Mild synthesis of protected α-D-glycosyl iodides

Article abstract of DOI:10.1002/(sici)1099-0690(199911)1999:11<3147::aid-ejoc3147>3.0.co;2-i

α-D-Glycosyl iodides are stereoselectively obtained by iodine replacement of the free anomeric hydroxyl group of fully protected sugars treated with a polymer bound triarylphosphane-iodine complex and imidazole. High yields and mild conditions, compatible

Full text of DOI:10.1002/(sici)1099-0690(199911)1999:11<3147::aid-ejoc3147>3.0.co;2-i

FULL PAPER
Mild Synthesis of Protected ␣--Glycosyl Iodides
Romualdo Caputo,*[a] Horst Kunz,*[b] Domenico Mastroianni,[a][b] Giovanni Palumbo,[a]
Silvana Pedatella,[a] and Francesco Solla[a]
Keywords: Glycosyl iodides / Triphenylphosphane-iodine complex / Carbohydrates / Glycosides / Polymer bound
reagents
α-
D
-Glycosyl iodides are stereoselectively obtained by iodine
conditions, compatible with all the common protecting
groups used in carbohydrate chemistry, characterize the
conversion.
replacement of the free anomeric hydroxyl group of fully
protected sugars treated with a polymer bound triarylphos-
phane-iodine complex and imidazole. High yields and mild
Glycosyl iodides were first prepared by the reaction of were necessary, probably due to the fact that, in the aprotic
glycosyl bromides with sodium iodide in acetone.[1] Dec- dipolar dichloromethane solvent, the protonated imidazole
ades later, glycosyl acetates, methyl glycosides, 1,6-anhydro- is still an appreciably strong acid and, therefore, a hydrogen-
sugars and glycosyl acetals were reported[2] to lead to α- bonded dimer of imidazole could be the actual neutralis-
glycosyl iodides by reaction with iodotrimethylsilane. Other ing species.
procedures include the reaction of anomeric hydroxyls with
iodoenamines[3] as well as reaction of anomeric acetates
with hydroiodic acid in glacial acetic acid.[4] In situ genera-
tion of glycosyl iodides, from activated donors, and their
subsequent glycosylation has also been reported.[5] In these
Scheme 1. Formation of protected α--glycosyl iodides
reactions, both α- and β--glycosyl iodides have been pro-
posed as intermediates, depending upon the stereochemical
In fact, the addition of a protected sugar to a mixture of
outcome. α--Glycosyl iodides have served as glycosyl do-
polymer bound triarylphosphane-iodine complex and imi-
nors in only a few cases,[6][7] and the general consensus has
dazole, at room temperature, results in the quick formation
been that these compounds are too reactive to be syntheti-
of the corresponding iodide. In all the experiments only α-
cally useful.[8] Until a rather recent report,[9] β--glycosyl
-glycosyl iodides were obtained without any traces of their
iodide formation had never been directly observed.
β-anomers (see Table 1). This happens even if assisting
Under these circumstances, it seems to be of interest to
groups like OAc or OBz are present at the C-2 position
report a new and efficient synthesis of α--glycosyl iodides
of the starting sugar and, therefore, the process should be
which are obtained under smooth conditions and in quite
thermodynamically controlled, eventually leading to the
satisfactory yields using a polymer-bound reagent that
more stable α-glycosyl iodide anomer.
makes any critical and time consuming workup and purifi-
The α--glycosyl iodides thus obtained can often be used
cation procedures unnecessary.
as isolated after filtration through Celite of the precipi-
The reagent is a complex of polystyryl diphenylphos-
tated excess imidazole and the solid polymer-bound phos-
phane and iodine prepared in situ in anhydrous dichloro-
phane oxide which is the only side-product of the reaction.
methane at room temperature. This species acts as a good
Their stability is greatly dependent on the sugar protecting
electrophile toward oxygen nucleophiles and has been ex-
groups and, as a matter of fact, O-benzyl protected α--
tensively used in our lab for various synthetic transform-
glycosyl iodides are rather unstable at room temperature
ations.[10] Its efficacy, however, is enhanced by the presence
and have to be used immediately after the preparation.
