Synthesis of the sulfonate analogue of seminolipid via horner-wadsworth- emmons olefination

Article abstract of DOI:10.1021/jo1008788

(Figure presented) The first synthesis of the sulfonate analogue of seminolipid, the main sulfoglycolipid in mammalian sperm, is reported. Installation of the sulfonate unit was accomplished by a quite unexplored strategy based on Horner-Wadsworth-Emmons olefination on a 3 ′-keto-galactoside, followed by stereoselective double bond reduction.

Full text of DOI:10.1021/jo1008788

pubs.acs.org/joc
Synthesis of the Sulfonate Analogue of Seminolipid
via Horner-Wadsworth-Emmons Olefination
Laura Franchini,*,† Federica Compostella,†
Diego Colombo,† Luigi Panza,*,‡ and Fiamma Ronchetti†
†Dipartimento di Chimica, Biochimica e Biotecnologie
FIGURE 1. Structure of SGG 1a and SGG sulfonate 1b.
ꢀ
per la Medicina, Universita di Milano, Via Saldini 50,
20133-Milano, Italy, and Dipartimento di Scienze Chimiche,
‡
SGG is an integral component of sperm lipid rafts2 and
participates on recognition events taking place during sperm-
egg interaction. According to a postulated model2 SGG and its
binding protein arylsulfatase-A (AS-A) form a complex that
engages in multivalent binding with the glycan moiety of the
zona pellucida (ZP), a family of sulfated egg glycoproteins with
sperm-binding ability.3 Whether the interaction between SGG
and AS-A is crucial for fertilization is actually a matter of
investigation,4 although analogous involvement of gangliosides
in cell-adhesion events is acknowledged.5
ꢀ
Alimentari, Farmaceutiche e Farmacologiche, Universita del
Piemonte Orientale, Via Bovio 6, 28100-Novara, Italy
laura.franchini@unimi.it; luigi.panza@pharm.unipmn.it
Received May 5, 2010
Seminolipid could also be significant in the frame of
sexually transmitted diseases. HIV-1 viral entry into a host
cell involves binding of the envelope-glycoprotein gp-120 to
CD4 receptor, chemokine coreceptors, and several galactose-
containing cell surface glycolipids6 such as galactosyl-
ceramide,7 GM3 ganglioside,7 sulfatide,8 and globotriaosylcer-
amide.9 SGG exhibits the same receptor functions showing
high affinity for gp-12010 and interaction with viruses and other
pathogen microbials.11 In this context the possibility to inhibit
the interaction between gp-120 and glycolipid receptors has
driven the development of HIV-1 entry inhibitors with a
simplified glycolipid structure.12
The first synthesis of the sulfonate analogue of seminolipid,
the main sulfoglycolipid in mammalian sperm, is reported.
Installation of the sulfonate unit was accomplished by a
quite unexplored strategy based on Horner-Wadsworth-
Emmons olefination on a 3 0-keto-galactoside, followed by
stereoselective double bond reduction.
Inhibitors and probes based on SGG structure would be
very useful as tools for studying how SGG acts in combina-
tion with AS-A as an adhesion molecule and to discover new
SGG analogues with HIV-1 entry inhibitor activity. To this
aim we developed the first synthesis of SGG sulfonate 1b, a
mimetic of SGG resistant to hydrolysis, in which the ester
oxygen of the sulfate moiety is replaced with a CH2 unit. The
replacement of the sulfate ester with a C-sulfonate results in a
Seminolipid SGG 1a (Figure 1) is a sulfated glycolipid
found in mammalian spermatozoa and testes in substantial
amount.1a SGG from mammalian spermatozoa is mainly a
single molecular species: 1-O-alkyl-2-O-acyl-3-O-(3-O-sulfo-
β-D-galactopyranosyl)-sn-glycerol, distinguishedby analmost
homogeneous composition in acyl (hexadecanoyl) and alkyl
(hexadecyl) chains.1b
(3) (a) Dunbar, B. S.; Timmons, T. M.; Skinner, S. M.; Prasad, S. V. Biol.
Reprod. 2001, 65, 951–960. (b) Prasad, S. V.; Skinner, S. M.; Carino, C.;
Wang, N.; Cartwright, J.; Dunbar, B. S. Cells Tissues Organs 2006, 166, 148–
164. (c) Wassarman, P. M. J. Biol. Chem. 2008, 283, 24285–24289.
(4) Tanphaichitr, N.; Carmona, E.; Bou Khalil, M.; Xu, H.; Berger, T.;
Gerton, G. L. Front. Biosci. 2007, 12, 1748–1766.
Seminolipid 1a was first synthesized by Gigg1c and its proper-
ties confirmed the structure of the natural material. Also, the
synthesis of deuterium-labeled SGG isotopomers for the quan-
tification of SGG in biological samples has been reported.1d
(5) Yu, S.; Kojima, N.; Hakomori, S. I.; Kudo, S.; Inoue, S.; Inoue, Y.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 2854–2859.
(1) (a) Tanphaichitr, N.; Bou Khalil, M.; Weerachatyanukul, W.; Kates,
M.; Xu, H.; Carmona, E.; Attar, M.; Carrier, D. Physiological and Biophy-
sical Properties of Male Germ Cell Sulfogalactosylglycerolipid.In Lipid
Metabolism and Male Fertility; De Vriese, S., Ed.; AOCS Press: Champaign,
IL, 2003. (b) For evidence that in mouse testis, besides the major molecule 1a,
some (C16:0-alkyl-C14:0-acyl), (C14:0-alkyl-C16:0-acyl), and (C17:0-alkyl-
C16:0-acyl) species are expressed during testicular maturation, see: Goto-
Inoue, N.; Hayasaka, T.; Zaima, N.; Setou, M. Glycobiology 2009, 19, 950–
957. (c) Gigg, R. J. Chem. Soc., Perkin Trans. 1 1978, 712–718. (d) Franchini,
L.; Panza, L.; Kongmanas, K.; Tanphaichitr, N.; Faull, K. F.; Ronchetti, F.
Chem. Phys. Lipids 2008, 152, 78–85.
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Puri, A.; Blumenthal, R. J. Virol. 2000, 74, 6377–6385.
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Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 7131–7134.
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Wiles, J.; Murray, G. J.; Brady, R. O.; Blumenthal, R. Proc. Natl. Acad. Sci.
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W.; Buhr, M.; Berger, T.; Carmona, E.; Vuong, N.; Kumarathasan, P.;
Wong, P. T. T.; Carrier, D.; Tanphaichitr, N. Dev. Biol. 2006, 290, 220–222.
(b) Weerachatyanukul, W.; Rattanachaiyanont, M.; Carmona, E.; Furims-
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DOI: 10.1021/jo1008788
r
Published on Web 07/14/2010
J. Org. Chem. 2010, 75, 5363–5366 5363
2010 American Chemical Society

