Selective synthesis of 4,6-O-alkenylidene and -benzylidene acetals from unprotected sucrose by lanthanide(III) resin-catalyzed transacetalization

Article abstract of DOI:10.1055/s-2000-6364

Lanthanide cation exchanged resins were used as catalysts for the acetalization of sucrose. Ytterbium(III)- and erbium(III)-exchanged resins promote the transacetalization of dialkyl acetals of α,β-unsaturated aldehydes. A two-step one-pot procedure provides direct access from the unsaturated aldehyde to a series of sucrose 4,6-acetals in good yields from unprotected sucrose without concomitant glycosidic bond cleavage.

Full text of DOI:10.1055/s-2000-6364

PAPER
525
Selective Synthesis of 4,6-O-Alkenylidene and -Benzylidene Acetals from
Unprotected Sucrose by Lanthanide(III) Resin-Catalyzed Transacetalization
S. Porwanski,1 P. Salanski,2 G. Descotes, A. Bouchu, Y. Queneau*
Laboratoire de Sucrochimie CNRS - Béghin-Say (UMR 143), c/o Eridania Béghin-Say, 27 Bd du 11 novembre 1918, B.P. 2132, 69603
Villeurbanne Cedex, France
Fax +33(0)472442991; E-mail: queneau@univ-lyon1.fr
Received 1 October 1999; revised 27 December 1999
though a higher temperature (80°C) is required as com-
Abstract: Lanthanide cation exchanged resins were used as cata-
pared to the room temperature reaction using pyridinium
p-toluenesulfonate (PPTS). The ytterbium(III) and the er-
bium(III) resins are more effective than the lantha-
lysts for the acetalization of sucrose. Ytterbium(III)- and erbi-
um(III)-exchanged resins promote the transacetalization of dialkyl
acetals of a,b-unsaturated aldehydes. A two-step one-pot procedure
provides direct access from the unsaturated aldehyde to a series of num(III) one. Filtration of the reaction mixture, followed
sucrose 4,6-acetals in good yields from unprotected sucrose without
concomitant glycosidic bond cleavage.
by washing and drying provided a recycled catalyst which
was still active, especially in the case of erbium. Under the
Key words: sucrose, acetals, lanthanides, heterogeneous catalysis,
carbohydrates, resins, transacetalization
same conditions, ytterbium(III) triflate, as an homoge-
neous catalyst, led to significant acidic cleavage of the in-
tersaccharidic bond, yielding the glucose acetal 4a (this
compound can be obtained from glucose in 54% yield un-
der Yb-resin catalysis). However, a stronger acidity is re-
quired to achieve direct acetalization of citral (1a) which
leads to extensive concomitant glycosidic cleavage.
The ability of some lanthanide cation exchanged resins3 to
catalyze the acetalization of carbohydrates has been eval-
uated.4 As carbohydrates are involved in increasingly
complex structures, there is a need for mild methods
which preserve the acid-sensitive parts of the substrates,
and notably the glycosidic bond such as the fragile fructo-
furanosidic linkage in sucrose. Sucrose can provide ace-
tals by acid-catalyzed reaction with dimethoxyalkanes or
methoxyalkenes (4,6-O-acetals),5-6 or under basic condi-
tions with benzylidene dihalides (4,6-O-acetals)7 or a-
chloromethylketones (2,3-O-acetals).8 The lanthanide
cation salts, known to be compatible with alcoholic (and
even aqueous) media,3,9 were compared with other acidic
catalysts, with respect to the preservation of both the di-
saccharidic structure and the non-carbohydrate moiety,
especially in the particular case of a,b-unsaturated dialkyl
acetals which exhibit sufficient reactivity under mild acid-
ic conditions.10,11
Using the most available resin prepared from ytterbium
sulfate, a series of a,b-unsaturated or aromatic aldehyde
acetals were transformed to their corresponding sucrose
acetals 3b-g (Scheme 2, Table 2) which were character-
ized as their peracetylated derivatives 5b-g. The 4,6-se-
lectivity was unambiguously assigned by NMR spec-
troscopy.11 Aromatic aldehyde acetals reacted also under
these conditions. However, the transacetalization of a,b-
unsaturated ketone acetals, such as ionone acetals 2h,i
leading to acetals 3h,i, was much slower.3 Reaction of
methyl a-D-glucopyranoside with citral dimethylacetal or
3,3-diethoxypropene could proceed in moderate yields,
whereas the corresponding galactoside, missing the trans
relationship between OH-4 and CH2OH-6, underwent a
much slower reaction.
