Catalytic asymmetric synthesis of phthioceranic acid, a heptamethyl-branched acid from Mycobacterium tuberculosis

Article abstract of DOI:10.1021/ol071078o

The first total synthesis of phthioceranic acid (1) has been achieved by an iterative catalytic asymmetric 1,4-addition protocol. This method provides a robust and high-yielding route for the preparation of 1,3-oligomethyl (deoxypropionate) arrays. After the desired number of methyl groups has been introduced, these arrays can be further functionalized at both ends to polymethyl-substituted lipids such as phthioceranic acid, a heptamethyl-branched fatty acid from the virulence factor Sulfolipid-I (2), found in Mycobacterium tuberculosis.

Full text of DOI:10.1021/ol071078o

ORGANIC
LETTERS
2007
Vol. 9, No. 16
3013-3015
Catalytic Asymmetric Synthesis of
Phthioceranic Acid, a
Heptamethyl-Branched Acid from
Mycobacterium tuberculosis
Bjorn ter Horst, Ben L. Feringa,* and Adriaan J. Minnaard*
Department of Organic and Molecular Inorganic Chemistry, Stratingh Institute for
Chemistry, UniVersity of Groningen, Nijenborg 4, 9747AG,
Groningen, The Netherlands
b.l.feringa@rug.nl; a.j.minnaard@rug.nl
Received May 9, 2007
ABSTRACT
The first total synthesis of phthioceranic acid (1) has been achieved by an iterative catalytic asymmetric 1,4-addition protocol. This method
provides a robust and high-yielding route for the preparation of 1,3-oligomethyl (deoxypropionate) arrays. After the desired number of methyl
groups has been introduced, these arrays can be further functionalized at both ends to polymethyl-substituted lipids such as phthioceranic
acid, a heptamethyl-branched fatty acid from the virulence factor Sulfolipid-I (2), found in Mycobacterium tuberculosis.
Mycobacterium tuberculosis is still one of the most lethal
infectious diseases in the world, with approximately three
million casualties annually.1 The rising number of human
immunodeficiency virus (HIV) infected people is a major
risk factor for the development of tuberculosis. Although
efforts are made to control tuberculosis, mutations within
the bacterium and irresponsible use of antibiotics are blocking
a final stop to the disease.1,2 New drugs are currently under
investigation.1,2
known to induce an immune response in the human body
and are therefore potential candidates for vaccines or
adjuvants.4 A compound class that is found in the cell wall
of the bacterium comprises the Sulfolipids.4,5 Sulfolipids are
acyl-substituted trehaloses containing a sulfate functionality.
Sulfolipid-I (SL-I) (2, Figure 1) is of particular interest
because it is known to induce an immune response.5,6 SL-I
(2,3,6,6′-tetraacyltrehalose 2′-sulfate, 2) and its acyl groups
were characterized by Goren decades ago7,8 and shown to
One of the reasons that M. tuberculosis is so hard to
destroy is the unusual thick cell wall of the bacteria.3 The
cell wall of M. tuberculosis consists partly of long chain
lipids which create a hydrophobic barrier for antibiotics and
other molecules. Components of that cell wall however are
(4) (a) Mycobacterial lipids: Chemistry and Biologic Activities. Tuber-
culosis; Youmans, G. P., Ed.; W.B. Saunders Company: Philadelphia,
London, Toronto, 1979; Chapter 4. (b) Lipids: Complex Lipids, Their
Chemistry, Biosynthesis and Roles. The Biology of the Mycobacteria:
Physiology, Identification and Classification; Ratledge, C., Stanford, J., Eds.;
Academic Press Inc: London, 1982; Chapter 4, Vol. 1. (c) Microbial Lipids;
Ratledge, C., Wilkinson, S. G., Eds.; Academic Press, London, 1988; Vol.
1, pp 248-250.
(5) (a) Gilleron, M.; Stenger, S.; Mazorra, Z.; Wittke, F.; Mariotti, S.;
Bo¨hmer, G.; Prandi, J.; Mori, L.; Puzo, G.; Gennaro De Libero, G. J. Exp.
Med. 2004, 199, 649-659. (b) De Libero, G.; Mori, L. Nat. ReV. Immunol.
