Catalytic asymmetric synthesis of mycolipenic and mycolipanolic acid

Article abstract of DOI:10.1002/ejoc.200901120

The first asymmetric synthesis of mycolipenic acid and mycolipanolic acid by using an improved iterative procedure involving catalytic asymmetric conjugate addition of MeMgBr as the key step is described. Mycolipenic and mycolipanolic acid are obtained in 11 steps with perfect stereocontrol, and both acids are identical to their counterparts from natural sources.

Full text of DOI:10.1002/ejoc.200901120

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200901120
Catalytic Asymmetric Synthesis of Mycolipenic and Mycolipanolic Acid
Bjorn ter Horst,[a] Jeroen van Wermeskerken,[a] Ben L. Feringa,*[a] and
Adriaan J. Minnaard*[a]
Keywords: Mycolipenic acid / Mycolipanolic acid / Asymmetric synthesis / Mycobacterium tuberculosis
The first asymmetric synthesis of mycolipenic acid and myco- acid are obtained in 11 steps with perfect stereocontrol, and
lipanolic acid by using an improved iterative procedure in-
volving catalytic asymmetric conjugate addition of MeMgBr
as the key step is described. Mycolipenic and mycolipanolic
both acids are identical to their counterparts from natural
sources.
Introduction
Mycobacterium tuberculosis (M. tuberculosis), the bacte-
rium responsible for the disease tuberculosis is still one of
the most lethal pathogens causing an annual death rate of
nearly two million people worldwide.[1] The cell wall of the
bacterium contains very complex lipids exclusively found in
pathogenic mycobacteria.[2] Pentaacyltrehalose (PAT, 1)
and diacyltrehalose (DAT, 2) are examples of cell-wall lipids
that are only found in virulent strains of M. tuberculosis
(Figure 1).[3b] Besides the fact that these lipids can poten-
tially be used as biomarkers, they are also known to elicit
an immune response in human, which makes them candi-
dates for vaccination studies.[3–6]
PAT (1) and DAT (2) are two trehalose-based glycolipids
esterified with mycolipenic (3) and mycolipanolic acid (4),
respectively (Figure 2). Mycolipenic acid, the major acyl
residue found in PAT and some forms of DAT, has shown
to be a potent inhibitor of leukocyte migration (pointing at
an immune response), in vitro.[3b,7] It has also been shown
that 3 acts as a B-cell antigen.[4,8,12]
Mycolipenic acid (phthienoic acid, 3) and mycolipanolic
acid (4) are both considered to be produced by polyketide
synthase Pks2 (polyketide synthase 2), a member of the
same enzyme class involved in the synthesis of phthiocer-
anic and hydroxyphthioceranic acid.[3a,10] This accounts for
the stereochemical relationship of the methyl substituents,
which is all-syn and of the (S) configuration at the ste-
reocenters, identical to phthioceranic acid and hydroxy-
phthioceranic acid.[11] In addition, the biosynthetic pathway
of mycolipenic and mycolipanolic acid are closely related,
as both are most likely derived from the same precursor.
Figure 1. Penta- and diacyltrehalose PAT (1) and DAT (2) from M.
tuberculosis.
Figure 2. Methyl-branched acids mycolipenic (3) and mycolipan-
olic acid (4).
The dehydratase unit of Pks2 can eliminate water from my-
colipanolic acid resulting in α,β-unsaturated mycolipenic
acid.
Mycolipenic acid (3) was first isolated and characterized
in the 1950s by Polgar.[12,13] The compound was synthesized
in 1958 by a lengthy route, comprising a kinetic resolution,
[a] Stratingh Institute for Chemistry, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Fax: +31-50-363-4278
E-mail: a.j.minnaard@rug.nl
b.l.feringa@rug.nl
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901120.
38
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 38–41

