O-Isopropylidenation of carbohydrates catalyzed by vanadyl triflate

Article abstract of DOI:10.1016/j.carres.2006.04.001

Vanadyl triflate has been identified as a mild and efficient catalyst for the chemoselective O-isopropylidenation of functionalized carbohydrates with acetone and acetone equivalents. The current protocol is compatible with a diverse array of protecting groups and the products can be readily isolated by simple aqueous wash.

Full text of DOI:10.1016/j.carres.2006.04.001

Carbohydrate Research 341 (2006) 1948–1953
Note
O-Isopropylidenation of carbohydrates catalyzed
by vanadyl triflate
a,b
Chun-Cheng Lin,a,b, Mi-Dan Jan, Shiue-Shien Weng,c
*
Chang-Ching Lina,b and Chien-Tien Chenc,*
aInstitute of Chemistry and Genomic Research Center, Academia Sinica, Taipei, Taiwan
bDepartment of Chemistry, National Tsing Hua University, Hsinchu, Taiwan
cDepartment of Chemistry, National Taiwan Normal University, Taipei, Taiwan
Received 12 December 2005; received in revised form 27 March 2006; accepted 2 April 2006
Available online 15 May 2006
Abstract—Vanadyl triflate has been identified as a mild and efficient catalyst for the chemoselective O-isopropylidenation of func-
tionalized carbohydrates with acetone and acetone equivalents. The current protocol is compatible with a diverse array of protecting
groups and the products can be readily isolated by simple aqueous wash.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: O-Isopropylidenation; Vanadyl triflate; Saccharides; Acetonides; Chemoselectivity
The protection of 1,2- and 1,3-diols as O-isopropylidene
derivatives (acetonides) is important in multi-step syn-
theses in organic, medicinal, and carbohydrate chemis-
try.1 Although many approaches have been displayed
for protecting diols and, particularly, polyols with con-
tiguous hydroxyl groups, O-isopropylidenation consti-
tutes one of the most important tactics during the
synthesis of oligosaccharides.2,3 The O-isopropyliden-
ation of a diol is typically performed by using acetone
under anhydrous conditions in the presence of an acid
catalyst—generally p-toluenesulfonic acid (TsOH) at ele-
vated temperature. In addition, this condensation reac-
tion can be performed by using other acetonide-forming
agents, including 2,2-dimethoxypropane (DMP) and 2-
methoxypropene (MP) in anhydrous solvents such as
N,N-dimethylformamide (DMF) or dimethylsulfoxide
with acid catalysis. So far, a diverse array of catalysts
have been employed in the isopropylidenation of saccha-
rides preferably by using the latter approach, including
pyridinium p-toluenesulfonate (PPTS),4 TESOTf,5
HBF4,6 di-p-nitrophenyl hydrogen phosphate,7 Lewis
acidic metal salts (e.g., ZnCl2,8 AlCl3,9 ceric ammonium
nitrate (CAN),10 CuSO4,11 FeCl3,12 SnCl2,13 and
PdX214), iodine,15 and heterogeneous media (e.g., ion
exchange resins,16 morillonite clay,17 zeolites,18 and
hetero-polyacids (HPA)19). However, the aforemen-
tioned reagents are either moisture sensitive or sluggish
in terms of reactivity at ambient temperature. Therefore,
dried solvents, harsher reaction conditions, or cumber-
some workup and purification are sometimes required
during synthetic manipulation, Thus, an ideal neutral
and water tolerant catalyst that meets the demands of
offering greater safety, re-usability, functional substrate
compatibility, and, most importantly, chemoselectivity
remains to be explored.
The synthetic utility of vanadium-containing com-
pounds has been widely explored and utilized as
reagents or catalysts for redox-type C–C bond forming
reactions20,21 and oxidation processes when combined
with suitable co-oxidants.22 For the past five years, we
have identified several water-tolerant vanadyl and other
oxometallic species as recoverable, amphoteric catalysts
for nucleophilic acyl substitutions (NAS) of anhy-
drides,23a,b methyl esters,23c and carboxylic acids23d
with protic nucleophiles with high functional group
*
Corresponding authors. E-mail: cclin@chem.sinica.edu.tw
0008-6215/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.04.001

C.-C. Lin et al. / Carbohydrate Research 341 (2006) 1948–1953
1949
compatibility and chemoselectivity and the direct atom-
efficient benzylidenation24 of saccharides with equimolar
aldehydes. In continuation of our related works in catal-
ysis25 and as part of an ongoing effort to explore vanadyl
triflate as a neutral, water-tolerant, and recoverable
reagent in new catalytic reactions, we report herein a
handy and efficient method for chemoselective O-
isopropylidenation of carbohydrates catalyzed by vana-
dyl triflate (VO(OTf)2ÆxH2O) at ambient temperature.