2005, 5, 485-496. (c) Daffe´, M.; Papa, F.; Laszlo, A.; David, H. L. J.
Gen. Microbiol. 1989, 135, 2759-2766.
(1) Tripathi, R. P.; Tewari, N.; Dwivedi, N.; Tiwari, V. K. Med. Res.
ReV. 2005, 25, 93-131.
(2) (a) Pasqualoto, K. F. M.; Ferreira, E. I. Curr. Drug Targets 2001, 2,
427-437. (b) Zhang, Y. Ann. ReV. Pharmacol. Toxicol. 2005, 45, 529-
564.
(3) Minnikin, D. E.; Kremer, L.; Dover, L. G.; Besra, G. S. Chem. Biol.
2002, 9, 545-553.
10.1021/ol071078o CCC: $37.00
© 2007 American Chemical Society
Published on Web 07/14/2007

acids.6d,g The hydroxy group in hydroxyphthioceranic acid
is assumed to be D (R), but no direct evidence exists to this
date.
There is a broad interest in the synthesis of 1,3-polymethyl
arrays because of their presence in many natural products
such as fatty acids and lipids, antibiotics, and marine natural
products.9-11 Most of the current synthetic approaches are
based on iterative chiral auxiliary strategies. For compounds
containing multiple methyl groups, such as 1, these ap-
proaches are however laborious and less practical. An
efficient catalytic asymmetric approach is therefore highly
warranted.
Recently, we showed that the asymmetric conjugate
addition of MeMgBr catalyzed by a [4‚CuBr] complex
comprises a very powerful iterative method for the prepara-
tion of deoxypropionates.14,16,17 On the basis of this method,
we prepared mycoceranic acid, a compound related to 1 but
of opposite stereochemistry and containing an array of four
methyl groups.14,15
An alternative method based on an asymmetric zirconium-
catalyzed carboalumination, palladium-catalyzed cross-
coupling has been reported by Negishi and co-workers. This
method was used in the synthesis of a four-methyl-branched
fatty acid from the graylag goose.12 Very recenty, Burgess
and co-workers reported an asymmetric hydrogenation ap-
proach to deoxypropionates starting with building blocks
based on the Roche ester.13
To put the robustness and efficiency of our iterative
catalytic conjugate addition methodology to the test, and as
a prelude to the enantioselective total synthesis of Sulfolipid-
I, we decided to embark on the first synthesis of phthioce-
ranic acid, a heptamethyl-substituted fatty acid from M.
tuberculosis.
Figure 1. Sulfolipid-I (SL-I).
contain one palmitic acid, one phthioceranic acid, and two
hydroxyphthioceranic acid residues.
Phthioceranic (1) and hydroxyphthioceranic acid are hepta-
and octamethyl-branched dextrorotatory deoxypropionates,
respectively, containing a palmitoyl chain and a hydroxy
functionality at C17 for the latter. There is strong evidence
that the stereochemistry at the methyl branches is all-L,4a,7
e.g., all-S for phthioceranic acid and 2S,4S,6S,8S,10R,12R,-
14R,16R for hydroxyphthioceranic acid. The absolute ster-
eochemistry of tetramethyl-substituted laevorotatory myco-
cerosic acid has been determined as all-D (R) by degradation
studies.4a Phthioceranic acids were compared to mycocerosic
acid and all-S dextrorotatory mycolipenic acids (2,4,6-
trimethyl-tetracos-2-enoic acid).4a More recent studies showed
that mycocerosic acid is produced by the enzyme myco-
cerosic acid synthase (MAS), whereas phthioceranic and
hydroxyphthioceranic acid are produced by polyketide syn-
thase, Pks2.3,6g This accounts for the opposite stereochemistry
observed. The biosynthesis of the mycolipenic acids is
genetically closely related to that of the phthioceranic
The synthesis of 1 starts with 314 that was submitted to
an enantioselective 1,4-addition with MeMgBr, catalyzed by
1 mol % of 4‚CuBr in t-BuOMe at -78 °C (Scheme 1).