of imidazole which, in principle, should act as a proton trap
Otherwise, their O-acetyl and O-benzoyl analogues are qu-
for the hydrogen ions released during the reaction, although
ite stable and can be stored for one to two weeks under a
it has also been reported[11] to play an active role in other
nitrogen atmosphere in the refrigerator without appreci-
reactions involving triphenylphosphane-iodine reagent. In
able decomposition.
our procedure two imidazole equivalents per phosphane
Replacing the polymer-bound triarylphosphane by sol-
uble triphenylphosphane resulted in somewhat lower yields
[a]
`
Dipartimento di Chimica Organica e Biologica, Universita di
(e.g. in the case of the compounds 1b, 2b and 4b, as shown
in Table 1). In addition, the crude reaction products need
to be chromatographed on silica gel in order to remove the
oxide that accompanies the α--glycosyl iodides.
Due to the mild reaction conditions, a broad range of
protecting groups such as ethers (products 1b, 2b, 4b), esters
Napoli Federico II,
Via Mezzocannone, 16 I-80134 Napoli (Italy)
E-mail: ctsgroup@cds.unina.it
[b]
Institut für Organische Chemie, Johannes Gutenberg Universi-
tät,
Duesberweg 10Ϫ14, D-55099 Mainz (Germany)
E-mail: hokunz@mail.uni-mainz.de
Eur. J. Org. Chem. 1999, 3147Ϫ3150
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/1111Ϫ3147 $ 17.50ϩ.50/0
3147

R. Caputo, H. Kunz, D. Mastroianni, G. Palumbo, S. Pedatella, F. Solla
Table 1. α--Glycosyl iodides from protected sugars and polystyryl diphenylphosphane-iodine complex
FULL PAPER
Sugar
Protection
Product
m.p.°C[a]
[α]D
Yield (%)
-Glucopyranose
2,3,4,6-tetra-O-acetyl
1a
Љ
1b
Љ
1c
2a
Љ
2b
Љ
106Ϫ107
231.1
Љ
100.4
Љ
97
Љ
Љ
Љ
80[c]
95
Љ
Љ
Љ
2,3,4,6-tetra-O-benzyl
Љ
2,3,4,6-tetra-O-benzoyl
2,3,4,6-tetra-O-acetyl
Љ
2,3,4,6-tetra-O-benzyl
Љ
2,3,4,6-tetra-O-benzoyl
3,4,6-tri-O-acetyl
amorphous
Љ
65[c]
96[c]
95
132Ϫ134
169.2
236.9
-Galactopyranose
Љ
Љ
Љ
oily
Љ
Љ
80[c]
92
[b]
amorphous
Љ
Љ
Љ
Љ
67[c]
97[c]
90[c]
Љ
2c
3
138.5
173.5
2-Deoxy-2-azido--
galactopyranose
-Mannopyranose
2,3,4,6-tetra-O-acetyl
4a
Љ
4b
Љ
4c
5
oily
190.3
95
Љ
Љ
Љ
Љ
82[c]
94
[b]
Љ
Љ
Љ
2,3,4,6-tetra-O-benzyl
Љ
2,3,4,6-tetra-O-benzoyl
2,3:5,6-di-O-isopropylidene
amorphous
Љ
amorphous
45Ϫ47
Љ
66[c]
96[c]
98
45.3
141.7
-Mannofuranose
[a]
[b]
[c]
Solvent, hexane/ethyl acetate 1:1. Ϫ
Not determined due to the product instability. Ϫ Reaction carried out with triphenylpho-
sphane.
