Desymmetrization of myo-inositol derivatives by lanthanide catalyzed phosphitylation with C2-symmetric phosphites

Article abstract of DOI:10.1016/j.bmc.2015.03.023

Desymmetrization by phosphorylation represents a promising method with potential impact in many different areas of research. C₂-Symmetric phosphoramidites have been used to desymmetrize myo-inositol derivatives by functionalization at different

Full text of DOI:10.1016/j.bmc.2015.03.023

Bioorganic & Medicinal Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Desymmetrization of myo-inositol derivatives by lanthanide
catalyzed phosphitylation with C2-symmetric phosphites
⇑
Michael Duss, Samanta Capolicchio, Anthony Linden, Nisar Ahmed, Henning J. Jessen
University of Zürich, Department of Chemistry, Winterthurerstrasse 190, 8057 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Desymmetrization by phosphorylation represents a promising method with potential impact in many
different areas of research. C2-Symmetric phosphoramidites have been used to desymmetrize myo-inosi-
tol derivatives by functionalization at different positions. With this method, 1:1 mixtures of diastere-
omers are obtained that can be separated subsequently. In this work, activation of a C2-symmetric
phosphoramidite is achieved by addition of pentafluorophenol (PFP) and leads to a reactive PFP phos-
phite, which can then be coupled to protected myo-inositol derivatives with reactive OH groups at the
1, 3, 4 and 6 positions. This strategy enhances the diastereoselectivity of the coupling reaction with a pre-
ference towards phosphitylation at position 6 (up to 3:1) or position 3 (up to 2:1). The concept of atten-
uative activation of phosphoramidites via in situ generated pentafluorophenol phosphite triesters is thus
proven in these studies. It is further shown that Lewis–Acid catalysis enhances the rate of phosphite tri-
ester coupling without affecting the diastereoselectivity. This novel strategy improves access to different
phosphorylated myo-inositol derivatives and will thus enable further studies into the function of these
important intracellular second messengers.
Received 23 January 2015
Revised 4 March 2015
Accepted 5 March 2015
Available online xxxx
Keywords:
Inositol
Phosphorylation
Diphosphoinositol polyphosphates
Chiral auxiliary
Phosphite
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
in high demand. Since myo-inositol is a meso-compound with a
plane of symmetry dissecting the 2- and 5-hydroxy groups, desym-
Phosphate esters of myo-inositol play crucial roles in different
biological myo-Inositol-1,4,5-trisphosphate
processes.1
metrization is a key step in the synthesis of many phosphorylated
inositol derivatives in enantiomerically pure form.13
(Ins(1,4,5)P3) 1 acts as a second messenger that regulates Ca2+-re-
lease from intracellular stores (Fig. 1).2 This discovery provided the
first proof of the importance of inositol phosphates in biology.
Among the 63 possible isomers of myo-inositol phosphates, includ-
ing 24 enantiomeric pairs and 15 meso-compounds, many turned
out to be of biological importance.3
The discovery of P-anhydride containing ‘energetic’ diphospho-
inositol polyphosphates X-PP-InsP5 (X denotes the position of the
anhydride on the myo-inositol scaffold; also generally referred to
as InsP7, Fig. 1) and X,Y-PP2-InsP4 (also referred to as InsP8) in
Dictyostelium (6-PP-InsP5)4 and in mammalian cells (1-PP-InsP5,
5-PP-InsP5),5,6 led to intensified research in this field. It was shown
that these molecules play a crucial role in many cellular processes
and diseases like, for example, telomere length control, vesicular
trafficking and diabetes.7–12
Different approaches for the desymmetrization of myo-inositol
have been reported.14 A very frequently used method relies on
the use of chiral auxiliaries. Riley et al., for example, converted
myo-inositol into camphanate esters15 and camphanylidene acet-
als have also been used frequently.16 Recently, another method
for the desymmetrization of 4,6-protected myo-inositol was devel-
oped based on an enantioselective acylation with a phosphinite
catalyst.17 In another approach, lanthanide triflates were used in
order to catalyze the diastereoselective acylation of myo-inositol
1,3,5-orthoformate with camphanic acid anhydride.18
A very efficient strategy relies on the desymmetrization of the
myo-inositol core by phosphorylation, as it directly enables the
introduction of a phosphate group, which potentially leads to short
reaction sequences (also referred to as ‘step economy’19). However,
only a few examples using this approach have been reported so far;
in 2001, Miller et al. published the development of a ‘kinase mimic’
facilitating the synthesis of myo-inositol-1-phosphate in a catalytic
manner.20 The Miller group also extended this approach from P(V)
to P(III) chemistry.21 Importantly, diastereoselective phos-
phitylation with synthetically useful selectivity has not yet been
reported.
In order to further investigate the diverse biological functions of
phosphorylated inositol derivatives, such as the nonsymmetric X-
PP-InsP5, efficient synthetic routes to access these molecules are
⇑
Corresponding author.
