Conformational and configurational study of 2-phenyl-4-aryl-1,3,2- dioxaphosphonates derived from xylofuranose derivatives. the anomeric effect in the spontaneous isomerization of cyclic phosphonates

Article abstract of DOI:10.1016/j.tetlet.2012.06.103

A series of 2-phenyl-4-aryl-1,3,2-dioxaphosphonates derived from xylofuranose derivatives were synthesized in two-steps from diacetone-d-glucose in order to carry out conformational and configurational studies. The absolute stereochemistry at the phosphorus atom was assigned by analyzing the ¹H NMR chemical shifts for each diastereoisomeric pair. This empirical assignment criterion was corroborated by X-ray crystallographic studies. Additionally, in the case of two cyclic phosphonates, spontaneous conversion into two more stable diastereoisomeric phosphonates occurred either in column chromatography or in solution (approx. 300 h in solution CDCl ₃). This is attributed to the cleavage of the O3-C4 bond within the P-heterocyclic ring, which is induced by the anomeric effect (n_O → π_PO).

Full text of DOI:10.1016/j.tetlet.2012.06.103

Tetrahedron Letters 53 (2012) 4727–4732
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Conformational and configurational study of
2-phenyl-4-aryl-1,3,2-dioxaphosphonates derived from xylofuranose
derivatives. The anomeric effect in the spontaneous isomerization of cyclic
phosphonates
Leticia Quintero a, Herbert Höpfl b, Martha Sosa-Rivadeneyra a, Aurelio Ortiz a,b, Rosa L. Meza-León a,
a,
Silvano Cruz-Gregorio a, , Fernando Sartillo-Piscil
⇑
⇑
a Centro de Investigación de la Facultad de Ciencias Químicas, BUAP, 14 Sur Esq. San Claudio, San Manuel, 72570 Puebla, Mexico
b Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, 62209 Cuernavaca, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-phenyl-4-aryl-1,3,2-dioxaphosphonates derived from xylofuranose derivatives were synthe-
Received 5 June 2012
Revised 22 June 2012
Accepted 24 June 2012
Available online 28 June 2012
sized in two-steps from diacetone-D-glucose in order to carry out conformational and configurational
studies. The absolute stereochemistry at the phosphorus atom was assigned by analyzing the 1H NMR
chemical shifts for each diastereoisomeric pair. This empirical assignment criterion was corroborated
by X-ray crystallographic studies. Additionally, in the case of two cyclic phosphonates, spontaneous con-
version into two more stable diastereoisomeric phosphonates occurred either in column chromatography
or in solution (approx. 300 h in solution CDCl3). This is attributed to the cleavage of the O3–C4 bond
Keywords:
Conformation
Configuration
Anomeric effect
Prodrugs
within the P-heterocyclic ring, which is induced by the anomeric effect (nO
?
p⁄P@O).
Ó 2012 Elsevier Ltd. All rights reserved.
Phosphonates
1,3,2-Dioxaphosphorinanes and related six-membered ring
compounds have attracted attention in recent decades due to their
relevance in the field of physical organic chemistry (stereochemis-
try and conformational)1 and in the area of medicinal chemistry
with clinical applications (e. g., prodrugs).2 Because of this, our re-
search groups have been interested in the conformational and con-
figurational study of 1,3,2-dioxaphosphorinane analogues of cAMP
and cGMP,3 such as 1,3,2-dioxaphosphorinanes derived from car-
bohydrate derivatives.4 We were able to observe two major confor-
mational equilibria: C1 ꢀ B1 and C1 ꢀ T, in solution and solid
state. For cyclic phosphates having RP configuration and an equa-
torially oriented phenyl group at the C-4 position, a chair-boat
equilibrium (C1 ꢀ B1) was proposed, whereas a chair-twist equi-
librium (C1 ꢀ T) was suggested for the corresponding SP-epimers
(Scheme 1).4 Conformers C2 and B2, of which B2 has been
proposed in related cyclic phosphates,5 were not observed. The
conformational equilibrium C1 ꢀ B1 is supported by a particular
solid-state structure, in which both conformers have been trapped
within the same crystal lattice (for R = Ph equatorially oriented).4a
Additionally, the structures of conformers C1 and T have been
determined by crystallographic studies,4c,e,f including the case of
a cyclic nucleotide that presented a chair conformation bearing
an axially oriented aryl group at C-4.4f
The above-mentioned conformational study with an aryl group
at C-4 position gained relevance for concrete applications thanks
to the publications of Erion and coworkers on the use of cyclic phos-
ph(on)ate prodrugs for the treatment of liver diseases such as hep-
atitis B and C (HepDirect prodrugs).2 Erions group found that the
´
biologically most active prodrugs carry an aryl group at C-4 and
are cis oriented with respect to the substituent at the phosphorus
atom. In the attempt to associate the biological activity of prodrugs
(prodrug cleavage) with the conformational behavior of cyclic
phosphates having a 4-methoxylphenyl group at C-4, we found that
some of them (A, B, and C) undergo a rapid, selective, and spontane-
ous conversion into the corresponding stable phosphate (E). This
conversion (or isomerization) occurs at different rate via a common
intermediate D, which is formed by the cleavage of the O3–C4 bond.
