An efficient synthesis of 2,3-dideoxy-α,β-unsaturated carbohydrate enals by mixed Lewis acid (HfCl4 and ZnI2) catalyzed hydration of glycals

Article abstract of DOI:10.1016/j.carres.2006.02.031

A new, efficient method has been developed for converting acyl-, arylalkyl- and alkyl-protected glycals into corresponding 2,3-dideoxy-α,β-unsaturated carbohydrate enals utilizing the in situ generated push-pull effect resulting from the synergistic combi

Full text of DOI:10.1016/j.carres.2006.02.031

Carbohydrate Research 341 (2006) 1052–1056
Note
An efficient synthesis of 2,3-dideoxy-a,b-unsaturated
carbohydrate enals by mixed Lewis acid (HfCl4 and ZnI2)
catalyzed hydration of glycals I
Mohammad Saquib, Ram Sagar and Arun K. Shaw*
Division of Medicinal and Process Chemistry, Central Drug Research Institute (CDRI), Lucknow 226 001, India
Received 22 December 2005; received in revised form 21 February 2006; accepted 23 February 2006
Available online 29 March 2006
Abstract—A new, efficient method has been developed for converting acyl-, arylalkyl- and alkyl-protected glycals into correspond-
ing 2,3-dideoxy-a,b-unsaturated carbohydrate enals utilizing the in situ generated push–pull effect resulting from the synergistic
combination of HfCl4 and ZnI2 in catalytic amounts. This new procedure eliminates the use of highly toxic Hg2+ ions and acidic
conditions (0.01–0.02 N H2SO4), besides radically shortening the reaction time.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: a,b-Unsaturated carbohydrate enals; Glycals; Hafnium tetrachloride; Zinc iodide; Push–pull effect
Carbohydrates are ubiquitous and vital biomolecules
that can be fashioned into a variety of versatile inter-
mediates used for the synthesis of biodynamic molecules.
2,3-Dideoxy-a,b-unsaturated sugar aldehydes (2), com-
monly known as Perlin aldehydes,1 constitute an
increasingly important class of carbohydrate derivatives
that have been used as precursors for the syntheses of
many biologically important compounds during the last
three decades.1c,2–4 Until now, Perlin aldehydes have
been prepared from the corresponding glycals either
by Perlin’s method1a or its many variants1b,c involving
Hg2+ ion and 0.01N–0.02 N H2SO4 to effect the trans-
formation. All these methods thus suffer from the draw-
back of using the highly toxic Hg2+ ion which is not
recommended in medicinal chemistry and is also unpop-
ular due to waste-disposal problems. Further, the use of
0.01–0.02 N H2SO4 requires the neutralization of the
reaction mixture with a base, which in the case of acet-
yl-protected Perlin aldehydes invariably results in a
1:1 mixture of isomeric products which seriously limits
their synthetic utility.2c,3c,5 In addition (2E)-4,6-di-O-
benzyl-2,3-dideoxy-aldehydo-D-erythro-hex-2-enose (2a),
an important intermediate, could not be synthesized
by reported methods in more than 50% yield.1c Our re-
search group has been using Perlin aldehydes 2 for prep-
aration of antitubercular compounds.2e–g Therefore,
it has long been our desire to develop an alternative
method for synthesis of Perlin aldehydes 2. We herein
report an efficient and quicker method for the synthesis
of Perlin aldehydes from acyl- and alkyl/arylalkyl-pro-
tected glycals 1 using benign reagents. Lewis acids are
known to efficiently catalyze a number of varied reac-
tions of glycals, including the Ferrier rearrangement.6
This prompted us to explore Lewis acids as possible cat-
alysts for the hydration of glycals to Perlin aldehydes.
