The first synthesis of epoxy-mycolic acids

Article abstract of DOI:10.1016/j.tetlet.2010.01.078

We report the synthesis of single enantiomers of two epoxy-mycolic acids containing an α-methyl-trans-alkene.

Full text of DOI:10.1016/j.tetlet.2010.01.078

Tetrahedron Letters 51 (2010) 1698–1701
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The first synthesis of epoxy-mycolic acids
*
Dakhil Z. Al Kremawi, Juma’a R. Al Dulayymi, Mark S. Baird
School of Chemistry, Bangor University, 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:
We report the synthesis of single enantiomers of two epoxy-mycolic acids containing an
alkene.
a-methyl-trans-
Received 1 December 2009
Revised 13 January 2010
Accepted 22 January 2010
Available online 28 January 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Mycolic acids (MAs), 1 (Fig. 1), are the major constituents of the
cell envelope of Mycobacterium tuberculosis and other mycobacte-
ria, some of which are pathogenic to animals and humans.1–3 Their
presence is thought to be linked to the resistance of these organ-
isms to most current antibiotics and other chemotherapeutic
agents.4
mycolates to produce the trans-homologues.13 The relative and
absolute stereochemistry of epoxy-mycolates containing a proxi-
mal cis- or
a-methyl-trans-alkene has been probed by two meth-
ods (Scheme 1).14 Firstly, opening of the epoxide 2 by acetolysis
followed by saponification and oxidative cleavage of both the de-
rived 1,2-diol and the alkene led to three products, including (R)-
ester 3. Secondly, reductive ring-opening of the epoxide 2 followed
by oxidative cleavage of the proximal alkene, saponification and
The two stereocentres at the
a and b-positions relative to the
carboxylic group have both been found to be in the R-configuration
for all the mycolic acids examined, irrespective of the other func-
tional groups. In each case ‘a–d’ represent long alkyl chains, and
generally each Mycobacterium contains a mixture of several homo-
logues. In the common classes of MA, the proximal group is often a
cyclopropane or an alkene and the distal group is a cyclopropane
OH
O
[X] = Distal group
[Y] = Proximal group
[X]
[Y]
α
β
)
OH
(
( )a
( )b
(
a
a
-MA), an
-keto-b-methyl fragment (keto-MA).2,3 In 1981, Daffé et al. re-
ported the identification of a new kind of mycolic acid in Mycobac-
terium fortuitum containing an -methyl epoxy-group at the distal
a-methoxy-b-methyl fragment (methoxy-MA) or an
c
( )d
a
Meromycolate chain
Hydroxy acid
position and a cis-alkene at the proximal position (2, R = H)
(Scheme 1).5 Minnikin et al. described the presence of similar mol-
ecules in M. fortuitum, Mycobacterium farcinogenes and Mycobacte-
rium senegalense; in these cases the major isomers had a cis-alkene
at the proximal position, but there was a minor (around 30% by
Figure 1.
R
NMR spectroscopy) component (2, R = Me) containing an
a-
CH3(CH2)15CH
CH3
(CH2)a CH CH CH (CH2)b CH CH (CH2)21CH3
methyl-trans-alkene at the proximal position; the major isomer
of the latter contained 78 carbons.6–8 Epoxy MAs are also present
in Mycobacterium chitae,8 Mycobacterium giae,8 Mycobacterium
peregrinum,7–9 Mycobacterium porcinum and M. senegalense,10 and
in a mutant strain of M. tuberculosis.11 Epoxy-mycolic acids have
also been detected directly by MALDI-TOF mass spectrometry;
the major isomers containing a trans-alkene at the proximal posi-
tion are reported to be C78 and C80 molecules.12,13 Studies using
Mycobacterium smegmatis have identified the function of the pro-
tein MSMEG0913 in adding the methyl branch adjacent to both
an alkene and a cyclopropane at the proximal position of epoxy-
OH COOH
2
O
(iii) oxidative cleavage
(i) acetolysis
(iv) methylation
(ii) saponification
H
CH3
(CH2)a COOCH3
CH3(CH2)15
CH3OOC
R
COOCH3
4
3
a = 11-15
R
CH3OOC CH (CH2)b CH CH (CH2)21CH3
5
OH COOCH3
R = H
b = 14, 16, 18, 20
R = Me b = 17, 19
* Corresponding author.