In a similar manner, cyclic phosphonate F was also spontaneously
converted (via intermediate H) very slowly into phosphonate G
(Scheme 2).4e The responsibility for the weakening of the O3–C4
bond was attributed to the anomeric effect, which induces the bond
cleavage and promotes the isomerization (Scheme 3).
⇑
Corresponding author. Tel.: +52 2222295500x7391.
E-mail addresses: fernando.sartillo@correo.buap.mx, silvano.cruz@correo.
Two routes were proposed for the conversions: the r⁄- and
the p⁄-route (Eqs. 1 and 2, respectively), both referring to the
buap.mx (F. Sartillo-Piscil).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.103

4728
L. Quintero et al. / Tetrahedron Letters 53 (2012) 4727–4732
Y
O
O
O
R
RP
O
P
O
R
O
Y
P
O
Y
O
O
O
P
O
O
O
R
O
O
O
O
C1
B1
O
C2
R = Aryl and alkyl. Y = OPh, nucleoside
NOT OBSERVED
R
O
O
R
O
O
O
SP
R
O
O
Y
O
P
O
O
O
P
O
O
O
O
O
P
Y
O
O
B2
Y
T
C1
Scheme 1. Conformational equilibria for 4-substituted-1,3,2-dioxaphosphorinanes derived from xylofuranose derivatives.
O
O
O
O
P
O
O
O
OPh
O
A
t85>45 h
O
O
O
O
O
O
O
4
O
O3
O
O
t85=5 h
P
O
O
O
P
P
O
O
O
OPh
O
O
O
E
OPh
OPh
B
D
O
t85=6.5 h
O
O
O
O
OPh
P
O
O
C
O
O
O
O
O
O
4
O
O
t60>30 days
O
O3
O
O
P
P
P
O
O
O
O
O
O
O
O
O
H
F
G
Scheme 2. Selective isomerization of cyclic phosph(on)ates.
molecular orbital involved in the hyperconjugation process
(Scheme 3).4e,f It was suggested that for cyclic phosphates (A–C),
both routes (r⁄ and p⁄) are involved, whereas for phosphonate F
only the p⁄ route was invoked. As these studies might be useful
for understanding the mechanistic course for the selective cleavage
of cyclic phosph(on)ate prodrugs,2a we now report on the confor-
mational behavior of 2-phenyl-4-aryl-1,3,2-dioxaphosphonates
derived from xylofuranose derivatives (cyclic phosphonate ana-
logues to phosphonate prodrugs), and provide further evidence
for the influence of the anomeric effect (via the p⁄-route) on the
spontaneous isomerization of cyclic phosphonates.
X
X
P
X
P
P
eq. 1
O
O
O
O
O
O
C4
O
O3
O
..
nO
*
σ
O
P
O
P
O
P
O
R
R
O
eq. 2
O
R
C4
O3
O
O
..
nO
*
π
Phosphonates 4a–c to 7a–c were prepared in good yields by
phosphorylation of the corresponding 1,3-diols 2a–c and 3a–c
Scheme 3. Anomeric effect involved in the O3–C4 bond cleavage of cyclic
phosphates (Eq. 1 and 2) and cyclic phophonates (Eq. 2).

