P-chiral oligonucleotides. 2D ROESY NMR assignment of absolute configuration at phosphorus and conformational analysis of 5′-O-monomethoxytrityl-(2′-O-deoxyribonucleoside) 3′-O-[O-(4-nitrophenyl)]methanephosphonates

Article abstract of DOI:10.1081/NCN-120003181

Fast and simple methodology for the assignment of the absolute configuration at the phosphorus atom in diastereomerically pure R_p and S_p 5′-O-monomethoxytrityl-2′-O-deoxynucleoside 3′-O-(O-4-nitrophenyl) methanephosphonate (3) was es

Full text of DOI:10.1081/NCN-120003181

Nucleosides, Nucleotides and Nucleic Acids
ISSN: 1525-7770 (Print) 1532-2335 (Online) Journal homepage: http://www.tandfonline.com/loi/lncn20
P-CHIRAL OLIGONUCLEOTIDES. 2D ROESY NMR
ASSIGNMENT OF ABSOLUTE CONFIGURATION
AT PHOSPHORUS AND CONFORMATIONAL
ANALYSIS OF 5′-O-MONOMETHOXYTRITYL-
(2′-O-DEOXYRIBONUCLEOSIDE) 3′-O-[O-(4-
NITROPHENYL)]METHANEPHOSPHONATES
Stefan Jankowski , Agnieszka B. Olejniczak & Zbigniew J. Lesnikowski
To cite this article: Stefan Jankowski , Agnieszka B. Olejniczak & Zbigniew J. Lesnikowski
(2002) P-CHIRAL OLIGONUCLEOTIDES. 2D ROESY NMR ASSIGNMENT OF
ABSOLUTE CONFIGURATION AT PHOSPHORUS AND CONFORMATIONAL ANALYSIS
OF 5′-O-MONOMETHOXYTRITYL-(2′-O-DEOXYRIBONUCLEOSIDE) 3′-O-[O-(4-
NITROPHENYL)]METHANEPHOSPHONATES, Nucleosides, Nucleotides and Nucleic Acids, 21:2,
177-190, DOI: 10.1081/NCN-120003181
To link to this article: http://dx.doi.org/10.1081/NCN-120003181
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Date: 05 August 2017, At: 21:45

NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS, 21(2), 177–190 (2002)
P-CHIRAL OLIGONUCLEOTIDES. 2D ROESY
NMR ASSIGNMENT OF ABSOLUTE
CONFIGURATION AT PHOSPHORUS AND
CONFORMATIONAL ANALYSIS OF
50-O-MONOMETHOXYTRITYL-(20-O-
DEOXYRIBONUCLEOSIDE) 30-O-[O-(4-
NITROPHENYL)]METHANEPHOSPHONATES
Stefan Jankowski,1 Agnieszka B. Olejniczak,2 and
Zbigniew J. Lesnikowski2,*
1Technical University of Lodz, Department of Chemistry,
Lodz 90-924, Poland
2Laboratory of Molecular Virology and Biological Chemistry,
Center of Microbiology and Virology, Polish Academy of Sciences,
Lodz 93-232, Poland
ABSTRACT
Fast and simple methodology for the assignment of the absolute con-
figuration at the phosphorus atom in diastereomerically pure RP and
SP 50-O-monomethoxytrityl-20-O-deoxynucleoside 30-O-(O-4-nitrophenyl)
methanephosphonate (3) was established. The method utilizes 2D
ROESY NMR and can be used for the stereochemical analysis of other
P-chiral mononucleotides. Configurational analysis shows that the major
conformation of the sugar residue in 3 is of the S (South) type. This
study will facilitate synthesis of stereoregular methylphosphonate oligo-
nucleotide analogues via the transesterification method.
*Corresponding author. Center of Microbiology and Virology PAS, 106 Lodowa St.,
Lodz 93-232, Poland. Fax: (+48-42)-677-1230; E-mail: zlesnik@wirus.cmiwpan.lodz.pl,
zlesnik@hotmail.com
177
Copyright # 2002 by Marcel Dekker, Inc.
www.dekker.com

