A stereocontrolled synthesis of the hydrophobic moiety of rhamnolipids

Article abstract of DOI:10.1016/j.tetlet.2015.01.091

A new approach toward the synthesis of the hydrophobic moiety of rhamnolipid derivatives has been developed, involving two key cross-metatheses and an unusual Mitsunobu reaction. Small structural variations of the side chains should enable a better understanding of the role of the lipid moiety in immunostimulatory and plant defense eliciting properties.

Full text of DOI:10.1016/j.tetlet.2015.01.091

Tetrahedron Letters 56 (2015) 1159–1161
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A stereocontrolled synthesis of the hydrophobic moiety of
rhamnolipids
Boudjema Menhour a, Patrick Mayon a, Karen Plé b, Sandrine Bouquillon a, Stéphan Dorey c,
Christophe Clément c, Magali Deleu d, Arnaud Haudrechy a,
⇑
a Institut de Chimie Moléculaire de Reims, UMR CNRS 7312, Université de Reims, 51687 Reims Cedex, France
b Institut de Chimie Organique et Analytique, UMR CNRS 7311, Université d’Orléans, 45067 Orleans Cedex, France
c Unité de Recherche Vignes et Vins de Champagne, EA 4707, Université de Reims, 51687 Reims Cedex, France
d Centre de Biophysique Moléculaire Numérique, Université de Liège, Passage des Déportés, 2, 5030 Gembloux, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
A new approach toward the synthesis of the hydrophobic moiety of rhamnolipid derivatives has been
developed, involving two key cross-metatheses and an unusual Mitsunobu reaction. Small structural
variations of the side chains should enable a better understanding of the role of the lipid moiety in
immunostimulatory and plant defense eliciting properties.
Received 19 December 2014
Revised 11 January 2015
Accepted 13 January 2015
Available online 19 January 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Rhamnolipids
Enzymatic resolution
Mitsunobu reaction
Cross metathesis reaction
Introduction
subsequently submitted to cross-metathesis conditions in order to
produce a versatile library of derivatives (Scheme 1).
Bacterial rhamnolipids,1 especially those produced by Pseudo-
monas aeruginosa2 and Burkholderia plantarii3 have shown a great
deal of promise for the environmental remediation of oil spills4
and toxic metals.5 They have also been described as immunostimu-
lators6 and have been reported for their antibacterial, mycoplas-
macidal, and antiviral activities.7 More recently, rhamnolipids
have been shown to efficiently induce a local resistance against
Botrytis cinerea and are promising as potential elicitors (Fig. 1).8
Typically, rhamnolipids are bioproduced as a mixture of com-
pounds with various chain lengths.9 Synthetic approaches are
scarce in the literature.6c,10 Most of them involve a selective reduc-
tion of a b-ketoester using Noyori conditions,6c,10c appearing early
in the overall synthesis, which can be a serious drawback in gain-
ing access to a wide range of compounds.
In a previous study, a smart solid phase strategy was used to
produce a glycolipid library. Immunostimulatory structure–activ-
ity relationship analysis indicated a specific recognition based
mode of action.6c Encouraged by a synthesis of (ꢀ)-(3S,6R)-3,
6-dihydroxy-10-methylundecanoic acid by Sabitha et al.11, we
investigated a different approach with a common key intermediate
Results and discussion
We focused on the use of the well-known acetoxyester 5,12
easily prepared on a convenient scale (more than five grams, with
a 94% ee, measured by chiral HPLC with an IC column) using PS
Amano lipase as a biocatalyst for an enantioselective acetylation
(Scheme 2). The loss of half of the starting material, inherent with
an enzymatic resolution, was an initial drawback, but this fact later
proved beneficial.
Selective deprotection of the acetate group of compound 5 was
then accomplished using standard conditions to give the 4(S)
hydroxyester while treatment with trifluoroacetic acid gave the
carboxylic acid 6 in nearly quantitative yields (Scheme 3).