L. Quintero et al. / Tetrahedron Letters 53 (2012) 4727–4732
4729
O
O
O
O
HO
O
O
1
refs. 4a and 4e
HO
R
HO
O
O
O
R
HO
HO
3a-c
O
O
2a-c
a: R = p-F-Ph
b: R = p-Cl-Ph
PhP(O)Cl2
/NEt3 in CH2Cl2
c: R = p-OMe-Ph
R
R
O
R
R
O
O
P
O
O
O
P
O
O
O
Ph
O
O
P
O
Ph
O
O
O
+
P
O
O
Ph
O
+
O
O
Ph
O
O
O
6a-c
O
7a-c
4a-c
5a-c
6a (40%)
7a (48%)
7b (48%)
4a (56%)
4b (65%)
4c (60%)
5a (29%)
5b (22%)
5c (30%)
6b (37%)
6c (41%)*
7c (41%)*
*Yields determined by 1H NMR
Scheme 4. Synthesis of cyclic phosphonates.
Cyclic phosphonates 4a-c
Cyclic phosphonates 5a-c
3
3
δ (ppm) and JH5-P (Hz) for H5
5a = 5.31 and <1.0
δ (ppm) and JH5-P (Hz) for H5
4a = 5.98 and <1.0
4b = 5.97 and <1.0
4c = 5.94 and <1.0
5b = 5.31 and <1.0
5c = 5.27 and <1.0
31P δ (ppm)
H5
Ar
H5
Ar
31P δ (ppm)
5a = 11.7
5b = 11.8
5c = 11.8
O
O
4a = 16.9
4b = 17.0
4c = 16.8
O
O
O
O
Ph
Ph
P
P
O
O
O
O
H3
O
H3
O
3
3
δ (ppm) and JH5-P (Hz) for H3
δ (ppm) and JH5-P (Hz) for H3
5a = 4.87 and 3.6
5b = 4.86 and 4.4
5c = 4.85 and 3.5
4a = 5.18 and <1.0
4b = 5.16 and <1.0
4c = 5.14 and <1.0
Cyclic phosphonates 6a-c
Cyclic phosphonates 7a-c
3
3
δ (ppm) and JH5-P (Hz) for H5
δ (ppm) and JH5-P (Hz) for H5
7a = 5.59 and 6.4
7b = 5.59 and 6.4
7c = 5.58 and 6.3
6a = 5.45 and 10.4
6b = 5.43 and 9.9
6c = 5.46 and 10.4
31P δ (ppm)
6a = 13.7
6b = 13.8
6c = 13.1
H5
Ar
H5
31P δ (ppm)
7a = 17.2
7b = 17.3
7c = 16.7
Ar
O
O
O
O
O
O
Ph
Ph
P
P
O
O
O
O
H3
O
H3
O
3
3
δ (ppm) and JH5-P (Hz) for H3
7a = 4.62 and 3.6
δ (ppm) and JH5-P (Hz) for H3
6a = 5.04 and 3.2
6b = 5.02 and 3.3
6c = 5.05 and 3.3
7b = 4.62 and 3.6
7c = 4.63 and 4.2
Chart 1. Relevant NMR data for cyclic phosphonates 4a–c, 5a–c, 6a–c, and 7a–c (a: Ar = p-F-Ph; b: Ar = p-Cl-Ph; c: Ar = p-OMe-Ph).

4730
L. Quintero et al. / Tetrahedron Letters 53 (2012) 4727–4732
(which were prepared in a sequential ‘one-pot’ procedure from
1,2:5,6-O-diisopropyliden-
-glucofuranose 1),4a,e and phenyl-
strong
1,3-steric interactions
a-D
phosphonic dichloride in the presence of triethylamine as base at
0 °C (Scheme 4).
H
H
O
O
Ar
O
P
P
O
O
O
O
O
O
The resulting cyclic phosphonates, except for 6c and 7c which
both spontaneously were converted into phosphonates 4c and 5c
(vide infra), were efficiently separated by flash chromatography.
The diastereoisomeric ratios were determined by analyzing the
reaction crude with 1H and 31P NMR techniques, and are in concor-
dance with the obtained yields. Since the stereochemistry of the
1,3-diol precursors is known, we could proceed to assign the abso-
lute stereochemistry at the P-atom of the cyclic phosphonates and
establish the respective conformations. Relevant NMR data such as
the 1H and 31P NMR chemical shifts and the 3JHH and 3JHP coupling
constants are depicted in Chart 1.
