An efficient synthesis of analogues of unsymmetrical archaeal tetraether glycolipids

Article abstract of DOI:10.1039/a802338g

Unsymmetrical tetraether analogues of glycolipids found in archaebacteria, possessing either two different carbohydrate units or a saccharidic moiety and a phosphate group as polar heads and a quasi-macrocyclic lipid core, are efficiently synthesized from

Full text of DOI:10.1039/a802338g

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An efficient synthesis of analogues of unsymmetrical archaeal tetraether
glycolipids
Gre´gory Lecollinet,a Rachel Auze´ly-Velty,a Thierry Benvegnu,*a† Grahame Mackenzie,b John W. Goodbyb
and Daniel Plusquelleca
a Ecole Nationale Supe´rieure de Chimie de Rennes, Laboratoire de Synthèses et Activations de Biomole´cules, associe´ au CNRS,
Avenue du Ge´ne´ral Leclerc, PO Box, 35700 Rennes, France
b The Department of Chemistry, Faculty of Science and the Environment, The University of Hull, Hull, UK HU6 7RX
Unsymmetrical tetraether analogues of glycolipids found in
archaebacteria, possessing either two different carbohydrate
units or a saccharidic moiety and a phosphate group as polar
heads and a quasi-macrocyclic lipid core, are efficiently
synthesized from versatile chiral building blocks.
(R)-dihydrocitronellyl chains having a combined length equal to
that of the bridging unit, attached to glycerol respectively at sn-3
and sn-2 positions, (ii) either a
D
-galactofuranose and a
phosphate group or the former and a lactosyl unit as polar head
groups at opposite ends of the lipid core, and (iii) an S
stereogenic center at each glycerol backbone similar to that of
the corresponding natural glycolipids 1 and 2.5 These synthetic
bolaamphiphiles might show similar behaviour to cyclic lipids
when aggregated or inserted into membranes.8
The Archaea domain, composed of a variety of extremophilic
microorganisms, belongs to a third kingdom of life which is
clearly distinct from the well-known Bacteria and Eucarya
domains.1–3 Preservation of the membrane function under
extreme conditions such as high temperatures and strong acidity
lies with the unique structure of archaebacterial lipids, which
fundamentally differ from those of their bacterial and eucaryotic
counterparts.4 Of particular interest is the unusual molecular
architecture of the macrocyclic tetraether lipids found in
methanogenic and thermoacidophilic species.5 These lipids are
characterized by a bipolar structure with two head groups linked
together by two C40 polyisoprenoid chains. Recently, Gräther
and Arigoni demonstrated that several natural lipids were in fact
nearly statistical mixtures of regioisomers differing in the
relative orientation of their glycerol units.6 The dimensions of
the tetraether lipids are therefore sufficient to form monolayered
membranes5 and the presence of two head groups with different
sizes at opposite sides of the monolayer is expected to induce
membrane curvature.4b The presence of galactofuranose units in
some glycolipids is puzzling since hexoses appear only in the
pyranose form in mammalian glycoconjugates. The unique
chemical design of these archaeal lipids represents an inter-
esting opportunity to build exceptionally thermal stable vesicles
which might be suitable for nano-scale technologies and for
