The synthesis of archaebacterial lipid analogues

Article abstract of DOI:10.1016/S0040-4039(00)00283-5

An efficient and convenient route to two novel quasimacrocyclic archaebacterial lipid analogues is presented. The target compounds 2 and 3 are prepared in seven and four steps, respectively, from known starting materials, and are useful for the study of s

Full text of DOI:10.1016/S0040-4039(00)00283-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2971–2974
The synthesis of archaebacterial lipid analogues
Burkhard Raguse,a,b,∗ Peter N. Culshaw,a,b Jognandan K. Prashar a,c and Kiran Raval a,d
aThe Cooperative Research Centre for Molecular Engineering and Technology, 126 Greville St, Chatswood,
NSW 2067, Australia
bCSIRO Telecommunication and Industrial Physics, PO Box 218, Lindfield, NSW 2070, Australia
cThe School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia
dAmbri Pty Ltd, 126 Greville St, Chatswood, NSW 2067, Australia
Received 12 November 1999; revised 7 February 2000; accepted 14 February 2000
Abstract
An efficient and convenient route to two novel quasimacrocyclic archaebacterial lipid analogues is presented.
The target compounds 2 and 3 are prepared in seven and four steps, respectively, from known starting materials, and
are useful for the study of synthetic tethered bilayer membranes. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: lipids; membranes; membrane spanning lipids; archaebacteria; biosensors.
We recently reported a novel, generic biosensor based on an ion channel switch embedded in a
synthetic lipid membrane tethered onto a planar gold electrode.1 In order for the ion channel switch
to function in a real environment such as whole blood or plasma, we required a stable, fluid bilayer
membrane. The formation of such a membrane was achieved through the use of synthetic lipids based on
the structural motifs found in archaebacterial lipids. Microorganisms belonging to the Archaea domain
typically thrive in environments usually considered too hostile for conventional life such as areas of
high temperature, high salt content and/or pH extremes.2 The lipid components of membranes of the
archaebacteria are essential for this extraordinary stability, and as such there have been numerous studies
concerning the isolation, structural elucidation and synthesis of these lipidic compounds.3 Additionally,
these lipids are of interest for applications such as drug delivery, separations and photochemical energy
conversion.4
Typically, archaebacterial lipids consist of a branched hydrocarbon (such as the isoprenoid phytanol)
connected to a glycerol backbone via an ether linkage, in contrast to the lipids of eubacteria and eu-
karyoates which possess ester-linked straight chain fatty acids. Furthermore, archaebacterial membranes
often contain macrocyclic lipids with ring sizes up to 72 atoms, such as the tetraether 1 obtained from
the archaebacterium Sulfolobus solfataricus.5
∗
Corresponding author. E-mail: burkhard.raguse@tip.csiro.au (B. Raguse)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00283-5
tetl 16563

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In this paper we report efficient synthetic routes for two novel quasimacrocyclic lipidic species (2 and
3), that have been used in the formation of tethered lipid membranes.1
The compounds designed in this work (2 and 3) were modelled closely on macrocyclic archaebacterial
lipids in that they contained phytanyl groups ether linked to glycerol moieties. The glycerol units were
linked together by non-branched alkyl chains connected via a central biphenyl group, allowing the entire
construct to span a bilayer membrane (∼38 Å), whilst the phytanyl groups were required in order to
ensure that the lipids remained in a liquid crystalline phase. The biphenyl group was employed so that
the lipid could not readily adopt a U-shape (i.e. in one leaflet of a bilayer membrane) as has also been
reported for other quasimacrocyclic lipid species.6
Central to the synthesis of both quasimacrocycles 2 and 3 was the preparation of a monophytanyl-
ated glycerol unit. Commercially available 1-O-benzyl glycerol was silylated at the primary hydroxyl
(TBDMSCl, imidazole, DMF, 85%) and reacted with phytanyl bromide7,8 (NaH, THF, 81%) to afford
the differentially protected monophytanyl glycerol derivative (Scheme 1). While desilylation of 4 could
be affected, the yields of the product were variable and so an alternative, more robust, route to this
material was sought.
Scheme 1. (i) TBDMSCl, imidazole, DMF; (ii) Phytanyl bromide, NaH, THF
Commercially available 1,3-di-O-benzyl glycerol was reacted with phytanyl bromide (KOH, PhCH3,
61%) and the product subsequently hydrogenolysed (Pd/C, H2, MeOH, 97%) to afford the monophytanyl
glycerol 5 as a colourless oil (Scheme 2). Condensation with benzaldehyde (cat. p-TSA, PhCH3) afforded
the six-membered ring ketal 6 which was smoothly reduced to the required 1-O-benzyl-2-O-phytanyl
glycerol 7 by either DIBALH (CH2Cl2, PhCH3, 69%) or in situ-generated borane (KBH4, BF3·Et2O,
44%). The intermediate 6 could also be prepared by reaction of 2-phenyl-5-hydroxy-1,3-dioxane with
phytanyl bromide; however, the low yield for the preparation of the starting dioxane9 made this route
unattractive.
Scheme 2. (i) Phytanyl bromide, KOH, PhCH3; (ii) H2, Pd/C, MeOH; (iii) PhCHO, p-TSA, PhCH3; (iv) DIBALH,
PhCH3/CH2Cl2