D-Galactose was first selected as a test mono-saccha-
ride substrate in the O-isopropylidenation (Table 1,
entry 1) by using acetone and common acetonide-form-
ing agents (DMP and MP) in the presence of a catalytic
amount of VO(OTf)2. The test reaction procedure in-
volved mixing 1 mmol of a given sugar with 5 mL of
acetone or 10 equiv of DMP or MP (as a 0.1 M solution
in CH3CN) in the presence of 5 mol % of VO(OTf)2 at
ambient temperature for 2–10 h. Of these three aceto-
nating reagents examined, 1,2:5,6-di-O-isopropylidene-
galactopyranoside—5 was the only isolable product
with yields in the range of 85–93%. The same chemose-
lective acetonide-protection product—6 was formed in
high yields (92–95%) when L-fucose was used as the
starting sugar (entry 2). Interestingly, the isoprop-
ylidenation of D-glucose and mannose led to only
thermodynamically favored di-isopropylidene-protected
furanosides 7 and 8,26 regardless of the acetonating re-
agents used (entries 3 and 4). As noted in entries 1 and
3 of Table 1, the straight acetone and VO(OTf)2 recipe
normally requires a longer reaction time to afford simi-
larly good yields of products 5 and 7 presumably due to
the poorer solubility of the starting materials 1 and 3 in
acetone. Additionally, the current new catalytic protocol
poses a very simple workup and purification procedure.
Simple extraction of the water-quenched reaction mix-
ture by ethyl acetate led to direct separation of the prod-
uct from the water-soluble catalyst. And the catalyst can
be readily recovered from the aqueous phase after con-
centration in vacuo.
To expand the substrate scope, a series of monosac-
charides with sulfur-(S–Ar) and oxygen-(OMe and
OPh) containing substituents at the anomeric position
were further examined with the optimal catalytic proto-
col (Table 2, entries 1–6). In general, isopropylidena-
tions by using DMP and MP led to better yields with
shorter reaction time than those by using acetone. Nota-
bly, glucopyranosides 9–11 (entries 1–3) furnished the
corresponding 4,6-O-isopropylidene derivatives 15–17
exclusively due to slower rate of trans ring-fused aceto-
nide formation. Whereas only 3,4-O-isopropylidene
derivatives 18 and 19 were obtained starting from galac-
topyranosides 12 and 13 (entries 4 and 5), respectively.
On the other hand, using a slight excess (1.2 equiv) of
MP in the case of benzylidene-protected 14 provided
trans-2,3-isopropylidene glucopyranoside 20 in moder-
ate yield (Table 2, 64%, entry 6). However, the trans-iso-
propylidene of 20 was unstable and easily decomposed.
With the preliminary success on the use of vanadyl tri-
flate in the chemoselective isopropylidenation of simple
monosaccharides, the generality of the protocol was fur-
ther extended to more complexed carbohydrates. As
shown in Table 3, chemoselective isopropylidenations
of functionalized monosaccharides (21, 24, and 26)
and disaccharides (22, 23, and 25) also proceeded
Table 1. Protection of free sugar acetonide with VO(OTf)2
Entry
Substrate
Product
Reagenta
Time (h)
Yield (%)
OH
O
HO
O
OH
DMP
MP
Acetone
2
2
10
93
89
85
O
1
HO
O
OH
HO
1
O
O
5
HO
OH
O
O
OH
OH
2
3
4
DMP
MP
Acetone
2
2
3
95
92
92
O
O
O
6
2
O
O
O
OH
O
OH
O
HO
DMP
MP
Acetone
2
2
10
90
90
89
HO
HO
3
O
OH
OH
O
7
O
HO
HO
O
O
O
HO
OH
DMP
MP
Acetone
2
2
3
93
92
92
HO
O
8
O
4
a DMP/MP 10 equiv or acetone 5 mL in CH3CN.