178
JANKOWSKI, OLEJNICZAK, AND LESNIKOWSKI
INTRODUCTION
Methylphosphonate (PMe) oligonucleotides are among the first analogs
of oligonucleotides considered as antisense agents1–3. The PMe-internucleo-
tide group, in contrast to the natural phosphodiester internucleotide linkage,
is uncharged and contains a chirality center at phosphorus.
The family of modified non-ionic oligonucleotide containing a non-
bridging P-C bond, in addition to the methylphosphonate modification,
includes among others, methylphosphonothioates4, hydroxymethylphospho-
nates5,6, benzylphosphonates7,8, (4,40-dimethoxytrityl)phosphonate9, carbor-
anylmethylphosphonates10, octylphosphonates11, phenylphosphonates12, and
phenylphosphonothioates13.
PMe-oligonucleotides are characterized by high lipophilicity, resistance
to nucleases, weak interactions with plasma proteins, wide distribution in
various tissues and promotion of easy nonreceptor-mediated cellular uptake.
The major drawback of these analogs is that they do not activate RNase-H.
This however is compensated to some extent by the fact that PMe-oligonu-
cleotides are relatively low in toxicity which allows administration of higher
doses of PMe-oligomers. Another strategy to avoid the problems related to
lack of RNase H induction by PMe-oligonucleotides while capitalizing on
their unique aspects is to use short PMe-oligomers as a part of chimerical or
hybrid oligonucleotide constructs.
The P-diastereomerism of PMe-oligonucleotides and the influence of
P-chirality on the physicochemical properties of PMe-oligomers have
been shown in the pioneering work of Miller14,15 and further emphasized
by Lesnikowski and Stec16. It is likely that only some of the diastereo-
mers will have desirable physicochemical characteristics facilitating an effi-
cient cellular uptake17 and the most favorable biological effects. This causes a
serious concern regarding the optimization of potential oligonucleotide
pharmaceuticals based on PMe chemistry and creates a need for
effective chirotechnology for P-chiral P-stereodefined oligonucleotide
synthesis.18
Most of the stereocontrolled methods for the formation of the PMe
internucleotide linkages are based upon the transesterification of P-stereo-
defined diastereomeric nucleotide monomers19–22. The first successful method
for the stereocontrolled synthesis of PMe-oligonucleotides was based on
the process of stereospecific nucleophilic substitution at a pentavalent
phosphorus atom in P-chiral, diastereomerically pure monomer type of 50-O-
monomethoxytrityl-20-O-deoxynucleoside 30-O-(O-4-nitrophenyl)methane-
phosphonate (3)19. The substitution of 4-nitrophenyl group by the 5-OH
group of the nucleoside component, activated with Grignard reagent
(tBuMgCl), proceeds with an inversion of absolute configuration at phos-
phorus. This method was successfully used for the step by step, in solution
synthesis of short, stereoregular PMe oligonucleotides ranging from dimers

P-CHIRAL OLIGONUCLEOTIDES
179
to pentanucleotides23, phosphorothioate oligodeoxynucleotides24 and phos-
phorothioate oligoribonucleotides25.
The assignment of the absolute configuration at phosphorus in PMe-
oligonucleotides is crucial for the discussion of the relationship between the
stereochemistry of the internucleotide linkage and the physicochemical and
biological properties of PMe-oligomers. For the assignment of absolute
configuration in PMe-dinucleotides X-ray crystallography ½SP-d(APMeT)26,
RP-d(CPMeG)27, d(TP(S)MeT)28,], 1H-NMR analysis29–31, mobility shift in RP-
HPLC9,32, and chemical correlation19–21,32,33 were employed. It should be
emphasized however, that the direct assignment of absolute configuration of
the internucleotide PMe group becomes very difficult or impractical for
longer than dimer oligomers.
In the transesterification methods used for the stereoselective synthesis
of PMe-oligonucleotides, the formation of the internucleotide linkage occurs
with defined stereochemistry19–21,34,35. Thus, in the case of monomer type of
3 and Grignard reagent assisted coupling, the reaction takes place with the
inversion of absolute configuration at phosphorus of 3. Consequently, the
absolute configuration at phosphorus of the PMe-internucleotide linkage
can be deduced for each subsequent linkage of the growing oligonucleotide
chain if the absolute configuration at phosphorus of monomer is
known20,23
.
The absolute configuration in diastereomeric 3 was assigned so far by
means of the chemical correlation method19. Briefly, after 50-deprotection of
3, and conversion of the resulting 20-O-deoxynucleoside 30-[O-(4-nitrophe-
nyl)]methanephosphonate to 20-O-deoxynucleoside (30,50) cyclic methylpho-
sphonate, the absolute configuration of the cyclic product was determined
by means of 31P-NMR36,37. Since the cyclization reaction is stereospecific,
and occurs with inversion of configuration at phosphorus, the absolute
configuration in the acyclic precursor, and consequently in 3 could be
assigned.
Herein we present a convenient and fast method based on 2D NMR
for assignment of absolute configuration at phosphorus and preliminary
results of configurational analysis of base-protected monomers type 3 (Fig. 1
and 2).
RESULTS AND DISCUSSION
Synthesis of Base Protected 50-O-Monomethoxytrityl-20-O-deoxynucleoside
30-O-(O-4-nitrophenyl)methylphosphonates (3)
For the synthesis of base protected 50-O-monomethoxytrityl-20-O-
deoxynucleoside 30-O-(O-4-nitrophenyl)methylphosphonates (3) (B ¼ Thy,
CytBz, AdeBz, GuaiBu) the method previously developed for the preparation