The analysis of the chemical shift displacements for the hydro-
gen atoms in the xylofuranose moiety revealed that, for the series
5a–c and 7a–c the H-3s hydrogen atoms are centered at higher
fields than the H-3s of the corresponding P-epimers 4a–c and
6a–c. This suggests that, these hydrogen atoms are affected by a
shielding effect from the diamagnetic current induced by the aro-
matic ring of the phenyl group attached to the phosphorus atom,6
and therefore, it can be established that phenyl group is
cis-oriented to H-3. With these data, the absolute configuration
at the phosphorus atom can be assigned as SP for 5a–c and 7a–c,
and as RP for the corresponding diastereomeric P-epimers
(Fig. 1). Although 31P NMR is commonly utilized for the determina-
tion of the configuration at the phosphorus atom for six-membered
ring phosph(on)ates (Gorenstein’s criteria),7 it has been reported
that misleading assignments can occur when the substituent at
O
Ar
O
O
T
C1
7a-c
1,3-steric interactions
O
P
O
P
H
H
O
O
O
Ar
O
O
O
O
O
Ar
O
O
C1
6a-c
T
Scheme 5. Proposed conformational equilibria for the cyclic phosphonates 7a–c
and 6a–c.
O
Ar
Figure 3. Perspective view of the molecular structure of 4c, showing
conformation.
a C1
O
Ar
O
Ph
O
O
O
P
O
O
RP
O
P
O
SP
O
H3
O
H3
δ ~ 4.6-4.8 ppm
5a-c and 7a-c
the P-atom is not permanently axially oriented.4e Therefore we
decided not to use that criteria.
Fortunately, a crystallographic study of the cyclic phosphonate
7a could be carried out, showing that the stereochemical model
depicted in Figure 1 was correct.8 This crystallographic study re-
vealed a twisted conformation, which is in accordance with the
H5-P vicinal coupling constant (3JH5-P = 6.4 Hz). Very similar values
δ ~ 5.1ppm
4a-c and 6a-c
Figure 1. Stereochemical model for the assignment of the absolute configuration at
the phosphorus atom in the cyclic phosphonates 4a–c, 5a–c, 6a–c, and 7a–c.
of H5-P coupling constants were observed for 7b and 7c
3
(3JH5-P = 6.4 Hz and JH5-P = 6.3 Hz, respectively), suggesting
a
C1 ꢀ T conformational equilibrium, which is shifted toward the T
O
Ar
Ar
O
O
O
O
O
P
O
O
P
O
O
O
O
H3
T
H3
O
C1
4a-c
O
Ar
O
O
P
O
O
O
O
Ar
P
O
O
H3
C1
5a-c
O
O
H3
B1
nO π*
Figure 2. Perspective view of the molecular structure of 7a, showing
conformation.
a
T
Scheme 6. Proposed conformational equilibria for the cyclic phosphonates 4a–c
and 5a–c.

L. Quintero et al. / Tetrahedron Letters 53 (2012) 4727–4732
4731
O
O
O3
O
C4
Ar
O
O
O
O
O
P
O
P
O
O
Ar
J
O
C1
6c
Ar
O
O
O
O
P
O
4c
O
O
O
O
O
P
O
I
O
O
Ar
O
O
O
O
O
P
Ar O
Ar O
O
O
O
C
5c
O
O
4
P
O
O3
P
O
O
O
K
T
7c
Scheme 7. Conversion of 6c and 7c into 4c and 5c via the p⁄-route.
conformation in order to release the twofold syn-1,3-steric interac-
tions (Fig. 2 and Scheme 5).
intermediary I. Then, collapsing of the contact ion pair (I) affords
directly 4c, meanwhile 5c is formed, first by scrambling and then
by collapsing of the contact ion pair (Scheme 7). It is important
to mention that, phosphonate 4c does not undergo conversion into
phosphonate 5c (or viceversa), even after a very long time
(months).
In conclusion, the results described herein confirm that the use
of a xylofuranose carbohydrate template facilitates the conforma-
tional and configurational study of cyclic six-membered ring phos-
phonates. In consequence, this study provides further evidence
that the anomeric effect plays a key role in both: the conforma-
tional behavior and the isomerization process of six-membered
ring phosphonates. These results support the proposal regarding
the beneficial influence of the anomeric effect in the cyclic
phosph(on)ate prodrugs’ cleavage.