The procedure described by Yu et al.3 was followed for
the preparation of three lanthanide cation exchange resins,
using lanthanum, ytterbium and erbium salts. The acetal-
ization of sucrose (in excess) with citral dimethylacetal
(2a) in anhydrous DMF was first studied (Scheme 1, Ta-
ble 1). Good yields (based on the starting dimethyl acetal)
of the sucrose 4,6-acetal 3a were obtained using these cat-
alysts (ca. 5 mol% in lanthanide cation equiv),3 even
Thus, in a two-step one-pot procedure, a,b-unsaturated al-
dehydes could be transformed to the corresponding su-
crose acetals via the dimethyl acetal obtained by treatment
with methanol at room temperature in the presence of res-
in and triethyl orthoformate, followed by evaporation
(carefully for low boiling point acetals) and reaction with
a solution of sucrose in anhydrous DMF at 80°C (ca. 24
6
O
catalyst
sucrose
R
O
O
O
OH
O
O
+
4
HO
HO
3
OH
O
OH
OH
OH
Table 1
2
O
OH
OH
3a
4a
R = CHO
R = CH(OMe)2
citral (1a)
CDMA (2a)
Scheme 1
Synthesis 2000, No. 4, 525–528 ISSN 0039-7881 © Thieme Stuttgart · New York

526
S. Porwanski et al.
PAPER
Table 1 Lanthanide(III)-Mediated Acetalization of Citral with Su-
crose
Table 2 Sucrose Acetalization with a,b-Unsaturated Aldehydes,
Ketones, or Acetals Catalyzed by Yb(III)-Exchanged Amberlyst Res-
in (Method A: From Isolated Dialkylacetal; Method B: Two-Step
One-Pot Procedure)a
Substratea Catalystb
Temp. (°C) Time (h) Yieldsc of 3a
(+ 4a)
Sub- Methodc
strateb
Prod- Yield Acetylated Yield
f
[a]D
uctd
(%) Acetalse
(%)
2a
2a
Yb-resin
Yb-resin
(recycled)
Yb-resin
Yb-resin
(two-step)
Yb(OTf)3
Yb(OTf)3
La-resin
80
80
24
24
66
55
2a
1a
100
80
24
24
58
69
2b
2c
1d
1e
1f
1g
2h
2i
A
Ag
B
B
B
3b
3c
3d
3e
3f
78
53
75
37
60
80
13
32
8
5b
5c
5d
5e
5f
5g
–
66
76
93
71
74h
54h
–
+75
+54
+60
+41
+44
+32
–
2a
2a
2a
2a
2a
25
60
80
80
80
46
24
24
24
24
38 (+ 9)
48 (+ 14)
44
77
76
B
3g
Er-resin
Er-resin
A (4 d, 80°C) 3hi
A (4 d, 50°C) 3ii
–
–
–
–
–
–
(recycled)
Yb-resin
Yb(OTf)3
Yb(OTf)3
1i
B (Er-resin,
3ii
1a
1a
1a
20-80
20
60
24
24
24
Traces
15 (+ 8)
31 (+ 13)
6d, 40°C)
a 0.5 Equiv compared to sucrose. Yields obtained are based on the
starting carbonyl compound.
b Resins were prepared following the procedure described in the liter-
ature.3
a 2.3 mmol for 4.6 mmol of sucrose.
b 100 mg of lanthanide resin (5 mol % in lanthanide ions) or 2 mol %
of Yb(OTf)3.
c 24 h at 80°C except otherwise specified.
d Products were characterized as peracetylated derivatives 5.
e Satisfactory microanalyses obtained: C 0.31, H 0.18.
f Temp. 20°C, c = 1, CHCl3.
c Based on 1a or 1b.
g As its diethyl acetal.
h Acetylated also at the phenolic OH.
h). Filtration, evaporation, and direct chromatography
then provided often higher yields of sucrose acetals.
i Characterized by comparison with authentic samples.11
In conclusion, lanthanide cation-exchanged resins were
shown to catalyze smoothly the acetalization of unprotect-
ed sucrose by a,b-unsaturated and aromatic aldehydes in
a two-step one-pot procedure. This method should be use-
ful in the case of acid sensitive substrates, either the poly-
ol or the carbonyl compound. Such sucrose acetals might
find applications based on their solubility in water, the
temporary protection of the reactive carbonyl centre, and
their ability to release the active carbonyl compound, such
as citral, vanillin or ionones, under mild and controllable
conditions.