E-mail address: m.baird@bangor.ac.uk (M.S. Baird).
Scheme 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.078

D. Z. Al Kremawi et al. / Tetrahedron Letters 51 (2010) 1698–1701
1699
2
O
O
S
O
N
(iii) saponification
(iv) methylation
(i) reduction
(ii) oxidative cleavage
N
N
+
Br(CH2)12
H
OH
15
[α]20
O
N
= +53.5
CH3(CH2)15CH CH2 CH (CH2)a COOMe
R
D
CH3(CH2)15
16
14
(CH2)a+1COOCH3
Ph
R
CH3
OH
6
(i), (ii)
O
CH3
7
H
H
S
Br
O
R
CH3(CH2)15
R
CH3(CH2)15
(CH2)aR'
13
17
(CH2)aR'
R
S
R
8
9
15
CH3
H
O
H
CH3
Scheme 2.
(iii), (iv)
O
N
N
N
O
methylation led to the two acids 6 and 7 (Scheme 2). The latter was
shown to have R,R-stereochemistry by comparison to a model
compound. On this basis the authors assigned all the stereocentres
in the epoxy-mycolic acid as R.14
S
N
13
O
15
[α]18
Ph
= -8.45
D
18
However, it seems clear in fact that the result actually suggests
that the epoxy fragment is R,S,S as in 8 rather than R,R,R as in 9, the
priorities in the epoxide being different from those in the ring-
opened alcohol.
O
S
N
N
O
N
13
= +6.3
N
O
15
We have recently reported the syntheses of an
and of methoxymycolic acids with absolute stereochemistry either
at the cis-cyclo-propane or at
-methyl-b-methoxy fragment.16 We
have also reported the synthesis of meromycolate fragments con-
taining the
-methyl-trans-cyclo-propane unit,17 and of hydroxy-
and ketomycolic acids containing an (R)- -methyl-trans-alkene
unit.18 We now report the synthesis of two stereoisomeric
epoxy-mycolic acids 28 and 29 containing an (R)- -methyl-trans-
a
-mycolic acid15
[α]20
Ph
D
19
a
Scheme 4. Reagents and conditions: (i) LiBSA, THF, ꢀ10 °C (71%); (ii) KO2CN@N-
CO2K, AcOH, THF, MeOH (93%); (iii) 1-phenyl-1H-tetrazole-5-thiol, acetone, K2CO3
(82%); (iv) H2O2, Mo7O24(NH4)6ꢁH2O, IMS, THF (67%).
a
a
a
N
N
alkene at the proximal position; these have the chain lengths re-
ported for one mycolic acid in M. fortuitum,19 and the total carbon
number is consistent with those observed for one component of a
mixture from M. smegmatis by MALDI mass spectrometry.12,13
The trans-epoxide unit was obtained from the acetal 10, itself
O
O
N
O
+
S
O
O
(CH2
)
14
OMe
O
Ph
21
20
derived from D-mannitol. Hydrogenation and cleavage of the acetal
(i)
led to aldehyde 12 which could be converted into alcohol 13 by
standard reactions (Scheme 3). Asymmetric epoxidation led to
the two diastereoisomers 14 and 15.
O
O
O
The aldehyde 14 was chain extended by reaction with the sul-
fone 16 and base, followed by saturation of the derived alkene
using di-imide. The resulting bromide 1720 was then converted
(CH2
)
13
OMe
(ii)-(v)
O
O
O
N
O
(i)
N
N
O
O
15
(CH2
)
S
17
12
[α]2D2
[α]2D2
= +17.6
N
11
10
OH
= +4.7
(ii)
Ph
[α]20D= +12.6
22
O
(iii), (iv)
Scheme 5. Reagents and conditions: (i) LiBSA, ꢀ10 °C (61%); (ii) H2, Pd/C, IMS, THF
(93%); (iii) LiAlH4, THF (91%); (iv) NBS, Ph3P, CH2Cl2 (96%); (v) 1-phenyl-1H-
tetrazole-5-thiol, K2CO3, acetone (90%).
