Microwave-induced, Montmorillonite K10-catalyzed Ferrier rearrangement of tri-O-acetyl-D-galactal: Mild, eco-friendly, rapid glycosidation with allylic rearrangement

Article abstract of DOI:10.1016/S0040-4039(02)01517-4

Montmorillonite K10 was found to catalyze, under microwave irradiation, rapid O-glycosidation of 3,4,6-tri-O-acetyl-D-galactal to afford exclusively the alkyl and aryl 2,3-dideoxy-D-threo-hex-2-enopyranosides with very high α-selectivity and without the formation of the 2-deoxy-D-lyxo-hexopyranosides. Under these conditions, 3,4,6-tri-O-acetyl-D-glucal as usual also underwent the Ferrier rearrangement.

Full text of DOI:10.1016/S0040-4039(02)01517-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6795–6798
Microwave-induced, Montmorillonite K10-catalyzed Ferrier
rearrangement of tri-O-acetyl- -galactal: mild, eco-friendly,
D
rapid glycosidation with allylic rearrangement
Bhagavathy Shanmugasundaram,a,b Ajay K. Boseb and Kalpattu K. Balasubramaniana,*
aIndian Institute of Technology Madras, Chennai 600 036, India
bStevens Institute of Technology, Hoboken, NJ 07030, USA
Received 21 May 2002; revised 1 July 2002; accepted 19 July 2002
Abstract—Montmorillonite K10 was found to catalyze, under microwave irradiation, rapid O-glycosidation of 3,4,6-tri-O-acetyl-
D
-galactal to afford exclusively the alkyl and aryl 2,3-dideoxy-
without the formation of the 2-deoxy-
underwent the Ferrier rearrangement. © 2002 Elsevier Science Ltd. All rights reserved.
D
-threo-hex-2-enopyranosides with very high a-selectivity and
D
-lyxo-hexopyranosides. Under these conditions, 3,4,6-tri-O-acetyl- -glucal as usual also
D
1. Introduction
glycosidation, which is the only synthetically viable
method for the Ferrier rearrangement of 3,4,6-tri-O-
2,3-Unsaturated glycosides (pseudoglycals) are versatile
synthetic intermediates and also constitute the struc-
tural units of several antibiotics.1 Allylic rearrangement
of glycals, otherwise known as the Ferrier rearrange-
ment,2 in the presence of a nucleophile generally leads
to the formation of 2,3-unsaturated glycosides. 2,3-
Unsaturated O-aryl glycosides, in turn can be trans-
formed to C-aryl glycosides through [1,3] sigmatropic
rearrangement.3 The Ferrier rearrangement of 3,4,6-tri-
acetyl-D-galactal for the synthesis of both alkyl and aryl
galactosides, the other methods are restricted to the
synthesis of only the 2,3-unsaturated alkyl galactosides.
Even in the SnCl4-catalyzed reaction, the Ferrier rear-
rangement is accompanied by the formation of minor
amounts of the aryl 2-deoxy-D-lyxo-hexopyranosides
and a bicyclic aryl 2-deoxy galactoside derivative.8
Herein, we report the Mont. K10-catalyzed, microwave-
O-acetyl-
3,4,6-tri-O-acetyl-
D
-galactal is not as easy a reaction as that of
-glucal, since it leads to 2-deoxy-
induced Ferrier rearrangement of 3,4,6-tri-O-acetyl-
galactal with alcohols and phenols in an open vessel,
giving 4,6-di-O-acetyl-2,3-dideoxy- -threo-hex-2-enopy-
D-
D
D-
lyxo-hexopyranosides by the addition of the OH nucle-
D
ophile to the galactal double bond.4
ranosides with very high a-selectivity (Scheme 1).
A wide range of acid reagents5 such as BF3·Et2O,
cation-exchange resin, InCl3, Yb(OTf)3, Sc(OTf)3,
Montmorillonite K10 (Mont. K10) and neutral
reagents6 such as DDQ, I2, and N-iodosuccinimide
have been described for the glycosidation of glucals.
But there are only a few catalysts available for the
As a part of our endeavor to develop an efficient, mild,
rapid, eco-friendly method for O- and C-glycosidation,
the use of microwaves for Ferrier rearrangement was
explored. Although Mont. K10-catalyzed glycosidation
of 3,4-di-O-acetyl-
L
-rhamnal and 3,4,6-tri-O-acetyl-D-
AcO
AcO
Ferrier rearrangements of 3,4,6-tri-O-acetyl-
like SnCl4,7 DDQ,6b LiBF46c in contrast to other known
Lewis acid catalyst, which mostly lead to 2-deoxy-
D-galactal
R2
R2
O
O
Mont. K10
D
-
OAc
+
ROH
3 a-j
OR
PhCl
lyxo-hexopyranosides. Except for the SnCl4 method of
R1
R1
MWI*
4 a-j R1 = H, R2 = OAc
5 a-j R1 = OAc, R2 = H
1
2
R1 =H , R2 = OAc
R1 = OAc, R2 = H
Keywords: galactal; glucal; Montmorillonite K10; Ferrier rearrange-
ment; microwave irradiation.
* Corresponding author. Present address: Shasun Drugs and Chemi-
cals Ltd, Velachery, Chennai 600 042, India. Tel.: 91-44-245 1106;
fax: 91-44-245 2462; e-mail: kkbalu@hotmail.com
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01517-4