All reactions were conducted in anhyd DMF (Aldrich) under N2. All
1
reagents were purchased from Aldrich. H and 13C NMR spectra
were recorded at 200 and 50 MHz in CD3OD (unprotected acetals)
or in CDCl3 (acetylated acetals) on a Bruker AC 200 spectrometer
at the Université Claude Bernard Lyon 1. TLC were performed on
silica gel 60 F254 aluminium plates (Merck) as well as flash chroma-
tography. Optical rotations (Sodium D line) were measured at 20°C
with a Perkin-Elmer 241 digital polarimeter. Elemental analyses
were performed by the Service Central de Microanalyse of the
CNRS (Solaize). Compounds 3a, 5a, 3h, and 3i were characterized
by comparison with authentic samples.11
R1
O
R2
O
catalyst
sucrose
OR
O
RO
R1
R1
OR
O
OMe
OMe
OR
O
R2
R2
O
Table 2
OR
OR
3b-i R = H
5b-i R = Ac
1b-i
2b-i
b: R1 = H, R2
=
e: R1 = H, R2
=
h: R1 = Me, R2
i: R1 = Me, R2
=
=
MeO
OH(OR)
MeO
c: R1 = H, R2
d: R1 = H, R2
=
=
f: R1 = H, R2
g: R1 = H, R2
=
MeO
(RO)HO
=
Scheme 2
Synthesis 2000, No. 4, 525–528 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Selective Synthesis of 4,6-O-Alkenylidene and -Benzylidene Acetals from Unprotected Sucrose
527
Preparation of the Catalysts3
13C NMR: d = 13.68 (C-12), 20.61-20.84 (6 COCH3), 27.76 (C-
To a solution of ytterbium sulfate octahydrate (1.00 g) in H2O (10
mL) was added Amberlyst 15W resin (1.00 g) and the suspension
was stirred at r.t. for 24 h. The solid was filtered, washed with H2O
and EtOH, and dried at 60°C at the rotatorary evaporator for 6 h,
then at r.t. for 48 h at 1 Torr.
11), 34.08 (C-10), 63.12, 63.39 (C-1',6'), 63.52 (C-5), 68.06 (C-6),
68.71 (C-2), 71.09 (C-3), 74.77 (C-4'), 75.57 (C-3'), 78.49 (C-4),
79.10 (C-5'), 90.46 (C-1), 101.48 (C-7), 103.98 (C-2'), 125.54 (C-
8), 137.29 (C-9), 169.81-170.57 (C=O).
3d
Transacetalization with Dimethyl Acetals; General Procedure
Method A: A solution of sucrose (1.57 g, 4.6 mmol), dimethyl acetal
2 (2.3 mmol) and the catalyst (100 mg) in anhyd DMF (7 mL) was
stirred at 25-100°C (Table 1). When all the starting acetal had re-
acted (TLC, Et2O/petroleum ether, 1:1), the mixture was filtered
and the filtrate was evaporated under reduced pressure. The residue
is then submitted to flash chromatography using CH2Cl2/acetone/
MeOH/H2O (56:20:20:4) as eluent, providing the sucrose acetal 3
(Table 1).
13C NMR: d = 11.34, 13.23 (2 CH3), 63.13, 64.04 (C-1', 6'), 64.75
(C-5), 69.54 (C-6), 71.81 (C-3), 73.97 (C-2), 75.40 (C-4'), 79.18 (C-
3'), 82.42 (C-5', 83.86 (C-4), 94.39 (C-1), 105.54 (C-2'), 106.85 (C-
7), 125.55 (C-9), 134.41 (C-8).
5d
1H NMR: d = 1.61 (s, 6 H, 2 × CH3), 2.02-2.17 (6 s, 18 H, 6
COCH3), 3.47 (t, J = 9.8 Hz, 1 H, H-6a), 3.55 (t, J = 9.8 Hz, 1 H, H-
6b), 3.98-4.25 (m, 7 H, H-4, 5, 1’a, 1’b, 5’, 6’a, 6’b), 4.76 (br s, 1 H,
H-7), 4.80 (dd, 1 H, H-2, J = 10.0, 3.8 Hz), 5.28 (d, J = 2.9 Hz, 1 H,
H-9), 5.32-5.48 (m, 3 H, H-3, 3’, 4’), 5.63 (d, J = 3.6 Hz, 1 H, H-1).