Franchini et al.
JOCNote
SCHEME 1. Retrosynthesis of 1b
SCHEME 2. Synthesis of 30-Keto-β-galactosylglycerolipid 3
stable, isosteric mimic of SGG that preserves the negative
charge of the sulfate.
Examples concerning the synthesis of sulfonate analogues of
bioactive carbohydrates are reported in literature, including the
sialyl Lewis X and sialyl Lewis A tetrasaccharides,13a,b glucose-6-
sulfate,13c N-acetylneuraminic acid,13d heparin,13e nucleosides,13f
and mannose-6-phosphate.13g
SCHEME 3. HWE Olefination: Synthesis of Sulfonates 2a,b
Most of these approaches rely on the “mesylate anion
chemistry”13f (addition of the methanesulfonate ester carb-
anion to a carbonyl function), while strategies based on the
Horner-Wadsworth-Emmons (HWE) olefination are still
quite unexplored.14a,b
Retrosynthetic analysis of the target sulfonate 1b is shown
in Scheme 1. The synthesis involves the suitably protected 30-
keto-β-galactosylglycerolipid 3 as key intermediate, origi-
nated from the β-glycidylgalactoside 4; a HWE olefination to
the R,β-unsaturated sulfonate 2, followed by the stereoselec-
tive double bond reduction should give access to 1b.
Ketone 3, the key building block for the HWE olefination,
was obtained starting from β-glycidyl galactoside 415-17
(Scheme 2). A regioselective epoxide opening by hexadecyl
alcohol promoted by boron trifluoride diethylether complex,
followed by conventional silylation, gave the 1-O-hexadecyl-
ether 5 in 58% overall yield. At this stage benzoyl groups
were exchanged for benzyls to avoid any problems during the
planned HWE olefination of ketone 3, and carefully controlled
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(13) (a) Borbas, A.; Szabovik, G.; Antal, Z.; Feher, K.; Csavas, M.;
Szilagyi, L.; Herczegh, P.; Liptak, A. Tetrahedron: Asymmetry 2000, 11, 549–
ꢁ
ꢁ
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Zemplen transesterification of compound 5, followed by ben-
ꢁ
ꢁ
566. (b) Jakab, Z.; Fekete, A.; Borbas, A.; Liptak, A.; Antus, S. Tetrahedron
2010, 66, 2404–2414. (c) Navidpour, L.; Lu, W.; Taylor, S. D. Org. Lett.
ꢁ
2006, 8, 5617–5620. (d) Szabo, Z. B.; Borbas, A.; Bajza, I.; Liptak, A.; Antus,
S. Tetrahedron Lett. 2008, 49, 1196–1198. (e) Herczeg, M.; Lazar, L.; Borbas,
zylation of the crude triol afforded the desired product 6.18
Compound 6 was next deprotected by selective removal of
the PMB group, and Dess-Martin oxidation of the 3-OH of
galactose finally furnished ketone 3.
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ꢁ
ꢁ ꢁ
ꢁ
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A.; Liptak, A. Org. Lett. 2009, 11, 2619–2622. (f) Musicki, B.; Widlanski,
T. S. J .Org. Chem. 1990, 55, 4231–4233. (g) Jeanjean, A.; Gary-Bobo, M.;
Nirde, P.; Leiris, S.; Garcia, M.; Morere, A. Bioorg. Med. Chem. Lett. 2008,
18, 6240–6243.
(14) (a) Carretero, J. C.; Davies, J.; Marchand-Brynaert, J.; Ghosez, L.
Bull. Soc. Chim. Fr. 1990, 127, 835–842. (b) Musicki, B.; Widlanski, T. S.
Tetrahedron Lett. 1991, 32, 1267–1270.
(15) β-Galactoside 4 was prepared by benzoylation of phenyl 3-O-(4-
methoxybenzyl)-1-thio-β-D-galactopyranoside (see ref 16) followed by β-
glycosylation with S-glycidol under DMTST-activation according to Kon-
radsson’s procedure (see ref 17). The compound was produced exclusively as
the β-anomer; the anomeric configuration was readily inferred from the
Horner-Wadsworth-Emmons olefination is an extremely
versatile reaction providing access to a variety of alkene com-
pounds bearing different functional groups.19 Previous investi-
gations14b,20 demonstrated that different sulfonate-stabilized
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(18) Zemplen transesterification, performed with 1 molar eq. of sodium
methoxide and stopped after 48 h, avoided TBS cleavage; in these conditions,
15% of the byproduct benzylated at position 4 and 6 but retaining the
benzoate at position 2 was recovered after flash chromatography.
(19) Maryanoff, B. E. Chem. Rev. 1989, 89, 863–927.
(20) Carretero, J. C.; Demillequand, M.; Ghosez, L. Tetrahedron 1987,
43, 5125–5134.
0
0
coupling costant: J1 ,2 = 8.0 Hz.
(16) Kanie, O.; Ito, Y.; Ogawa, T. Tetrahedron Lett. 1996, 37, 4551–4554.
(17) Lindberg, J.; Svensson, S. C. T.; Pahlsson, P.; Konradsson, P.
Tetrahedron 2002, 58, 5109–5117.
5364 J. Org. Chem. Vol. 75, No. 15, 2010