drug-delivery applications.7
The general synthetic pathway for 3 and 4 (Scheme 1)
involved the preparation of the key diol intermediate 9,
subsequent monoprotection of one of the hydroxy groups
and sequential introduction of the polar heads. The crucial
glycosylation or phosphorylation step of the monoglycosylated
compound 12 should be performed under extremely mild
conditions in order to preserve the highly hydrolysable b- -
D
galactofuranosidic linkage. Hemimacrocyclic diol 9 was ob-
tained from (S)-(+)-2,2-dimethyl-1,3-dioxolan-4-ylmethanol
[(S)-(+)-solketal],‡ hexadecane-1,16-diyl ditriflate 5, and
(R)-(2)-citronellyl bromide as the starting materials. After
experimentation, we found that reaction of hexadecane-
1,16-diol with Tf2O in the presence of the hindered base
2,6-lutidine resulted in the efficient formation of the expected
ditriflate 5. Separation of the ditriflate from the base was easily
accomplished by flash chromatography, thus affording com-
pound 5 in 80% yield. Alkylation of two (S)-solketal units by 5
proceeded efficiently (83%) in refluxing CH2Cl2 for 24 h using
1,8-bis(dimethylamino)naphthalene (proton sponge: PS) as a
base. Removal of the isopropylidene groups in the presence of
a Dowex acidic resin in refluxing MeOH gave the tetraol (73%)
which was selectively protected by p-methoxytritylation of the
primary hydroxy groups. Williamson coupling of the remaining
secondary hydroxy groups using commercially available
(R)-(2)-citronellyl bromide provided the unsaturated tetraether
8 (80% yield). Palladium-catalyzed hydrogenation of 8 led to
the formation of byproducts due to the unexpected cleavage of
the ether linkages at the sn-2 glycerol sites. Addition of 1 equiv.
of triethylamine in the hydrogenation media avoided this side
reaction and furnished quasi-quantitatively the corresponding
saturated lipid. Finally, the key diol 9 was readily prepared by
removal of the trityl groups under acidic conditions.9
We report here the first synthesis of the chiral unsymmetrical
quasi-macrocyclic tetraethers 3 and 4 as models of the natural
OR1
H
O
O
O
H
O
OH
O
O
OH
OH
O
OH
OH
HO
HO
O
O
P
HO
OH
O
1 R1
=
2 R1
=
OH
–
HO
HO
O
OH
O
At this stage, the next challenging problem involved the
selective protection of 9 so as to introduce two different polar
head groups at opposite ends. After experimentation, we found
that the protocol reported by Bouzide and Sauve´10 was the most
efficient methodology for the monoprotection of the diol.
Treatment of 9 with Ag2O and BnBr in CH2Cl2 led to the
formation of the monobenzylated product 10 (50% yield) in
addition to the dibenzylated derivative (10% yield) and the
H
(R)
OR2
O
O
O
O
(S)
H
(S)
O
OH
(R)
O
OH
OH
OH
OH
O
OH
O
HO
O
HO
4 R2 = 2 Na+ O–
P
O–
3 R2
=
O
OH
OH
OH
unreacted recyclable diol (36%). The introduction of the b- -
D
lipids 1 and 2 isolated from Methanospirillum hungatei, which
is a methanogenic species.5 The most characteristic structural
features of the target molecules lies with (i) a lipid core
consisting in a linear hexadecanemethylene spacer and two
galactofuranoside unit was performed stereospecifically by way
of n-pentenyl glycoside (NPG) glycosylation11 from the key
galactofuranosyl donor 11.12 After hydrogenolysis of the
benzyloxy group, we envisaged either the glycosylation or the
Chem. Commun., 1998
1571

