An efficient and versatile chemical synthesis of bioactive glyco-glycerolipids

Article abstract of DOI:10.1016/j.tetlet.2011.12.030

Synthesis of β-glyco-1,2-diacylglycerols is achieved by a versatile and simple procedure based on trichloro-acetimidate methodology and use of peracetate sugar substrates. The chemical strategy was tested through stereoselective preparation of β-galacto- and β-gluco-lipid derivatives capable to trigger immune system response. The synthetic approach is designed to obtain enantiomerically pure regio- and stereo-isomers including derivatives containing poly-unsaturated fatty acids.

Full text of DOI:10.1016/j.tetlet.2011.12.030

Tetrahedron Letters 53 (2012) 879–881
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Tetrahedron Letters
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An efficient and versatile chemical synthesis of bioactive glyco-glycerolipids
⇑
Emiliano Manzo , Maria Letizia Ciavatta, Dario Pagano, Angelo Fontana
Istituto di Chimica Biomolecolare, CNR, Via Campi Flegrei 34, I 80078 Pozzuoli (Na), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of b-glyco-1,2-diacylglycerols is achieved by a versatile and simple procedure based on
trichloro-acetimidate methodology and use of peracetate sugar substrates. The chemical strategy was
tested through stereoselective preparation of b-galacto- and b-gluco-lipid derivatives capable to trigger
immune system response. The synthetic approach is designed to obtain enantiomerically pure regio-
and stereo-isomers including derivatives containing poly-unsaturated fatty acids.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 19 September 2011
Revised 5 December 2011
Accepted 9 December 2011
Available online 16 December 2011
Keywords:
Glycoglycerolipids
Galactodiacylglycerols
Glucodiacylglycerols
Synthesis
Glycoglycerolipids are ubiquitous structural and signaling mol-
ecules of living organisms.1–3 Within the family, mono- (MGDG)
and di- (DGDG) galactosyl diacylglycerols represent 70–80% of
membrane lipids in photosynthetic cells4 and comprise the most
abundant lipid class in the biosphere. In the last years, natural
and synthetic analogs of MGDGs and DGDGs have attracted the
interest of the bio-medical community for the biological properties
including antiviral,5 anti-tumor,6 anti-inflammatory, and immuno-
suppressive activities.7 In particular, innate immune response trig-
gered by glycolipids is emerging as a key process against bacterial
and viral infections, as well as tumor immunosurveillance and
autoimmunity.8 Immunogenic properties of lipids depend on the
structural characteristics of the molecule. Very recently, Kinjo
et al. showed that recognition of MGDG and DGDG by natural killer
T cells is strictly dependent on fatty acid composition of the diac-
ylglycerol residue.9 The high specificity of this process requires
accurate purification of the antigenic lipids and, for this reason,
the synthetic preparation is regarded as a crucial step for address-
ing evaluation and development of these immune-modulatory
lipids.
in good yields (about 81% and 82%, respectively). Acetate group
at C-20 of the sugars oriented the equatorial attack with very high
stereoselectivity, as proved by 1H NMR analysis that showed only
signals for the b-isomers (d = 4.51, J = 7.7 Hz, H-10). The two-step
strategy proved to be more efficient than the traditional
Koenigs–Knorr conditions11 that are generally employed for the
synthesis of glycolipids from acetate-protected pyranose sug-
ars.12,13 Diols 5 and 16 were quantitatively obtained through selec-
tive hydrolysis of the isopropylidene intermediates by zinc nitrate
hexahydrate in acetonitrile.14 DCC-mediated condensation with
two or one equivalents of free fatty acids (i.e., linolenic or stearic)
yielded alternatively di- and mono-acyl derivatives 6 and 8,
respectively. Notably, equimolar amounts of reagents gave only
esterification at C-10, as indicated by NMR spectra (e.g., 8) that
showed deshielding of H2-1 signals at d 4.10, long range correlation
of H2-1 to the carboxylic ester at d 173.3, and resonances of H-2 at
3.86 ppm. Although esterification of the secondary hydroxyl group
of glycerol has been sometimes reported,15 2-acyl-glycerol deriva-
tives were not observed under these experimental conditions. The
procedure allows step-wise introduction of different acyl residues
at C-1 and C-2 of glycerol, thus permitting to control fatty acid reg-
iospecificity of the synthetic glycolipid (e.g., 10 and 100). Selective
removal of acetyl groups on the sugar unit with hydrazine hydrate
led to b-galactosyl-diacylglycerols (7 and 11) and b-galactosyl-
monoacylglycerols (9). Usually this reaction cannot be applied to
unsaturated substrates because of partial hydrogenation of double
bonds,12,16–18 but we found that this drawback could be efficiently
solved (75–88% yield for the linolenoyl derivatives 7, 11, 18, and
22) by reducing the amount of hydrazine (2.4 mol per acetate unit)
and keeping the temperature reaction below 45 °C.
Here we discuss an improved and versatile strategy for the syn-
thesis of b-galactosyl- and b-glucosyl-mono- and di-acylglycerols
containing both saturated and unsaturated fatty acids.
As shown in Fig. 1, the synthesis starts with acetylation of the
sugar unit, followed by selective deacetylation of the anomeric hy-
droxyl group with benzylamine (compounds 2 and 13). The cou-
pling with 1,2-O-isopropylidene glycerol by trichloroacetimidate
methodology10 gave galactose- (4) and glucose- (15) derivatives
In conclusion, access to natural and synthetic analogs of b-gly-
co-1,2-diacyl-glycerols was achieved by a versatile and simple
⇑
Corresponding author. Tel.: +39 0818675310; fax: +39 0818041770.
E-mail addresses: emanzo@icb.cnr.it, emanzo@icmib.na.cnr.it (E. Manzo).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.030