All our attempts to couple 4(S) and 6 via diverse esterification
procedures were unsuccessful (DCC/DMAP, EDCI, and TBTU with
diverse solvents). Surprisingly, compound 5 was the only observed
product as previously described by Duynstee et al.,10b While
attempting various coupling reactions, work was also in progress
to recycle the unwanted enantiomer 4(R). Reviewing the literature
showed that Mitsunobu reactions on allylic alcohols can be
performed with ‘non-aryl’ carboxylic acids.13 One particularly
interesting case was reported with a more complex structure.13c
⇑
Corresponding author. Tel.: +33 (0)326913236; fax: +33 (0)326913166.
E-mail address: arnaud.haudrechy@univ-reims.fr (A. Haudrechy).
http://dx.doi.org/10.1016/j.tetlet.2015.01.091
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

1160
B. Menhour et al. / Tetrahedron Letters 56 (2015) 1159–1161
6
Ph3P (1.5 eq)
4(R)
5 (68%, ee = 96% measured by chiral HPLC)
COOH
DEAD (1.5 eq)
6
O
R = H: RL-1,210
AcOH (3 eq)
Toluene, 1h, -30°C to rt
OH
O
O
O
COOtBu
OH
HO
R =
: RL-2,210
HO
OR
Ph3P (1.9 eq)
O
4(R)
...
O
OH
O
DEAD (1.9 eq)
6 (1.2 eq)
Toluene, 1h, rt
Figure 1. Two examples of natural rhamnolipids used as elicitors.8
AcO
7 (65%)
COOtBu
Scheme 4. Mitsunobu reactions on hydroxylester 4(R) (1).
n
COOH
O
m
O
O
m
m
O
1)
m
AcO
cat. Grubbs II reagent
CuI, Et2O, 40°C
2
AcO
COOtBu
COOtBu
1
AcO
2) H2, PtO2, AcOEt
AcO
COOtBu
COOtBu
m
5
COOH
8 (m = 2 to 17)
+
Yield from 57 to 76%
AcO
HO
4 (R)
AcO
5
3
Scheme 5. Cross metathesis-hydrogenation of double ester 5.
Scheme 1. Retrosynthetic approach to the hydrophobic moiety of rhamnolipid
derivatives 1 (m and n varying from 2 to 17).
COOtBu
3 (m = 2) (1.25 eq)
Ph3P (1.9eq)
O
4(R)
2
PS Amano lipase
COOtBu
COOtBu
O
DEAD (1.9 eq)
Toluene, -30°C to rt
Vinyl acetate (3 eq)
Pentane, 4Å MS
30°C, 18h
HO
AcO
AcO
4
2a (58%)
5 (42%, ee = 94%)
+ 4(R) (45%)
Scheme 6. Mitsunobu reactions on hydroxylester 4(R) (2).
Scheme 2. Enzymatic resolution using PS Amano lipase.
Table 1
Cross metathesis of ester 2 (m = 2)
n
5
n
1)
20% CF3COOH in CH2Cl2
2h, rt
K2CO3 (2 eq), 0°C
MeOH, 30 min, rt
COOtBu
COOtBu
O
cat. Grubbs II reagent
CuI, Et2O, 40°C
O
m
2
COOtBu
COOH
O
O
2) H2 / PtO2 / AcOEt
AcO
AcO
HO
AcO
2 (m = 2)
9a-e (n = 2 to 6)
CF3COOH
CH2Cl2
4(S) (88%)
6 (95%)
Scheme 3. Orthogonal deprotection of acetoxyester 5.
n
Two preliminary tests gave very promising results (Scheme 4). The
first one allowed us to recycle the undesired enantiomer 4(R),
while the second one solved two problems, the desired inversion
of configuration and coupling to furnish compound 7, a precursor
of double chain derivatives 1. This second example proved to be
an esthetic alternative to the desired previously unsuccessful cou-
pled reaction.
COOH
O
2
O
AcO
1a-e (n = 2 to 6)
We then tested the cross-metathesis conditions on the pivotal
substrate 514 with sixteen different alkenes using conditions
recently described by Voigtritter et al.15 (Scheme 5). As the separa-
tion of Z and E-alkenes proved to be difficult in certain cases, we
decided to directly hydrogenate the crude mixture in a one-pot
sequence.
n
2
3
4
5
6
Yield in 9 (%)
Yield in 1a–e (%)
60
76
53
86
48
70
54
73
52
88
to a Mitsunobu coupling with hydroxylester 4(R) giving the
expected compound 2 (m = 2) in 58% yield (Scheme 6).