Asymmetric Synthesis of Mycolipenic and Mycolipanolic Acid
which confirmed the stereochemistry to be all-(S) and the
In our previous reported reduction/olefination sequence,
double bond to have (E) stereochemistry.[13] Mycolipenic a Fukuyama reduction with Pd/C and Et3SiH was applied,
acid has been synthesized as a racemate by Minnikin et al. followed by Wittig olefination. This typically resulted in a
in 1992.[9]
yield of 70% over two steps.[11] It turned out, however, that
Mycolipanolic acid (4) was isolated and characterized by reduction of thioester 6 with DIBAL-H followed by ole-
degradation studies in 1968 by Polgar.[14] A racemic total fination with Horner–Wadsworth–Emmons (HWE) reagent
synthesis of mycolipanolic acid (as a mixture of dia- (EtO)2P(O)CH2COSEt, afforded unsaturated thioester 7 in
stereomers) has been reported by Minnikin in 1996.[15]
80% yield over two steps (typically 80–90%). This improved
Because only pathogenic M. tuberculosis contains 1 and sequence makes the methodology, as the steps are iterative,
2, isolation of these lipids and acids from M. tuberculosis is considerably more efficient.
undesirable. Besides the danger of working with virulent M.
The second asymmetric 1,4-addition reaction was per-
tuberculosis, the extremely slow growth of the bacteria (cell formed under the same conditions as the first addition; syn
division every 22–24 h[16] compared to 20 min for E. coli) product 8 was obtained in 90% yield with an excellent
makes it difficult to obtain sufficient quantities for bio- syn/anti ratio of 97:3. Thioester 8 was subsequently reduced
logical studies. Moreover, most of the glycolipids occur as to alcohol 9 with DIBAL-H in 89%. Tosylation of alcohol
mixtures of analogues, which differ in the number of acyl 9 to give 10 (85%), followed by a copper-catalyzed Grig-
chains on trehalose, the number of methyl substituents and nard cross coupling with C16H33MgBr resulted in silyl ether
the chain length. To study the biological activity of PAT 11 (95%). Subsequent deprotection with tetrabutylammo-
(1) and DAT (2) it is important to obtain pure and fully nium fluoride (TBAF) in THF resulted in alcohol 12 in
characterized material, and therefore a synthetic approach 87% yield. Alcohol 12 was oxidized to aldehyde 13 under
is highly desirable. Nevertheless, as these lipids contain mul- neutral conditions with N-methylmorpholine oxide (NMO)
tiple stereocenters, their efficient and enantioselective prep- and catalytic tetrapropylammonium perruthenate (TPAP)
aration is far from trivial. As part of our ongoing research in 90% yield.[19] No epimerization of the α-methyl
on the cell-wall constituents of M. tuberculosis[11,17] com- stereocenter was observed during this transformation. Alde-
prising the synthesis of PAT (1) and DAT (2), we therefore hyde 13 was treated with Wittig reagent 14, which resulted
decided to embark on the enantioselective synthesis of my- in the corresponding olefin with an (E)/(Z) ratio of 9:1; the
colipenic (3) and mycolipanolic acid (4).
desired (E) isomer 15 was isolated in 65% yield.[20] This
reagent has been used previously in the synthesis of poly-
methylated fatty acids.[21] In the final step, ethyl ester 15
was hydrolyzed in a water/THF mixture with LiOH, and
Results and Discussion
We started the synthesis of mycolipenic acid with a care- mycolipenic acid (3) was obtained in 85% yield (Scheme 1).
ful optimization of our iterative copper-catalyzed asymmet- The optical rotation of the synthetic material {[α] = +16.4
ric 1,4-addition protocol we reported previously.[11,18] Start- (c = 1.96, CHCl3)} matches well with that reported in litera-
ing from 5 and using 1 mol-% of (R,SFe)-Josiphos/CuBr as ture for the natural product isolated from M. tuberculosis
the catalyst in combination with 1.2 equiv. of MeMgBr, we ([α] = +19).[13] The methyl ester of mycolipenic acid was
obtained compound 6 as the (S) enantiomer in excellent subsequently prepared by treatment of 3 with (trimethyl-
yield (95%) and enantiomeric excess (ee = 98%).
silyl)diazomethane in methanol, and its optical rotation {[α]
Scheme 1. Total synthesis of mycolipenic acid (3).
Eur. J. Org. Chem. 2010, 38–41
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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39

