Highly efficient and stereoselective synthesis of β-glycolipids

Article abstract of DOI:10.1039/b718521a

β-Galactosylceramide and glycolipid analogues were prepared in high yield and with complete chemo and stereoselectivity by reaction of α-iodo glycosides with stannyl ceramides, formed in situ. TBAI was used to activate both the iodogalactose and the stannyl ether. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b718521a

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Highly efficient and stereoselective synthesis of b-glycolipids†
Jose´ Antonio Morales-Serna, Omar Boutureira, Yolanda D´ıaz, M. Isabel Matheu and Sergio Castillo´n*
Received 30th November 2007, Accepted 30th November 2007
First published as an Advance Article on the web 11th December 2007
DOI: 10.1039/b718521a
b-Galactosylceramide and glycolipid analogues were pre-
pared in high yield and with complete chemo and stere-
oselectivity by reaction of a-iodo glycosides with stannyl
ceramides, formed in situ. TBAI was used to activate both
the iodogalactose and the stannyl ether.
In these cases the stereochemistry of the reaction is determined
by the participation of the neighbouring groups. Recently, it has
been shown that glycosyl iodides18 are excellent glycosyl donors
for glycosylation reactions and they have been successfully used in
the synthesis of a-glycosphingolipids.19
One of the drawbacks in the synthesis of GSLs is that the
yields in the direct glycosylation of ceramides are low. This has
been attributed to the low nucleophilicity of sphingosines and
ceramides, and has been circumvented by using azidosphingosine
2 (Scheme 1b), a precursor of ceramide, instead of the ceramide
1 (Scheme 1a). This strategy allows obtaining high yields in the
glycosylation step but increases the number of steps. Consequently,
the development of simple and direct synthesis of glycolipids is
still a challenge. In this study, we report a simple and very efficient
procedure for the glycosylation of b-amidoalcohols and ceramides
by using glycosyl iodides as glycosyl donors.
In recent years there has been a rapid increase in the knowledge
of cellular processes in which lipids are involved. In particular,
glycosphingolipids (GSLs) are widely distributed in the membrane
of eukaryotic cells1 and play a critical role in many stages
of the cell cycle including growth, proliferation, differentiation,
adhesion, senescence and apoptosis. GSLs serve a variety of
functions through interaction with many biofactors2 by inhibiting
or interfering with the physiological effects of these factors or
cells.3 GSLs can also interact in the cell surface with toxins,
viruses and bacteria.4 In addition, some intermediates in the GSLs
metabolism, such as ceramide, sphingosine and phosphorylated
derivatives, have been identified as signals.5
GSLs (Fig. 1) and related compounds have mainly been
investigated in reference to storage diseases,6 which are a group
of genetic diseases where, most commonly, GSLs accumulate
due to specific defects in lysosomal hydrolases. However, in
recent years, these compounds have been studied as a strategy
for pharmacological prevention of microbial infections,7 cancer
chemotherapy,8 modifying the activity of receptors for insulin,9
epidermal growth factor10 and nerve growth factor11 which may
have potential effects in Alzheimer’s12 and Parkinson’s13 diseases.
Scheme 1 Retrosynthetic analysis of b-galactosylceramide.
Our approach uses stannyl ethers,20 derived from ceramides
with the purpose of increasing the nucleophilicity of oxygen
without significantly modifying the basicity (Scheme 2). We
selected glycosyl iodides as the glycosyl donors because they can
be activated by tetrabutylammonium iodide (TBAI), which can
also activate the stannyl ether.
Fig. 1 Naturally occurring b-glycosphingolipids.