Franchini et al.
JOCNote
FIGURE 2. Selected 1H NMR signals of sulfonates 2a and 2b (CDCl3).
SCHEME 4. Double Bond Reduction, Acylation, and Deprotections:
Synthesis of 1b
phosphonates are efficient olefinating agents allowing the pre-
paration of R,β-unsaturated sulfonates from both aldehydes
and ketones. Toward our target sulfonate 1, the reactivity of
isobutyl sulfonylphosphonate 7 was examined (Scheme 3).
Deprotonation of 7 was effected with BuLi at -78 °C. The
produced anion reacted smoothly with ketone 321 and R,β-
unsaturated sulfonates 2a,b were isolated as a mixture in 75%
yield after chromatographic purification.22
1H NMR analysis of the 2a,b mixture ascertained the
presence of two geometric sulfonate isomers that were found
in different conformations. However, the interpretation of
the spectroscopic data was not trivial, as the 1H NMR
spectrum (Figure 2) showed some not easily assignable
couples of signals having a similar shape but different
integrations and correlations in COSY spectrum. Careful
examination of the HSQC spectrum allowed the correct
assignment of the signals. NMR data, together with a very
preliminary conformation analysis of compounds 2a and 2b
suggested that whereas the pyranose ring of E-isomer 2a
adopts a chairlike conformation, the Z-isomer 2b prefers a
twist boatlike conformation (see Scheme 3), attributable to
an allylic strain between sulfonate and 20-OBn, with a
significant change of both H-10 chemical shift and J1 ,2
0
0
value.23 Diagnostic NMR signals and J values corroborating
this assignment for 2a and 2b are reported in Table 1 (see
Supporting Information).
Hence attention was focused on the double bond reduc-
tion, which had to afford stereoselectively from both con-
formers 2a and 2b the 30-equatorially arranged isomer with
the correct galacto-configuration. The best choice was cata-
lytic hydrogenation, and the best results were obtained using
Raney nickel, which gave exclusively galactoside 8 in satis-
factory 85% yield24 (Scheme 4). The conformations of both
isomers account for the observed stereoselectivity of the double
bond reduction as a 40 substituent for 2a and both 10 and 40
substituents for 2b hamper the attack from the top side.
The configuration at C-30 was deduced from the vicinal
(21) Preliminary attempts on tetradecanal or cyclohexanone models
afforded the corresponding R,β-unsaturated sulfonates in 60-70% yield.
(22) For our purposes the very difficult separation of sulfonates 2a and 2b
was not required as both isomers had to stereoselectively converge to a single
reduction product.
(23) E/Z ratio was 2:1 according to integration of the olefinic proton
(δ 6.78 ppm for E-isomer and δ 6.68 ppm for Z-isomer).
(24) Attempts to reduce compounds 2a,b by catalytic hydrogenation in
the presence of Pt/C resulted in an incomplete reaction, and sodium
borohydride showed stereoselectivity in favour of the gulo-isomer. More-
over, a difference of reactivity between 2a and 2b was observed in the
hydrogenation reaction where TLC analysis showed a faster reduction of
compound 2a with respect to compound 2b.
0
0
0
0
coupling constants (J2 ,3 = 10.5 Hz and J3 ,4 = 3.0 Hz), and
NOE contacts for compound 8, corroborating this assign-
ment, are shown in Figure 3.
Completion of the synthesis was then accomplished by
means of selective desilylation of the glycerol 2-OH, followed
by acylation with palmitoyl chloride, which produced com-
pound 9 in high yield (90%). Nucleophilic displacement on
isobutyl sulfonate ester 9 by tetrabutylammonium bromide
J. Org. Chem. Vol. 75, No. 15, 2010 5365