H
15
[α]2D5
15
[α]2D6
= -15.0
12
13
= -11.4
O
(vii), (vi)
(v), (vi)
into the corresponding sulfone 18. In the same manner, diastereo-
isomer 15 was converted into 19 (Scheme 4).
O
O
O
The
a-methyl-trans-alkene unit was obtained by chain exten-
H
sion of the aldehyde 20, prepared as described earlier,18 with the
sulfone 21 and base, followed by saturation of the derived mixture
of alkenes and then conversion into the sulfide 22 by standard
methods (Scheme 5).
15
H
15
15
[α]2D8
[α]20
14
= -63.5
= +53.5
D
Scheme 3. Reagents and conditions: (i) H2, Pd/C, ethanol (92%); (ii) HIO4 (77%); (iii)
Ph3P@CHCOOMe, toluene (65%); (iv) DIBAL, CH2Cl2 (95%); (v) -(+)-diethyl tartrate,
Ti(Oi-Pr)4, t-BuOOH, CH2Cl2, ꢀ20 °C (75%); (vi) PCC, CH2Cl2 [82% (14), 75% (15)];
(vii)
-(ꢀ)-diethyl tartrate, Ti(Oi-Pr)4, t-BuOOH, CH2Cl2, ꢀ20 °C (67%).
The reaction of the sulfone 23, derived by the oxidation of 22,
with aldehyde 2418,21 and base, followed by saturation of the
alkenes produced, led to the acetal 25. This could be transformed
L
D

1700
D. Z. Al Kremawi et al. / Tetrahedron Letters 51 (2010) 1698–1701
into aldehyde 26 by changing the protecting group to acetate fol-
lowed by oxidative cleavage (Scheme 6).
Finally, coupling of 26 and 18 in the presence of base led to the
protected epoxy-mycolic acid 27, using the method described ear-
lier.18 Compound 27 could be deprotected to 28 (Scheme 7). In the
same way 29 was obtained from 19. Compound 28 showed signals
for the two epoxide hydrogens at d 2.73 (1H, dt, J 2.2, 5.4 Hz) and
2.43 (1H, dd, J 2.2, 7.3 Hz) and for the two methyl signals at d 1.0
and 0.94. The carbons of the epoxide appeared at d 63.99 and
59.00 in the 13C NMR spectrum.22 Epoxy-mycolates present in M.
smegmatis are reported to show a double doublet at d 2.43 and a
broad triplet at d 2.72 and methyl signals at d 1.02 and 0.92.23
An earlier paper reports the chemical shifts as d 2.39 and 2.695,
with the signal for the methyl adjacent to the epoxide appearing
at d 0.98 and that adjacent to the alkene at d 0.92; the epoxide car-
bons are reported to appear at d 63.7 and 58.6.6 Compound 29
showed similar spectra to those of 28 but showed differences in
detailed chemical shifts;24 thus the epoxide carbons appeared at
d 63.91 and 57.60 and the methyl group signals corresponding to
those mentioned above occurred at d 0.94 and 0.92.
tBuMe2SiO
O
O
N
N
N
CO2Me
S
O
(CH2)17
O
+
O
N
(CH2)21CH3
20
24
23
α
[
]
D= +12.5
Ph
(i), (ii)
This method produces an epoxy-MA 28 with the same chain
lengths as those reported for one natural example, and molecular
rotations for fragments which are consistent with those reported.
Moreover, by simple variation the method can be adjusted to pro-
vide an epoxy-MA of any necessary chain length and varying abso-
lute stereochemistry.
tBuMe2SiO
O
O
CO2Me
18
(CH2)21CH3
25
20
α
]
[
D= +4.4
Acknowledgement
(iii)-(v)
We thank the Government of Iraq for the award of a grant to
D.K. Al Z.
OAc
O
CO2Me
References and notes
H
18
25
(CH2)21CH3
1. Minnikin, D. E. In The Biology of the Mycobacteria; Ratledge, C., Stanford, J., Eds.;
Academic Press: San Diego, 1982; pp 95–184.
