Crystal structures, circular dichroism spectra and absolute configurations of some L-ascorbic acid derivatives

Article abstract of DOI:10.1016/j.molstruc.2003.09.019

Chiral 2,3-O,O-dibenzyl ethers of L-ascorbic acid with 4-(5,6-epoxy)- (4) and 6-O-tosyl- (8) functional groups were studied by X-ray crystallography and circular dichroism (CD) spectroscopy. The stereostructure of 2,3-O,O-dibenzyl-5,6-isopropylidene-L-ascorbic acid (6) and 8 was determined by X-ray crystal structure analysis. Comparison of the CD-spectra of 4 and 8 with the CD-spectra of their synthetic precursors (2-3, 5-7) and L-ascorbic acid (1) itself, as well as crystal structures of 6 and 8 permitted to deduce the absolute configuration of 4. Thus, the chiral atoms C-4 and C-5 in 4 have R and S configurations, which is consistent with the configuration of 1.

Full text of DOI:10.1016/j.molstruc.2003.09.019

Journal of Molecular Structure 687 (2004) 101–106
www.elsevier.com/locate/molstruc
Crystal structures, circular dichroism spectra and absolute configurations
of some L-ascorbic acid derivatives
a
Karlo Wittine , Tatjana Gazivoda , Marko Markus , Draginja Mrvos-Sermek ,
a
b
ˇa
Antonija Hergold-Brundic , Mario Cetina , Dinko Ziher , Vesna Gabelica ,
ˇ
b
c
d
d
ˇ
´
Mladen Mintas , Silvana Raic-Malic
a
a,
*
´
´
a
´
Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb, Croatia
bLaboratory of General and Inorganic Chemistry, Faculty of Science, University of Zagreb, Zvonimirova 8, 10000 Zagreb, Croatia
cFaculty of Textile Technology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia
d
´
Pliva Pharmaceutical Industry, Inc., Research Division, Prilaz Baruna Filipovica 25, 10000 Zagreb, Croatia
Received 27 June 2003; accepted 19 September 2003
Abstract
Chiral 2,3-O,O-dibenzyl ethers of L-ascorbic acid with 4-(5,6-epoxy)- (4) and 6-O-tosyl- (8) functional groups were studied by X-ray
crystallography and circular dichroism (CD) spectroscopy. The stereostructure of 2,3-O,O-dibenzyl-5,6-isopropylidene-L-ascorbic acid (6)
and 8 was determined by X-ray crystal structure analysis. Comparison of the CD-spectra of 4 and 8 with the CD-spectra of their synthetic
precursors (2–3, 5–7) and L-ascorbic acid (1) itself, as well as crystal structures of 6 and 8 permitted to deduce the absolute configuration of
4. Thus, the chiral atoms C-4 and C-5 in 4 have R and S configurations, which is consistent with the configuration of 1.
q 2003 Elsevier B.V. All rights reserved.
Keywords: L-Ascorbic acid derivatives; Single crystal X-ray diffraction; Circular dichroism spectroscopy; Cotton effect
1. Introduction
cytostatic activity against murine leukemia and mammary
carcinoma cells [9,10].
The circular dichroism (CD) spectroscopy has been
found useful for determination of absolute configuration of
variety of chiral molecules [11] including the pharmaco-
logically active ones [12] and has also been applied in
combination with X-ray crystallography to the present
system. The present study deals with the chiral 2,3-O,O-
dibenzyl derivatives of L-ascorbic acid containing 4-(5,6-
epoxy)- (4) and 6-O-tosyl- (8) functional groups and it was
undertaken to determine the absolute configuration of these
compounds which is of importance for elucidation of
stereochemical course of their condensation reaction with
nucleoside bases [9,10] and evaluation of their biological
properties.
L-Ascorbic acid (vitamin C) is one of the naturally
occurring antioxidants and radical scavengers that protect
cellular components against oxidative damage by free
radicals and active oxygen species [1,2]. There is consider-
able evidence that vitamin C is important in the prevention
of a large number of chronic diseases such as cancer,
cerebral apoplexy, diabetes, atopic dermatitis, myocardial
infarction and AIDS [3–5]. In addition to that, some
derivatives of L-ascorbic acid have been shown to possess
important pharmacological properties. Thus, 5,6-O,O-dis-
ubstituted L-ascorbic acid derivatives have been found to be
clinically effective antitumor agents against various human
cancers [6], while C-2-alkylated derivatives possess
immuno-stimulant activities [7]. The C-2-O- and C-3-O-
alkylated lipid soluble derivatives of L-ascorbic acid are
known to protect against the lipid peroxidation of the
biomembranes [8]. We have found that 5-fluorouracil
derivative of L-ascorbic acid exhibited rather pronounced
2. Results and discussion
2.1. Compounds preparation
The syntheses of compounds 4 and 8, outlined in the
Scheme 1, were performed from the starting L-ascorbic acid
*
Corresponding author. Tel.: þ385-1-4597-214; fax: þ385-1-4597-250.
´ ´
E-mail address: silvana.raic@pierre.fkit.hr (S. Raic-Malic).
0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.09.019