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The alternative regioisomer of 7 was prepared by starting with known diol 8.7 Monobenzylation of
8 was achieved through alkylation (BnBr, CH2Cl2/THF, 68%) of the in situ-derived dibutylstannoxane
derivative,10 itself formed by treatment of 8 with dibutyltin oxide (Bu2SnO, PhCH3), to afford 1-O-
phytanyl-2-O-benzyl-glycerol 9 (Scheme 3).
Scheme 3. (i) Bu2SnO, PhCH3; (ii) BnBr, CH2Cl2/THF
With efficient routes to two regioisomers of mono-phytanylated mono-benzylated glycerol, we next
explored linking each unit via a membrane spanning tether. Thus, the known dibromide 1011 was reacted
under basic conditions with either 7 or 9 to afford the bromolipid species 11 and 12, respectively, in 52
and 70% yield (Scheme 4).
Scheme 4. (i) 9, KOH, PhCH3; (ii) 7, NaH, THF
Coupling of the bromolipid 11 was achieved by reaction with 4,40-biphenol (NaH, DMF) to give the
dibenzylated membrane spanning lipid in good yield (65%). Removal of the protecting groups under
hydrogenolytic conditions (Pd/C, H2, MeOH, 55%) afforded the desired membrane spanning lipid 2
as a wax. Reaction of the alternative bromolipid 12 under identical conditions afforded the isomeric
membrane spanning lipid 3, again in fair yield (45% for the dibenzylated compound, 95% yield for the
debenzylation) (Scheme 5).
Scheme 5. (i) 4,40-biphenol, NaH, DMF; (ii) H2, Pd/C, MeOH/CH2Cl2
The synthetic procedures detailed above were carried out on a scale such that multigram quantities of
these membrane spanning lipids were obtained. For instance, diol 2 was produced in >100 g quantities
from 7 prepared by using the method shown in Scheme 2. All new materials prepared in this work have
been fully characterised12 and the product diols 2 and 3 have been subsequently coupled to disulfide
containing species for attachment to gold and use in tethered membrane system.1 Further studies on
these and related species will be reported in due course.
Acknowledgements
We gratefully acknowledge financial support from the Australian Government through the Cooperative
Research Centre for Molecular Engineering and Technology and to Dr C. J. Burns for help in preparing
this manuscript.