In a similar way but to a different extension, the somewhat
larger values for the H5-P coupling constants of the cyclic phos-
phonates 6a–c (3JH5-P ꢀ10 Hz) suggest similar conformational
equilibria (C1 ꢀ T), but now with a more equimolar distribution
of the chair and twist conformers (Scheme 5). This proposal is sup-
ported by a recent report regarding a cyclic phosphate, in which
the chair conformation C1 (with an aryl group axially oriented at
C-4 position), was observed in solid state, and the T conformation
in solution.4f
In contrast, for the cyclic phosphonates 4a–c and 5a–c the vic-
inal H5-P coupling constants had values less than 1 Hz, suggesting
that the aryl group at C-5 spends more time in equatorial position.
Therefore, is proposed that the C1 conformation is predominant in
solution, especially for series 4a–c. X-ray crystallographic studies
of the cyclic phosphonate 4c confirmed the NMR assignment and
the proposed conformation (Fig. 3).9 Therefore, for 4a–c a C1 ꢀ T
and for 5a–c a C1 ꢀ B1 conformational equilibrium is proposed.
The reason for the proposal of a B1 conformation for series 5a–c,
instead of a twisted conformation (like the other phosphonates),
is because in this conformation, the pseudo-axial orientation of
Acknowledgments
We thank Consejo Nacional de Ciencia y Tecnología (CONACyT)
and Benemérita Universidad Autónoma de Puebla-Programa de
Mejoramiento del Profesorado (BUAP-PROMEP) for financial
support.
the P@O bond is prone to nO
?
p⁄ orbital interactions (anomeric
effect, see Scheme 6).10
Supplementary data
As mentioned above, during the separation by column chroma-
tography, conversion (or isomerization) of phosphonates 6c and 7c
into phosphonates 4c and 5c occurred. Contrary to the isomeriza-
tion process, where only one phosphonate is spontaneously
converted into a more stable phosphonate,4e in this case, two phos-
phonates in a 1:1 ratio are converted into two stable phosphonates
in a 4:1 ratio (6c and 7c into 4c and 5c, respectively), either in
chromatographic column or very slowly in solution (more than
300 h in CDCl3). Like previous report, we propose that the conver-
sion proceeds via the p⁄-route (see Eq. 2 and Scheme 3). In this
case, a common intermediate I is formed by the cleavage of the
O3–C4 bond, which is induced by the anomeric effect (Scheme
7). Thus, the cyclic phosphonate 6c having C1 conformation and
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
06.103.
References and notes
1. (a) Frank, É.; Wolfling, J. Curr. Org. Chem. 2007, 11, 1610; (b) Gallagher, M. J. In
Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J. G., Quin,
L., Eds.; VCH: Deerfield Beach, FL, 1987; (c) Bentrude, W. G.; Setzer, W. N.
Stereospecificity in 31P-Element Coupling Proton-Phosphorus Coupling. In
Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J. G., Quin,
L. D., Eds.; VCH: Weinheim, 1987; (d) Bentrude, W. G. Steric and
Stereoelectronic Effects in 1,3,2-Dioxaphosphorinanes. In Methods in
Stereochemical Analysis; Juaristi, E., Ed.; VCH: New York, 1995.
the cyclic phosphonate 7c having
nO hyperconjugation to give mesomers J and K, respec-
tively, which weaken the respective O3–C4 bond to generate
T conformation enable
2. (a) Erion, M. D.; Reddy, K. R.; Boyer, S. H.; Matelich, M. C.; Gomez-Galeno, J.;
Lemus, R. H.; Ugarkar, B. G.; Colby, T. J.; Schanzer, J.; van Poelje, P. D. J. Am.
Chem. Soc. 2004, 126, 5154; (b) Erion, M. D.; van Poelje, P. D.; Mackenna, D. A.;
?
p⁄
P@O

4732
L. Quintero et al. / Tetrahedron Letters 53 (2012) 4727–4732
Colby, T. J.; Montag, A. C.; Fujitaka, J. M.; Linemeyer, D. L.; Bullough, D. A. J.
6. Frank, E.; Körtvélyesi, T.; Czugler, M.; Mucsi, Z. Keglevich. Steroids 2007, 72, 437.
7. (a) Gorenstein, D. G. In Phosphorus-31 NMR Principles and Applications;
Gorenstein, D. G., Ed.; Academic Press: New York, 1984; (b) Gorenstein, D.