The corresponding thioester 5 was isolated in an excellent
(9) (a) Williams, D. R.; Nold, A. L.; Mullins, R. J. J. Org. Chem. 2004,
69, 5374-5382. (b) Cooksey, J. P.; Kocienski, P. J.; Li, Y.; Schunk, S.;
Snaddon, T. N. Org. Biomol. Chem. 2006, 4, 3325-3336. (c) Evans, D.
A.; Dow, R. L.; Shih, T. L.; Takacs, J. M.; Zahler, R. J. Am. Chem. Soc.
1990, 112, 5290-5313. (d) Duffey, M. O.; LeTiran, A.; Morken, J. P. J.
Am. Chem. Soc. 2003, 125, 1458-1459. (e) Nagamitsu, T.; Takano, D.;
Marumoto, K.; Fukuda, T.; Furuya, K.; Otoguro, K.; Takeda, K.; Kuwajima,
I.; Harigaya, Y.; Omura, S. J. Org. Chem. 2007, 72, 2744-2756.
(10) For a recent review on deoxypropionate units, see: Hanessian, S.;
Giroux, S.; Mascitti, V. Synthesis 2006, 7, 1057-1076.
(11) Review: Modern aldol methods for the total synthesis of polyketides.
Schetter, B.; Mahrwald, R. Angew. Chem. Int. Ed. 2006, 45, 7506-7525.
(12) (a) Negishi, E.; Tan, Z.; Liang, B.; Novak, T. Proc. Natl. Acad.
Sci. 2004, 101, 5782-5787. (b) Novak, T.; Tan, Z.; Liang, B.; Negishi, E.
J. Am. Chem. Soc. 2005, 127, 2838-2839. (c) Liang, B.; Novak, T.; Tan,
Z.; Negishi, E. J. Am. Chem. Soc. 2006, 128, 2770-2771.
(13) (a) Zhou, J.; Burgess, K. Angew. Chem., Int. Ed. 2007, 46, 1129-
1131. (b) Zhou, J.; Zhu, Y.; Burgess, K. Org. Lett. 2007, 9, 1391-1393.
(14) ter Horst, B.; Feringa, B. L.; Minnaard, A. J. Chem. Commun. 2007,
489-491.
(15) Cox, J. S.; Chen, B.; McNeil, M.; Jacobs, W. R., Jr. Nature 1999,
402, 79-83.
(16) (a) Schuppan, J.; Minnaard, A. J.; Feringa, B. L. Chem. Commun.
2004, 792-793. (b) Feringa, B. L.; Badorrey, R.; Pen˜a, D.; Harutyunyan,
S. R.; Minnaard, A. J. Proc. Natl. Acad. Sci. 2004, 16, 5834-5838.
(17) (a) Des Mazery, R.; Pullez, M.; Lo´pez, F.; Harutyunyan, S. R.;
Minnaard, A. J.; Feringa, B. L. J. Am. Chem. Soc. 2005, 127, 9966-9967.
(b) Lo´pez, F.; Minnaard, A. J.; Feringa, B. L. Acc. Chem. Res. 2007, 40,
179-188.
(6) For the biosynthesis of SL-I, see the following references: (a) Schelle,
M. W.; Bertozzi, C. R. ChemBioChem 2006, 7, 1516-1524. (b) Fernandes,
N. D.; Kolattukudy, P. E. J. Biol. Chem. 1998, 273, 2823-2828. (c)
Converse, S. E.; Mougous, J. D.; Leavell, M. D.; Leary, J. A.; Bertozzi, C.
R.; Cox, J. S. Proc. Natl. Acad. Sci. 2003, 100, 6121-6126. (d) Bhatt, K.;
Gurcha, S. S.; Bhatt, A.; Besra, G. S.; Jacobs, W. R., Jr. Microbiology
2007, 153, 513-520. (e) Mougous, J. D.; Petzold, G. J.; Senaratne, R. H.;
Lee, D. H.; Akey, D. L.; Lin, F. L.; Munchel, S. E.; Pratt, M. R.; Riley, L.