880
E. Manzo et al. / Tetrahedron Letters 53 (2012) 879–881
Y
OAc
Thrichloroacetonitrile
Benzylamine
THF
Y
Y
Y
OH
OAc
OAc
DBU/CH2Cl2 / 0°C
Ac2O/pyr
O
O
O
Z
O
Z
Z
Z
AcO
HO
AcO
AcO
OAc
OH OH
OAc
OH
OAc
OAc
O
NH
D-galactose:Y=OH, Z=H
1: Y=OAc, Z=H, 92%
2: Y=OAc, Z=H, 74%
3: Y=OAc, Z=H,79%
D-glucose: Y=H, Z=OH
12: Y=H, Z=OAc, 95%
13: Y=H, Z=OAc, 71%
14: Y=H, Z=OAc, 81%
CCl3
HO
O
CH2Cl2/BF3OEt2
-10°C
O
DCC/DMAP
Zn(NO3)2
CH3CN/50°C
carbox. acid (2.2 eq)
Y
OAc
Y
OAc
Y
OAc
O
CH2Cl2
O
O
O
O
Z
O
Z
Z
O
O
OH
R
O
AcO
AcO
AcO
O
O
OH
OAc
OAc
R
OAc
O
6: Y=OAc, Z=H, R= linolenoyl, 94%
6': Y=OAc, Z=H, R=stearoyl, 92%
17: Y=H, Z=OAc, R= linolenoyl, 91%
17': Y=H, Z=OAc, R=stearoyl, 93%
5:Y=OAc, Z=H, 77%
16: Y=H, Z=OAc, 79%
4: Y=OAc, Z=H, 81%
15: Y=H, Z=OAc, 82%
DCC/DMAP
carboxylic acid-1 (1 eq)
NH2NH2
EtOH (85%)
44°C
-5°C
CH2Cl2
DCC/DMAP
Y
OAc
Y
Y
OAc
OH
O
carbox. acid-2 (1.2 eq)
O
O
O
O
Z
CH2Cl2
O
O
Z
O
Z
HO
O
O
O
R
AcO
O
R
OH
AcO
O
R
OAc
O
OAc
O
OH
R1
R
O
8: Y=OAc, Z=H, R= linolenoyl, 86%
8': Y=OAc, Z=H, R=stearoyl, 83%
19: Y=H, Z=OAc, R= linolenoyl, 84%
19': Y=H, Z=OAc, R=stearoyl, 87%
10: Y=OAc, Z=H, R= linolenoyl, R1= stearoyl, 92%
10': Y=OAc, Z=H, R=stearoyl, R1= linolenoyl, 89%
21: Y=H, Z=OAc, R= linolenoyl, R1= stearoyl, 94%
21': Y=H, Z=OAc, R=stearoyl, R1= linolenoyl, 86%
7: Y=OH, Z=H, R= linolenoyl, 75%
7': Y=OH, Z=H, R=stearoyl, 86%
18: Y=H, Z=OH, R= linolenoyl, 88%
18': Y=H, Z=OH, R=stearoyl, 81%
NH2NH2
EtOH (85%)
44°C
NH2NH2
EtOH (85%)
44°C
1
Y
OH
Y
OH
6'
5'
2
O
3
O
O
O
Z
O
O
O
Z
4'
HO
O
R
HO
2'
3'
1'
OH
R
OH
O
R1
OH
O
9: Y=OH, Z=H, R= linolenoyl, 82%
9': Y=OH, Z=H, R=stearoyl, 84%
20: Y=H, Z=OH, R= linolenoyl, 81%
20': Y=H, Z=OH, R=stearoyl, 79%
11: Y=OH, Z=H, R= linolenoyl, R1= stearoyl, 80%
11': Y=OH, Z=H, R=stearoyl, R1= linolenoyl, 77%
22: Y=H, Z=OH, R= linolenoyl, R1= stearoyl, 82%
22': Y=H, Z=OH, R=stearoyl, R1= linolenoyl, 75%
Figure 1. Synthesis of b-galactosyl- and b-glucosyl-1,2-mono- and di-acylglycerols.
chemical synthesis involving stereoselective glycosylation of tri-
chloroacetimidate-sugar donor and 1,2-O-isopropylidene glycerol
acceptor. Unlike other methods based on benzyl-derivatives,19,20
the use of acetate as protecting group of the sugar units allows
the synthesis of derivatives containing poly-unsaturated fatty
acids (e.g., 7 and 11) in very high yields. The synthetic strategy is
also more convenient than methods based on acetobromo deriva-
Acknowledgments
The authors thank Mrs. D. Melck of ICB-NMR service, M. Zampa
of ‘Servizio di Spettrometria di Massa’ for HRESIMS, and C. Iodice
for spectrophotometric measurements.
Supplementary data
tives of a-D-galactose or a-D-glucose, which suffers for high toxic-
ity and low stability of bromo glycosides, as well as for the high
costs of the promoters associated to the Koenigs–Knorr reac-
tion.12,13,21 Furthermore, in addition to the simplification of the
synthetic steps, the use of acetate determines at very high extent
the stereoselectivity of the coupling reaction that leads exclusively
to beta-anomers.
The sequence of reactions is of general application and gives ac-
cess to regio- and stereo-pure compounds. Enantiopure 1,2-O-iso-
propylidene glycerols were not tested in this work, but their use is
compatible with the trichloroacetimidate/etherate boron trifluor-
ide coupling step that, unlike Koenigs–Knorr and galactosyl-bro-
mide procedures,13b,22 avoid epimerization at C-2 of these chiral
substrates.19 We are currently applying the above synthesis for
the preparation of natural variants of glycolipid binders to CD1
receptors of natural killer T cells.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.030.
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