11.7 Hz, 1 H, benzyl(i)b-H}, 4.55 {d, J ϭ 11.1 Hz, 1 H, benzyl(ii)a-
H}, 4.68 {d, J ϭ 11.7 Hz, 1 H, benzyl(iii)a-H}, 4.72 {d, J ϭ
11.7 Hz, 1 H, benzyl(iv)a-H}, 4.74 {d, J ϭ 11.7 Hz, 1 H, benzyl(-
iii)b-H}, 4.84 {d, J ϭ 11.7 Hz, 1 H, benzyl(iv)b-H}, 4.93 {d, J ϭ
11.1 Hz, 1 H, benzyl(ii)b-H}, 6.93 (d, J ϭ 3.8 Hz, 1 H, 1-H),
7.22Ϫ7.39 (m, 20 H, aromatic H).
(products 1a,c, 2a,c, 3, 4a,c), acetals (product 5) and the
azido group (product 3) are preserved under the treatment
with the triarylphosphane-iodine complex. It is noteworthy
that even rather unstable α--glycosyl iodides, like 2b, can
be obtained in good yield by this procedure.
Under the same conditions, the following α--glycosyl iodides were
also prepared:
Experimental Section
2,3,4,6-Tetra-O-acetyl-␣-D-galactopyranosyl iodide (2a): (95%), oily.
General: 1H and 13C NMR spectra: Bruker AM-250 and DRX-400
spectrometers, CDCl3 as solvent, chemical shifts in ppm (δ), TMS
as internal standard. Ϫ Optical rotations: Perkin-Elmer 141 polar-
imeter (1.0 dm cell length), CHCl3 as solvent. Ϫ Combustion
analyses: Perkin-Elmer Series II 2400, CHNS analyzer. Ϫ TLC
analyses: silica gel Merck 60 F254 plates (0.2 mm layer tickness). Ϫ
Column chromatography: Merck Kieselgel 60 (70Ϫ230 mesh). Ϫ
Dry dichloromethane was distilled from P2O5 immediately before
use.
25
25
Ϫ [α]D ϭ ϩ236.9 (c ϭ 1.2) {ref.[12] [α]D ϭ ϩ235 (c ϭ 1.7),
CHCl3}. Ϫ C14H19IO9 (458.2): calcd. C 54.29, H 4.50; found C
54.71, H 4.38. Ϫ H NMR (400 MHz): δ ϭ 1.98, 2.03, 2.08, 2.12
1
(4 s, 4 ϫ 3 H, 4 ϫ CH3), 4.08 (dd, J ϭ 6.3 Hz, 10.6 Hz, 1 H, 6b-
H), 4.15Ϫ4.21 (m, 2 H, 5-H, 6a-H), 4.32 (dd, J ϭ 4.1 Hz, 10.5 Hz,
1 H, 2-H), 5.25 (dd, J ϭ 3.3 Hz, 10.5 Hz, 1 H, 3-H), 5.45 (d, J ϭ
2.4 Hz, 1 H, 4-H), 7.05 (d, J ϭ 4.1 Hz, 1 H, 1-H). Ϫ 13C NMR: δ ϭ
20.75, 20.45, 20.40 (CH3), 60.51 (C-6), 66.51, 67.42, 69.61 73.60
(C-2, C-3, C-4, C-5), 75.24 (C-1), 169.50, 169.69, 169.71, 170.18
(CϭO).
Formation of ␣-D-Glycosyl Iodides by Polystyryl Diphenylphos-
phane-I2 Complex: 2,3,4,6-tetra-O-Benzyl-␣-
D-galactopyranosyl
2,3,4,6-Tetra-O-benzyl-␣-D-glucopyranosyl iodide (1b): (95%),
Iodide (2b). ؊ General Procedure A: To a magnetically stirred sus-
pension of dry polystyryl diphenylphosphane (0.65 g, ca. 1.9 mmol)
in anhydrous dichloromethane (6 mL) at room temp., a solution of
I2 (0.50 g, 1.9 mmol) in the same solvent (6 mL) was added drop-
wise in the dark and under dry argon (or nitrogen) atmosphere.