180
JANKOWSKI, OLEJNICZAK, AND LESNIKOWSKI
Figure 1. Synthesis of 50-O-monomethoxytrityl-20-O-deoxynucleoside 30-O-(O-4-nitrophenyl)-
methanephosphonates (3), monomers for the preparation of P-stereodefined methanepho-
sphonate oligonucleotide analogues. (i) 2, 1,2,4-triazole, Et3N=CH3CN. (ii) Seperation of SP-3
and RP-3 diastereomers of 3.
of base unprotected 3 was employed19,38. Thus, suitable 50-O-mono-
methoxytrityl-N-protected 20-O-deoxynucleosides were treated with prepared
in situ [O-(4-nitrophenyl)methylphosphonotriazolidate in CH3CN solution.
The resulting mixture of diastereomeric 3, after standard work up, was
separated into individual diastereomers by means of silica gel chromato-
graphy. Diastereomerically pure 3 were then analyzed by means of TLC, UV
1
spectroscopy31, P- and H-NMR (Fig. 1, Table 1–3).
NMR Measurements
The absolute configuration at phosphorus in dinucleoside methylpho-
sphonates was assigned using NOE’s of the PCH3 group derived from
ROESY experiments31, taking advantage of the fact that different config-
urations at phosphorus lead to a different environment of the PCH3 group.
The Rp configuration in dinucleotides was unequivocally derived from the
ROESY spectra on the basis of two diagnostic crosspeaks of similar intensity
for most dinucleotides as expected. In the case of d(TPMeT) and d(TPMeA)
dimers, however, only one ROE signal to H30 was detected within the sen-
sitivity limits of the experiment. Two diagnostic crosspicks were observed for
dimers with tentatively assigned SP configuration, though the crosspeak to
H40 was characterized by a much lower intensity. The ROE signals were

P-CHIRAL OLIGONUCLEOTIDES
181
Figure 2. Schematic view of RP and SP, 50-O-monomethoxytrityl-20-O-deoxyadenosine 30-O-
(O-4-nitrophenyl)methanephosphonate (3). (a) RP50-O-Monomethoxytrityl-20-deoxyadeno
sine30-O-(O-4nitrophenyl)methanephosphonate (3, B ¼ AdeBz). (b) SP50-O-Monomethoxy-
trityl-20-deoxyadenosine30-O-(O-4nitrophenyl)methanephosphonate (3, B ¼ AdeBz).
integrated and the distances between the PCH3 group and H30 or H40 were
estimated for d(APMeT), d(APMeA) and d(TPMeA) dimers. The average sum
of the distances PCH3=H30 and PCH3/H40 is equal to 626 pm. For the SP
isomers both distances were equal within the experimental error, and for the
RP isomers the distance of the PCH3 group to H30 is shorter by more than
50 pm31.
We applied the same method to the deoxyribonucloside methylphos-
phonates 3, keeping in mind that the flexibility of mononucleosides is
supposed to be higher than the flexibility of the dimers due to lower steric
hindrance and lack of stacking stabilization. In all cases ROEs were visible.
The diastereomers 3-SLOW (the SLOW descriptor is used for the diaster-
eomer characterized by the lower mobility on a TLC plate) were assigned as
the RP isomers on the basis of two diagnostic crosspeaks between PCH3
and H30 and H40. For the FAST diastereomer, including 20-O-deox-
yadenosine and thymine derivatives, only ROE to H30 was detected, as
expected for the opposite Sp isomer (Table 4). 20-O-Deoxycytidine and 20-
O-deoxyguanosine methylphosphonates 3-FAST isomers are giving ROE
signals to H40 weaker than to H30, but recognizable. This observation