α
[
]
D=+4.9
26
2. Watanabe, M.; Aoyagi, Y.; Ridell, M.; Minnikin, D. E. Microbiology 2001, 147,
1825–1837.
Scheme 6. Reagents and conditions: (i) LiBSA, THF, ꢀ10 °C (89%); (ii) H2, Pd/C, IMS,
THF (95%); (iii) HFꢁpyridine, pyridine, THF (80%); (iv) Ac2O, pyridine, toluene (98%);
(v) HIO4, Et2O (64%).
3. Watanabe, M.; Aoyagi, Y.; Mitome, H.; Fujita, T.; Naoki, H.; Ridell, M.; Minnikin,
D. E. Microbiology 2002, 148, 1881–1902.
4. Heath, R. J.; White, S. W.; Rock, C. O. Appl. Microbiol. Biotechnol. 2002, 58, 695–
703.
5. Daffé, M.; Lanéelle, M. A.; Puzo, G.; Asselineau, C. Tetrahedron Lett. 1981, 22,
4515–4516.
6. Minnikin, D. E.; Minnikin, S. M.; Goodfellow, M. Biochim. Biophys. Acta 1982,
712, 817–822.
N
N
O
(CH)13SO2
7. Minnikin, D. E.; Minnikin, S. M.; O’Donnell, A. G.; Goodfellow, M. J. Gen.
Microbiol. 1984, 2, 243–249.
N
CH3(CH2)15
N
18
8. Minnikin, D. E.; Minnikin, S. M.; Parlett, J. H.; Goodfellow, M.; Magnusson, M.
Arch. Microbiol. 1984, 139, 225–231.
9. Minnikin, D. E.; Minnikin, S. M.; Hutchinson, I. G.; Goodfellow, M.; Grange, G.
M. J. Gen. Microbiol. 1984, 130, 363–367.
10. Luquin, M.; Lanéelle, M. A.; Ausina, V.; Barcelo, G. M.; Belda, F.; Alonso, C.;
Prats, G. Int. J. Syst. Bact. 1991, 41, 390–394.
OAc
O
Ph
(CH2)19
OMe
(CH2)21CH3
OHC
+
26
(i)
11. Dinadayala, P.; Laval, F.; Raynaud, C.; Lemassu, A.; Lanéelle, M.-A.; Lanéelle, G.;
Daffé, M. J. Biol. Chem. 2003, 278, 7310–7319.
12. Laval, F.; Lanéelle, M.-A.; Deon, C.; Monsarrat, B.; Daffé, M. Anal. Chem. 2001,
73, 4537–4544.
O
OAc
O
OMe
(CH2)21CH3
(CH2)19
13. Laval, F.; Haites, R.; Movahedzadeh, F.; Lemassu, A.; Wong, C. Y.; Stoker, N.;
Billman-Jacobe, H.; Daffe, M. J. Biol. Chem. 2008, 283, 1419–1427.
14. Lacave, C.; Lanéelle, M.-A.; Montrozier, H.; Rols, M.-P.; Asselineau, C. Eur. J.
Biochem. 1987, 163, 369–378.
(CH2)12
CH3(CH2)15
27
25
α
[
]
D = -2.0
15. Al Dulayymi, J. R.; Baird, M. S.; Roberts, E. Chem. Commun. 2003, 228–229; Al
Dulayymi, J. R.; Baird, M. S.; Roberts, E. Tetrahedron 2005, 61, 11939–11951.
16. Al Dulayymi, J. R.; Baird, M. S.; Roberts, E.; Deysel, M.; Verschoor, J. Tetrahedron
2007, 63, 2571–2592.
(ii)
OH
O
17. Al-Dulayymi, J. R.; Baird, M. S.; Mohammed, H.; Roberts, E.; Clegg, W.
Tetrahedron 2006, 62, 4851–4862.
18. Koza, G.; Rowles, R.; Theunissen, C.; Al Dulayymi, J. R.; Baird, M. S. Tetrahedron
Lett. 2009, 50, 7259–7262.