Deprotection of derivative 8 (m = 2) with CF3COOH in CH2Cl2
gave the carboxylic acid 3 (m = 2 in 89% yield) which was subjected

B. Menhour et al. / Tetrahedron Letters 56 (2015) 1159–1161
1161
Baillieul, F.; Dorey, S. Plant Cell Environ. 2009, 32, 178–193; (b) Vatsa, P.;
Sanchez, L.; Clément, C.; Baillieul, F.; Dorey, S. Int. J. Mol. Sci. 2010, 11, 5095–
5108; (c) Sanchez, L.; Courteaux, B.; Hubert, J.; Kauffmann, S.; Renault, J.-H.;
Clément, C.; Baillieul, F.; Dorey, S. Plant Physiol. 2012, 160, 1630–1641; (d)
Delaunois, B.; Farace, G.; Jeandet, P.; Clément, C.; Baillieul, F.; Dorey, S.;
Cordelier, S. Environ. Sci. Pollut. Res. 2014, 21, 4837–4846.
Finally, to demonstrate the generality of our approach, a second
cross-metathesis reaction was performed on derivative 2 (m = 2)
with five different alkenes, followed by direct hydrogenation of
the reaction mixture (Table 1). In all cases, dimeric structures com-
ing from the alkenes and approximately 15% of starting materials
could be easily removed by silica gel column chromatography. Acid
hydrolysis then gave the desired acids in yields varying between
70% and 88%.
9. (a) Koch, A. K.; Kappeli, O.; Fiechter, A.; Reiser, J. J. Bacteriol. 1991, 173, 4212–
4219; (b) Bodour, A. A.; Drees, K. P.; Maier, R. M. Appl. Environ. Microbiol. 2003,
69, 3280–3287.
10. (a) Westerduin, P.; de Haan, P. E.; Dees, M. J.; van Boom, J. H. Carbohydr. Res.
1988, 180, 195–205; (b) Duynstee, H. I.; Van Vliet, M. J.; van der Marel, G. A.;
van Boom, J. H. Eur. J. Org. Chem. 1998, 2, 303–307; (c) Coss, C.; Carrocci, T.;
Maier, R. M.; Pemberton, J. E.; Polt, R. Helv. Chim. Acta 2012, 95, 2652–2659.
11. Sabitha, G.; Reddy, S. S. S.; Bhaskar, V.; Yadav, J. S. Synthesis 2010, 7, 1217–
1222.
Conclusion
We have developed a short and efficient strategy for the synthe-
sis of the hydrophobic moiety of rhamnolipids. A first metathesis/
hydrogenation sequence gives access to a large number of func-
tionalized alkyl side chains. Mitsunobu conditions were then used
to couple the acid and the alcohol fragments. A second metathesis/
hydrogenation sequence was then applied to give the desired lipid
esters which were hydrolyzed to give the corresponding acids.
Subsequent rhamnosylation will then allow us to obtain hybrid
structures, in order to better understand the structure–activity
relationships of this fascinating class of elicitors. Work is in pro-
gress toward this goal.
12. (a) Tan, C.-H.; Holmes, A. B. Chem. Eur. J. 2001, 7, 1845–1854; (b) Souto, J. A.;
Vaz, E.; Lepore, I.; Pöppler, A.-C.; Franci, G.; Alvarez, R.; Altucci, L.; de Lera, A. R.
J. Med. Chem. 2010, 53, 4654–4667; (c) Kaji, E.; Komori, T.; Yokoyama, M.; Kato,
T.; Nishino, T.; Shirahata, T. Tetrahedron 2010, 66, 4089–4100.
13. (a) Kwon, Y.-U.; Chung, S.-K. Org. Lett. 2001, 3, 3013–3016; (b) Kobayashi, Y.;
Matsuumi, M. Tetrahedron Lett. 2002, 43, 4361–4364; (c) Shen, R.; Lin, C. T.;
Bowman, E. J.; Bowman, B. J.; Porco, J. A., Jr J. Am. Chem. Soc. 2003, 125, 7889–
7901; (d) Ainai, T.; Matsuumi, M.; Kobayashi, Y. J. Org. Chem. 2003, 68, 7825–
7832; (e) Alvarez, L. D.; Veleiro, A. S.; Baggio, R. F.; Garland, M. T.; Edelsztein, V.