E-mail address: henningjacob.jessen@chem.uzh.ch (H.J. Jessen).
http://dx.doi.org/10.1016/j.bmc.2015.03.023
0968-0896/Ó 2015 Elsevier Ltd. All rights reserved.
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2
M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
O
P
O
O
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
P
P
P
P
P
P
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
O
O
O
P
O
O
O
O
O
P
P
O
O
O
O
O
O
O
OH
O
O
O
P
HO
O
O
O
2
ent-2
O
OH
P
O
O
P
O
O
O
P
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
O
P
P
O
O
O
O
O
O
O
O
O
O
O
P
P
P
P
P
P
P
O
O
O
O
O
O
O
O
1
O
O
O
O
O
O
O
O
P
O
O
P
O
O
O
P
O
O
O
O
O
O
O
O
3
ent-3
Figure 1. Structure of myo-InsP3 1, and four representatives of the X-PP-InsP5: 4-PP-InsP5 2, 6-PP-InsP5 ent-2, 1-PP-InsP5 3 and 3-PP-InsP5 ent-3.
Problem:
N
N
HN
N
R-OH
R
N
N
N
N
RO
RO
RO
RO
RO
RO
P
P
OR
P
N
slow activation
fast coupling
R
very reactive,
not observable
intermediate
Solution:
RO
F
F
F
F
F
RO
R-OH
P
F
O
F
F
OH
RO
RO
P
OR
P
N
F
RO
RO
fast activation
slow coupling
F
reactive
but observable
intermediate
Scheme 1. Common activation process (top) and attenuated activation process with PFP 5 (bottom). Relative reaction rates are indicated underneath the arrows. Problem:
low diastereoselectivity due to slow activation but fast coupling. Solution: inversion of the relative rates leading to fast activation and slow coupling.
Recently, C2-symmetric P-amidite 4 (Scheme 2) has been devel-
oped to desymmetrize inositol derivatives.22–24 This novel reagent
enables the synthesis of all four unsymmetric diphospho-inositol
polyphosphates (Fig. 1) with improved step economy, because
desymmetrization and phosphitylation are carried out in one sin-
gle step, which is followed by in situ oxidation to the phosphate.
Importantly, the chiral auxiliary additionally serves as a base labile
protecting group as it is a chiral derivative of the well-established
b-cyanoethyl group. The desymmetrization of meso-diol 7 with
free hydroxy groups at the 4- and 6-positions (Scheme 2), resulted
in a mixture of phosphate triesters (either modified at the 4- or 6-
position) with a diastereomeric ratio of 1.0:0.8 slightly favouring
position 4.22 For further applications, the preferential synthesis of
only one of the products would be advantageous, even though sep-
aration of the diastereomers was possible by crystallization and
flash chromatography. However, as activated P-amidites are highly
reactive, initial attempts did not improve the diastereoselectivity
(Scheme 1, Problem).
Herein, the results of an optimization process are reported,
enabling the desymmetrization of protected myo-inositol 7–9 by
phosphorylation in either the 4 or 6 position using pentafluorophe-
nol as the activator of phosphoramidite 4, pentafluorophenolate as
an attenuated leaving group and Yb(OTf)3 as a Lewis acid catalyst.
The results of this screening are then applied to the desymmetriza-
tion of protected myo-inositol 16, in which either the 1 or 3 posi-
tion can be phosphorylated.
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M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
3
2. Results and discussion
1. HC(OMe)3
pTsOH, DMF, 91%
O
O
O
R1O
2.1. Pentafluorophenol (PFP) as an activator and leaving group
in P-amidite chemistry
2. R-Cl, Lutidine, DCM,
R1 = TES: 7, 73%
OH
HO
OH
R1 = TBS. 8, 58%
HO
HO
OH
OH
2
1
3
4
An obvious experiment to favour diastereoselective phos-
phitylation preferentially at the 4- or the 6-position of meso-com-
pounds 7, 8 and 9 (Scheme 2, different protecting group patterns)
would be lowering of the temperature during the reaction.
However, at low temperatures (À78 °C), activation of phospho-
ramidite 4 with 4,5-dicyanoimidazole and other common activa-
tors, such as, for example, 1H-tetrazole, does not take place and,
as a consequence, no product is formed. This can be attributed to
the very slow activation at low temperature (Scheme 1, Problem).
The reactions were then run at a temperature at which activa-
tion of the phosphoramidite with common activators proceeded
efficiently. However, this did not lead to significant selectivity
(ca. 1.0:0.8), as reported earlier.22 In order to solve this problem,
an activator was required, which is acidic enough to enable fast
activation of the P-amidite but subsequently serves as an attenu-
ated leaving group compared to an imidazolide or tetrazolide, since
the latter are too readily displaced leading to low selectivity.