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B. Shanmugasundaram et al. / Tetrahedron Letters 43 (2002) 6795–6798
glucal has been previously reported,9 the authors have not
It is interesting to note that the reaction of 3,4,6-tri-O-
acetyl- -galactal, benzyl alcohol 3b and Mont. K10,
when performed without microwave irradiation, but
under reflux conditions in 1,2-dichloroethane, not only
required longer time, viz. 24 hours, but resulted in a
low yield (30%) of benzyl 4,6-di-O-acetyl-2,3-dideoxy-
extended this reaction either with tri-O-acetyl-
D
-galactal
D
or to phenols. Earlier, our laboratory reported10 the
solvent-free microwave-induced Ferrier rearrangement of
glucals with phenols in a sealed vessel which always carries
the hazard of explosion. We have now adopted the
microwave oven-induced reaction enhancement (MORE)
technique,11 wherein the reaction is carried out in an open
vessel.
D
-threo-hex-2-enopyranoside 4b. No reaction was
observed in the absence of Mont. K10 both under
microwave irradiation and reflux conditions. Also, the
galactal 1 with phenol 3g and Mont. K10 under reflux
conditions in 1,2-dichloroethane for about 30 hours,
3,4,6-Tri-O-acetyl- -galactal 1 in the presence of alco-
D
hols/phenols 3 and Mont. K10 (100% w/w) in chlorobenz-
yielded phenyl 4,6-di-O-acetyl-2,3-dideoxy-D-threo-hex-
ene, when irradiated with microwaves at power level (PL)
2-enopyranoside 4g, albeit in very low yield (ꢀ20%)
and the reaction did not go to completion. Thus we find
that the Mont. K10-catalyzed reaction of 3,4,6-tri-O-
2, yielded exclusively the alkyl/aryl 2,3-dideoxy-D-threo-
hex-2-enopyranosides 4a–j in reasonably good yields with
very high a-selectivity as summarized in Table 1. The
reactions were completed in a very short period, in general
acetyl- -galactal 1 with alcohols and phenols, generally
D
leads to easy allylic rearrangement. It is also observed
that the reaction time is greatly reduced under
microwave irradiation when compared to the conven-
tional reflux method.
to afford the alkyl or aryl 2,3-dideoxy-D-threo-hex-2-
enopyranosides. All these compounds were characterized
1
by H NMR, 13C NMR, IR and mass spectral data.
Table 1. Glycosidation of 3,4,6-tri-O-acetyl-D-galactal 1

