Chem. Soc. 1962, 84, 1091−1095.
(24) (a) Stawin
33, 3185−88. (b) Skowron
D.; Trippett, S. Tetrahedron Lett. 1980, 21, 321−322.
́
ski, J.; Stromberg, R.; Zain, R. Tetrahedron Lett. 1992,
́
ska, A.; Pakulski, M.; Michalski, J.; Cooper,
7872
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