1950
C.-C. Lin et al. / Carbohydrate Research 341 (2006) 1948–1953
Table 2. Protection of sugar acetonide with VO(OTf)2
Entry
1
Substrate
Product
Reagenta
Time (h)
Yield (%)
OH
DMP
MP
Acetone
1
4
11
92
90
85
O
O
O
HO
HO
O
HO
HO
HO
OMe
OMe
9
15
OH
O
O
O
HO
HO
O
STol
OPh
2d
DMP
MP
Acetone
4
6
24
89
82
50b
STol
OPh
HO
OH
10
OH
O
OH
16
17
O
O
HO
HO
O
3
DMP
MP
Acetone
4
6
24
88
86
50b
HO
OH
11
OH
OH
HO
HO
OH
O
O
O
O
SPh
4
DMP
MP
Acetone
4
6
24
92
94
50b
SPh
OH
12
OH
18
OH
O
HO
HO
O
O
OH
O
5
DMP
MP
Acetone
1
6
11
91
87
80
HO
HO
OMe
OMe
13
19
STol
O
O
O
O
O
Ph
O
O
STol
6c,d
DMP
MP
20
4
Trace
64
HO
Ph
O
OH
14
20
a DMP/MP 1.5 equiv or acetone 5 mL in CH3CN.
b 50% starting material was recovered.
c DMP: 10 equiv and MP: 1.2 equiv in 0.5 M CH3CN.
d Tol = p-methyl phenyl.
smoothly within 6 h and in good yields. Functional
groups such as NHTroc (Troc: C(O)O(CH)2CCl3, entry
1), alkene (entry 3), NHTFA (TFA: C(O)CF3, entries 4
and 5), azido, tert-butyl ester, and Fmoc (a carbamate,
entry 5) remained intact under optimal VO(OTf)2-cata-
lyzed conditions. Notably, chemoselective formations
of the thermodynamically favored 1,2-acetonides (30
and 31) over the corresponding 1,3- or 2,3-acetonides
were observed at the terminal triol side chain of sialic
acids (entries 4 and 5). Strikingly, in the case of
azido-galactoside-26 (entry 6), 74% of the expected
3,4-O-isopropylidene 32 was formed along with 4,6-O-
isopropylidene protected side product (15%).
In summary, a new versatile and neutral protocol for
the chemoselective O-isopropylidenation of functional-
ized carbohydrates has been demonstrated, which in-
volves water-tolerant vanadyl triflate as the recoverable
catalyst. The current system offers an efficient and
alternative variant over existing methods in terms of
operational simplicity and compatibility to diversed
functional groups, auguring well for its potential applica-
tions in organic and carbohydrate synthesis.
1. Experimental
1.1. General methods
1H and 13C NMR spectra were recorded on Bruker AM-
1
400 or 500 MHz. Assignment of H NMR spectra was
achieved using 2D methods (COSY). Chemical shifts
were expressed in parts per million using residual CHCl3
as reference. High-resolution mass spectra were ob-
tained by means of a Micromass (Autospec) mass spec-
trometer. Analytical thin-layer chromatography (TLC)
was performed on precoated plates (silica gel 60
F-254). Silica gel 60 (E. Merck Co.) was employed
for all flash chromatography. All reactions were car-
ried out in oven-dried glassware (120 °C) under an
atmosphere of nitrogen unless indicated otherwise. All
solvents were dried and distilled by standard tech-
niques.
Compounds 5,27 7,28 8,29 15,30 18,31 19,32 20,24 27,33
29,34 and 3235 have previously been reported and the
NMR spectral data are in good agreement with the
literature data.