After 30 min solid imidazole (0.24 g, 3.5 mmol) was also added in
one portion and stirring continued for a few more minutes. Then,
a solution of 2,3,4,6-tetra-O-benzyl-α--galactopyranose (0.54 g,
1.0 mmol) in the same solvent (6 mL) was slowly added to the sus-
pension. After 2.5 h the starting protected sugar was completely
consumed (TLC monitoring). The reaction mixture was then di-
luted with anhydrous Et2O (5 mL) to precipitate the excess imidaz-
ole and filtered through Celite (5 g, layer thickness ca. 3 cm). The
resulting clear solution was finally evaporated under reduced press-
ure to give a residue consisting of the nearly pure, rather unstable,
amorphous. Ϫ C34H35IO5 (650.6): calcd. C 62.77, H 5.42; found C
25
63.15, H 5.33. Ϫ [α]D ϭ ϩ100.4 (c ϭ 1.2). Ϫ 1H NMR
(400 MHz): δ ϭ 2.80 (dd, J ϭ 3.9 Hz, 9.0 Hz, 1 H, 2-H), 3.61 (d,
J ϭ 9.7 Hz, 1 H, 6a-H), 3.76Ϫ3.80 (m, 3 H, 4-H, 5-H, 6b-H), 3.88
(t, J ϭ 8.9 Hz, 1 H, 3-H), 4.43 {d, J ϭ 11.9 Hz, 1 H, benzyl(i)a-H},
4.47 {d, J ϭ 10.7 Hz, 1 H, benzyl(ii)a-H}, 4.54 {d, J ϭ 11.9 Hz, 1
H, benzyl(i)b-H}, 4.61 {d, J ϭ 11.7 Hz, 1 H, benzyl(iii)a-H}, 4.68
{d, J ϭ 11.7 Hz, 1 H, benzyl(iii)b-H}, 4.78 {d, J ϭ 10.9 Hz, 1 H,
benzyl(iv)a-H}, 4.82 {d, J ϭ 10.9 Hz, 1 H, benzyl(iv)b-H}, 4.94 {d,
J ϭ 10.7 Hz, 1 H, benzyl(ii)b-H}, 6.82 (d, J ϭ 3.9 Hz, 1 H, 1-H),
7.22Ϫ7.40 (m, 20 H, aromatic H). Ϫ 13C NMR: δ ϭ 67.51 (C-6),
72.45, 73.42, 75.15, 75.63 (benzyl CH2), 75.55 (C-1), 77.85, 79.04,
80.73, 83.69 (C-2, C-3, C-4, C-5).
2,3:5,6-Di-O-isopropylidene-␣-D-mannofuranosyl iodide (5): (98%),
25
iodide 2b (0.60 g, 92%), amorphous. Ϫ C34H35IO5 (650.6). Ϫ 1H m.p. 45Ϫ47°C (dec.) (from hexane/AcOEt). Ϫ [α]D ϭ ϩ141.7
NMR (400 MHz): δ ϭ 3.18 (dd, J ϭ 3.9 Hz, 9.6 Hz, 1 H, 2-H), (c ϭ 1.4). Ϫ C10H19IO5 (346.2): calcd. C 34.70, H 5.53; found C
1
3.53Ϫ3.56 (m, 2 H, 5-H, 6a-H), 3.60 (dd, J ϭ 7.6 Hz, 9.4 Hz, 1 H,
34.53, H 5.62. Ϫ H NMR (400 MHz): δ ϭ 1.29, 1.36, 1.43, 1.46
6b-H), 3.82 (dd, J ϭ 2.6 Hz, 9.7 Hz, 1 H, 3-H), 3.92Ϫ3.99 (m, 1 (4 s, 4 ϫ 3 H, 4 ϫ CH3), 3.91Ϫ3.96 (m, 2 H, 4-H, 6a-H), 4.06 (dd,
H, 4-H), 4.41 {d, J ϭ 11.7 Hz, 1 H, benzyl(i)a-H}, 4.48 {d, J ϭ J ϭ 6.2 Hz, 8.9 Hz, 1 H, 6b-H), 4.47Ϫ4.52 (m, 1 H, 5-H), 4.83 (dd,
3148
Eur. J. Org. Chem. 1999, 3147Ϫ3150

Mild Synthesis of Protected α-D-Glycosyl Iodides
FULL PAPER
J ϭ 3.7 Hz, 5.8 Hz, 1 H, 3-H), 5.33 (d, J ϭ 5.8 Hz, 1 H, 2-H), 6.65 Under the same conditions, the following α--glycosyl iodides were
(d, J ϭ 0.7 Hz, 1 H, 1-H). Ϫ 13C NMR: δ ϭ 24.25, 24.95, 25.64,
also obtained and found to be identical with the samples prepared
26.57 (CH3), 66.29 (C-6), 73.14 (C-2), 79.45, 79.79 (C-3, C-5), 85.34 according to procedure A:
(C-4), 100.91 (C-1), 108.96, 112.41 (Me2C).