Phenols and their conjugated phenolates, respectively, could fulfil
these criteria, as they can be relatively acidic depending on the
substitution pattern, thus facilitating the activation process. Due
to the oxophilic properties of phosphorus, the replacement of the
phenolate was expected to be slower compared to the highly reac-
tive N-bound tetrazolides or imidazolides. In fact, P-tetrazolides
are very difficult to observe in NMR studies due to their high
reactivity.25,26 Thus, following established protocols for P-amidite
activation, a slow activation process precedes a fast nucleophilic
displacement reaction, which is incompatible with the goal of
increased selectivity (Scheme 1) at low temperature.
6
5
Ph
1. PhC(OMe)3
CSA, DMSO, 94%
OH
16
O
O
O
R1O
2. TBS-Cl, Lutidine,
DCM, 70%,
HO
OH
R1 = TBS:9
Scheme 3. Protecting group strategy for the synthesis of meso-diol 7, 8 and 9.
Abbreviations: CSA = camphorsulfonic acid, TES = triethylsilyl, TBS = tert-butyl-
dimethylsilyl.
As a second goal, the substrate scope of desymmetrization by
phosphitylation using phosphoramidite
4 (Scheme 1) was
improved. Towards this aim, different orthogonal protecting
groups were introduced, which could be used to access different
phosphorylated myo-inositol derivatives. Thus, myo-inositol
derived meso-diols 7, 8 and 9, containing different silyl protecting
groups in the 2-position (triethylsilyl, TES; tert-butyl dimethylsilyl,
TBS) and ortho-esters in the 1, 3, 5 positions (ortho-formate, ortho-
benzoate), were synthesized. The hydroxy groups at the 1-, 2-, 3-
and 5-position were protected as reported previously.27 Briefly, 7,
8 and 9 were synthesized starting from myo-inositol 16. After acid
catalyzed transesterification with the corresponding ortho-ester
(Scheme 3), the triols were selectively TES-protected (diol 7) or
TBS-protected (diol 8 and 9) in the equatorial position. The more
stable TBS-group was chosen for the optimization process, while
the TES-protected diol 7 has been employed in the previous syn-
thesis of 4-PP-InsP5 2 and 6-PP-InsP5 3.22
F
F
F
F
F
F
F
F
F
F
OH
NC
CN
N
P
PFP (5), 5 eq.
NC
CN
O
P
MeCN, r.t.
O
O
O
O
4
6
R2
O
R2
R2
O
O
O
O
O
1.
6
O
O
3
O
1
5
R1O
R1O
Ph
R1O
Ph
4
2
HO
OH
HO
O
O
P O
O
O
P O
O
OH
DMF
7, 8, 9
O
NC
NC
2. mCPBA
CN
CN
Ph
Ph
R1 = TES, R2 = H: 7
R1 = TBS, R2 = H: 8
R1 = TBS, R2 = Ph: 9
R1 = TES, R2 = H: 10
R1 = TBS, R2 = H: 11
R1 = TES, R2 = H: 13
R1 = TBS, R2 = H: 14
R1 = TBS, R2 = Ph: 12
R1 = TBS, R2 = Ph: 15
Scheme 2. Activation of P-amidite 4 with pentafluorophenol (PFP) 5 leads to an activated PFP–phosphite triester 6 that still undergoes coupling with hydroxy groups in meso-
diols 7, 8 or 9. The relative reaction rates are inverted: the initial activation is faster than the coupling, which enables lowering of the temperature to potentially increase
selectivity.
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M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
Figure 2. Top: 31P NMR analysis of in situ generated PFP–phosphite 6. Pentafluorophenol 5 is acidic enough to serve as an activator of P-amidite 4, which is quantitatively
converted to 6 within less than 1 h at room temperature. Bottom: 31P NMR analysis after coupling of 6 with diol 8 and oxidation to triesters 11 and 14 resulting in a product
ratio of 1:1.9.
The desymmetrization of meso-diol 7 with free hydroxy groups
at the 4- and 6-positions using P-amidite 4 resulted in a diastere-
omeric mixture of phosphate triesters 10 and 13 with a ratio of
1.0:0.8, as reported previously.22 The absolute configuration of
the products 10 and 13 has been unambiguously assigned by X-
ray crystal structure analysis and thus the products of the reaction
and their ratio can readily be assigned by 31P NMR analysis as the
chemical shifts of the P-triesters are available.22
Phosphoramidites need to be activated under slightly acidic
conditions.28 After protonation of the amino group by the activator,
nucleophilic attack of the conjugated base of the activator to phos-
phorus occurs. The activated species is then coupled to the hydroxy
group, as, for example, in meso-diols 7, 8 or 9 (Scheme 1). Two
main requirements can be defined for the desired activator: firstly,
lower reactivity of the activated P(III) intermediate and therefore
slower reaction rate to increase the selectivity. Secondly, the
amount of side product in which both hydroxy groups are phos-
phorylated should be decreased: twofold phosphitylation necessi-
tated the use of 2.5 equiv of meso-diol 7 in the previous report in
order to suppress this unwanted side reaction.22 When diols 8
and 9 were used in this study, comparable results were obtained
and in order to suppress twofold phosphitylation, 2 equiv of
meso-diol were required.
