A biomimetic approach to the synthesis of a mycolic acid motif

Article abstract of DOI:10.1016/j.tetlet.2009.12.105

A new method for the stereoselective synthesis of the (R,R)-β-hydroxy-α-alkyl fatty acid fragment of mycolic acids, via an asymmetric anti-aldol reaction is reported. The 'mycolic acid motif' fragment was prepared in three steps and >98% ee.

Full text of DOI:10.1016/j.tetlet.2009.12.105

Tetrahedron Letters 51 (2010) 1185–1186
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Tetrahedron Letters
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A biomimetic approach to the synthesis of a mycolic acid motif
Cathryn H. S. Driver a, Mohammed O. Balogun b, Gianna Toschi a, Jan A. Verschoor b, Mark S. Baird c,
Lynne A. Pilcher a,
*
a Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa
b Department of Biochemistry, University of Pretoria, Pretoria 0002, South Africa
c Department of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A new method for the stereoselective synthesis of the (R,R)-b-hydroxy-a-alkyl fatty acid fragment of
mycolic acids, via an asymmetric anti-aldol reaction is reported. The ‘mycolic acid motif’ fragment was
prepared in three steps and >98% ee.
Received 17 November 2009
Revised 4 December 2009
Accepted 16 December 2009
Available online 24 December 2009
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Tuberculosis
Mycolic acid
Mycolic acid motif
Asymmetric anti-aldol reaction
Mycolic acids 1 (MAs) are located in the protective wax coat of
the cell wall of Mycobacterium tuberculosis and other mycobacte-
ria.1 MAs consist of two main parts, a mycolic acid motif with a
22 or 24 carbon
a-alkyl chain and a b-hydroxy group, and a
mero-mycolate chain with variable functional groups.2 The myco-
lic acid motif, common to MAs from all mycobacteria, usually has
an R,R configuration.3 These complex waxes are recognized by anti-
bodies4 and therefore show potential for use in TB therapy and
new diagnostic techniques such as biosensor assays.5 Furthermore,
they have been shown to re-programme the macrophages to pro-
mote a telerogenic response in experimental asthma and are being
explored for their potential in the prevention and treatment of
asthma.6
Natural mycobacterial MAs are present as complex mixtures
containing different functionalities X and Y, and a range of homo-
logues with different chain lengths.1,2 To fully understand the bio-
logical role of MAs there is a need for the synthesis of individual
acids with known stereochemistry. In the biosynthesis of mycolic
Scheme 1. Mycolic acid structure1,2 and retrosynthetic analysis.
method developed by Kurosu and Lorca,10 to couple saturated car-
boxylic acids with short chain aldehydes for the synthesis of 2,
containing a terminal alkene. This can be used to extend the a-al-
kyl chain in the mycolic acid motif to its full length of 22 or 24 car-
bons as described by Toschi and Baird,9 and can be extended to the
full MA by reaction at the THPO-group.11
The norephedrine-based auxiliary 6 was attached to both the
required unsaturated acid 5a and a saturated acid 5b to give chiral
esters 3a and 3b (Scheme 2). The corresponding zirconium eno-
lates were prepared by treatment with LDA and transmetalation
with a zirconium complex (Cp2ZrCl2). Coupling of these enolates
to a variety of aldehydes 4a–d showed that chain length and func-
tionality in the aldehyde had little effect on the outcome of the
reaction, but the introduction of the terminal alkene in the enolate
led to lower yields (30–45% for 3a with 4a–d vs 50% for 3b with 4b/
d). Using freshly prepared LDA, strictly anhydrous conditions, a
constant temperature of À78 °C (monitored by an internal probe)
and dropwise addition of the reagents, the anti-aldol product 7a
was obtained from 3a and 4a on a 1 g scale in 45% yield with high
acids, the
a and b carbons are joined in a Claisen-type condensa-
tion to give a b-keto product which is selectively reduced.7 In pub-
lished syntheses of the mycolic acid motif, two stereocentres are
inserted consecutively in three to five steps.8,9 We now report a
more biomimetic approach (Scheme 1) whereby an auxiliary-med-
iated anti-aldol reaction gives the
a-alkyl-b-hydroxy product 2 in
the required R,R configuration in one step. We have adapted the
* Corresponding author. Tel.: +27 12 420 5384; fax: +27 12 420 4687.
E-mail address: lynne.pilcher@up.ac.za (L.A. Pilcher).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.12.105