C.-C. Lin et al. / Carbohydrate Research 341 (2006) 1948–1953
Table 3. Protection of sugar acetonide with DMPa/VO(OTf)2
1951
Entry
Substrate
Time (h)
Product
Yield (%)
OH
O
O
O
HO
HO
O
1b
5
91
STol
STol
HO
NHTroc
21
TrocHN
27
HO
O
OH
OH
O
OH
O
OH
O
O
O
2b
STol
O
5
6
89
88
O
HO
O
STol
O
HO
HO
HO
HO
HO
HO
28
22
HO
HO
O
OH
O
OH
O
OH
O
OH
O
O
O
3
O
HO
HO
HO
HO
HO
HO
29
23
OH
HO
HO
O
O
CO2Me
O
CO2Me
O
4
5
6
6
92
89
O
HO
TFAHN
O
TFAHN
HO
HO
24
30
OH
HO
O
O
CO2Me
CO2Me
HO
TFAHN
O
HO
TFAHN
OH
O
O
OH
O
CO2Me
O
HO
CO2Me
HO
HO
HO
O
TFAHN
O
25
O
TFAHN
HO
31
HO
O
O
HO
HO
OH
O
OH
O
N
3 O
N
3 O
6
5
74
t
t
OBu
OBu
FmocHN
FmocHN
26
O
O
32
a DMP: 10 equiv in 0.5 N CH3CN.
b Tol = p-methyl phenyl.
1.2. General procedure for the acetonation
1.2.1. 1,2:3,4-Di-O-isopropylidine-1-thio-L-fucopyranose
(6). Syrup; 1H NMR (500 MHz, CDCl3): 5.52 (d,
1H, J = 4.8 Hz), 4.59 (dd, 1H, J = 8.0 Hz, J = 2.4 Hz),
4.29 (dd, 1H, J = 5.2 Hz, J = 2.4 Hz), 4.08 (dd, 1H,
J = 8.0 Hz, J = 2.0 Hz), 3.94–3.89 (m, 1H), 1.53 (s,
3H), 1.47 (s, 3H), 1.35 (s, 3H), 1.33 (s, 3H), 1.26 (d,
3H, J = 6.4 Hz); 13C NMR (125 MHz, CDCl3): 108.83,
108.12, 96.45, 73.44, 70.86, 70.27, 63.38, 25.94, 25.93,
24.80, 24.34, 15.82; TLC: Rf = 0.25 (EtOAc/hexane,
1/3); HRMS (FAB) calcd for C12H21O5 [M+H]+:
245.1382. Found: 245.1389.
A 50 mL two-necked round bottomed flask, under dry
nitrogen atmosphere, was charged with the vanadyl tri-
flate, VO(OTf)2 (0.05 mmol), followed by addition of
anhydrous acetonitrile (10 mL). To the above solution
of catalyst, DMP (10 mmol), MP (10 mmol), and ace-
tone (5 mL) were slowly added at ambient temperature
and the resulting solution was stirred for 10 min. The
starting sugar (1.0 mmol) was then added to the above
green solution and the resulting mixture was stirred at
ambient temperature for a period of time. The reaction
was monitored by TLC. After completion of the reac-
tion, to the reaction mixture were sequentially added
0.5 mL triethylamine and cold saturated sodium bicar-
bonate solution. The reaction mixture was extracted with
30 mL ethyl acetate. The organic layer was washed with
brine solution, dried over magnesium sulfate, and fil-
tered. The organic solvent was removed in vacuo and
the crude product was purified by flash chromatography.
1.2.2. 4-Methyl phenyl-3,4-O-isopropylidine-1-thio-b-D-
glucopyranoside (16). Syrup; 1H NMR (400 MHz,
CDCl3): 7.41 (d, 2H, J = 8 Hz), 7.14 (d, 2H, J =
8 Hz), 4.58 (d, 1H, J = 9.6 Hz), 3.94 (dd, 1H,
J = 10.4 Hz, J = 5.2 Hz), 3.76 (t, 1H, J = 10.8 Hz),
3.68 (t, 1H, J = 8.4 Hz), 3.51 (t, 1H, J = 9.6 Hz), 3.41–
3.27 (m, 2H), 3.10 (br, 1H, OH), 2.90 (br, 1H, OH),
2.61 (s, 3H, CH3), 1.49 (s, 3H, CH3), 1.42 (s, 3H,

1952
C.-C. Lin et al. / Carbohydrate Research 341 (2006) 1948–1953
CH3); 13C NMR (100 MHz, CDCl3): 138.68, 133.43,
129.84, 127.50, 99.80, 88.80, 74.91, 72.89, 72.66, 71.52,
61.97, 28.94, 21.12, 19.10; TLC: Rf = 0.20 (ethyl ace-
tate/hexane, 1/2); EI MS (70 eV) calcd for C16H22O5S:
326. Found: 326 (M+, 24), 202 (28), 122 (100), 91 (89),
69 (34). Anal. Calcd for C16H22O5S: C, 58.87; H, 6.79.