Two-step One-pot Procedure for the Acetalization of Aldehydes
Method B: To a solution of aldehyde 1 (2.3 mmol) in MeOH (2 mL)
containing the catalyst (100 mg) was added trimethyl orthoformate
(0.5 mL, 4 mmol). The mixture was stirred at r.t. typically for 1 h
(TLC, Et2O/petroleum ether 1:1). MeOH and the remaining ortho-
formate were then evaporated (rotarotary evaporator) before adding
a solution of sucrose (1.57 g, 4.6 mmol) in anhyd DMF (7 mL). The
reaction was then proceeded as above.
13C NMR: d = 11.05 (CH3), 12.94 (CH3), 20.56-20.85 (6 COCH3),
63.24, 63.31 (C-1’, 6’), 63.64 (C-5), 68.04 (C-6), 68.76 (C-2), 71.06
(C-3), 74.76 (C-4’), 75.62 (C-3’), 78.43 (C-4), 79.14 (C-5’), 90.39
(C-1), 103.99 (C-2’), 104.91 (C-7), 124.37 (C-9), 132.29 (C-8),
169.76-170.55 (6 C=O).
3e
13C NMR: d = 57.36 (OCH3), 64.57, 64.44 (C-1’,6’), 66.16 (C-5),
71.13 (C-6), 73.25 (C-3), 75.38 (C-2), 76.84 (C-4’), 80.61 (C-3’),
84.22 (C-5’), 85.39 (C-4), 95.82 (C-1), 103.63 (C-7), 106.97 (C-2’),
116.05 (CHarom), 130.49 (CHarom), 133.10 (Carom), 163.19 (Carom).
Acetylation of Sucrose Acetals
For characterization purposes, acetylation was achieved by reaction
with Ac2O in pyridine (1/3 vol., 1.4 mL per mmol of substrate) at
0°C for 24 h followed by coevaporation with toluene and flash chro-
matography (EtOAc/hexane, 1:1). Acetylated acetals 5b-g were ob-
tained as oils in 54 to 93% yield.
5e
1H NMR: d = 2.05-2.19 (6 s, 18 H, 6 COCH3), 3.65 (t, J = 9.7 Hz,
1 H, H-6a), 3.72 (t, J = 9.7 Hz), 3.80 (s, 3 H, OCH3, H-6b), 4.13-
4.43 (m, 7 H, H-4, 5, 1’a, 1’b, 6’a, 6’b, 5’), 4.86 (dd, J = 10.0, 3.8 Hz,
1 H, H-2), 5.35-5.58 (m, 4 H, H-3, 3’, 4’, 7), 5.69 (d, J = 3.8 Hz, 1
H, H-1), 6.85-7.38 (m, 4Harom).
The NMR data for compounds 3b-i and 5b-i are given below.
3b
13C NMR: d = 19.12, 25.91 (2 CH3), 63.12, 63.94 (C-1',6'), 64.64
(C-5), 69.46 (C-6), 71.75 (C-3), 73.86 (C-2), 75.39 (C-4'), 79.10 (C-
3'), 82.32 (C-5'), 83.86 (C-4), 94.33 (C-1), 100.57 (C-7), 105.48 (C-
2'), 123.03 (C-8), 140.83 (C-9).
13C NMR: d = 55.30 (OCH3), 63.16, 63.42 (C-1’,6’), 63.61 (C-5),
68.42 (C-6), 68.76 (C-2), 71.10 (C-3), 74.84 (C-4’), 75.61 (C-3’),
78.85 (C-4), 79.23 (C-5’), 90.50 (C-1), 101.62 (C-7), 104.07 (C-2’),
113.57 (CHarom), 127.54 (CHarom), 129.48 (Carom), 160.14 (Carom),
169.80-170.63 (6 C=O).