1186
C. H. S. Driver et al. / Tetrahedron Letters 51 (2010) 1185–1186
Scheme 2. Synthesis of chiral esters 3a and 3b and subsequent anti-aldol reaction with various aldehydes. Reagents and conditions: (i) EDCI, DMAP, CH2Cl2 (3a = 89%,
3b = 91%); (ii) LDA, Cp2ZrCl2, THF, À78 °C; (iii) Na(s), MeOH (70%).
omy of the process. By applying the method directly to the aldol
O
OMTPA
H
reaction of mero-mycolate aldehydes and long chain acids, it is
hoped that more efficient syntheses of a range of homologues of
natural mycolic acids can be achieved.
9
MeO
–0.050
OTHP
+0.045
–0.024
–0.037
Acknowledgements
–0.076
–0.010
The authors thank Mr. E. Palmer for the NMR spectra. Financial
support was provided by the University of Pretoria and the Na-
tional Research Foundation of South Africa.
Figure 1. MTPA ester of compound 2 (DdH values in ppm).
Table 1
Supplementary data
Comparison of the 13C NMR alkene signals (in ppm) of diastereomers 2 with the
literature compound 811
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.12.105.
OH
O
OH
O
THPO
PvO
OMe
OMe
10
10
References and notes
8
2
1. Minnikin, D. E. Lipids: Complex lipids, Their Chemistry, Biosynthesis and Roles.
In The Biology of the Mycobacteria; Ratledge, C., Stanford, J., Eds.; Academic
Press: San Diego, 1982; pp 95–184.
2. Watanabe, M.; Aoyagi, Y.; Mitome, H.; Fujita, T.; Naoki, H.; Ridell, M.; Minnikin,
D. E. Microbiology 2002, 148, 1881–1902.
Carbon
2
epi-2
anti-8
134.9
117.1
135.6
116.6
134.9
117.1
HC@CH2
HC@CH2
3. Minnikin, D. E.; Polgar, N. Chem. Commun. 1966, 648–649.
4. Schleicher, G. K.; Feldman, C.; Vermaak, Y.; Verschoor, J. A. Clin. Chem. Lab. Med.
2002, 40, 882–887.
5. Thanyani, S. T.; Roberts, V.; Siko, D. G.; Vrey, P.; Verschoor, J. A. J. Immunol.
Methods 2008, 332, 61–72.
6. Korf, J. E.; Pynaert, G.; Tournoy, K.; Boonefaes, T.; Van Oosterhout, A.;
Ginneberge, D.; Haegeman, A.; Verschoor, J. A.; De Baetselier, P.; Grooten, J.
Am. J. Respir. Crit. Care Med. 2006, 174, 152–160.
diastereoselectivity (>98%) (determined as described below) after
separation from the two syn-diastereomers by flash chromatogra-
phy.12 The chiral auxiliary was cleaved from aldol product 7a by
transesterification with sodium methoxide, rather than reduc-
tively,10 to yield the methyl ester 2 (70%) (Scheme 2). The configu-
ration of the hydroxy group in 2 was determined using Mosher’s
method, by conversion into the (R)- and (S)-MTPA esters.13 The
7. Lee, R. E.; Armour, J. W.; Takayama, K.; Brennan, P. J.; Besra, G. S. Biochim.
Biophys. Acta 1997, 1346, 275–284.
8. Al-Dulayymi, J.; Baird, M. S.; Roberts, E. Tetrahedron 2005, 61, 11939–11951.
9. Toschi, G.; Baird, M. S. Tetrahedron 2006, 62, 3221–3227.
10. Kurosu, M.; Lorca, M. J. Org. Chem. 2001, 66, 1205–1209.
11. Al-Dulayymi, J. R.; Baird, M. S.; Roberts, E.; Deysel, M.; Verschoor, J. A.
Tetrahedron 2007, 63, 2571–2592.
D
dH values for the protons in the left and right segments (Fig. 1),
respectively, indicated an R configuration.
12. Ester 3a (1.1 g, 2.3 mmol) was added to a solution of freshly prepared LDA
(16 mL, 1.0 M) and Cp2ZrCl2 (0.7 mmol) in THF (8 mL) at À78 °C under
nitrogen. After 2 h, Cp2ZrCl2 (5.8 mmol) in THF (5 mL) was added and 0.5 h
later the aldehyde 4a (670 mg, 2.6 mmol) in THF (3.5 mL) was added. The
mixture was stirred at À78 °C for 2 h, then the reaction was quenched with 1 M
On standing in CDCl3 at room temperature for a week, the solu-
tion of 2 partly epimerized at the alpha position giving a mixture of
anti- and syn-diastereomers. A comparison of the 13C NMR data for
the two diastereomers with those published for mycolic acid motif
811 demonstrated that the relative stereochemistry of 2 was anti
(Table 1).
This synthesis of the mycolic acid motif methyl ester 2 repre-
sents a novel approach to a key intermediate that has been used
to prepare mycolic acids.11 The use of anti-aldol methodology re-
duces the number of steps required from six,11 to two. Prior to
the removal of the auxiliary, the diastereomeric products are read-
ily separated giving the desired R,R-product in >98% ee. The chiral
auxiliary can be recovered and recycled, increasing the atom econ-
HCl. Flash chromatography gave product 7a in 45% yield. m
max/cmÀ1: 3509,
2924, 1722; dH (500 MHz, CDCl3): 7.22–7.06 (7H, m), 6.90 (2H, d, J = 7.0 Hz),
6.74 (2H, s), 6.01 (1H, d, J = 8.0 Hz), 5.60–5.40 (1H, m), 4.91–4.70 (2H, m), 4.60
(1H, s), 3.90–3.80 (1H, m), 3.75–3.60 (4H, m), 3.54–3.48 (3H, m), 3.44–3.38
(1H, m), 3.37–3.30 (1H, m), 2.65–2.50 (2H, m), 2.49–2.44 (1H, m), 2.26 (3H, s),
2.08 (3H, s), 2.03 (6H, s), 1.80–1.30 (27H, m); dC (75.4 MHz, CDCl3): 174.2,
139.2, 138.5, 137.4, 136.7, 136.2, 134.4, 131.3, 130.9, 130.0, 128.9, 127.8, 126.9,
125.3, 117.2, 98.8, 77.2, 71.5, 67.7, 62.3, 55.5, 50.9, 50.4, 47.3, 35.5, 33.6, 30.8,
29.6, 29.5, 29.3, 26.2, 25.8, 25.5, 22.7, 20.8, 20.0, 19.7, 19.2, 14.1, 9.8; HRMS
(+ESI) calcd for C48H70NO5 [M+H]+ = 740.5254, found: 740.5248.
13. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113,
4092–4096.