With this stimulating biological background and the compli-
cated availability of GSLs from natural sources, many organic
chemists have focused on developing methods for synthesizing
GSLs. Crucial steps in these protocols involve the formation of the
glycosidic bond between a properly protected carbohydrate and
sphingosine or azidosphingosine.14 The most classical and efficient
glycosylation procedures15 that have been used in the GSLs’
synthesis include glycosyl trichloroacetimidates16 and fluorides.17
Departamento de Qu´ımica Anal´ıtica y Qu´ımica Orga´nica, Universitat Rovira
i Virgili, C/Marcel.li Domingo s/n, 43007 Tarragona, Spain. E-mail:
sergio.castillon@urv.net; Fax: +34 977 558446; Tel: + 34 977 559556
† Electronic supplementary information (ESI) available: Experimental
details and characterization data of all the new compounds. See DOI:
10.1039/b718521a
Scheme 2 Glycosylation–isomerization reactions of BnOSnBu3 4 with
the iodogalactose 3 in the presence of TBAI and TBDMSOTf to give 6.
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Table 1 Synthesis of compound 11 by glycosylation of stannyl amide 8
with iodides 3 and 7 in the presence of TBAI followed by isomerizationa
Initially we studied the reaction of tetra-O-acetyl-a-
iodogalactose 3, prepared by reaction of penta-O-acetylgalactose
with TMSI, with the stannyl ether 4. We tried the reaction in
toluene at 80 ◦C using TBAI as the catalyst, and we obtained the
orthoester 5 (Scheme 2), which is the product usually obtained
when the reaction is driven in neutral conditions. Treatment of 5
with tert-butyldimethylsilyl triflate (TBDMSOTf)21 afforded the
b-O-glycoside 6 in 90% overall yield. In the absence of TBAI,
and in similar conditions, the reaction doesn’t evolve. When the
reaction was carried out at room temperature, only the starting
material was recovered and at 150 ◦C, decomposition products
were observed in the crude reaction mixture. The reaction was
then tried in CH2Cl2 at reflux in the presence of catalytic amounts
of TBAI obtaining the orthoester 5, which after treatment with
TBDMSOTf gave 6 in 95% yield. Interestingly, the reaction also
evolved at room temperature providing similar yields although in
a longer reaction time. In both cases, the presence of TBAI was
necessary for the reaction to take place.
Finally, we tried the reaction with the benzyl alcohol as the
glycosyl acceptor in boiling dichloromethane and toluene, and
neither the glycosylated product or the orthoester were observed.
It can be concluded that the use of TBAI and stannyl ether
derivatives allow the efficient activation of disarmed glycosyl
iodides affording the corresponding orthoester in excellent yields.
This orthoester can be isomerized to the b-glycoside in excellent
yields by using TBDMSOTf or BF3·OEt2.
Entry
Solvent
Temp/◦C
Yield (%)b
Ratio (a–b)
1
2
CH2Cl2
CH2Cl2
Toluene
Toluene
Rt
Reflux
80
62
88
93
—
0 : 1
0 : 1
0 : 1
—
3
4c
80
a Reaction conditions: 3 (1.2 mmol), 8 (1 mmol), Bu4NI (0.10 mmol),
toluene, 80 ◦C, 18 h and then BF3·Et2O (3 mmol), CH2Cl2 (20 mL), 0 ◦C,
30 min. b Yields of isolated product after chromatographic purification
over two steps. c In absence of Bu4NI.
