Determination of the configuration of an Archaea membrane lipid containing cyclopentane rings by total synthesis

Article abstract of DOI:10.1002/anie.200250629

The fat controller: Thermoacidophilic Archaea possess membranes containing lipids consisting of mixtures of macrocyclic, 72-membered tetraethers composed of saturated isoprenoid chains linked to glycerol or higher sugars. Lipids of this type form highly s

Full text of DOI:10.1002/anie.200250629

Angewandte
Chemie
Archaea Membrane Lipids
In fact, the configuration of the macrocyclic tetraether A has
been determined by total synthesis of the diol C and
comparison with a sample of C obtained by degradation of
natural A.[5] In the meantime, a synthesis of A has also been
carried out.[6]
Determination of the Configuration of an
Archaea Membrane Lipid Containing
Cyclopentane Rings by Total Synthesis**
In contrast, the configurations of the compounds contain-
ing five-membered rings are largely unknown and, as a
consequence, stereoselective synthesis of a lipid containing
cyclopentane units has not been undertaken. De Rosa et al.
obtained various pure diols with five-membered rings, among
them 1, by degradation of natural etherlipids and determined
Elvira Montenegro, Bert Gabler, Gesa Paradies,
Matthias Seemann, and Günter Helmchen*
Dedicated to Professor Kurt Mislow
on the occasion of his 80th birthday
8
The Archaea are microorganisms that proliferate under
extreme environmental conditions, such as high acidity, high
temperature and/or high salt concentration.[1] Archaea are
classified into three phenotypes on the basis of their living
habitats: methanogens, halophiles, and thermoacidophiles.
Among the distinctive features of the thermoacidophiles are
their membranes. These contain lipids consisting of mixtures
of macrocyclic, 72-membered tetraethers composed of satu-
rated isoprenoid chains linked to glycerol or higher sugars.[2]
Furthermore, compounds with up to eight five-membered
rings were isolated. Typical tetraethers are compounds A and
B.[3] Note that there is a relationship between A and B in that
the five-membered rings of B can formally be generated by
connecting CH3 and CH2 groups of A, for example C18 and
C10.
20'
15'
19'
9
7
1
3
5
16
H
17'
10
18'
11
19
15
20
13
OH
H
H
OH
18
17
10'
16'
H
1'
7'
9'
8'
1
their constitution by mass spectrometry and NMR spectro-
scopy.[7] Important features, revealed by 13C NMR spectro-
scopy, are the C2 symmetry of the structure of 1 and the trans
configuration of the rings which was deduced by comparison
of the 13C NMR spectroscopy data with those of cis- and trans-
1,3-dimethylcyclopentane.[8] Herein we report the determi-
nation of the complete relative and absolute configuration of
the diol 1 by a stereoselective synthesis.
During the last decade syntheses of ethers related to
archaea membranes have been reported by several groups.[4]
Before embarking on the considerable effort of the
synthesis of 1, it appeared wise to exclude possible config-
urations by comparing naturally derived C40
diol 1 with C20 model compounds. Exploiting
the C2 symmetry of 1, the diastereomers 2a–d
16
10
1
7
HOH2C
O
O
O
17
18
(Scheme 1) were chosen as model compounds.
The methylated center at C3 was assumed to be
analogous to the all-methylated chain C, but all
possible configurations in the 1,3-trans-disub-
stituted ring and the a-methyl group were
considered.
Our route to these compounds relies on
lactones (S,S)-3 and (R,R)-3 as starting materi-
als, available enantiomerically pure on a 100 g
scale by asymmetric allylic substitution.[9] The
CH3 group corresponding to C19 was intro-
duced by formation of the enolate (lithium
diisopropylamide (LDA), THF, ꢀ788C) and
alkylation with methyl iodide which proceeded
CH2OH
O
A
1
7
HOH2C
O
O
10
18
O
O
CH2OR
B
C
HO
OH
with diastereoselectivity of 4a:4b = 6:1; both lactones can be
obtained in pure form by column chromatography.[10]
[*] Prof. Dr. G. Helmchen, Dr. E. Montenegro, Dr. B. Gabler,
G. Paradies, Dr. M. Seemann
Compounds 2a–d were synthesized by analogous routes.
As a representative example, the synthesis of alcohol 2a is
described in Scheme 2. Bromide 5 was prepared from
citronellol (98% ee), and transformed into an organocopper
reagent which was coupled with lactone (þ)-4a by an Sn2’
reaction.[11] Reduction of the resulting acid 6 with LiAlH4
gave the corresponding alcohol which was transformed into
the tosylate 7. Chain elongation by cross coupling tosylate 7
with 3-methylbutylmagnesium bromide, catalyzed by
Li2CuCl4,[12] furnished the unsaturated precursor of 8 in high
Organisch-chemisches Institut
Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-4205
E-mail: en4@ix.urz.uni-heidelberg.de
[**] We thank the Alexander von Humboldt Foundation for a postdoc-
toral fellowship to E. Montenegro, T. Weiss and W. Haseloff for
recording the NMR spectra, K. Brödner and O. Tverskoy for
experimental assistance, Dr. C. Kühl (Bernina Biosystems) for a
sample of tetraether membrane lipids from Archaea, and Dr. T.
Netscher (Hoffmann-La Roche) for (R)-citronellal.
Angew. Chem. Int. Ed. 2003, 42, 2419 – 2421
DOI: 10.1002/anie.200250629
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2419