O
(CH2)19
OH
(CH2)21CH3
(CH2)12
CH3(CH2)15
20
α
]
[
D = -5.5
28
19. Asselineau, C.; Asselineau, J.; Lanéelle, G.; Lanéelle, M.-A. Prog. Lipid Res. 2002,
41, 501–523.
OH
specific rotation of ½ ꢂ ꢀ13.1 (c 1.2, CHCl3),
a 2D0
O
20. The bromide 17 gave
a
corresponding to an [M]D of ꢀ98. Since the chiral centres are flanked by long
chains, this may be used as a means of predicting [M]D and hence ½a D20
ꢂ
values
(CH2)19
O
OH
(CH2)21CH3
(CH2)12
of molecules containing this fragment.
CH3(CH2)15
21. Koza, G.; Theunissen, C.; Al Dulayymi, J. R.; Baird, M. S. Tetrahedron 2009, 65,
29
20
10214–10229.
α
[
]
D = + 6.0
22. Compound 28, mp = 71–73 °C, ½a D20
ꢂ
ꢀ5.5, (c 0.74, CHCl3). Found [M+Na]+:
1204.1912; C80H156NaO4 requires: 1204.1896. dH (500 MHz, CDCl3): 5.33 (1H,
td, J 6.3, 15.1 Hz), 5.24 (1H, dd, J 7.6, 15.1 Hz), 3.74–3.70 (1H, m), 2.73 (1H, dt, J
2.2, 5.4 Hz), 2.49–2.45 (1H, m), 2.43 (1H, dd, J 2.2, 7.3 Hz), 2.11–2.01 (2H, m),
Scheme 7. Reagents and conditions: (i) KBSA, 1,2-dimethoxyethane, ꢀ5 °C (26%);
(ii) LiOH, THF, H2O, MeOH, 45 °C (70%).

D. Z. Al Kremawi et al. / Tetrahedron Letters 51 (2010) 1698–1701
1701
1.96 (2H, q, J 6.7 Hz), 1.78–1.06 (134H, br m), 1.0 (3H, d, J 6.3 Hz), 0.94 (3H, d, J
6.7 Hz), 0.89 (6H, t, J 6.6 Hz); dC (125 MHz, CDCl3): 178.70, 136.46, 128.41,
72.13, 63.99, 59.00, 50.72, 37.24, 36.69, 36.02, 35.54, 33.77, 32.58, 32.24, 31.92,
29.86, 29.77, 29.70, 29.58, 29.53, 29.50, 29.42, 29.36, 29.14, 27.34, 27.21, 26.07,
dt, J 6.6, 15.1 Hz), 5.24 (1H, dd, J 7.6, 15.2 Hz), 3.77–3.69 (1H, m), 2.69–2.67
(1H, m), 2.49–2.45 (2H, m), 2.05–2.02 (2H, m), 1.96 (2H, q, J 6.6 Hz), 1.79–1.03
(134H, br m), 0.94 (3H, d, J 6.7 Hz), 0.92 (3H, d, J 6.6 Hz), 0.88 (6H, t, J 6.1 Hz); dC
(125 MHz, CDCl3): 178.19, 136.45, 128.41, 72.11, 63.91, 57.60, 50.63, 37.23,
36.68, 35.81, 35.55, 34.59, 32.57, 32.18, 31.92, 29.93, 29.77, 29.65, 29.59, 29.57,
29.52, 29.50, 29.44, 29.41, 29.35, 29.23, 29.12, 27.32, 26.88, 26.13, 25.71, 22.67,
25.71, 22.68, 20.95, 17.28, 14.11; mmax
1048 cmꢀ1
23. Yuan, Y.; Barry, C. E. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 12828–12833.
24. Compound 29, mp: 54–55 °C,
+6.0 (c 0.52, CHCl3). Found [M+Na]+:
1204.1843; C80H156NaO4 requires: 1204.1896. dH (500 MHz, CDCl3): 5.33 (1H,
: 3368, 2922, 2851, 1686, 1463,
.
20.95, 15.93, 14.10; m .
max: 3392, 2924, 2853, 1748, 1464, 1070 cmꢀ1
½ ꢂ
a 2D0