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K. Wittine et al. / Journal of Molecular Structure 687 (2004) 101–106
Scheme 1. Reagents: (i) HBr/CH3COOH; (ii) benzyl chloride, K2CO3; (iii) sat. aq. solution Na2CO3; (iv) acetyl chloride (v) benzyl chloride, K2CO3; (vi) 50%
CH3OOH; (vii) toluene-4-sulfonylchloride, pyridine.
by using selective protection and deprotection of functional
groups [9].
the lactone moiety are conjugated to the carbonyl group.
The lone electron pairs of the two hydroxyl groups attached
to the C-2 and C-3 are conjugated additionally to the enone
CyC–CyO system. The UV-spectra of enones are charac-
terized by an intense absorption band (K-band) in the 215–
250 nm region [13]. L-ascorbic acid (1) gives UV-band at
240 nm resulting from the p–pp transition. This band also
appears in the UV-spectra of 2,3-O,O-dibenzylated L-
ascorbic acid derivatives 3, 4 and 6–8, but at shorter
wavelengths than that of the parent compound 1 and 2 and 5
containing free hydroxyl groups at C-2 and C-3. Such
hypsochromic effect was also found in the spectrum of 2,3-
O,O-dibenzyl-6-O-tosyl derivative of L-ascorbic acid (8,
Table 1).
This study shows that compounds 4 and 8 exist in 4R, 5S
enantiomeric form, that is, all reaction steps (Scheme 1)
proceeded with retention of configuration.
2.2. X-ray crystal structure study
Perspective view of the molecules 6 and 8 with atom
numbering is displayed in Figs. 1 and 2.
The skeleton of 6 and 8 contains five membered lactone
ring with two benzyloxy groups attached to the atoms C-2
and C-3. The X-ray crystal structure data of 6 and 8 show
that the carbon atoms C-4 and C-19 (designated as C5 in the
Scheme 1) have R and S configurations.
In the CD-spectrum of 1 (Table 1), a positive CD-
band is recorded at 210 nm (band III, D1 ¼
0.25 mmol21 cm2), intense but negative Cotton effect is
found at 227 nm (band II, D1 ¼ 21.1 mmol21 cm2) and
2.3. UV- and CD-spectra
a
positive CD-band at 247 nm (band I, D1 ¼
The L-ascorbic acid (cf. Scheme 1) consists of a g-
lactone ring in which the ene-diol portion and the oxygen of
0.37 mmol21 cm2).

K. Wittine et al. / Journal of Molecular Structure 687 (2004) 101–106
103
Fig. 1. The molecular structure and labelling of the compound 6. Displacement ellipsoids are drawn at the 20% probability level.
Fig. 2. The molecular structure and labelling of the compound 8. Displacement ellipsoids are drawn at the 20% probability level.
Cotton effects in the range 230–250 nm (bands I and
II) arises from p–pp transition (K-band), while the one
found at 210 nm can be derived from the n–pp transition
(R-band). This assignment is in accord with that of
structurally related cyclopentenone systems [13]. The
CD-spectra of 2,3-O,O-dibenzyl-4-(5,6-epoxy)-L-ascorbic
acid (4, Fig. 3) and 2,3-O,O-dibenzyl-6-O-tosyl-L-
ascorbic acid (8, Fig. 4) are quite similar.
In the spectra of both compounds, the sign of the Cotton
effect which can be ascribed to cyclic conjugated enones is
negative at approximately 265 nm and positive around
240 nm. This indicates that compounds 4 and 8 have the
same stereostructure. Since the configuration of C-4 (R) and
C-5 (S) in 8 was determined by its crystallographic analysis,
the configurations of the chiral carbon atoms C-4 and C-5 in
both compounds 4 and 8 had to be the same that is R and S.
3. Conclusion
The absolute configuration of the 4-(5,6-epoxy) (4) and
6-O-tosyl (8) derivatives of L-ascorbic acid can be deduced
on the basis of comparison of their CD-spectra with the CD-
spectra of their synthetic precursors (2, 3 and 5–7) and L-
ascorbic acid (1) itself, as well as crystal structures
determined for 6 and 8. Thus, it was found that C-4 and
C-5 in 4 have R and S configurations.
4. Experimental
UV spectra were recorded on a Hewlett–Packard 8452A
Diode Array spectrophotometer, while CD-spectra were
measured on JASCO J-810 spectropolarimeter.