Nurminen et al. published an excellent review summarizing dif-
ferent activators for phosphoramidites.25 These can be classified as,
for example, tetrazole derivatives, imidazole derivatives, pyri-
dinium halides and phenols. Several activators from every class
were tested in this study to analyse their potential to promote
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M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
5
Table 1
diastereoselective phosphitylations. While tetrazole derivatives
and imidazole derivatives did activate P-amidite 4, the consecutive
coupling was too rapid to occur with significant selectivity.
Pyridinium halides and nitrophenols did not activate P-amidite 4
at all or not sufficiently in order to be of synthetic use (see
Supporting information). Significantly, initial attempts to activate
phosphoramidite 4 with pentafluorophenol (PFP) 5 delivered
promising results. Within less than one hour, the starting material
4 was consumed, quantitatively delivering the observable and thus
stable PFP–phosphite 6. The species can be assigned easily due to
the multiplicity of a signal in the 31P NMR spectrum: a triplet in
the 1H-decoupled mode caused by phosphorus–fluorine coupling
was observed at 138 ppm (Fig. 2). This is in contrast with P-tetra-
zolides, which are very difficult to observe by NMR-spectroscopy.25
Due to their attenuated reactivity as a result of their increased sta-
bility, PFP–phosphites are promising candidates for diastereoselec-
tive phosphitylations. Indeed, after addition of meso-diol 7 or 8 to
the PFP–phosphite 6, phosphitylation of the OH groups occurred
via displacement of the leaving group pentafluorophenolate. The
product ratio obtained was 1:1.9 after 80 min at room temperature
(Fig. 2).
In contrast to 2-nitrophenol and 2,4-dinitrophenol, which can
only be introduced via prior protonation of the phosphoramidite
4 with dicyanoimidazole (DCI), activation of 4 with 5 equiv of
PFP 5 was complete after 30–60 min without the requirement for
an additional proton source (Supporting information). Why the
activation of 4 using only 2-nitrophenol and 2,4-dinitrophenol
does not occur, even though 2,4-dinitrophenol (pKa: 4.125) is more
acidic than PFP 5 (pKa: 5.429), is unclear. Moreover, 2,4-dinitrophe-
nol is only commercially available in a stabilized form that con-
tains significant amounts of water, which leads to hydrolysis of
the activated phosphite 6 to an H-phosphonate (Supporting infor-
mation) and thus necessitates prior drying. This requirement is
inconvenient for synthetic applications.
Summary of the temperature and solvent screening with PFP 5 as the activator of 4
according to Scheme 2
Entry Solvent Temp
Selectivity*
10/14
Reaction time Conversion*
(°C)
(h)
(%)
1d
MeCN
MeCN
MeCN
DMSO
DCM
DCM
THF
rt
0
1.0:1.6
1.0: 1.6
1.0:1:7
1.0:1.7
1.0:1.4
1.0:1.5
1.0:1.3
1.0:1.2
—
1.0:1.5
1.0:1.9
1.0:2.4
1.0:2.7
1.25
9
70
2
4.5
120
18
1.5
72
18
3
20
23
24
18
6
34
23
10
—
2b,d
3b,d
4c,e
5b,c,d
6e
À20
rt
0?rt
À20
rt
7c,d
8b,d
9d
THF
THF
0
À20
10a,b,e DMF
rt
8
11a,e
12a,e
DMF
DMF
rt
70
21
18
À20
72
72
13a,b,e DMF
À40
Compound 8 was phosphitylated to a mixture of 11/14.
Bold values indicate the best conditions identified.
*
Selectivity and product formation (11/14) were determined from 31P NMR by
integration of the product signals and comparison with the H-phosphonate signal
from the hydrolysis product of unreacted PFP–phosphite 6; 5 equiv of pentafluo-
rophenol were used for the activation.
a
Activation was carried out in MeCN because of reduced hydrolysis and faster
reactions, phosphitylation was then carried out in DMF.
b
Reaction was stopped before completion.
Phosphoramidite 4 was not completely activated.
3 equiv of meso-diol 8 were used.
1.5 equiv meso-diol 8 were used.
c
d
e
obtained. Thus, the solvent screen identified DMF as the most suit-
able solvent.
2.3. PFP–phosphites in diastereoselective phosphitylation:
catalyst screen
A problem arising from the low reaction temperature was the
extended coupling time. After 3 days at À40 °C, only 20% of the
PFP–phosphite 6 were consumed (Table 1, entry 13). It thus
became necessary to enhance the reaction rate, ideally without
affecting the improved diastereoselectivity, in order to develop a
synthetically useful procedure.
2.2. PFP–phosphites in diastereoselective phosphitylation:
solvent screen
As the activated PFP–phosphite 6 has attenuated reactivity
compared to tetrazolides or imidazolides, the phosphitylation
reaction requires more time to go to completion (3 h in DMF).