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Wieczorek, L. SPIE Proc. 1998, 3270, 50; Woodhouse, G. E.; King, L. G.; Wieczorek, L.; Cornell, B. A. Faraday Discuss.
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(Archaebacteria); Kates, M.; Kushner, D. J.; Matheson, A. T., Eds.; Elsevier: Amsterdam, 1993; pp. 261.
4. The Achaebacteria: Biochemistry and Biotechnology; Danson, M. J.; Hough, D. W.; Lunt, G. G., Eds.; Portland Press:
Chapel Hill, 1992; Gambacorta, A.; Gliozzi, A.; De Rosa, M. World J. Microbiol. Biotechnol. 1995, 11, 115; De Rosa, M.;
Lanzotti, V.; Nicolaus, B.; Trincone, A.; Gambacorta, A. In Microbiology of Extreme Environments and its Potential for
Biotechnology; Da Costa, M. S.; Duarte, J. C.; Williams, R. A. D., Eds.; Elsevier: London, 1989; pp. 131.
5. Yamauchi, K.; Moriya, A.; Kinoshita, M. Biochim. Biophys. Acta 1989, 1003, 151.
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7. Joo, C. N.; Shier, T.; Kates, M. J. Lipid Res. 1968, 9, 782.
8. In this work racemic glycerol and a mixture of stereoisomers of phytanol were employed. For routes to stereochemically
pure phytanol, see: Sita, L. R. J. Org. Chem. 1993, 58, 5285; Burns, C. J.; Field, L. D.; Hashimoto, K.; Petteys, B. J.; Ridley,
D. D.; Rose, M. Aust. J. Chem. 1999, 52, 387.
9. Crich, D.; Beckwith, A. L. J.; Chen, C.; Yao, Q.; Davison, I. G. E.; Longmore, R. W.; Anaya de Parrodi, C.; Quintero-Cortes,
L.; Sandoval-Ramirez, J. J. Am. Chem. Soc. 1995, 117, 8757.
10. Aragozzini, F.; Maconi, E.; Ponteza, D.; Scolastico, C. Synthesis 1989, 225.
11. Nineham, A. W. J. Chem. Soc. 1953, 2601. In the present work the dibromide 10 was prepared in two steps from ω-
pentadecalactone by reductive ring-opening (LiAlH4, THF, 90%) and reaction of the generated diol with HBr (H2SO4,
95%).
12. Spectroscopic data for key compounds prepared in this work. Compound 5: 1H NMR (CDCl3, 200 MHz) δ 0.80–0.95 (15H,
m, phytanyl CH3s), 0.95–1.72 (24H, m, phytanyl CH2s and CHs), 3.40–3.80 (7H, m, CH–O and CH2–O); m/z (CI): 372.64
1
(M+H+). Found: C, 73.6; H, 12.9; C23H48O3·0.2H2O requires: C, 73.4; H, 13.0%. Compound 6: H NMR (CDCl3, 200
MHz) δ 0.81–0.93 (15H, m, phytanyl CH3s), 0.93–1.69 (24H, m, phytanyl CH2s and CHs), 3.51–3.70 (7H, m, CH2–O),
4.36, 4.42 (1H, m, CHPh, cis:trans, 1:1), 7.36, 7.45 (5H, m, CHPh, cis:trans, 1:1); m/z (CI): 461.4 (M+H+). Found: C, 78.2;
H, 11.4; C30H52O3 requires: C, 78.2; H, 11.4%. Compound 7: 1H NMR (CDCl3, 200 MHz) δ 0.80–0.92 (15H, m, phytanyl
CH3s), 0.92–1.70 (24H, m, phytanyl CH2s and CHs), 3.48–3.80 (7H, m, CH–O and CH2–O), 4.55 (2H, m, CH2Ph), 7.3
(5H, m, CH2Ph); m/z (CI): 463.3 (M+H+). Found: C, 78.0; H, 12.1; C30H54O3 requires: C, 77.9; H, 11.8%. Compound 9: 1H
NMR (CDCl3, 200 MHz) δ 0.82–0.88 (15H, m, phytanyl CH3s), 1.08–1.37 (24H, m, phytanyl CH2s and CHs), 2.32 (1H,
s(br), OH), 3.50 (6H, m, CH2–O), 3.98 (1H, m, CH–O), 4.55 (s, 2H, O–CH2–Ph), 7.32 (m, 5H, CH2–Ph); m/z (EI) (HR)
found: 462.4086; C30H54O3 requires: 462.4072. Compound 11: 1H NMR (CDCl3, 200 MHz) δ 0.80–0.90 (15H, m, phytanyl
CH3s), 0.90–1.62 (52H, m, phytanyl CH2s and CHs), 3.39–3.66 (9H, m, CH–O and CH2–O), 4.58 (2H, s, CH2Ph), 7.35 (5H,
m, CH2Ph); m/z (CI): 753.4 (M+H+). Found: C, 71.8; H, 11.6; C45H83BrO3 requires: C, 71.9; H, 11.1%. Compound 12: 1H
NMR (CDCl3, 200 MHz) δ 0.82–0.88 (15H, m, phytanyl CH3s), 1.18–1.59 (52H, m, phytanyl CH2s, CHs and C15 chain
CH2s), 3.36–3.57 (9H, m, CH–O and CH2–O), 4.55 (2H, s, CH2–Ph), 7.32 (m, 5H, aromatic protons); m/z (EI) 752 (M+).
Found: C, 72.1; H, 11.2; C45H83BrO3 requires: C, 71.9; H, 11.1%. Compound 2: 1H NMR (CDCl3, 200 MHz) δ 0.80–0.95
(30H, m, phytanyl CH3s), 0.95–1.70 (100H, m, phytanyl CH2s and CHs), 3.40–3.80 (18H, m, CH–O and CH2–O), 3.98 (4H,
t, CH2OAr), 6.93, 7.43 (8H, AA0XX0 multiplets, arom. –H); m/z (ES): 1370 (M+Na+). Found: C, 78.3; H, 12.2; C88H162O8
requires: C, 78.4; H, 12.1%. Compound 3: 1H NMR (CDCl3, 200 MHz) δ 0.82–0.88 (30H, m, phytanyl CH3s), 1.07–1.73
(100H, m, phytanyl CH2s, CHs and C15 chain CH2s), 3.47–3.62 (18H, m, CH–O and CH2–O), 3.98 (4H, CH2–O–Ar), 6.95,
7.46 (8H, AA0XX0 multiplets, arom. –H); m/z (ESI) 1348 (M+). Found: C, 78.1; H, 12.1; C88H162O8 requires: C,78.4; H,
12.1%.