G.; Rowell, R.; Findlay, J. J. Am. Chem. Soc. 1980, 102, 5077; (c) Majoral, J. P.;
Pujol, R.; Navech, J. Bull. Soc. Chim. Fr. 1972, 606.
Pharmacol. Exp. Ther. 2005, 312, 554; (c) Hecker, S. J.; Reddy, K. R.; van Poelje, P.
D.; Sun, Z.; Huang, W.; Varkhedkar, V.; Reddy, M. V.; Fujitaki, J. M.; Olsen, D. B.;
Koeplinger, K. A.; Boyer, S. H.; Linemeyer, D. L.; MacCoss, M.; Erion, M. D. J. Med.
Chem. 2007, 50, 3891; (d) Borch, R. F.; Millard, J. A. J. Med. Chem. 1987, 30, 427;
(e) Hu, L.; Yu, C.; Jiang, Y.; Han, J.; Li, Z.; Browne, P.; Race, P. R.; Knox, R. J.;
Searle, P. F.; Hyde, E. I. J. Med. Chem. 2003, 46, 4818; (f) Li, Z.; Han, J.; Jiang, Y.;
Browne, P.; Knox, R. J.; Hu, L. Bioorg. Med. Chem. 2003, 11, 4171.
8. Crystal data for C20H20FO6P (7a), Mr = 406.33, 0.07 Â 0.31 Â 0.55 mm3,triclinic,
space group P1, a = 9.833(2), b = 9.913(2), c = 10.914(3) Å,
a = 101.397(4),
b = 96.714(4),
2hmax = 50.00, 9432 reflections measured, 6895 unique (Rint = 0.039),
R1 = 0.0677 for 3827 reflections with I > 2 (I) and wR2 = 0.1579 for all data,
c qcalcd = 1.343, T = 293 K,
= 102.291(4) °, V = 1004.8(4) Å3, Z = 2,
3. Nelson, K. A.; Bentrude, W. G.; Setzer, W. N.; Hutchinson, J. P. J. Am. Chem. Soc.
1987, 109, 4058.
r
4. (a) Sartillo-Piscil, S.; Cruz-Gregorio, S.; Sánchez, M.; Höpfl, H.; Anaya de Parrodi,
C.; Quintero, L. Tetrahedron 2003, 59, 4077; (b) Sartillo-Piscil, F.; Cruz-Gregorio,
S.; Sánchez, M.; Quintero, L. Tetrahedron 2004, 60, 3001; (c) Cruz-Gregorio, S.;
Sánchez, M.; Clara-Sosa, A.; Bérnes, S.; Quintero, L.; Sartillo-Piscil, F. J. Org.
Chem. 2005, 70, 7107; (d) Romero, M.; Hernández, L.; Quintero, L.; Sartillo-
Piscil, F. Carbohydr. Res. 2006, 341, 2883; (e) Cruz-Gregorio, S.; Rodrıguez-
Palacios, V.; Höpfl, H.; Quintero, L.; Sartillo-Piscil, F. J. Org. Chem. 2009, 74, 197;
(f) Quintero, L.; Sánchez-Vázquez, M.; Cruz-Gregorio, S.; Sartillo-Piscil, F. J. Org.
Chem. 2010, 75, 5852.
509 parameters, GOF = 0.938. CCDC-660541.
9. Crystal data for C21H23O7 P (4c), Mr = 418.36, 0.24 Â 0.27 Â 0.30 mm3, triclinic,
space group P212121, a = 13.2510(8), b = 19.6905(12), c = 7.6822(5) Å,
V = 2004.4(2) Å3, Z = 4,
q
calcd = 1.386, T = 293 K, 2hmax = 50.00, 19525
reflections measured, 3534 unique (Rint. = 0.038), R1 = 0.0395 for 3232
reflections with I > 2 (I) and wR2 = 0.1014 for all data, 265 parameters,
r
GOF = 1.066. CCDC-880585.
10. Hernández, J.; Ramos, R.; Sastre, N.; Meza, R.; Hommer, H.; Salas, M.; Gordillo,
B. Tetrahedron 2004, 60, 10927.
5. (a) Hermans, R. J. M.; Buck, H. M. J. Org. Chem. 1988, 53, 2077; (b) Neeser, J.-T.;
Tronchet, J. M. J.; Charollais, E. J. Can. J. Chem. 1983, 61, 13; (c) Morr, M.; Ernts,
L.; Mengel, R. Liebigs Ann. Chem. 1982, 651.