The reactions were similar for the diols 7–9 and further optimiza-
tion regarding solvent and temperature in order to increase the
diastereomeric ratio of the product mixture was performed using
PFP 5 as an activator. Most of the reactions were stopped before
completion, since the initial goal was to improve the product ratio,
which was readily determined by analysing crude mixtures by 31P
NMR spectroscopy (for an example see Fig. 2, bottom). The yields
were optimized in a second screening. Conversion of the starting
material to the products can be assigned easily by comparing the
integral of the product triesters after oxidation (e.g., 11/14) with
the integral of hydrolysed and thus unreacted PFP–phosphite 6.
Hydrolysis produces the corresponding H-phosphonate, which is
sometimes also oxidized to a phosphate diester (see Supporting
information).
The activation and coupling were initially carried out in ace-
tonitrile at different temperatures. No significant increase of selec-
tivity was obtained (Table 1, entries 1–3). The first attempt to
perform the reaction in DMSO led to a product ratio of 1:1.7.
However, the activation was sluggish, a problem that was also
observed with THF (Table 1, entries 7 and 9). When the reaction
was performed in DMF, a selectivity of up to 1.0:1.9 was obtained
at room temperature (Table 1, entry 11). In contrast to other stud-
ied solvents, the diastereoselectivity was much more temperature-
dependent. At À40 °C, for example, a product ratio of 1.0:2.7 was
Lanthanide trifluoromethanesulfonates have recently been used
as Lewis–Acid catalysts in regioselective acylations of myo-inositol
1,3,5-ortho-formate.18 Moreover, they have been applied in the
activation of P(V) esters.30 Lanthanide salts might therefore also
promote P(III)-based phosphitylations by Lewis–Acid catalysis.
Lanthanide triflates indeed accelerated the phosphitylation
reactions at low catalyst loading (1%) without significantly affect-
ing the selectivity (Table 2). Three different lanthanide triflates
(Ytterbium3+-, Samarium3+- and Europium3+) were compared to
one another in terms of their effect on the reaction rate and selec-
tivity in DMF. At room temperature, approximately twofold
acceleration compared to the rate with the uncatalyzed phos-
phitylation was observed for the three lanthanides. Yb(OTf)3 was
chosen for further investigation as it showed slightly better selec-
tivity than Eu(OTf)3 and Sm(OTf)3.
Further experiments at lower temperatures using Yb(OTf)3
were then performed. At À10 °C, a diastereomeric mixture of prod-
ucts (11/14) with a ratio of 1.0:2.0 was obtained in 80% yield after
only 19 h reaction time. Importantly, only 1.2 equiv of meso-diol 8
were used and even under these almost equimolar conditions, dou-
ble phosphitylation at both the 4- and 6-OH was less than 5%,
which is a significant improvement compared to the initially
reported conditions. When the reaction was performed with
Yb(OTf)3 in DMF at À40 °C, the ratio of the diastereomeric product
mixture was 1.0:2.5 (entry 7, Table 2). In order to investigate,
whether the reaction is Lewis- or Brönsted acid catalyzed, a control
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M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
Table 2
2:1 could be achieved in the desymmetrization (see Supporting
information) with a preference for phosphitylation in the 3-
position.
Conditions and results of Ln3+ catalyzed phosphitylations in DMF according to
Scheme 2
Entry
Temp
Selectivity*
Catalyst
Reaction
time (h)
Conversion*
(%)
(°C)
11/14
2.5. Assignment of the regioselectivity
1
rt
rt
rt
rt
À10
À10
À40
1.0:1.8
1.0:1.6
1.0:1.5
1.0:1.5
1.0:2.0
1.0:2.0
1.0:2.5
—
1.5
1.5
1.5
1.5
18
21
30
67
52
55
80
66
2b
3
1%Yb(OTf)3
1% Eu(OTf)3
1% Sm(OTf)3
1% Yb(OTf)3
1% Yb(OTf)3
1% Yb(OTf)3
The 31P chemical shifts of the two diastereomers 10 and 13
allowed an unambiguous assignment of the structures based on a
previously reported X-ray crystal structure in combination with
NMR data.22 Under the screened conditions, compound 13 was
preferentially formed, which is phosphorylated at position 6.
Compounds 11 and 14 are closely related to 10 and 13; however,
no crystal structure could be obtained for these molecules. It can
be assumed that there is also an analogous preference for phos-
phitylation at the 6-position in these compounds. Importantly,
compound 13 is the precursor for 6-PP-InsP5 3, which is the natu-
rally occurring enantiomer. In the case of diol 9, the initially
obtained diastereomeric mixture of 12 and 15 (1.0:2.5, overall
yield 80%) was separated by flash chromatography and single crys-
tals of the minor isomer were obtained for an X-ray crystal struc-
ture analysis (Fig. 3). According to this analysis, the minor
diastereomer was compound 12 and consequently the major
diastereomer 15 is phosphorylated at position 6. This also corrobo-
rated the assignment for compounds 11 and 14, as there seems to
be a general preference for PFP–phosphite 6 to functionalize the 6-
position in rigid ortho-esters of inositol, such as 7–9.