C.; Coirini, H.; Burton, G. Bioorg. Med. Chem. 2008, 16, 3831–3838.
14. Wohlrab, A.; Lamer, R.; VanNieuwenhze, M. S. J. Am. Chem. Soc. 2007, 129,
4175–4177.
15. Voigtritter, K.; Ghorai, S.; Lipshutz, B. H. J. Org. Chem. 2011, 76, 4697–4702
Spectral data of selected new compounds:
20
(R)-3-(((R)-3-acetoxyoctanoyl)oxy)octanoic acid 1a: [
a
]
ꢀ1.0° (c 0.25, CHCl3);
D
1H NMR (600 MHz, CDCl3): d 5.24–5.16 (m, 2H, 2CH-O), 2.66–2.50 (m, 4H,
a-
Acknowledgments
CH2, a0-CH2), 2.02 (s, 3H, COCH3), 1.68–1.52 (m, 4H, 2CH2), 1.38–1.19 (m, 12H,
6CH2), 0.87 (t, 6H, 2CH3); 13C NMR (150 MHz, CDCl3): d 175.6, 170.6, 169.9,
70.8, 70.7, 39.4, 38.4, 34.0, 33.9, 31.7, 31.6, 24.9, 22.6, 21.2, 14.1; HRMS (ESI+)
m/z calcd for C18H32O6Na [M+Na]+ 367.2097, found 367.2079.
Financial support from the CNRS (Centre National de la Recher-
che Scientifique), the Ministry of Higher Education and Research
(MESR: Ministère de l’Enseignement Supérieur et de la Recherche)
are gratefully acknowledged.
(R)-3-(((R)-3-acetoxyoctanoyl)oxy)nonanoic acid 1b: [
a]
20 +1.1° (c 0.9, CHCl3);
D
1H NMR (500 MHz, CDCl3): d 5.75 (br s, 1H, OH), 5.26–5.15 (m, 2H, 2CH-O),
2.66–2.49 (m, 4H, a
-CH2, a0-CH2), 2.02 (s, 3H, COCH3), 1.69–1.50 (m, 4H, 2CH2),
1.38–1.18 (m, 14H, 7CH2), 0.87 (t, 6H, 2CH3); 13C NMR (125 MHz, CDCl3): d
175.3, 170.6, 169.9, 70.9, 70.7, 39.4, 38.8, 34.0, 31.8, 31.7, 29.1, 25.2, 24.9, 22.7,
22.6, 21.2, 14.2, 14.1; HRMS (ESI+) m/z calcd for C19H34O6Na [M+Na]+ 381.2253,
References and notes
found 381.2249.
20
1. Abdel-Mawgoud, A. M.; Lépine, F.; Déziel, E. Appl. Microbiol. Biotechnol. 2010,
86, 1323–1336. and references cited therein.
(R)-3-(((R)-3-acetoxyoctanoyl)oxy)decanoic acid 1c:
[
a
]
ꢀ1.4° (c 0.85,
D
CHCl3); 1H NMR (600 MHz, CDCl3): d 7.38 (br s, 1H, OH), 5.25–5.15 (m, 2H,
2. (a) Jarvis, F. G.; Johnson, M. J. J. Am. Chem. Soc. 1949, 71, 4124–4126; (b) Hauser,
G.; Karnovsky, M. L. J. Bacteriol. 1954, 68, 645–655; (c) Edwards, J. R.; Hayashi, J.
A. Arch. Biochem. Biophys. 1965, 111, 415–421; (d) Johnson, M. K.; Boese-
Marazzo, D. Infect. Immunol. 1980, 29, 1028–1033; (e) Hirayama, T.; Kato, I.
FEBS Lett. 1982, 139, 81–85; (f) Syldatk, C.; Lang, S.; Wagner, F.; Wray, V.; Witte,
L. Z. Naturforsch., Teil C 1985, 40, 51–60.
3. (a) Häussler, S.; Nimtz, M.; Domke, T.; Wray, V.; Steinmetz, I. Infect. Immunol.