2,3,4,6-Tetra-O-acetyl-␣-D-glucopyranosyl iodide (1a): (80%)
2,3,4,6-Tetra-O-benzyl-␣-
D
-glucopyranosyl iodide (1b): (65%)
2,3,4,6-Tetra-O-acetyl-␣-
D-glucopyranosyl iodide (1a): (97%), m.p.
106Ϫ107°C (dec.) (from hexane/AcOEt) [ref.[12] m.p. 108°C]. Ϫ
2,3,4,6-Tetra-O-acetyl-␣-D-galactopyranosyl iodide (2a): (80%)
2,3,4,6-Tetra-O-benzyl-␣-
25
25
[α]D ϭ ϩ231.1 (c ϭ 1.1) {ref.[2] [α]D ϭ ϩ233 (c ϭ 1), CHCl3}.
Ϫ C14H19IO9 (458.2): calcd. C 54.29, H 4.50; found C 54.81, H
4.43. Ϫ 1H NMR (400 MHz): δ ϭ 2.02, 2.04 (2s, 2 ϫ 3 H,
2 ϫ CH3), 2.09 (s, 6 H, 2 ϫ CH3), 3.98Ϫ4.29 (m, 3 H, 2-H, 5-H,
6a-H), 4.33 (dd, J ϭ 12.8 Hz, 3.5 Hz, 1 H, 6b-H), 5.17 (t, J ϭ
9.9 Hz, 1 H, 4-H), 5.46 (t, J ϭ 9.8 Hz, 1 H, 3-H), 6.98 (d, J ϭ
3.8 Hz, 1 H, 1-H). Ϫ 13C NMR: δ ϭ 20.51, 20.72 (CH3), 60.70 (C-
2), 66.73 (C-6), 70.12, 71.56, 72.88 (C-3, C-4, C-5), 74.75 (C-1),
169.33, 169.44, 169.69, 170.33 (CϭO).
D-galactopyranosyl iodide (2b): (67%)
2,3,4,6-Tetra-O-acetyl-␣-D-mannopyranosyl iodide (4a): (82%)
2,3,4,6-Tetra-O-benzyl-␣-
D-mannopyranosyl iodide (4b): (66%)
Also under the same conditions:
2-Deoxy-2-azido-3,4,6-tri-O-acetyl-␣-D-galactopyranosyl iodide (3):
25
(90%), amorphous. Ϫ [α]D ϭ ϩ173.5 (c ϭ 1.2). Ϫ C12H16IN3O7
(441.2): calcd. C 32.67, H 3.66; found C 32.80, H 3.71. Ϫ 1H NMR
(400 MHz): δ ϭ 2.04, 2.06, 2.11 (3 s, 3 ϫ 3 H, 3 ϫ CH3), 3.38 (dd,
J ϭ 10.6 Hz, 4.1 Hz, 1 H, 2-H), 4.08 (dd, J ϭ 6.4 Hz, 10.8 Hz, 1
H, 6a-H), 4.13Ϫ4.29 (m, 2 H, 5-H, 6b-H), 5.16 (dd, J ϭ 10.6 Hz,
3.3 Hz, 1 H, 3-H), 5.43Ϫ5.46 (m, 1 H, 4-H), 6.79 (d, J ϭ 4.1 Hz,
1 H, 1-H). Ϫ 13C NMR: δ ϭ 20.52, 20.43 (CH3), 58.48 (C-2), 60.50
(C-6), 66.10, 71.90, 73.90 (C-3, C-4, C-5), 74.70 (C-1), 169.38,
169.64, 170.21 (CϭO).