Of late HfCl4 has emerged as a versatile Lewis acid.7
Its high catalytic activity due to larger radii of Hf,8
low toxicity9a–c and relatively greater degree of water
tolerance induced us to explore this promising catalyst
for our current studies in water and acetonitrile.10
The study was initiated by stirring a mixture of 3,4,6-
tri-O-benzyl-D-glucal (1a, 104 mg, 0.25 mmol) and
HfCl4 (8 mg, 0.025 mmol) at 110–120 ꢁC in 1:1
CH3CN–H2O. The reaction was completed in ꢀ2.5 h
q CDRI Communication No. 6855.
*
Corresponding author. E-mail: akshaw55@yahoo.com
0008-6215/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.02.031

M. Saquib et al. / Carbohydrate Research 341 (2006) 1052–1056
1053
1
(TLC). The H NMR spectrum of the crude product
and ZnI2 (7 mg, 0.022 mmol) in 7:3 CH3CN–H2O as sol-
vent in a preheated oil bath at 130–135 ꢁC for 2 h (Table
1, entry 2).
showed that Perlin aldehyde 2a was formed only in
traces. We performed several trial experiments using
different quantities of HfCl4 in different proportions of
CH3CN–H2O mixtures at different temperatures, but
better results were not obtained.
Acetylation of the crude product mixture, followed by
column chromatography, gave 43% of the Perlin alde-
hyde 2b and 45% of the 2-deoxy product 3b as their acet-
yl derivatives. Under identical conditions, 3,4,6-tri-O-
methyl-D-galactal 1e furnished 2e (53%) and 3e (29%)
as their acetyl derivatives. Higher temperature did not
lead to any increase in the proportion of Perlin alde-
hyde; on the contrary, the overall yield was decreased,
probably due to degradation of the starting material.
Lowering the oil bath temperature to below 120 ꢁC
led to a noticeable decrease in the proportion of Perlin
aldehyde 2. Any further increment of the amount of
the reagents in the same ratio had no visible effect on
the product formation. It is worth reporting that none
of the alkyl/arylalkyl-protected glycals underwent the
title reaction to furnish the desired aldehyde, either in
the presence of HfCl4 or ZnI2 alone.
This protocol was also extended to acyl-protected gly-
cals. Since the acyl group is a better leaving group than
either the OBn or OMe group, we first wanted to see the
effect of HfCl4 alone on the acyl-protected glycals. On
stirring a mixture of 3,4,6-tri-O-acetyl-D-glucal 1f
(136 mg, 0.5 mmol) and HfCl4 (18 mg, 0.056 mmol) in
CH3CN in a temperature range of 105–110 ꢁC, the reac-
tion was completed (TLC) in 90 min. The 1H NMR
spectrum of the worked-up product showed exclusive
formation of 2f. When the same reaction was carried
out using 3,4,6-tri-O-acetyl-D-glucal 1f (136 g,
0.5 mmol), HfCl4 (18 mg, 0.056 mmol) and ZnI2 (7 mg,
0.022 mmol), and the temperature of the oil bath was
raised rapidly from room temperature to 110 ꢁC, the
reaction was completed in only 6–8 min (TLC). 1H
NMR of the worked-up reaction mixture showed exclu-
sive formation of 2f that could be used as such for fur-
ther reactions. As the reaction was carried out in very
weakly acidic conditions, washing of the reaction mix-
ture with water sufficed eliminating the need for the
use of alkali for neutralization of the reaction mixture
that solved the problem of acyl group migration. Acetyl-
ation of the worked-up product 2f, followed by column
chromatography, furnished its pure triacetate derivative
in 74% yield (Table 1, entry 6). The reaction did not pro-
ceed with ZnI2 alone. Similar results were obtained for
the other acyl-protected glycals 1g and 1h by adopting
the same methodology as mentioned above for 1f (Table
1, entries 7 and 8).