Franchini et al.
JOCNote
isobutylsulfonylphosphonate 7 (see Supporting Information)
(0.13 g, 0.45 mmol) in dry THF (1.2 mL), kept under argon
and cooled at -78 °C, was added dropwise n-BuLi (0.17 mL of a
2.5 M solution in hexanes, 0.43 mmol). After 20 min a solution
of ketone 3 (0.30 g, 0.35 mmol) in THF (3.0 mL) was slowly
added. The mixture was stirred at -78 °C for 1 h and then
allowed to warm to room temperature. Stirring was continued at
room temperature for 20 h; the pale yellow solution was diluted
with EtOAc (40 mL) and washed with water (20 mL), dried over
Na2SO4, and evaporated under reduced pressure. Purification
of the crude material by flash chromatography (n-hexane/
EtOAc 8:2) afforded sulfonates 2a,b (0.26 g, 75%) as a colorless
oil. Their E and Z configurations and the E/Z ratio (2:1) were
established by NMR analysis on the mixture of compounds
2a and 2b. ESI-MS m/z = 1017.5 [M þ Na]þ; Anal. Calcd.
for C57H90O10SSi: C 68.77, H 9.11; found C 68.51, H 9.33. In
Table 1 are reported significant 1H and 13C chemical shifts and
coupling constants for 2a,b. (see Supporting Information).
FIGURE 3. Diagnostic NOE contacts for galactoside 8.
gave compound 10, which was purified by flash column
chromatography and then subjected to hydrogenolysis in
the presence of palladium over charcoal to remove the benzyl
groups, affording the target sulfonate 1b (Scheme 4).
In summary, the first synthesis of the seminolipid sulfo-
nate analogue 1b was developed using a HWE olefination to
install a vinylsulfonate, followed by a highly stereoselective
reduction of unsaturated sulfonates 2a,b.
Variations on the lipid moiety of 10 will allow to generate
soluble or labeled or fluorescent sulfonate analogues of
SGG; moreover, the flexibility of this strategy could also
be exploited to obtain sulfonate analogues of other naturally
occurring sulfated glycolipids.
Acknowledgment. Prof. Nutch Tanphaichitr (University
of Ottawa, Ontario, Canada) is gratefully acknowledged for
very helpful support on defining the biological background.
We thank the University of Milan and the University of
Piemonte Orientale for financial support.
Experimental Section
3-O-[2,4,6-Tri-O-benzyl-3-deoxy-3-(E)-isobutylsulfonomethy-
lene-β-D-galactopyranosyl]-2-O-tert-butyldimethylsilyl-1-O-hexa-
decyl-sn-glycerol (2a) and 3-O-[2,4,6-Tri-O-benzyl-3-deoxy-3-(Z)-
isobutylsulfonomethylene-β-D-galactopyranosyl]-2-O-tert-butyl-
dimethylsilyl-1-O-hexadecyl-sn-glycerol (2b). To a solution of
Supporting Information Available: Experimental proce-
dures, full characterization of new compounds and proton,
carbon NMR spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
5366 J. Org. Chem. Vol. 75, No. 15, 2010