182
JANKOWSKI, OLEJNICZAK, AND LESNIKOWSKI
Table 1. UV and Chromatographic Characteristics of 50-O-Monomethoxytrityl-20-O-deoxy-
nucleoside 30-O-(O-4-Nitrophenyl)methanephosphonate (3) (B ¼ Thy, CytBz, AdeBz, GuaiBu
)
Isomer
SP-dT
RP-dT
Sp-dC
RP-dC
SP-dA
RP-dA
SP-dG
RP-dG
1
UV lmax
UV lmin
262.4
245.3
0.66
265.1
245.2
0.56
261.1
243.6
0.68
261.8
245.3
0.55
278.9
251.1
0.63
279.1
251.5
0.52
261.1
241.7
0.44
261.1
242.4
0.29
1
2
TLC ½RfŠ
1 In 95% C2H5OH. 2 Developing solvent system CHCl3:CH3OH ¼ 95:5.
Table 2. Chemical Shifts (ppm) at 25ꢀC
Sp-T Rp-dC Sp-dC Rp-dA Sp-dA
29.99 30.19 29.82 30.25
11.28 11.17 11.20
Rp-T
Rp-dG
Sp-dG
31P30.36
NH
NHBz=i
Bu
H8
H6
H5
H2
29.96
11.38
30.25
11.27
29.85
12.05
11.60
11.39
12.06
11.61
8.59
8.59
8.59
8.59
8.16
8.14
7.47
7.47
8.15
7.20
8.15
7.15
CH3
PCH3
H10
1.49
1.79
6.21
2.50
2.50
5.21
4.09
1.46
1.76
6.20
2.43
2.43
5.19
4.16
3.25
1.79
6.17
2.45
2.70
5.17
4.21
3.31
1.76
6.14
2.39
2.61
5.16
4.27
3.33
1.84
6.53
2.73
3.21
5.45
4.24
3.20
1.83
6.49
2.65
3.24
5.42
4.32
3.29
1.82
6.28
2.68
3.04
5.23
4.21
3.19
1.80
6.25
2.60
2.99
5.24
4.21
3.23
H20
H200
H30
H40
H50=
H50
3.17=3.
4-NO2- 7.352,6
Ar 8.183,5
4-MeO- 7.182,6
7.432,6
8.243,5
7.212,6
6.873,5
3.72
7.382,6
8.193,5
7.202,6
6.843,5
3.70
7.472,6
8.253,5
7.222,6
6.883,5
3.72
7.442,6
8.213,5
7.142,6
6.783,5
3.70
7.472,6
8.253,5
7.162,6
6.813,5
3.70
7.392,6
8.193,5
7.132,6
6.773,5
3.70
7.432,6
8.243,5
7.152,6
6.793,5
3.71
Ar
6.843,5
3.71
MeO
Ph
Ph(Bz)
7.21–7.36 7.24–7.40 7.22–7.35 7.25–7.40 7.16–7.34 7.16–7.35 7.16–7.30 7.18–7.34
8.002,6
7.503,5
7.624
7.992,6
7.503,5
7.624
8.042,6
7.543,5
7.644
8.042,6
7.543,5
7.644
i-Pr
1.11
(6H);
2.74
(1H)
1.12
(6H);
2.74
(1H)