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K. Wittine et al. / Journal of Molecular Structure 687 (2004) 101–106
Table 1
4.1. X-ray structure determination
UV and CD-maxima of L-ascorbic acid (1) and its derivatives (2–8)
Compound
l (nm) [1; D1 (mmol21 cm2)]
Colourless single crystals of 6 and 8 were grown at room
temperature by slow evaporation of a very dilute ethanol
solution. In spite of many attempts, we were not able to
obtain single crystal of the compound 4 suitable for its
absolute configuration determination by X-ray crystallo-
graphic analysis. Crystal data, data collection and refine-
ment of 6 and 8 are summarized in Table 2. The intensities
were collected on a Philips PW1100 diffractometer updated
by Stoe and Cie [14,15] using Mo Ka radiation
1
2
3
4
UV 240 (7189)
CD 210.0 (0.25), 227.0 (21.10), 247.4 (0.37)
UV 240 (8601)
CD 211.0 (0.05), 229.8 (21.52)
UV 234 (18134)
CD 213.4 (1.28), 234.2 (20.39), 254.4 (20.35)
UV 234 (20427)
CD 197.8 (1.01), 242.0 (1.24), 264.6
(20.20), 275 sh (20.15)
UV 238 (7367)
˚
(0.71073 A). The intensities were corrected for Lorentz
5
6
7
8
CD 205.4 (0.72), 228.6 (22.29)
UV 234 (16735)
and polarization effects, and for extinction. The crystal
structures were solved by direct methods. All non-hydrogen
atoms were refined anisotropically by full-matrix least-
squares calculations based on F 2. All hydrogen atoms in the
compound 6, except bonded to the carbon atoms C22 and
C23, were found in a difference Fourier map and their
coordinates and isotropic thermal parameters have been
refined freely. Hydrogen atom attached to the oxygen atom
CD 198.8 (4.01), 231.4 (21.28)
UV 234 (9639)
CD 212.4 (1.80), 253.6 (20.70)
UV 228 (11846)
CD 198.4 (1.67), 211.8 (1.30), 236.4
(1.59), 263.6 (20.46)
Solvent for 1–8 is acetonitrile.
Fig. 3. CD (—, c ¼ 3.015 £ 1023 mol dm23, l ¼ 0:01 cm) and UV (- -, c ¼ 1.621 £ 1025 mol dm23, l ¼ 1 cm) spectra of 4 in CH3CN.
Fig. 4. CD (—, c ¼ 1:003 £ 1023 mol dm23, l ¼ 0:01 cm) and UV (- -, c ¼ 3:096 £ 1025 mol dm23, l ¼ 1 cm) spectra of 8 in CH3CN.

K. Wittine et al. / Journal of Molecular Structure 687 (2004) 101–106
105
Table 2
The crystal data, data collection and refinement for the compounds 6 and 8
Compound
6
8
Formula
C
23H24O6
C27H26O8S
510.54
Formula weight
Temperature (K)
Crystal size (mm3)
Crystal colour
Crystal system
Space group
396.42
293(2)
293(2)
0.30 £ 0.68 £ 0.87
Colourless
Orthorhombic
P 21 21 21
8.022(1)
10.634(2)
23.492(4)
2004.0(6)
4
0.30 £ 0.45 £ 0.58
Colourless
Orthorhombic
P 21 21 21
5.918(1)
14.693(2)
29.239(3)
2542.4(6)
4
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
V (A )
Z
D
calc. (gcm23
)
1.314
1.334
m (mm21
F(000)
)
0.095
0.176
840
1072
Scan-mode
v
3.08–28.99
u 2 2u
3.10–30.01
u range for data collection (deg)
Index ranges
210 # h # 10
214 # k # 14
212 # l # 32
4594
28 # h # 8
220 # k # 20
221 # l # 41
7474
Collected reflections
Independent reflections/Rint.
Reflection number I $ 2sðIÞ
Refinement method
4195/0.0198
6467/0.0559
2749
2616
Full-matrix least-squares on F 2
4195/0/336
0.0682, 0
Full-matrix least-squares on F 2
6467/0/327
0.1000, 0
Data/restrains/parameters
Weighting parameters a,ba
Extinction coefficientb
Goodness-of-fit on F 2
R ½I $ 2sðIÞꢀ
0.0066(19)
0.963
0.0064(13)
0.815
0.0375
0.0511
wR ½I $ 2sðIÞꢀ
0.0919
0.1311
Flack parameter, x
˚
Max./min. electron density (e A
0.0(10)
-0.13(10)
23
)
0.144/20.146
0.195/20.248
a
2
w ¼ 1=½s2ðF02Þ þ ðaPÞ þ bPꢀ; where P ¼ ðF02 þ 2Fc2Þ=3:
b
SHELXL97 [17].
O5 in the compound 8 was found in a difference Fourier
map and included in the refinement with positional
parameters fixed. All other hydrogen atoms in both
compounds were treated using appropriate riding models.
Due to insufficient anomalous scatters, we were not able to
establish the absolute configuration of 6 and 8 with
complete reliability. However, the refined Flack parameters
and R-values are in agreement with the configuration of one
of the enantiomer (4R, 5S) of the compounds 6 and 8.
Programmes used for the structure solution, refinement and
analysis were: SHELXS97 [16], SHELXL97 [17] and PARST96
[18]. The molecular and crystal structure drawings were
prepared by the PLATON98 program [19]. Crystallographic
data excluding structure factors for the structures reported in
this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
numbers CCDC-212032 and CCDC-212033. Copies of data
can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: þ44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
We are grateful to the Ministry of Science and
Technology of Croatia for support of this work (Project
Number 0125003).
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