2,3,4,6-Tetra-O-acetyl-␣-D-mannopyranosyl iodide (4a): (95%), oily.
25
25
Ϫ [α]D ϭ ϩ190.3 (c ϭ 1.2) {ref.[12] [α]D ϭ ϩ190 (c ϭ 1),
CHCl3}. Ϫ C14H19IO9 (458.2): calcd. C 54.26, H 4.50; found C
1
53.83, H 4.47. Ϫ H NMR (400 MHz): δ ϭ 2.01, 2.08, 2.10, 2.17
(4s, 4 ϫ 3 H, 4 ϫ CH3), 3.96 (ddd, J ϭ 9.9 Hz, 4.7 Hz, 2.0 Hz, 1
H, 5-H), 4.14 (dd, J ϭ 2.0 Hz, 12.7 Hz, 1 H, 6a-H), 4.35 (dd, J ϭ
4.9 Hz, 12.6 Hz, 1 H, 6b-H), 5.38 (t, J ϭ 9.9 Hz, 1 H, 4-H), 5.48
(dd, J ϭ 3.5 Hz, 1.4 Hz, 1 H, 2-H), 5.80 (dd, J ϭ 3.5 Hz, 10.0 Hz,
1 H, 3-H), 6.71 (d, J ϭ 1.4 Hz, 1 H, 1-H). Ϫ 13C NMR: δ ϭ 21.34,
21.44, 21.53 (CH3), 62.03 (C-6), 66.10, 66.90, 69.30 (C-2, C-3, C-
4), 74.03 (C-5), 76.04 (C-1), 170.23, 170.41, 171.20 (CϭO).
2,3,4,6-Tetra-O-benzoyl-␣-D-mannopyranosyl iodide (4c): (96%),
25
25
amorphous. Ϫ [α]D ϭ ϩ45.3 (c ϭ 1.2) {ref.[4] [α]D ϭ ϩ45 (c ϭ
1, CHCl3)}. Ϫ C34H27IO9 (706.5): calcd. C 60.19, H 4.61; found C
1
60.54, H 4.69. Ϫ H NMR (400 MHz): δ ϭ 4.40 (dt, J ϭ 9.9 Hz,
2.8 Hz, 1 H, 5-H), 4.55 (dd, J ϭ 3.9 Hz, 12.5 Hz, 1 H, 6a-H), 4.76
(dd, J ϭ 2.4 Hz, 12.5 Hz, 1 H, 6b-H), 5.92 (dd, J ϭ 1.5 Hz, 3.1 Hz,
1 H, 2-H), 6.28 (t, J ϭ 9.9 Hz, 1 H, 4-H), 6.40 (dd, J ϭ 3.2 Hz,
10.2 Hz, 1 H, 3-H), 6.96 (d, J ϭ 1.5 Hz, 1 H, 1-H), 7.18Ϫ8.15 (m,
20 H, aromatic H). Ϫ 13C NMR: δ ϭ 61.45 (C-6), 65.86, 66.30,
69.65 (C-2, C-3, C-4), 73.98 (C-5), 75.39 (C-1), 164.65, 165.07,
165.12, 165.62 (CϭO).