1998, 66, 1588–1593; (b) Andrä, J.; Rademann, J.; Howe, J.; Koch, M. H. J.;
Heine, H.; Zähringer, U.; Brandenburg, K. Biol. Chem. 2006, 387, 301–310.
4. (a) Zhang, Y.; Miller, R. M. Appl. Environ. Microbiol. 1992, 58, 3276–3282; (b)
Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Eng. Geol. 2001, 60, 193–207.
5. (a) Desai, J. D.; Banat, I. M. Microbiol. Mol. Biol. Rev. 1997, 61, 47–64; (b) Ochoa-
Loza, F. J.; Artiola, J. F.; Maier, R. M. J. Environ. Qual. 2001, 30, 479–485.
6. (a) Goran, P.; Visnja, P. Chem. Abstr. 1996, 124, 106658; (b) Schwichtenberg, L.
Doctoral Thesis, Christian-Albrechts University Kiel, 2003, http://e-diss.uni-
kiel.de/diss 809/.; (c) Bauer, J.; Brandenburg, K.; Zähringer, U.; Rademann, J.
Chem. Eur. J. 2006, 12, 7116–7124.
2CH-O), 2.66–2.50 (m, 4H, a
-CH2, a0-CH2), 2.02 (s, 3H, COCH3), 1.68–1.50 (m,
4H, 2CH2), 1.38–1.20 (m, 16H, 8CH2), 0.87 (t, 6H, 2CH3); 13C NMR (150 MHz,
CDCl3): d 175.9, 170.8, 170.0, 70.9, 70.8, 39.4, 38.8, 34.0, 31.9, 31.7, 29.4, 29.3,
25.2, 24.9, 22.7, 22.6, 21.2, 14.2, 14.1; HRMS (ESI+) m/z calcd for C20H36O6Na
[M+Na]+ 395.2410, found 395.2404.
20
(R)-3-(((R)-3-acetoxyoctanoyl)oxy)undecanoic acid 1d:
[
a
]
+2.2° (c 1.25,
D
CHCl3); 1H NMR (600 MHz, CDCl3): d 8.0 (br s,1H, OH), 5.25–5.16 (m, 2H, 2CH-
O), 2.66–2.50 (m, 4H,
a
-CH2, a0-CH2), 2.02 (s, 3H, COCH3), 1.68–1.51 (m, 4H,
2CH2), 1.38–1.19 (m, 18H, 9CH2), 0.87 (t, 6H, 2CH3); 13C NMR (150 MHz,
CDCl3): d 175.9, 170.7, 169.9, 70.9, 70.7, 39.4, 38.8, 34.0, 31.9, 31.7, 29.6, 29.5,
29.3, 25.2, 24.8, 22.8, 22.6, 21.2, 14.2, 14.1; HRMS (ESI+) m/z calcd for
C
21H38O6Na [M+Na]+ 409.2566, found 409.2570.
20
(R)-3-(((R)-3-acetoxyoctanoyl)oxy)dodecanoic acid 1e:
[
a
]
ꢀ1.6° (c 0.65,
D
CHCl3); 1H NMR (600 MHz, CDCl3): d 5.25–5.16 (m, 2H, 2CH-O), 2.66–2.49 (m,
4H, a
-CH2, a0-CH2), 2.02 (s, 3H, COCH3), 1.68–1.50 (m, 4H, 2CH2), 1.38–1.19 (m,
20H, 10CH2), 0.87 (t, 6H, 2CH3); 13C NMR (150 MHz, CDCl3): d 175.4, 170.6,
169.9, 70.9, 70.7, 39.4, 38.8, 34.0, 32.0, 31.7, 29.7, 29.6, 29.5, 29.4, 25.2, 24.9,
22.8, 22.6, 21.2, 14.2, 14.1; HRMS (ESI+) m/z calcd for C22H40O6Na [M+Na]+
423.2712, found 423.2723.
7. Leisinger, T.; Margraff, R. Microbiol. Rev. 1979, 43, 422–442.
8. (a) Varnier, A. L.; Sanchez, L.; Vatsa, P.; Boudesocque, L.; Garcia-Brugger, A.;
Rabeoelina, F.; Sorokin, A.; Renault, J.-H.; Kauffmann, S.; Pugin, A.; Clément, C.;