B. ter Horst, J. van Wermeskerken, B. L. Feringa, A. J. Minnaard
SHORT COMMUNICATION
= +14.5 (c = 0.47, CHCl3)} corresponds well with the val- available for the synthesis of PAT (1) and DAT (2), which
ues of [α] = +14.7 and +16.8 found in the literature for the is currently under investigation in our laboratories.
methyl ester.[13,22]
Supporting Information (see footnote on the first page of this arti-
For the synthesis of mycolipanolic acid we envisioned an
Evans aldol reaction by using the boron enolate of enantio-
pure oxazolidone 16 with aldehyde 13 for the introduction
of the syn-hydroxymethyl unit.[23,24] To prevent over-oxi-
dation or epimerization at the α-stereocenter, aldehyde 13
was prepared freshly for the Evans aldol reaction
(Scheme 2). After the aldol reaction with 16 and workup,
17 was isolated in a moderate 45% yield with perfect ste-
reocontrol.
1
cle): Detailed experimental procedures, H and 13C NMR spectro-
scopic and analytical data of all compounds in Schemes 1 and 2.
Acknowledgments
We thank T. D. Tiemersma-Wegman (GC) and A. Kiewiet (MS)
(Stratingh Institute for Chemistry, University of Groningen) for
technical support and The Netherlands Organization for Scientific
Research (NWO-CW) for financial support.
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Scheme 2. Evans aldol reaction for the introduction of the syn-
vicinal hydroxymethyl motif in mycolipanolic acid (4).
Although a very small amount of a diastereomer of 17
was observed in the 13C NMR spectrum (see Supporting
[7] H. Husseini, S. Elberg, Am. Rev. Tuberc. 1952, 65, 655–672.
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ready present in 13 as a result of the two enantioselective
[9] G. S. Besra, R. C. Bolton, M. R. McNeil, M. Ridell, K. E.
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nikin, Biochemistry 1992, 31, 9832–9837.
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nately, we lost material during chromatography, and only
16% of pure 4 was isolated. Later it was found that purifi-
cation on silica with pentane/diethyl ether (5:1) containing
1% acetic acid gave perfect separation, and no problems
were observed during the isolation process. A sample of 4
was converted into the corresponding methyl ester by using
excess (trimethylsilyl)diazomethane in MeOH to compare
the optical rotation to the literature value of the natural
product. The optical rotation of the methyl ester of syn-
thetic 4 {[α] = –7.0 (c = 0.2, CHCl3)} is in perfect agreement
with the literature value of [α] = –7.19.[14]
[10] a) K. Bhatt, S. S. Gurcha, A. Bhatt, G. S. Besra, W. R. Ja-
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[20] The HWE equivalent of Wittig reagent 14 was also investi-
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[21] J. Guiard, A. Collmann, M. Gilleron, L. Mori, G. De Libero,
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Conclusions
Mycolipenic and mycolipanolic acid, two methyl-
branched fatty acids from M. tuberculosis, have been suc-
cessfully synthesized by using an improved iterative proto-
col for asymmetric conjugate addition of MeMgBr. Both
compounds have been prepared for the first time as a single
enantiomer. Mycolipenic acid was obtained with an overall
yield of 5% in 11 steps with an average of 84% yield per
step. The closely related mycolipanolic acid was synthesized
enantioselectively with an overall yield of 2% in 11 steps
with an average yield of 75% per step. Both acids are iden-
tical to their counterparts from natural sources and are now
40
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Eur. J. Org. Chem. 2010, 38–41

Asymmetric Synthesis of Mycolipenic and Mycolipanolic Acid
[22] N. Polgar, J. Chem. Soc. 1954, 1008–1010.
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Received: October 5, 2009
Published Online: November 18, 2009
Eur. J. Org. Chem. 2010, 38–41
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