Communications
HMBC, and DEPT experiments. Data
were compared with those of diol 1
which was prepared by degradation of
a mixture of tetraether lipids extracted
from Archaea Sulfolobus acidocaldar-
S
7
8
R
R
8
9
1
R
7
9
1
10
S
13
R
10
H
R
13
S
OH
18
H
17
OH
18
H
17
H
2a
2c
19
20
20
ius.[14] In Figure 1 the differences in 13
C
19
chemical shift values are plotted for
the cyclopentane ring and adjacent
CH(Me) units. From these data, con-
figurations 2b and 2d can be excluded.
The 13C chemical shift values of the
diastereomers 2a and 2c differ at most
by 0.18 ppm. Thus, a decision between
the two corresponding configurations
for 1 on the basis of this NMR
spectroscopy data is not possible.
Nevertheless, these compounds were
useful because it was found that the
diastereomeric alcohols 2a–d all dis-
play significantly different optical
rotations (Figure 1). On the basis of
this observation it was expected that
comparison of the optical rotations of
1, obtained by degradation and by
total synthesis, would allow conclusive
H
O
H
H
O
O
O
R1
R1
H
R2
R2
(R,R)-3 R1 = R2 = H
(-)-4a
1 = H, R2 = CH3
(-)-4b R1 = CH3, R2 = H
(S,S)-3 R1 = R2 = H
(+)-4a R1 = CH3, R2 = H
(+)-4b R1 = H, R2 = CH3
R
S
8
R
R
8
9
7
R
7
9
1
1
10 S
S
10
H
R
13
OH
R
H
18
OH
17
18
H
17
H
2b
19
20
2d
19
20
establishment
of
configuration.
Accordingly, a synthesis of diol 1 was
carried out.[15]
Scheme 1. Model compounds 2a–d and starting materials.
The synthesis of 1 starting from 7
involved chain elongation by a hydro-
isoprene unit followed by dimeriza-
tion (Scheme 3). For chain elongation,
a copper catalyzed cross-coupling of
Br
a)
b)
HO
(+)-4a
OBn
c)
5
(+)-(R)-Citronellol
tosylate
7 (see Scheme 2) with a
Grignard reagent prepared from bro-
mide 9 was carried out. After removal
of the protecting group, alcohol 10 was
obtained and transformed into the
corresponding bromide. Copper-cata-
lyzed dimerization, reduction of the
double bonds, and deprotection gave
the diol 1 in 32% yield from 7.
OTs
H
COOH
H
OBn
OBn
H
H
6
7
8 R = CH2Ph
2a R = H
d), e)
f)
H
OR
H
Samples of synthetic C40 diol 1 and
1 derived from archaea lipid displayed
identical 13C NMR spectra. All the
resonance signals were assigned by
2D NMR spectroscopic techniques.
Optical rotations of [a]24306 = + 14.7
(c = 1.00, CHCl3) for the synthetic
diol 1 and [a]24306 = + 15.5 (c = 1.00,
CHCl3) for the natural derived diol 1
were found. Thus, there is agreement
Scheme 2. Synthesis of model compound 2a. a) 1. BnBr, NaH, DME, 08C!reflux, 4 h; 2. O3,
NaBH4, MeOH/CH2Cl2 (1:1), ꢀ788C!RT, ꢁ12 h; 3. CBr4, Ph3P, Et2O, RT, ꢁ12 h, 70%
(3 steps); b) 1. Mg, THF, 658C; 2. CuBr·SMe2, THF/SMe2 (5:1), ꢀ788C; 3. (þ)-4a, ꢀ788C!
RT, ꢁ12 h, 80%; c) 1. LiAlH4, THF, 658C; 2. TsCl, pyridine, 08C, 77% (2 steps); d) 3-methylbu-
tyl magnesium bromide (3 equiv), 10 mol% Li2CuCl4, THF, ꢀ788C!08C, ꢁ12 h, 85%;
e) TsNHNH2, DME, NaOAc, H2O, reflux, 2 h; f) Pd(OH)2/C, H2, AcOEt/MeOH (1:1), 1.1 atm,
RT, ꢁ12 h, 88% (2 steps). Bn=benzyl, Ts=p-toluenesulfonyl, DME=dimethoxyethane.
yield. The double bond was reduced with diimine without the
isomerization[13] which had occurred in a variety of transition-
metal catalyzed hydrogenations. Finally, hydrogenation of
benzyl ether 8 furnished the desired model compound 2a in
32% overall yield from (þ)-(R)-citronellol.
within the range of precision of the measurement of optical
rotation. Considering also the large differences in the optical
rotations of the model compounds 2a–d the proposed
configuration of diol 1 is strongly supported.
In conclusion, the absolute and relative configuration of
an archaea membrane lipid containing five-membered rings
was determined for the first time. This was accomplished by
NMR spectra of isomers 2a–d were recorded and the
chemical shift assignments established by COSY, HMQC,
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www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 2419 – 2421