At this juncture, we decided to use a combination of
two Lewis acids hoping to invoke the push–pull effect11
for making this transformation more facile. Since glycals
are cyclic vinyl ethers having leaving group at C-3 either
in the form of an ester or an alkoxy group, we assumed
that HfCl4 like TiCl4 would coordinate with the acetoxy
or alkoxy group7a making it a better leaving group. This
process may become more pliant if a group or an atom
showing dual character of a good nucleophile and leav-
ing group concurrently adds on to C-1 of the glycal mole-
cule pushing the p bond from C-1–C-2 to C-2–C-3.
This would cause elimination of the leaving group at
C-3, which itself is being instantaneously substituted
by a molecule of water to form the hemiacetal, which
then equilibrates with the open-chain a,b-unsaturated
(Z) aldehyde that ultimately isomerizes to the more sta-
ble (E) enal.12 Accordingly for acting at the olefinic
bond we decided to use either CdI2 or ZnI2 (Zn, Cd and
Hg belong to the same group). Both of these showed
almost identical results, but our choice fell upon ZnI2
which is nontoxic9d and dissociates in water or polar
organic solvents to a greater extent to furnish Zn2+
and Iꢁ ions.
Considering all these aspects on carefully optimizing
the various parameters one by one, we found that on
stirring the substrate 1a (104 mg, 0.25 mmol) with HfCl4
(18 mg, 0.056 mmol) and ZnI2 (7 mg, 0.022 mmol) in a
7:3 CH3CN–H2O in a temperature range of 135–
140 ꢁC (oil bath) for 2.5 h furnished Perlin aldehyde 2a
and some other minor side products, including the 2-
deoxy derivative 3a in approximately 4:1 ratio (Table
1, entry 1). Acetylation of the crude product mixture,
followed by column chromatography, gave the acetyl
derivative of the Perlin aldehyde 2a in 72% yield
(Scheme 1, Table 1, entry 1).
In the case of benzyl-protected arabinal 1c and
methyl-protected glucal 1d, the Perlin aldehydes were
obtained exclusively by adopting the same methodology
as mentioned above for 1a (Table 1, entries 3 and 4).
Use of the same methodology in case of 3,4,6-tri-O-
benzyl-D-galactal (1b) yielded a 1:2 mixture of Perlin
aldehyde 2b and 2-deoxy pyranose 3b. After perform-
ing several trial experiments on 1b, the best result ob-
tained was the formation of a 1:1 mixture of Perlin
aldehyde 2b and 2-deoxy sugar derivative 3b by stirring
1b (104 mg, 0.25 mmol) with HfCl4 (18 mg, 0.056 mmol)
In conclusion, this paper describes a new and efficient
method for the preparation of 2,3-dideoxy-a,b-unsatu-
rated sugar aldehydes 2 from their corresponding glycals
1, eliminating the use of highly toxic Hg2+ and acidic
conditions by using inexpensive, benign reagents like
HfCl4 and ZnI2 and very mildly acidic conditions. The
simpler workup procedure solves the problem of acyl
The ratio was determined from the 1H NMR spectrum of the crude
product mixture.

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M. Saquib et al. / Carbohydrate Research 341 (2006) 1052–1056
Table 1. Results of Perlin hydrolysis of benzyl/methyl/acyl-protected glycals using HfCl4 (0.056 mmol) and ZnI2 (0.022 mmol) in a 7:3 CH3CN–H2O
solvent mixture
Entry
Substrate
Rxn. temp. (ꢁC)
Rxn. time
Product
Product ratioa 2:3
Yield (%) 2
Ref.b
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
135–140
130–135
135–140
135–140
130–135
Rt!110
Rt!110
Rt!110
2.5 h
2.0 h
2.3 h
2.5 h
2a and 3a
2b and 3b
2c
4:1
1:1
—
—
3:2
—
72c
43c
85
1c
1c
2c
1c
New
1a
2j
2d
2e and 3e
2f
90
2.0 h
53c
74c
73c
67c
6–8 min
6–8 min
6–8 min
1g
1h
2g
2h
—
—
2j
a The ratios were determined from the 1H NMR spectra of the crude product mixtures.
b References are to known compounds.
c Isolated yields after acetylation of the product mixture.