B. Shanmugasundaram et al. / Tetrahedron Letters 43 (2002) 6795–6798
6797
Similarly, we found that 3,4,6-tri-O-acetyl-
with alcohols/phenols 3 in the presence of Mont. K10
as catalyst and under microwave irradiation afforded
D
-glucal 2
2. General experimental procedure
2.1. Glycosidation using alcohols
the alkyl or aryl 4,6-di-O-acetyl-2,3-dideoxy-D-erythro-
hex-2-enopyranosides, 5a–j in very good yields as sum-
To a mixture of glycal 1 or 2 (1 mmol) and alcohols
(3.0 or 6–8 equiv. in the cases of low boiling alcohols
such as ethanol) in PhCl (5 mL) taken in a conical flask
(100 mL) was added Mont. K10 (100% w/w). Then a
funnel was placed on the top of the flask and the
contents were irradiated with microwaves at PL=2.
The course of the reaction was monitored using TLC.
After completion of the reaction, the solution was
filtered and the filtrate was concentrated in vacuo. The
product was purified using column chromatography.
marized in Table 2.
The yields and the stereoselectivity were in the range
observed previously with other catalysts. The predomi-
nant a-selectivity follows the anomeric effect.12
In summary, our present methodology has the follow-
ing advantages: (a) the catalyst, Mont. K10 clay can be
easily recovered and reused. (b) Short reaction times (in
order of minutes) and generally good yields. (c) No
need for perfectly dry solvents and reagents. In conclu-
sion, we have developed a mild and eco-friendly
approach for the glycosidation of glycals, particularly
2.2. Glycosidation using phenols
To a mixture of glycal 1 or 2 (1 mmol) and phenols (3.0
equiv.) in PhCl (5 mL) taken in a conical flask (100
mL) was added Mont. K10 (100% w/w). Then a funnel
was placed on the top of the flask and the contents were
for 3,4,6-tri-O-acetyl-D-galactal to afford the corre-
sponding alkyl and aryl 2,3-dideoxy-
enopyranosides.
D-threo-hex-2-
Table 2. Glycosidation of 3,4,6-tri-O-acetyl-D-glucal 2

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B. Shanmugasundaram et al. / Tetrahedron Letters 43 (2002) 6795–6798
irradiated with microwaves at PL=2. The course of the
reaction was monitored using TLC. After the comple-
tion of the reaction, the reaction mixture was filtered,
extracted with dichloromethane and washed with
aqueous NaOH (10%), followed by water. The organic
layer was then dried and evaporated. The crude
product was purified using column chromatography.
4. (a) Sabesan, S.; Neira, S. J. Org. Chem. 1991, 56, 5468–
5472; (b) Sobhana Babu, B.; Balasubramanian, K. K.
Carbohydr. Lett. 1999, 3, 339–342.
5. (a) Descotes, G.; Martin, J. C. Carbohydr. Res. 1977, 56,
168–172; (b) Sobhana Babu, B.; Balasubramanian, K. K.
Tetrahedron Lett. 2000, 41, 1271–1274; (c) Pearson, W.
H.; Schkeryantz, M. J. Org. Chem. 1992, 57, 2986–2987;
(d) Yadav, J. S.; Reddy, S.; Murthy, C. V. S. R.; Kumar,
M. G. Synlett 2000, 10, 1450–1451.
6. (a) Koreeda, M.; Houston, T. A.; Shull, B. K.; Klemke,
E.; Tuinman, R. Synlett 1995, 90–92; (b) Toshima, K.;
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Chem. Soc., Chem. Commun. 1993, 704–706; (c) Sobhana
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Acknowledgements
The authors thank CSIR, New Delhi for funding and
Dr. K. Vijayakumaran for useful discussions.
7. Grynkiewicz, G.; Priebe, W.; Zamojski, A. Carbohydr.
Res. 1979, 68, 33–41.
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