P-CHIRAL OLIGONUCLEOTIDES
Table 3. J(H,H) Coupling Constants (Hz) at 25ꢀC
183
J [Hz]
Rp-T
6.9
Sp-T
7.0
Rp-dC
Sp-dC
Rp-dA
Sp-dA
Rp-dG
Sp-dG
1020
10200
2030
6.2
6.4
6.1
3.7
6.2
6.4
6.7
3.7
6.9
6.9
6.1
3.2
6.7
6.7
6.0
3.2
6.0
7.7
6.3
2.2
6.1
7.3
6.0
3.1
6.9
6.3
7.0
6.4
20030
20200
30400
3.3
À 14.0
3.3
À 14.6
À 14.4
À 14.3
À 14.0
À 14.3
À 14.4
À 14.2
2.9
4.0
3.7
3.7
4.0
4.0
3.8
4.0
3.4
5.2
3.3
6.1
2.5
4.9
2.8
4.9
40500
405
4.0
3.7
*
17.8
7.5
4.0
4.0
*
17.7
7.8
5.2
6.1
4.9
4.9
À 14.5
50500
PCH3
30P8.2
À 12.2
17.7
7.0
À 12.3
17.7
7.0
À 10.4
17.8
8.1
À 15.1
17.7
7.9
À 14.5
17.8
7.0
17.7
* Overlaps with OH.
means that the PCH3 group is shifted towards H30, but still close enough to
H40 to generate ROE, as previously was detected for dinucleotides
d(TPMeT) and d(TPMeA)31.
The chirality at phosphorus is designed as RP and SP, according to the
Cahn-Ingold-Prelog rules. It should be pointed out that, because of the
replacement of the nucleosidyl residue in the dinucleotide discussed above for
the 4-nitrophenyl group in 3, and the substituent priority rules (4-nitrophe-
nyl > nucleosidyl), the same spatial orientation of the ligands at phosphorus
atom corresponds to different configurational descriptors according to the
R=S nomenclature. However, it seems that the rotational freedom around the
P-O and O-C bonds in 3 leads to the same relation between R=S config-
uration at P-atom and the relative intensities of P-CH3 and H30=H40 cross-
peaks in the ROESY experiments as for dinucleotides.
The crosspeaks of the PCH3 group were integrated and distances to H30
and H40 were estimated (Table 4) using the internal distance H20=H200
(180 pm) for the distance calibration39. For the thymidine derivative vicinal
20=200 protons are overlapping and distance estimation was impossible.
Table 4. Estimated Distances in Pm Between PCH3 and H30 or H40 for (3)
dGiBu
dABz
dCBz
Rp
Sp
RP
SP
RP
SP
PCH3=H30
PCH3H40
273
281
273
327
246
302
235
–
218
273
278
368

184
JANKOWSKI, OLEJNICZAK, AND LESNIKOWSKI
The estimated distances were used to build (HYPERCHEM 4.5) models
illustrating the position of the P-CH3 group in Sp and Rp isomers (Fig. 2). As
an example, diagnostic parts of the SP-dA 3 and RP-dA 3 spectra are shown
in Fig. 3 and 4.
The absolute configuration at phosphorus in 3 established by ROESY
experiments correlates well with 31PNMR chemical shifts and chromato-
graphic mobility on TLC plates. In all cases diastereomers with assigned SP
configuration and higher mobility on TLC (designed as FAST) were shifted
up-field by about 0.4 ppm in the 31PNMR spectra.
The sugar rings in nucleic acids exist in an equilibrium between two
conformers – called north (N) and south (S). We were interested if the
conformation on the phosphorus atom may affect this equilibrium in the case
of monomers 3. The conformational analysis of a five-membered ring is
facilitated by the Altona-Sundaralingam concept40,41. Each conformer is
described by three parameters: the puckering amplitude ðFmÞ, the phase angle
of pseudorotation (P) and its population. We analyzed 31PNMR decoupled
spectra by the program PSEUROT, which calculates the best fit of the five
pseudorotational parameters and defines the two-state equilibrium. The
puckering amplitudes were held constant and equal to one another, and in
Figure 3. The diagnostic part of RP-dA 3 ROESY spectrum.

P-CHIRAL OLIGONUCLEOTIDES
185
Figure 4. The diagnostic part of SP-dA 3 ROESY spectrum.
every fitting raised in 1ꢀ steps between 30ꢀ and 42ꢀ. The herein described
preliminary analysis was limited to the spectra recorded at 25ꢀC. It was found
that in all cases, independently of the Patom configuration, the south con-
formation dominates.
Table 5. Pseudorotational Parameters P and F (ꢀ), Percentage Populations North and South
and Rms Errors (Hz)
Deoxyribonucloside Methanephosphonate
Parameter Sp-dG
Rp-dG
Sp-dA
Rp-dA
Sp-dC
Rp-dC
Sp-T
179.4
31.0
40.3
31.0
Rp-T
SP200.0
SF
211.1
33.0
39.2
33.0
174.9
32.0
13.4
31.0
0.77
0.117
176.5
31.0
27.0
31.0
0.67
0.186
142.4
31.0
154.0
30.0
175.3
32.0
0.50
32.0
0.59
0.290
30.0
19.3
30.0
0.70
0.079
NP13.8
NF
%S
À 16.3
32.0
0.65
0.134
30.0
0.71
0.068
0.60
0.148
0.61
0.092
Rms error