4b
5a
6c
7a
19
60
Compound 8 was phosphitylated to a mixture of 11/14.
Bold values indicate the best conditions identified.
Selectivity and product formation (11/14) were determined from 31P NMR by
integration of the product signals and comparison with the H-phosphonate signal
from the hydrolysis product of unreacted PFP–phosphite 6; 5 equiv of PFP were
used. 1.5 equiv of meso-diol 8 were used.
*
a
Activation of P-amidite 4 to PFP–phosphite 6 was carried out in MeCN.
Reaction was stopped before completion.
1.2 equiv of meso-diol 8 were used.
b
c
experiment was carried out under identical conditions, but with 1%
of triflic acid instead of Yb(OTf)3. This experiment did not show any
acceleration of the reaction compared to the uncatalyzed control
experiment (results not shown). This reaction was then carried
out on a larger scale (100 mg P-amidite 4) with diol 7 and the
excellent results were reproduced (the major isomer 13 was iso-
lated in 58% yield).
3. Conclusions
2.4. Diastereoselective phosphitylation of meso-diol 16
The method described in this paper is the first using PFP 5 to
directly activate a phosphoramidite. Mizuguchi and Makino syn-
thesized phosphate triesters with a pentafluorophenolate as a leav-
ing group for subsequent transesterification.31 However, the
phosphoramidite that served as the starting material was first acti-
vated with 1H-tetrazole, which is not required, as shown herein.
Consequently, PFP 5 is a novel activator for phosphoramidites in
general and shows significant advantages over other activators in
the desymmetrization of meso-diols 7–9 by phosphitylation with
dissymmetric P-amidite 4. Attenuative activation of P-amidites
to increase the selectivity was applied in these studies.
To test, whether the PFP–phosphite coupling could be extended
to other substrates, meso-diol 16 with OH groups in the 1 and 3
positions was desymmetrized under the above-described condi-
tions (Scheme 4). This reaction could not be accelerated by the
addition of Yb3+. It has been proposed that the lanthanide-cat-
alyzed acylation requires two axial OH-groups in the myo-inositol
derivative to bind and activate the metal.18 In substrate 16, both
OH groups are equatorial, and this might explain failure of the cat-
alytic reaction. Notwithstanding, if the reaction was conducted at
room temperature without lanthanide catalyst, a selectivity of
F
F
F
F
F
F
F
F
F
F
NC
CN
OH
N
P
NC
CN
OR
PFP (5), 5 eq.
O
P
O
O
MeCN, r.t.
O
O
4
6
OPMB
1.
OR
HO
OH
2
1
3
4
HO
RO
OP(O)(Rχ)2
OR
OP(O)(Rχ)2
OH
OR
6
5
OPMB
PMBO
OPMB
16
RO
OR
17
OR
18
DMF
2. mCPBA
Ph
O
Rχ =
R=PMB
CN
Scheme 4. Activation of P-amidite 4 with pentafluorophenol (PFP) 5 leads to an activated PFP–phosphite triester 6 that undergoes coupling with hydroxy groups in meso-diol
16 at room temperature within 3 h. Under these conditions, derivative 18 is formed preferentially (2:1).
Please cite this article in press as: Duss, M.; et al. Bioorg. Med. Chem. (2015), http://dx.doi.org/10.1016/j.bmc.2015.03.023

M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
7
other phosphorylated representatives of the large inositol family
with improved yield and step-economy.
4. Experimental
4.1. Activation of P-amidite 4 with PFP 5
Phosphoramidite 4 (1 equiv) was coevaporated with MeCN
(2Â), dissolved in MeCN and pentafluorophenol 5 (5 equiv) was
added. The reaction mixture was stirred until PFP–phosphite 6
was formed completely (45 min, monitored by 31P NMR). Then,
MeCN was removed in vacuo and the residue was coevaporated
twice with MeCN. The solvent used in the phosphitylations was
then added.
4.2. Phosphitylation with PFP phosphite 6
In the appropriate solvent, PFP phosphite 6 (1.0 equiv) was stir-
red at the appropriate temperature in the presence of a meso-diol
7–9 (1.2 equiv). After complete consumption of 6 (monitored by
31P NMR), the intermediate phosphite triester was oxidized by
addition of mCPBA (1.2 equiv) and the reaction stirred for another
10 min. At this stage, the diastereomeric ratio was determined by
31P NMR. The solution was then diluted with EtOAc, washed with
brine (2Â), dried over MgSO4 and the solvents were removed in
vacuo. Excess diol was then removed by column chromatography
(MTBE/hexane 9:1) and the combined yield of both diastereomers
was determined. Next, the diastereomers were separated by flash
column chromatography. The diastereomeric ratio obtained after
chromatography was >95:5.