Angewandte
Chemie
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Figure 1. Differences in 13C NMR chemical shifts of samples of diol 1 obtained by
degradation of Archaea membrane lipids and of synthetic compound 2a–d
(125 MHz, CDCl3). The x axis gives the number of the carbon atom and y axis the
Dd (d(2)ꢀd(1)). In addition the optical rotations of 2a–d are given.
OSiPh2tBu
a)
OH
7
+
H
OBn
Br
H
10
9
b)
8
20'
15'
19'
5
9
7
3
1
16
H
17'
10
18'
11
19
15
20
13
OH
H
H
18
OH
17
10'
16'
H
1'
7'
9'
8'
[12] a) M. Tamura, J. Kochi, Synthesis 1971, 303 – 305;
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[13] A. G. Schultz, T. J. Guzi, E. Larsson, R. Rahm,
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7795 – 7805.
1
Scheme 3. Synthesis of diol 1. a) 1. 9, Mg, THF, 658C; 2. 5 mol% Li2CuCl4, THF,
ꢀ708C; 3. 7, ꢀ708C!RT, ꢁ12 h; 4. Bu4NF, THF, 85% (2 steps); b) 1. CBr4, Ph3P,
Et2O, RT, ꢁ12 h; 2. Mg (0.5 equiv), THF, 658C, 5 mol% Li2CuCl4, THF, 658C;
3. PtO2, EtOAc, 2 h; 4. Pd(OH)2/C, H2, 4 bar, 38% (4 steps). Ts=p-toluenesulfonyl,
DME=dimethoxyethane.
[14] The mixture of lipids was obtained from Bernina
Biosystems, Munich. Degradation was carried
out according to a method developed in the
Arigoni goup (O. W. Gräther, PhD thesis, ETH Zürich, 1994):
reaction with HI followed by treatment of the resultant iodides
with silver acetate to give acetates which were saponified.
synthesis of the four model compounds 2a–d and the diol 1
and comparison of their NMR spectroscopic and optical
rotation data with those of diol 1 derived from natural archaea
[15] B. Gabler, PhD thesis, Universität Heidelberg, 1997.
lipids.
Received: November 25, 2002 [Z50629]
Keywords: archaea membrane lipids · asymmetric synthesis ·
.
configuration determination · cross-coupling · natural products
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www.angewandte.org
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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