Found: C, 58.49, H, 6.47.
J = 4.7 Hz, H-3e), 1.75 (dd, 1H, J = 12.3 Hz,
J = 12.3 Hz, H-3a), 1.36 (s, 3H, CH3), 1.35 (s, 3H,
CH3); 13C NMR (100 MHz, MeOD): 170.19, 159.50,
135.65, 118.05, 117.02, 110.11, 100.50, 77.66, 74.46,
70.52, 68.31, 67.46, 66.48, 54.09, 54.04, 41.75, 27.21,
25.92; TLC: Rf = 0.63 (EtOAc/hexane, 1/1+15%
MeOH); HRMS (FAB) calcd for C18H26F3NO9
[M+H]+: 458.1638. Found: 458.1640.
1.2.3. Phenyl 4,6-O-isopropylidine-b-D-glucopyranoside
1
(17). Syrup; H NMR (400 MHz, CDCl3): 7.32–7.27
1.2.6. Methyl-(2-allyl-3,5-dideoxy-9-O-(methyl-3,5-dide-
oxy-5-trifluoroacetamido-D-glycero-a-D-galacto-non-2-
ulopyranosylonate)-5-trifluoroacetamido-D-glycero-a-D-
galacto-non-2-ulopyranoside) onate (31). Syrup; 1H
NMR (400 MHz, MeOD): 5.86 (dddd, 1H, J =
17.2 Hz, J = 10.4 Hz, J = 5.2 Hz, J = 5.2 Hz), 5.24
(dddd, 1H, J = 17.2 Hz, J = 1.6 Hz, J = 1.6 Hz, J =
1.6 Hz), 5.11 (dddd, 1H, J = 10.4 Hz, J = 1.6 Hz, J =
1.6 Hz, J = 1.6 Hz), 4.29 (dddd, 1H, J = 12.8 Hz,
J = 5.2 Hz, J = 1.6 Hz, J = 1.6 Hz), 4.22 (dd, 1H, J =
13.2 Hz, J = 6.0 Hz), 4.08 (dd, 1H, J = 8.4 Hz, J =
6.0 Hz), 4.02–3.94 (m, 5H), 3.92–3.83 (m, 3H), 3.82 (s,
3H), 3.81 (s, 3H), 3.77–3.70 (m, 1H), 3.72–3.68 (m,
2H), 3.53 (dd, 1H, J = 8.8 Hz, J = 1.2 Hz), 3.47 (d,
1H, J = 7.2 Hz), 2.69 (dd, 1H, J = 12.0 Hz, J =
4.4 Hz), 2.68 (dd, 1H, J = 12.0 Hz, J = 4.4 Hz), 1.78
(t, 1H, J = 12.0 Hz), 1.75 (t, 1H, J = 12.0 Hz), 1.34 (s,
3H), 1.33 (s, 3H); 13C NMR (100 MHz, MeOD):
170.73, 170.10, 159.78, 159.77, 135.46, 118.97, 118.96,
117.10, 110.14, 100.19, 100.09, 73.73, 73.68, 72.62,
71.22, 70.17, 69.65, 68.54, 68.51, 68.37, 66.38, 64.78,
53.95, 53.91, 53.40, 53.38, 41.65, 41.57, 27.25, 25.96;
TLC: Rf = 0.38 (EtOAc/hexane, 1/1+10% MeOH);
HRMS (FAB) calcd for C30H43N2F6O17 [M+H]+:
817.2466. Found 817.2451.
(m, 2H), 7.08–7.01 (m, 3H), 4.96 (d, 1H, J = 7.32 Hz),
3.94 (dd, 1H, J = 10.5 Hz, J = 5.4 Hz), 3.79 (t,1H,
J = 10.4 Hz), 3.75 (dd, 1H, J = 10.8 Hz, J = 5.4 Hz),
3.66 (dd, 1H, J = 7.32 Hz, J = 6.1 Hz), 3.49 (br, 1H,
OH), 3.41–3.35 (m, 2H), 1.90 (br, 1H, OH), 1.51 (s,
3H), 1.46 (s, 3H); 13C NMR (100 MHz, CDCl3):
156.85, 129.57, 123.13, 116.85, 101.15, 99.93, 74.36,
73.50, 72.97, 67.41, 61.97, 28.96, 19.04; TLC: Rf = 0.16
(ethyl acetate/hexane, 1/2). Anal. Calcd for C15H20O6:
C, 60.80; H, 6.80. Found: C, 60.92; H, 6.58.
1.2.4. 4-Methyl phenyl O-(3,4-O-isopropylidine-b-D-
galactopyranosyl)-(1!4)-1-thio-b-D-glucopyranoside (28).
White solid; mp 189–191 °C; 1H NMR (400 MHz,
MeOD): 7.49 (d, 2H, J = 8.4 Hz), 7.16 (d, 2H,
J = 8.0 Hz), 4.55 (d, 1H, J1,2 = 10 Hz, H-1), 4.39 (d,
1H, J1,2 = 8.4 Hz, H-10), 4.21 (dd, 1H, J = 5.6 Hz,
J = 2.4 Hz, H-40), 4.07 (dd, 1H, J = 7.8 Hz, J =
5.6 Hz, H-30), 3.97–3.75 (m, 5H, H-50, H-60a, H-60b,
H-6a, H-6b), 3.57–3.56 (m, 2H, H-3, H-4), 3.49–3.43
(m, 2H, H-5, H-20), 3.27 (dd, 1H, J = 10 Hz,
J = 7.8 Hz, H-2), 2.31 (s, 3H), 1.47 (s, 3H), 1.32 (s,
3H); 13C NMR (100 MHz, MeOD): 137.46, 132.35,
129.38, 129.08, 109.63, 102.58, 87.89, 79.38, 78.99,
78.96, 76.40, 73.89, 73.60, 72.98, 71.98, 60.96, 60.49,
26.94, 25.03, 19.65; TLC: Rf = 0.25 (MeOH/CHCl3,
1/3); HRMS (FAB) calcd for C22H32O10S [M+H]+:
489.1794. Found 489.1787.
Acknowledgment
This research was supported by the Academia Sinica,
National Taiwan Normal University and National
Science Council in Taiwan.
1.2.5. Methyl-(2-allyl-3,5-dideoxy-3,5-dideoxy-5-trifluoro-
acetamido-8,9-O-methylethyliden-a-D-galacto-non-2-ulo-
pyranosid)onate (30). Syrup; 1H NMR (400 MHz,
MeOD): 5.87 (dddd, 1H, J = 17.2 Hz, J = 10.5
Hz, J = 5.4 Hz, J = 5.4 Hz), 5.24 (dddd, 1H, J =
17.2 Hz, J = 1.7 Hz, J = 1.7 Hz, J = 1.7 Hz), 5.12
(dddd, 1H, J = 10.5 Hz, J = 1.7 Hz, J = 1.4 Hz, J =
1.4 Hz), 4.28 (dddd, 1H, J = 12.8 Hz, J = 5.4 Hz,
J = 1.7 Hz, J = 1.4 Hz), 4.23 (ddd, 1H, J = 6.3 Hz,
J = 6.3 Hz, J = 6.3 Hz, H-8), 4.10 (m, 1H), 4.08 (dd,
1H, J = 8.4 Hz, J = 6.3 Hz, H-9a), 4.00 (dd, 1H, J =
8.4 Hz, J = 6.3 Hz, H-9b), 3.97 (dd, 1H, J = 10.2 Hz,
J = 10.0 Hz, H-5), 3.80 (s, 3H), 3.78 (dd, 1H, J =
10.2 Hz, J = 1.1 Hz, H-6), 3.71 (ddd, 1H, J = 12.3 Hz,
J = 10.0 Hz, J = 4.7 Hz, H-4), 3.53 (dd, 1H, J =
6.3 Hz, J = 1.1 Hz, H-7), 2.67 (dd, 1H, J = 12.3 Hz,
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.carres.
2006.04.001.
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