W.; Leary, J. A.; Berger, J. M.; Bertozzi, C. R. Nat. Struct. Mol. Biol. 2004,
11, 721-729. (f) Sirakova, T. D.; Thirumala, A. K.; Dubey, V. S.; Sprecher,
H.; Kolattukudy, P. E. J. Biol. Chem. 2001, 276, 16833-16839. (g) Dubey,
V. S.; Sirakova, T. D.; Kolattukudy, P. E. Mol. Microbiol. 2002, 45, 1451-
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(7) (a) Goren, M. B. Biochim. Biophys. Acta 1970, 210, 116-126. (b)
Goren, M. B. Biochim. Biophys. Acta 1970, 210, 127-138. (c) Goren, M.
B.; Brokl, O.; Das, B. C.; Lederer, E. Biochemistry 1971, 10, 72-81. (d)
Goren, M. B.; Brokl, O.; Roller, P.; Fales, H. M.; Das, B. C. Biochemistry
1976, 15, 2728-2735. (e) D and L symbols are used in the sense defined
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3333-3335.
(8) There is some debate about the position of the acyl groups in SL-I;
see ref 6a.
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Org. Lett., Vol. 9, No. 16, 2007

Scheme 1
95% yield and 98% ee. This conjugate addition reaction was
) +6.2 (c ) 0.55, CHCl3), is in agreement with the value
reported for the isolated compound, which consists of a
mixture of phthioceranic acid homologues ([R]D ) +7.9 (c
) 2.03, CHCl3)).7c The mass spectra of the methyl esters of
both synthetic 1 and the natural product were compared and
are in agreement (M ) 564) (see Supporting Information
for details).
followed by a two-step transformation of 5 into 6 via
reduction of the thioester to its corresponding aldehyde and
subsequent Wittig reaction with Ph3PCHCOSEt (E/Z > 95:
5). By repeating this sequence of 1,4-addition, reduction, and
Wittig olefination, we introduced all seven methyl groups
in a 1,3-syn fashion with excellent stereoselectivity and very
high yield. Thus, heptamethyl-substituted thioester 7 was
synthesized in 19 steps with 8% overall yield starting from
3. The diastereoselectivity of all iterative conjugate addition
reactions was >96%, as calculated from the clearly observed
In summary, we have completed the first synthesis of
phthioceranic acid as a prelude to the synthesis of SL-I. By
applying the iterative 1,4-addition protocol, we were able to
introduce an unprecedented array of seven methyl groups in
a 1,3-syn fashion. Overall, 1 was prepared in 24 steps with
excellent stereoselectivity. With this synthesis, the structure
and stereochemistry of phthioceranic acid has been confirmed
and 1 is available in sufficient quantities for the synthesis
of 2. Currently, the iterative protocol is being explored in
our labs for the synthesis of hydroxyphthioceranic acid.
1
syn/anti isomers by H NMR.18
Thioester 7 was reduced with DIBALH19 to alcohol 8,
which was subsequently converted into tosylate 9 with 2
equiv of TsCl and pyridine. The long aliphatic chain was
introduced via a copper-catalyzed coupling reaction with
C14H29MgBr (3 equiv) and 20 mol % of CuBr‚SMe2. The
resulting silylether 10 was deprotected with TBAF to give
alcohol 11, which was finally oxidized to title compound 1
with catalytic RuCl3‚(H2O)x and NaIO4 in 90% over two
steps. The overall yield of the synthesis is 4% over 24 steps.
Acknowledgment. We thank T. D. Tiemersma-Wegman
(GC) and A. Kiewiet (MS) for technical support (Stratingh
Institute, University of Groningen). Financial support from
The Netherlands Organization for Scientific Research (NWO/
CW) and a generous gift of Josiphos ligand from Solvias,
Basel, are gratefully acknowledged.
1
No minor diastereomers of 1 could be observed by H or
13C NMR, most probably as a result of the chromatography
steps which remove traces of minor diastereoisomers. The
optical rotation of synthetic 1 was +4.60 ([R]D, c ) 1.12,
CHCl3). To compare our material with the available literature
data for the natural product, a sample of 1 was converted
into the corresponding methyl ester. Its optical rotation, [R]D
Supporting Information Available: Complete experi-
mental procedures and spectral data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(18) For full details, see Supporting Information.
(19) The thioester was reduced with DIBALH in two steps via the
aldehyde.
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Org. Lett., Vol. 9, No. 16, 2007
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