Next we explored the reaction of amide 8, a simplified model
of ceramides, with glycosyl donor 3 (Table 1). The reaction
was carried out in dichloromethane and toluene at different
temperatures and in the presence or absence of TBAI. When the
reaction was performed at room temperature in dichloromethane,
the orthoester 9 was obtained, and further treatment with
TBDMSOTf or BF3·OEt2 afforded 11 in 62% yield over two steps
(entry 1, Table 1). The yield increased to 88% (entry 2) when
the reaction was heated to reflux. The best result, 93% yield, was
obtained performing the reaction in toluene at 80 ◦C (entry 3). In
this case the presence of TBAI was also necessary for the reaction
to evolve (entry 4). These results contrast with those obtained in
the glycosylation of ceramides using other common leaving groups
such as trichloroacetimidate, iodide or bromide, where yields were
lower than 50%.15
Scheme 3 shows the proposed mechanism of the reaction, which
must start by the attack of an iodide anion on 3 to form the b-
iodo-intermediate I.22 In principle, it would be expected, according
to the well known mechanism for orthoester formation, that
intermediate II would be formed by intramolecular attack of the
acetate group. But the fact that benzyl alcohol doesn’t react in
these conditions seems to indicate that the attack of iodide is faster,
thereby making the reaction reversible. Then, it is reasonable to
think that iodide, removed from II or from TBAI, will attack the
tin in II or III to form the thermodynamically stable stannyl iodide,
driving the reaction to the orthoester 5 (Scheme 3). Consequently,
TBAI should have two functions, to form the reactive b-iodo-
glycoside and make the reaction irreversible.23,24
It has been reported that the use of pivaloyl protecting groups
avoids the formation of the orthoester.25 However, in our hands,
when starting from pivaloyl protected donor 7, this led to the
formation of a mixture of orthoester 10 and b-glycoside 12 similar
to that reported for related glycosyl donors.26
In order to examine the scope of the reaction we performed
different experiments with stannyl ethers 13–15 (Table 2). The
reaction of the stannyl derivative of ceramide analogue 13 with
donor 3 was carried out under the optimized reaction conditions
to initially give the corresponding orthoester which was treated
with BF3·Et2O to afford glycolipid 16 in excellent yield (entry 1,
Table 2). The azido-sphingosine and ceramide have two hydroxyl
groups and we considered the possibility of protecting both as
a stannyl acetal. Thus, compounds 14 and 15 were prepared by
reaction of azido-sphingosine and ceramide with dibutyltinoxide
with water exclusion. The glycosylation of 14 and 15 with a-
iodogalactose 3 under the optimized conditions provided, after
treatment with BF3·OEt2, excellent yields of glycolipid 17 (entry
2) and galactosylceramide 18 (entry 3). Glycosylation of 15 with
Scheme 3 Proposed reaction mechanism ion.
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Table 2 Glycosylation of ceramide and ceramide analoguesa
Entry
1c
Acceptor
Glycolipid
Yield (%)b
93
2c
94
90
3c
4d
90
a Reaction conditions: 3 (1.2 mmol), 13, 14 or 15 (1 mmol), Bu4NI (0.10 mmol), toluene, 80 ◦C, 18 h and then BF3·Et2O (3 mmol), CH2Cl2 (20 mL), 0 ◦C,
30 min. b Yields of isolated product after chromatographic purification over two steps. c 3 was used as glycosyl donor. d 20 was used as glycosyl donor.
hepta-O-acetyllactosyl iodide (20) also afforded corresponding
glycolipid 19 in excellent yield. The overall process takes place
with complete chemo and stereoselectivity. The yields obtained
are close to those obtained by enzymatic procedures by using
glycosyl fluorides as glycosyl donors.25 In addition, when the
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In conclusion, we have developed a new and highly efficient
protocol for the glycosylation of ceramides consisting in the
reaction of stannyl ethers with a-iodogalactose derivatives in
the presence of TBAI as an activator. The procedure provides
very efficient access to b-galactosyl-ceramide and derivatives,
and is fully chemo and stereoselective. Because the ceramide
acceptor does not need to be protected, and no special protecting
groups in the donor are required, this protocol can be readily
utilized for the simple and efficient preparation of glycolipids
with important biological properties. Furthermore, this direct
glycosylation protocol reduces the overall number of steps and
provides a rapid access to complex target molecules.
Financial support from DGESIC CTQ-2005–03124/BQU
(Ministerio de Educacio´n y Ciencia, Spain) is acknowledged. We
are also grateful to the Servei de Recursos Cientifics (URV) for
its technical assistance. Fellowship from DURSI (Generalitat de
Catalunya) and Fons Social Europeu to JAMS and OB is gratefully
acknowledged.
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