Figure 3. A view of the molecular structure of 12 (50% probability ellipsoids; only
the major disorder conformation of the silyl group, one phenyl ring and one cyano
group are shown) corresponding to the minor diastereomer obtained in the
coupling reactions of 6 with 9 after oxidation. Consequently, the major diastere-
omer is phosphorylated at position 6.
Phosphate triester 10: 1H NMR (500 MHz, 298 K, CDCl3, d/ppm):
7.49–7.37 (m, 8H), 7.32–7.27 (m, 2H), 5.67 (ddd, J = 6.6, J = 6.6,
J = 6.6, 1H), 5.44 (s, 1H), 5.45–5.39 (m, 2H), 4.93–4.84 (m, 1H),
4.54–4.41 (m, 1H), 4.20–4.11 (m, 1H), 4.10–4.02 (m, 2H), 4.01–
3.96 (m, 1H), 3.00 (d, J = 5.8, 2H), 2.79–2.56 (m, 3H), 0.98 (t,
J = 7.8, 9H), 0.64 (q, J = 7.8, 6H); 13C NMR (126 MHz, 298 K, CDCl3,
d/ppm): 136.3 (2Â d, J = 2.6), 130.3, 130.2, 129.5, 129.4, 126.3,
126.0, 116.0, 115.6, 102.7, 76.2 (d, J = 4.8), 76.1 (d, J = 4.9), 74.3,
72.9 (d, J = 5.5), 72.6 (d, J = 7.5), 68.8 (d, J = 3.4), 67.5, 60.1, 27.4
(d, J = 7.6), 26.9 (d, J = 8.0), 6.9, 4.8; 31P{1H} NMR (202 MHz,
298 K, CDCl3, d/ppm): À4.8; HRMS (ESI) calcd for 665.2060
(M+Na), found 665.2050; Rf (SiO2,solvent DCM/EtOAc 1:1) 0.6.
Phosphate triester 13: 1H NMR (400 MHz, 298 K, CDCl3, d/ppm):
7.44–7.31 (m, 8H), 7.29–7.22 (m, 2H), 5.59 (ddd, J = 6.6, J = 6.6,
J = 6.6, 1H), 5.50 (ddd, J = 6.1, J = 6.1, J = 6.1, 1H), 5.45 (s, 1H),
4.98–4.81 (m, 1H), 4.50–4.45 (m, 1H), 4.45–4.41 (m, 1H), 4.05–
3.92 (m, 2H), 3.79–3.64 (m, 1H), 3.38 (d, J = 5.1, 1H), 2.98 (d,
J = 6.1, 2H), 2.82–2.60 (m, 2H), 0.92 (t, J = 8.0, 9H), 0.57 (q, J = 7.9,
6H); 13C NMR (101 MHz, 298 K, CDCl3, d/ppm): 136.3 (2Â d,
J = 4.3), 130.0, 129.7, 129.2, 129.0, 126.1, 125.8, 115.9, 115.7,
102.5, 76.1 (d, J = 5.4), 75.5 (d, J = 5.0), 74.3, 72.6 (d, J = 6.0), 72.5
(d, J = 7.6), 68.9 (d, J = 2.4), 67.1, 60.0, 27.2 (d, J = 8.0), 26.8 (d,
J = 6.5), 6.7, 4.6; 31P{1H} NMR (162 MHz, 298 K, CDCl3, d/ppm):
À5.4; HRMS (ESI) calcd for 665.2060 (M+Na), found 665.2057; Rf
(SiO2,solvent DCM/EtOAc 1:1) 0.5.
Conceptually, the relative rates of the involved reactions were
inverted (slow activation and fast reaction for tetrazole/imidazole
activation as opposed to fast activation and slow reaction for PFP
5 activation). Due to the attenuated reactivity of PFP–phosphite
6, one can phosphpitylate myo-inositol derivatives with a diastere-
omeric ratio of 1.0:2.7 (27:73) at À40 °C. However, this reaction is
very slow. After 3 days, only one third of the starting material 6
was consumed. Since such long reaction times are not convenient
for laboratory use, an additive was identified to accelerate the reac-
tion. Catalytic amounts of Yb(OTf)3 (1%) led to completion of the
reaction (80% yield of the diastereomeric mixtures) within 3 days
and
a
comparable selectivity (29:71) at À40 °C in DMF.
Importantly, under these conditions twofold phosphitylation of
both hydroxy groups in, for example, diol 8 is avoided, a problem
that was encountered with standard activators, such as tetrazole
or DCI. Consequently, the required amount of inositol derivative
8
was reduced to 1.2 equiv, which represents a significant
improvement compared to the previously reported conditions, in
which more than 2 equiv of the respective diol had to be used.22
Furthermore, the method described herein provides the first
example of an Yb(OTf)3 accelerated PFP–phosphite coupling. The
diastereoselectivity of the reaction compared to activation with
tetrazole and its derivatives is increased by a factor of 2.5, favour-
ing phosphitylation of position 6 of the inositol derivative. The
obtained preferred product 13 is the precursor of the naturally
occurring 6-PP-InsP5 3. The obtained pairs of diastereomers, for
example 10/13 and 12/15, can be separated readily by FC and crys-
tallisation, thus enabling a straightforward synthesis of enan-
tiomerically pure inositol phosphates and diphosphoinositol
polyphosphates with improved yields. The concepts disclosed
herein will help to further develop the field of diastereoselective
phosphitylations in order to obtain myo-inositol phosphates and
Phosphate triesters 11 and 14 were not separated and only the
diastereomeric ratios were obtained by 31P NMR for optimization
purposes.