186
JANKOWSKI, OLEJNICZAK, AND LESNIKOWSKI
EXPERIMENTAL
Materials
Deoxynucleosides (B ¼ Thy, Cyt, Ade, Gua) were obtained from
Pharma Waldhof GmbH (Dusseldorf, Germany). Column chromatography
was performed on silica gel 230-400 mesh obtained from Sigma-Aldrich.
(Steinheim, Germany) or reverse phase silica gel C8 from Fluka (Steinheim,
Germany). TLC was performed on silica gel F254 plates purchased from
Sigma-Aldrich (Steinheim, Germany). Solvents were purchased in the highest
available quality. UV measurements were performed on GBC Cintra10e UV-
VIS spectrometer (Dandenong, Australia). NMR spectra were recorded with
a Bruker Avance DPX 250 spectrometer equipped with BB inverse probe-
head. Tetramethylsilane and 85% H3PO4 were used as external standards for
1H-NMR and 31P-NMR, respectively.
Methods
50-O-Monomethoxytrityl-20-O-deoxynucleoside(B ¼ Thy, CytBz, AdeBz,
GuaiBu) (1) were synthesized as described 42.
RP and SP 50-O-Monomethoxytrityl-20-O-deoxynucleoside 30-O-(O-4-
nitrophenyl)methyl-phosphonate (3) (B ¼ Thy, CytBz, AdeBz, GuaiBu). [O-
(4-nitrophenyl)]methyl-phosphonochloridate (2, 3.3 g, 14.0 mmol)38,
1,2,4-triazole (3.6 g, 52.0 mmol) and triethylamine (2.7 g, 3.7 mL, 26.5 mmol)
were mixed together in anhydrous CH3CN (50 mL) under nitrogen. After 15
min at room temperature, to the resultant clear solution of [O-(4-
nitrophenyl)]methylphosphonotriazolidate a solution of the proper O,N-
protected nucleoside 1 (5.6 mmol) was added under nitrogen. The reaction
progress was monitored by TLC (CHCl3:CH3OH ¼ 95:5). After completion
of the reaction (ca. 3 h), CH3CN was evaporated under reduced pressure,
then the oily residue was dissolved in CHCl3 (100 mL). The CHCl3
solution was washed with H2O ð3 Â 50 mL). The organic fraction was
separated, dried over MgSO4, and then CHCl3 was evaporated under
reduced pressure yielding crude 3 as a mixture of RP and SP diastereomers.
Monomer 3 was then purified and separated into individual diastereomers on
reverse phase C8 silica gel column using CHCl3 as eluting solvent system, or
alternatively on normal phase silica using a gradient of CH3OH in CHCl3.
The yield of 3 as SP; RP and SP þ RP fraction was 35–50%. The UV and
1
chromatographic characteristics are presented in Table 1, 31P- and H-
NMR data are shown in Tables 2 and 3.
NMR measurements. NMR spectra were recorded at 25ꢀC. Samples
were prepared by dissolving a quantity of 3 (10 mg) in DMSO-d6 (0.7 mL,
99.5% D, final concentration of 3 1.7–2.0 mM) and solutions were purged

P-CHIRAL OLIGONUCLEOTIDES
187
with dry Ar for 30 min. Spectra were referenced to the residual DMSO signal
at 2.49 ppm. ROESY experiments were carried out with an average field of
2.2 kHz and 350 ms duration. The transmitter was set to 6 ppm. The
coupling constants were derived from 1D homodecoupling experiments
and verified through the simulation procedure by the DAISY (Bruker)
procedure. Integration of 2D spectra was performed using X-WIN NMR
2.1 (Bruker) program subroutine. For the PSEUROT analyses [PSEUROT
6.2, J. van Wijk and C. Altona, Leiden Institute of Chemistry, Leiden
University, Netherlands], we have used the following l electro-negativites:
l
(C10) ¼ l(CH2OR) ¼ 0:68; l (C20) ¼ 0.72,
l
(C30) ¼ l(C40) ¼0.62,
l ¼ ðO40Þ ¼1:40; l(-O-P(O)(CH3)O-) ¼ 1.29 as for OPO(OEt)2 group40,41
.
ACKNOWLEDGMENTS
Supported in part by the Polish Foundation for the Promotion of Sci-
ence (FNP), Project Phare SCI-TECH II, grant number PL9611=98=03-02.9,
Polish Committee for Scientific Research (KBN) grant number 6 PO5F 023
20p02 and Technical University of Lodz.
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Received August 14, 2001
Accepted December 7, 2001