2,3,4,6-Tetra-O-benzyl-␣-D-mannopyranosyl iodide (4b): (94%),
amorphous. Ϫ C34H35IO5 (650.6). Ϫ 1H NMR (250 MHz): δ ϭ
3.55Ϫ3.70 (m, 2 H, 5-H, 6a-H), 3.85 (dd, J ϭ 4.1 Hz, 11.2 Hz, 1
H, 6b-H), 3.99 (dd, J ϭ 1.4 Hz, 3.0 Hz, 1 H, 2-H), 4.10 (t, J ϭ
9.7 Hz, 1 H, 4-H), 4.40 (dd, J ϭ 3.1 Hz, 9.6 Hz, 1 H, 3-H), 4.44
{d, J ϭ 11.0 Hz, 1 H, benzyl(i)a-H}, 4.52Ϫ4.70 (m, 6 H, benzyl
CH2), 4.90 {d, J ϭ 11.0 Hz, 1 H, benzyl(i)b-H}, 6.91 (d, J ϭ
1.4 Hz, 1 H, 1-H), 7.21Ϫ7.43 (m, 20 H, aromatic H). Ϫ 13C NMR:
δ ϭ 67.73 (C-6), 72.21, 72.53, 73.12, 73.73 (benzyl CH2), 74.74 (C-
4), 75.11 (C-1), 78.32, 78.61, 79.85 (C-2, C-3, C-5).
2,3,4,6-Tetra-O-benzoyl-␣-D-galactopyranosyl iodide (2c): (97%),
25
amorphous. Ϫ [α]D ϭ ϩ138.5 (c ϭ 1.2). Ϫ C34H27IO9 (706.5):
calcd. C 60.19, H 4.61; found C 60.32, H 4.59. Ϫ 1H NMR
(400 MHz): δ ϭ 4.32Ϫ4.36 (m, 1 H, 5-H), 4.49 (dd, J ϭ 5.5 Hz,
11.4 Hz, 1 H, 6a-H), 4.69 (dd, J ϭ 5.3 Hz, 11.4 Hz, 1 H, 6b-H),
5.02 (dd, J ϭ 4.3 Hz, 10.5 Hz, 1 H, 2-H), 5.95 (dd, J ϭ 3.2 Hz,
10.6 Hz, 1 H, 3-H), 6.10 (d, J ϭ 3.2 Hz, 1 H, 4-H), 7.19Ϫ7.68 (m,
21 H, 1-H, aromatic H). Ϫ 13C NMR: δ ϭ 61.51 (C-6), 67.70 (C-
5), 68.31 (C-4), 70.78 (C-3), 74.17 (C-2), 74.95 (C-1), 163.58,
164.80, 165.65, 166.11 (CϭO).
Formation of ␣-D-Glycosyl Iodides by Triphenylphosphane-I2 Com-
plex: 2,3,4,6-tetra-O-Benzoyl-␣-
D-glucopyranosyl Iodide (1c). ؊
General Procedure B: A magnetically stirred solution of I2 (0.50 g,
1.9 mmol) in dry dichloromethane (6 mL) at room temp. was ti-
trated with a solution of triphenylphosphane (0.50 g, 1.9 mmol) in
the same solvent (6 mL) in the dark and under dry argon (or nitro-
gen) atmosphere. After 30 min solid imidazole (0.24 g, 3.5 mmol)
was added, in one portion, to the pale yellow solution of triphenyl-
phosphaneϪI2 complex. After a few more minutes 2,3,4,6-tetra-
O-benzoyl-α--glucopyranose (0.60 g, 1.0 mmol), dissolved in the
same solvent (6 mL), was also added. Within 3.5 h the starting pro-
tected sugar was completely consumed (TLC monitoring) and the
reaction mixture was evaporated under reduced pressure. A quick
chromatography of the residue on silica gel (light petroleum ether/
AcOEt, 7:3) yielded the pure iodide 1c (0.68 g, 96%), m.p.
Acknowledgments
This work represents a partial fulfilment of the Dottorato di
Ricerca thesis by D.M. Financial support by Regione Campania
(Ricerca Scientifica) to G.P. is gratefully acknowledged.
25
132Ϫ134°C (dec.) (from hexane/AcOEt). Ϫ [α]D ϭ ϩ169.2 (c ϭ
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