Phosphate triester 12: 1H NMR (500 MHz, 298 K, CDCl3, d/ppm):
7.63–7.59 (m, 2H), 7.50–7.40 (m, 8H), 7.39–7.30 (m, 5H), 5.72 (ddd,
J = 6.2, J = 6.2, J = 6.2, 1H), 5.48 (ddd, J = 6.0, J = 6.0, J = 6.0, 1H),
5.05–4.97 (m, 1H), 4.67–4.51 (m, 1H), 4.32–4.27 (m, 1H), 4.27–
4.21 (m, 1H), 4.19–4.15 (m, 1H), 4.14–4.11 (m, 1H), 3.10–2.97
(m, 2H), 2.80–2.72 (m, 1H), 2.70–2.63 (m, 1H), 0.95 (s, 9H), 0.10
(2Â s, 6H); 13C NMR (126 MHz, 298 K, CDCl3, d/ppm): 136.8,
Please cite this article in press as: Duss, M.; et al. Bioorg. Med. Chem. (2015), http://dx.doi.org/10.1016/j.bmc.2015.03.023

8
M. Duss et al. / Bioorg. Med. Chem. xxx (2015) xxx–xxx
136.2 (2Â d, J = 2.8), 130.2, 130.1, 130.1 129.8, 129.5, 129.3, 129.2,
129.0, 128.0 126.2, 126.0, 125.4, 115.7, 115.5, 107.3, 76.3 (d,
J = 5.2), 75.8 (d, J = 4.8), 75.4, 73.7 (d, J = 7.6), 73.1 (d, J = 6.0), 69.6
(d, J = 3.0), 67.4, 59.3, 27.3 (d, J = 8.0), 26.8 (d, J = 7.7), 25.8, 18.2,
À4.6 (2Â); 31P{1H} NMR (202 MHz, 298 K, CDCl3, d/ppm): À4.7;
HRMS (ESI) calcd for 741.2373 (M+Na), found 741.2360; Rf (SiO2,
solvent DCM/EtOAc 3:1) 0.6.
References and notes
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Phosphate triester 15: 1H NMR (500 MHz, 298 K, CDCl3, d/ppm):
7.63–7.59 (m, 2H) 7.51–7.40 (m, 8H), 7.38–7.31 (m, 5H), 5.69 (ddd,
J = 6.2, J = 6.2, J = 6.2, 1H), 5.51 (ddd, J = 6.0, J = 6.0, J = 6.0, 1H),
5.06–5.00 (m, 1H), 4.65–4.57 (m, 1H), 4.51–4.46 (m, 1H), 4.25–
4.21 (m, 1H), 4.11–4.07 (m, 1H), 4.01–3.93 (m, 1H), 3.05 (d, J = 6.0,
2H), 2.77–2.60 (m, 2H), 0.95 (s, 9H), 0.10 (2Â s, 6H); 13C NMR
(126 MHz, 298 K, CDCl3, d/ppm): 136.8, 136.2 (2Â d, J = 2.8), 130.2,
130.0 (2Â), 129.5, 129.4, 129.3 (2Â), 127.9, 126.2, 125.9, 125.4,
115.7, 115.4, 107.3, 76.2 (d, J = 5.1), 75.7 (d, J = 4.7), 75.3, 73.7 (d,
J = 7.8), 73.0 (d, J = 6.0), 69.6 (d, J = 3.0), 67.4, 59.3, 27.2 (d, J = 8.1),
26.8 (d, J = 7.7), 25.8, 18.2, À4.6, À4.7; 31P{1H} NMR (202 MHz,
298 K, CDCl3, d/ppm): À5.2; HRMS (ESI) calcd for 741.2373
(M+Na), found 741.2362; Rf (SiO2, solvent DCM/EtOAc 3:1) 0.4.
The analytical data obtained for phosphate triesters 17/18 were
identical with those reported in the literature.22
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The authors want to thank Professor Jay Siegel and Professor
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Swiss
National
Science
Foundation
(Ambizione
Grant
PZ00P2_136816 to H.J.J.) and Novartis (Postgraduate Fellowship
to N.A.) is gratefully acknowledged.
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CCDC-1044370 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmc.2015.03.023.
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Please cite this article in press as: Duss, M.; et al. Bioorg. Med. Chem. (2015), http://dx.doi.org/10.1016/j.bmc.2015.03.023