5b
1H NMR: d = 1.65 (s, 3 H, CH3), 1.68 (s, 3 H, CH3), 2.00-2.13 (8
s,18 H, 6 COCH3), 3.46 (t, J = 9.6 Hz, 1 H, H-6a), 3.53 (t, J = 9.6
Hz, 1 H, H-6b), 4.02-4.25 (m, 7 H, H-4, 5, 1'a, 1'b, 5', 6'a, 6'b), 4.76
(dd, J = 10.0, 3.9 Hz, 1 H, H-2), 5.09 (d, J = 6 Hz, 1 H, H-7), 5.19
(d, J = 6 Hz, 1 H, H-8), 5.31-5.44 (m, 3 H, H-3, 3', 4'), 5.60 (d,
J = 3.8 Hz, 1 H, H-1).
3f
13C NMR: d = 58.30 (OCH3), 64.57, 65.42 (C-1’,5’), 66.19 (C-5),
71.50 (C-6), 73.25 (C-3), 75.27 (C-2), 76.82 (C-4), 80.59 (C-3’),
84.44 (C-5’), 85.31 (C-4), 95.80 (C-1), 100.83 (C-7), 106.93 (C-2’),
114.57 (CHarom), 121.83 (CHarom), 126.71 (Carom), 146.97 (Carom),
150.46 (Carom).
13C NMR: d = 18.72 (CH3), 20.37-20.79 (COCH3), 25.52 (CH3),
63.18, 63.31 (C-1', 6'), 62.47 (C-5), 68.11 (C-6), 68.76 (C-2), 71.06
(C-3), 74.68 (C-4'), 75.54 (C-3'), 78.56 (C-4), 79.04 (C-5'), 90.31
(C-1), 99.42 (C-7), 103.91 (C-2'), 121.42 (C-8), 139.86 (C-9),
169.78-170.52 (C=O).
5f
1H NMR: d = 2.05-2.31 (7 COCH3), 3.64 (t, J = 9.7 Hz, 1 H, H-6a),
3.70 (t, J = 9.7 Hz, 1 H, H-6b), 3.81 (s, 3 H, OCH3), 4.10-4.40 (m,
7 H, H-4, 5, 1’a, 1’b, 5’, 6’a, 6’b), 4.86 (dd, J = 10.1, 3.9 Hz, 1 H, H-
2), 5.37-5.51 (m, 3 H, H-3, 3’, 4’), 5.58 (s, 1 H, H-7), 5.68 (d, J = 3.9
Hz, 1 H, H-1), 6.94-7.27 (m, 3 Harom).
3c
13C NMR: d = 14.21 (C-12), 23.22 (C-11), 35.37 (C-10), 63.18,
64.00 (C-1',6'), 64.67 (C-5), 69.50 (C-6), 71.79 (C-3), 73.96 (C-2),
75.47 (C-4'), 79.18 (C-3'), 82.38 (C-5'), 84.29 (C-4), 94.38 (C-1),
102.79 (C-7), 105.56 (C-2'), 127.73 (C-8), 137.05 (C-9).
13C NMR (50 MHz, CDCl3) d = 20.43-20.76 (7 COCH3), 56.12
(OCH3), 63.20, 63.38 (C-1’,6’), 63.50 (C-5), 68.57 (C-2), 58.59 (C-
6), 71.05 (C-3), 74.76 (C-4’), 75.58 (C-3’), 78.46 (C-4), 79.20 (C-
5’), 104.46 (C-1), 97.77 (C-7), 104.04 (C-2’), 113.10 (CHarom),
118.50 (CHarom), 126.51 (CHarom), 130.17 (Carom), 137.82 (Carom),
151.13 (Carom), 168.06-170.63 (7 C=O).
5c
1H NMR: d = 0.89 (tq, J = 7.4 Hz, 3 H, CH3), 1.41 (t, J = 7.4Hz, 2
H, H-11), 1.97-2.19 (m, 20 H, 2 H-10 + 6 COCH3), 3.49 (t, J = 9.7
Hz, 1 H, H-6a), 3.56 (t, J = 9.7 Hz, 1 H, H-6b), 4.02-4.34 (m, 7 H,
H-4, 5, 1'a, 1'b, 5', 6'a, 6'b), 4.81 (dd, J = 10.0, 3.8 Hz, 1 H, H-2),
4.92 (d, J = 4.9 Hz, 1 H, H-7), 5.17-5.52 (m, 4H, H-8, 3,3',4'), 5.65
(d, J = 3.8 Hz, 1 H, H-1), 5.90 (dt, J = 15.6, 4.7 Hz, 1 H, H-9).