1H NMR and 13C NMR spectra were recorded on an
Avance Bruker instrument at 200 MHz for 1H and
50 MHz for 13C. Chemical shift values are expressed in
d ppm. Optical rotations were determined on Rudolph
Autopol III polarimeter using a 1-dm cell at 28 ꢁC;
concentrations mentioned are in g/100 mL. Commercial
grade CH3CN containing 0.2% water, purchased from
Ranbaxy Fine Chemicals Ltd (India), was used in all
experiments.
R2
R1
R2
R1
R3 R2
R1
Rxn. Condition
as in Table 1
O
O
R3
R4
R3
+
CHO
OR6
R4
OR6
R5
R5
2
1
R5
R3
R4
R6
R1
R2
OBn
OBn
H
OCH3
OCH3
a
b
c
H
OBn
H
H
OCH3
H
CH2OBn
CH2OBn
H
OBn
H
OBn
OCH3
H
OAc
H
OAc
H
H
H
H
H
H
H
OBn
H
d
e
f
CH2OCH3
CH2OCH3
CH2OAc
H
OAc
OAc
H
H
H
H
H
H
1.2. Typical procedure for the preparation of 2,3-dideoxy-
a,b-unsaturated carbohydrate enals
g
OAc
H
CH2OAc
H
OAc
h
1.2.1. (2E)-4,6-Di-O-benzyl-2,3-dideoxy-aldehydo-D-ery-
thro-hex-2-enose (2a). To a solution of the benzyl-pro-
tected glucal 1a (104.0 mg, 0.25 mmol) in a mixture of
7:3 CH3CN–H2O (10 mL) was added HfCl4 (18.0 mg,
0.056 mmol) and ZnI2 (7.0 mg, 0.022 mmol). The result-
ing reaction mixture was heated with stirring in an oil
bath at 135–140 ꢁC for 2.5 h. On completion of the reac-
tion, the reaction mixture was quenched with cold water
and neutralized with few drops of satd aq NaHCO3. The
aqueous layer was extracted with EtOAc (5 · 5 mL).
The combined organic layer was dried over Na2SO4
and evaporated in vacuo to obtain the crude product
mixture 2a and 3a in a 4:1 ratio (NMR). This was sub-
jected to acetylation with Ac2O and pyridine to obtain
the chromatographically pure acetyl derivative of 2a
(66.0 mg, 72%).
Scheme 1. Perlin hydrolysis of glycals.
group migration in case of acyl-protected glycals as
observed in the Hg2+-catalyzed Perlin hydrolysis.2c,3c,5
This method thus broadens the synthetic utility of these
carbohydrate enals, especially in the field of medicinal
chemistry.
1. Experimental
1.1. General methods
All reactions were monitored by thin-layer chromato-
graphy (TLC) over silica gel plates. The spots on the
TLC plates were visualized by warming the CeSO4
(1% in 2 N H2SO4) sprayed plates in an oven at
100 ꢁC. Silica gel (100–200 mesh) was used for column
chromatography, and the desired compounds were
eluted with an EtOAc–hexane mixture of increasing
polarity. IR spectra were recorded on a Perkin–Elmer
881 spectrophotometer, and values are expressed in
cmꢁ1. FAB mass spectra (FABMS) were recorded on
a JEOL SX 102/DA 6000 mass spectrometer using
argon/xenon (6 kV, 10 mA) as the FAB gas. HRMS data
was recorded on a JEOL MS model JMS-600H mass
spectrometer. The calibration was done by using PFK.
A similar reaction procedure was adopted for the
preparation of compounds 2c and 2d.