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O
O
O
i
ii
O
O
TfO(CH2)16OTf
HO(CH2)16OH
5
O
6
iii, iv
OH
PMTO
HO
OPMT
O
O
7
v
O
PMTO
O
O
OPMT
O
8
9
Fig. 1 Transmission electron micrograph of compound 4 vesicles negatively
stained with uranyl acetate. The bar is 1500 nm.
vi, vii
O
O
HO
O
O
OH
Purification by gel filtration chromatography with Sephadex
LH-20 furnished the targeted unsymmetrical glycolipid ana-
logue 4 in 85% yield.
Aqueous dispersions of glycolipids 3 and 4 were sonicated at
60 °C for 20 min. Transmission electron microscopy revealed
that phosphate 4 furnished spherical vesicles of 100–1000 nm
diameter stable for several weeks at ambient temperature as
shown in a representative micrograph (Fig. 1). Compound 3
provided myelin-type aggregates (not shown).
Further physicochemical measurements are under inves-
tigation.
We are grateful to the CNRS and the Re´gion Bretagne for
grants to R. A.-V. and G. L., respectively, to M. Lefeuvre for
assistance with NMR experiments, and to P. Guenot for mass
spectrometry measurements.
viii
O
HO
O
OBn
O
O
10
O
O
OAc
O
O
O
O
O
OAc
OH
ix, x
O
10 +
OAc
OAc
OAc
OAc
OAc
OAc
11
12
OAc
O
OAc
O
xi, xii
AcO
OAc
O
+
12
3
SEt
AcO
OAc
OAc
13
O
O
Notes and References
O
O
OPO(OBn)2
xiv, xv
O
OAc
xiii
† E-mail: thierry.benvegnu@ensc-rennes.fr
12
4
O
‡ (S)-(+)-Solketal and its (R)-(2)-isomer are currently available on a
kilogram scale. (S)-(+)-Solketal was obtained from Chemi S.p.A., 20092
Cinisello Balsamo, Italy.
OAc
OAc
OAc
14
PMT = 4-methoxytrityl
§ All yields reported herein refer to isolated pure materials which have 1H
and 13C NMR spectra, elemental analyses and high resolution mass spectral
characteristics in accordance with the proposed structures. Selected data for
Scheme 1 Reagents and conditions: i, Tf2O, 2,6-lutidine, CH2Cl2, 0 °C,
then room temp., 80%; ii, (S)-solketal, PS, CH2Cl2, reflux, 83%; iii, Dowex
H+ resin, MeOH, reflux; iv, 4-methoxytrityl chloride, pyridine, DMAP,
THF, reflux, 60% over two steps; v, (R)-citronellyl bromide, NaH, 130 °C,
80%; vi, H2, Pd/C, Et3N, AcOEt; vii, HCO2H, Et2O, 70% over two steps;
viii, Ag2O, BnBr, CH2Cl2, 50%; ix, NIS, Et3SiOTf, CH2Cl2; x, H2, Pd/C,
EtOH, 73% over two steps; xi, NIS, TESOTf, 4 Å molecular sieves,
CH2Cl2; xii, MeONa, MeOH, 65% over two steps; xiii, 1H-tetrazole,
(BnO)2PNPri2, CH2Cl2, then MCPBA, CH2Cl2, 240 to 0 °C, 80%; xiv,
MeONa, MeOH; xv, H2, Pd/C, MeOH, acetate buffer (pH 5) (3:1 v/v), then
Amberlite® IR-120 (Na+), MeOH, then gel-filtration on Sephadex LH-20
(1:2 CH2Cl2–MeOH), 85% over two steps
20
3: [a]D 226.7 (c 0.78 in MeOH); FABMS (m-nitrobenzyl alcohol matrix):
Calc. for [M + Na]+: 1195.7907. Found: 1195.7880. For 4: [a]D 226.4 (c
20
0.72 in MeOH); FABMS (m-nitrobenzyl alcohol matrix): Calc. for [M +
H]+: 973.6333. Found: 973.6331; Calc. for [M 2 Na + 2H]+: 951.6514.
Found: 951.6524.
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order to prevent any hydrolysis of the former b- -galactofur-
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D
anosidic bond. The glycosylation of 12 using lactosyl thioglyco-
side 13 as a donor13 was carried out under NIS–Et3SiOTf
conditions and provided the desired b-d-glycoside in 70% yield.
Deacetylation of the glycosyl hydroxy groups under standard
conditions gave the totally deprotected bipolar lipid 3 in 93%
yield.§
Having successfully prepared the bis-glycoside 3, we turned
our attention towards the introduction of a phosphate group onto
intermediate 12. Alkyl dibenzyl phosphate 14 was prepared by
reacting alcohol 12 with dibenzyl N,N-diisopropylphosphor-
amidite and 1H-tetrazole followed by in situ mild oxidation of
the resultant alkyl dibenzyl phosphite with MCPBA (80%
yield).14 The transformation of 14 into the phosphate salt 4 was
performed by sequential deacetylation of the galactosyl unit,
catalytic hydrogenolysis (Pd/C) in a buffered solvent mixture
(MeOH–AcOH–NaOAc, pH 5) avoiding the glycoside hydroly-
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Received in Glasgow, UK, 24th March 1998; 8/02338G
1572
Chem. Commun., 1998