3g
13C NMR: d = 55.69 (OCH3), 63.25, 64.07 (C-1’,6’), 64.78 (C-5),
69.90 (C-6), 71.85 (C-3), 73.92 (C-2), 76.49 (C-4), 80.31 (C-3’),
Synthesis 2000, No. 4, 525–528 ISSN 0039-7881 © Thieme Stuttgart · New York

528
S. Porwanski et al.
PAPER
(2) Present address: Polish Academy of Sciences, Institute of
93.84 (C-5’), 84.95 (C-4), 94.42 (C-1), 103.47 (C-7), 105.52 (C-2),
111.32, 115.65, 120.90 (CHarom)131.03, 148.40, 148.85 (Carom).
Organic Chemistry, ul. Kasprzaka 44, 01-224 Warsaw,
Poland.
(3) Yu, L.; Chen, D.; Li, J.; Wang, P. G. J. Org. Chem. 1997, 62,
3575, and references cited therein.
5g
1H NMR: d = 2.05-2.28 (7 s, 21 H, 7 COCH3 ), 3.64 (t, J = 9.7 Hz,
1 H, H-6a), 3.72 (t, J = 9.7 Hz, 1 H, H-6b), 3.83 (s, 3 H, OCH3),
4.16-4.39 (m, 7 H, H-4,5, 1’a, 1’b, 5’, 6’a, 6’b), 4.86 (dd, J = 10.0,
3.8 Hz, 1 H, H-2), 5.33-5.57 (m, 3 H, H-3, 3’, 4’), 5.46 (s, 1 H, H-
7), 5.68 (d, J = 3.7 Hz, 1 H, H-1), 6.98-7.27 (m, 3 Harom).
(4) For a review on carbohydrate acetalization, see: Calinaud, P.;
Gelas. J. In Preparative Carbohydrate Chemistry; Hanessian,
S., Ed.; Marcel Dekker: New York, 1997; p 3.
(5) Khan, R.; Mufti, K. S. Carbohydr. Res. 1975, 43, 247.
Khan, R.; Mufti, K. S.; Jenner, M. R. Carbohydr. Res. 1978,
65, 109.
Fanton, E.; Gelas, J.; Horton, D.; Karl, H.; Khan, R.; Lee, C.-
K.; Patel, G. J. Org. Chem. 1981, 46, 1057.
(6) Carbonel, S.; Fayet, C.; Gelas, J. Carbohydr. Res.1999, 319,
63.
13C NMR d = 20.68-20.84 (7 COCH3), 55.91 (OCH3), 63.11, 63.46
(C-1’,6’), 63.51 (C-5), 68.47 (C-6), 68.66 (C-2), 71.01 (C-3), 74.89
(C-4’), 75.63 (C-3’), 78.92 (C-4), 79.20 (C-5’), 90.54 (C-1), 101.16
(C-7), 104.13 (C-2’), 110.46, 118.79, 122.48 (CHarom), 135.78,
140.23, 150.93 (Carom), 168.89-170.59 (7 C=O).
(7) Khan, R. Carbohydr. Res. 1974, 32, 375.
(8) Giry-Panaud. N.; Descotes, G.; Bouchu, A.; Queneau, Y. Eur.
J. Org. Chem. 1999, 3393.
(9) For a review on the use of lanthanide triflates in aqueous
media, see: Kobayashi, S. Synlett 1994, 689.
(10) Fanton, E.; Fayet, C.; Gelas. J.; Jhurry, D., Deffieux, A.;
Fontanille, M. Carbohydr. Res. 1992, 226, 337.
Fanton, E.; Fayet C., Gelas, J. Carbohydr. Res. 1997, 298, 85.
(11) Salanski, P.; Descotes, G.; Bouchu, A.; Queneau, Y. J.
Carbohydr. Chem. 1998, 17, 129.
Acknowledgement
Financial support from Béghin-Say and CNRS is greatly acknow-
ledged. We thank the CNRS for post doctoral fellowships to SP and
PS.
References
(1) Present address: Laboratory of Organic and Applied
Chemistry, University of Lodz, ul. Naturowicza 68, 90-136
Lodz, Poland.
Article Identifier:
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Synthesis 2000, No. 4, 525–528 ISSN 0039-7881 © Thieme Stuttgart · New York