1.2.2. (2E)-4,6-Di-O-benzyl-2,3-dideoxy-aldehydo-D-threo-
hex-2-enose (2b). To a solution of the benzyl-protected
galactal 1b (104.0 mg, 0.25 mmol) in a mixture of 7:3
CH3CN–H2O (10 mL) was added HfCl4, (18.0 mg,
0.056 mmol) and ZnI2 (7.0 mg, 0.022 mmol). The result-
ing mixture was heated with stirring in a preheated oil
bath whose temperature was maintained between 130
and 135 ꢁC for 2 h. On completion of the reaction, the
mixture was quenched with cold water and neutralized

M. Saquib et al. / Carbohydrate Research 341 (2006) 1052–1056
1055
with a few drops of satd aq NaHCO3. After separation
of the organic layer, the aqueous layer was extracted
with EtOAc (5 · 5 mL). The combined organic layer
was dried over Na2SO4 and evaporated in vacuo to ob-
tain the crude product mixture 2b and 3b in a 1:1 ratio
(NMR). This was subjected to acetylation with Ac2O
and pyridine to obtain the chromatographically pure
acetyl derivative of 2b (40.0 mg, 43%).
ing the spectral data and Mr. Anoop Kishore Pandey,
for his technical assistance. M.S. and R.S. are thankful
to DOD, New Delhi, for financial support.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.carres.
2006.02.031.
A similar reaction procedure was adopted for the
preparation of compound 2e.
1.2.3. (2E)-4,6-Di-O-acetyl-2,3-dideoxy-aldehydo-D-ery-
thro-hex-2-enose (2f). To a solution of the acetyl-pro-
tected glucal 1f (136.0 mg, 0.5 mmol) in CH3CN
(7 mL) was added HfCl4 (18.0 mg, 0.056 mmol) and
ZnI2 (7.0 mg, 0.022 mmol). The resulting mixture was
heated with stirring in an oil bath whose temperature
was raised rapidly from room temperature to 110 ꢁC.
On completion of the reaction (6–8 min, TLC control),
the reaction was quenched with water. The organic layer
was separated, and the aqueous layer was extracted with
EtOAc (4 · 5 mL). The combined organic layer was
washed with water 3–4 times, dried over Na2SO4 and
evaporated in vacuo to obtain the desired aldehyde
(90%). This was subjected to acetylation with Ac2O
and pyridine to obtain the chromatographically pure
acetyl derivative 2f (101.0 mg, 74%).
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1.3. Physicochemical and spectral data
1.3.1. (2E)-5-O-Acetyl-4,6-di-O-methyl-2,3-dideoxy-alde-
hydo-D-threo-hex-2-enose (acetyl derivative of 2e). [a]D
+11.5 (c 0.130, CHCl3); Rf 0.45 (2:3 EtOAc–hexane);
IR (Neat, cmꢁ1): m 2988, 2931 (–C–H str), 2833
(–CHO str), 1744 (COCH3), 1693 (C@O), 1456 (C@C),
1374 (C–H def of COCH3), 1111 (C–O str). 1H
NMR (200 MHz, CDCl3): d 9.55 (d, 1H, J1,2 7.8 Hz,
H-1), 6.64 (dd, 1H, J3,2 15.7, J3,4 5.2 Hz, H-3), 6.28
(ddd, 1H, J2,3 15.7, J2,1 7.8, J2,4 1.2 Hz, H-2), 5.09 (m,
1H, H-5), 4.11 (td, 1H, J4,3 5.2, J4,2 1.3 Hz, H-4),
3.64–3.36 (m, 2H, H-6), 3.34 (s, 3H, OCH3), 3.29 (s,
3H, OCH3), 2.03 (s, 3H, COCH3). 13C NMR
(50 MHz, CDCl3): d 193.3 (C-1), 170.7 (COCH3),
152.4 (C-3), 134.0 (C-2), 79.4 (C-4), 72.8 (C-5), 70.6
(C-6), 59.6 and 58.9 (2 · OCH3), 21.3 (COCH3).
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