The synthesis of single enantiomers of trans-alkene containing mycolic acids and related sugar esters
Graphical abstract
Introduction
Mycolic acids (MAs) are characteristic components of the cell envelopes of mycobacteria, often covalently bound to arabinogalactan as a penta-arabinose tetramycolate.1, 2, 3 They are also seen as trehalose dimycolate (TDM—a potent signalling agent in the immune system),4, 5 trehalose monomycolate (TMM)6 and other sugar esters that are not bound to the cell wall,7, 8, 9 or as free mycolic acids.10 MAs comprise two elements (Fig. 1), the β-hydroxy-acid, for which d is generally 21 or 23, and the meromycolate. The latter is a long chain usually containing two groups X and Y.
In the most common classes of mycobacterial MA, the two groups are both cis-cyclopropanes (α-MA), or group X is an α-methyl-β-methoxy or α-methyl-β-keto group (methoxy and keto-MA, respectively). MAs are generally present as complex mixtures containing several different chain lengths of each class, and the detailed composition is dependent on the mycobacterium.11, 12 The presence and proportion of individual classes of MA is important for the virulence of diseases such as tuberculosis.13, 14 The MAs are themselves strongly bioactive and indeed individual synthetic MAs of different classes matching the structures of components of natural mixtures are selectively active.15 In addition, there are a range of generally less abundant MAs in mycobacteria containing other X and Y groups; these include molecules containing an α-methyl-trans-alkene unit.11, 12 Keto-mycolic acids 2 with a proximal trans-alkene substituent and a variety of chain lengths have been reported (Table 1).11, 12, 16, 17, 18 In these cases, the methyl group adjacent to the alkene is on the proximal side relative to the hydroxy-acid. The methyl group in the X position of keto- and methoxy-MA and at the Y-position in α-methyl-trans-cyclopropane containing MA, is distal from it.11, 12
A derivative of a hydroxy-MA with a proximal α-methyl-trans-alkene has also been reported.16 Mycobacterium smegmatis synthesizes keto-MAs containing about 33% of a trans-alkene. The same percentage is seen in the hydroxy-mycolate.19 In addition, related methoxy-MA 3 have been reported as in Table 2.11
The biosynthesis of these and other MA has been studied extensively.20, 21, 22, 23, 24, 25, 26 Labelling studies show the methyl branch of trans-MA from Mycobacterium tuberculosis are exclusively on carbons derived from the 2-position of acetate, while those from M. smegmatis, are exclusively derived from the 1-position.27
Another type of mycolic acid, isolated from M. smegmatis and Mycobacterium aurum,17 Mycobacterium chelonei,28 and Mycobacterium fortuitum,29 contains an α-methyl-trans-alkene at the proximal position and a cis-alkene at the distal position.30, 31, 32 In the latter case, the specific rotation of its methyl ester has been reported to be +1.4 (CHCl3),33 while that of the acetoxy methyl ester is reported as +3 (CHCl3), and that of a derivative of a mycolate containing only an α-methyl-trans-alkene chiral centre corresponds to a molecular rotation (MD) of −19.5.33 In addition, the specific rotation of a wax ester containing this unit has been reported to be +4.3 (CHCl3).17, 34, 35 This allows the contribution to the molecular rotation from the α-methyl-trans-alkene to be calculated, as the only other chiral centres in these cases are at the hydroxy-acid position, the contribution of which to the molecular rotation is known (+40°). This leads to a value of −25° for the α-methyl-trans-alkene (for 30% of methyl branched molecules); this in turn suggests this sub-unit has an (R)-stereochemistry, based on model compounds.18
As part of a study to determine the biological significance of specific MA structures,36, 37, 38, 39, 40, 41, 42 and to confirm regio- and stereochemistry, we now report the synthesis of hydroxy-MA, keto-MA and methoxy-MA containing a α-methyl-trans-alkene unit. Four sugar esters and one glycerol ester were prepared from one methoxy-MA, to provide the opportunity for a systematic comparison of their effects on cytokines and chemokines, as well as their application as antigens in the serodiagnosis of tuberculosis.
Section snippets
Results and discussion
As the first step in the synthesis of alkene-MA 23, 6-bromohexanal (4) was chain extended to the ester 6 and then converted into the sulfone 7 (Scheme 1):
In order to fix the (R)-stereochemistry of the α-methyl-trans-alkene fragment, aldehyde 8,37, 43, 44, 45 was chain-extended with compound 7 and base, using a modified Julia–Kocienski reaction,46, 47, 48 followed by hydrogenation of the derived alkenes to give 9. The pivaloate group was removed and the primary alcohol was converted into sulfone
General
Chemicals were obtained from commercial suppliers (Sigma, Aldrich, and Alfa Aeser) or prepared from them by the methods described. Solvents which were required to be dry, e.g., ether, tetrahydrofuran were dried over sodium wire and benzophenone under nitrogen, while CH2Cl2 was dried over calcium hydride. All reagents and solvents used were of reagent grade unless otherwise stated. Silica gel (Merck 7736) and silica gel plates used for column chromatography and thin layer chromatography were
Acknowledgements
HMA and RTH wish to thank the Government of Iraq for the award of PhD studentships. CR thanks the Welsh Government for support under the A4B programme. We wish to thank Dr. Paul Gates of Bristol University for carrying out accurate mass MALDI determinations, and Dr. A Ramsay and the Special Programme for Research and Training in Tropical Diseases at the WHO for access to sera from the TDR TB Specimen Bank.
References and notes (67)
- et al.
Prog. Lipid Res.
(1998) - et al.
Prog. Lipid Res.
(2012) Carbohydr. Res.
(2012)- et al.
J. Biol. Chem.
(2012) - et al.
Mol. Cell
(2000) - et al.
J. Biol. Chem.
(1998) - et al.
J. Biol. Chem.
(1995) - et al.
J. Biol. Chem.
(1996) - et al.
J. Biol. Chem.
(2001) - et al.
Prog. Lipid Res.
(2002)
Bioorg. Chem.
J. Biol. Chem.
J. Biol. Chem.
Tetrahedron
Tetrahedron
Tetrahedron
Tetrahedron Lett.
Tetrahedron
Tetrahedron
Tetrahedron Lett.
Tetrahedron
Tetrahedron
Tetrahedron Lett.
Chem. Phys. Lipids
Tetrahedron Lett.
Tetrahedron
J. Biol. Chem.
Tetrahedron Lett.
Tetrahedron
Tetrahedron
Crit. Rev. Eukaryot. Gene Expr.
Microbiology-SGM
Cited by (8)
Mycobacterium alvei (ω-1)-methoxy mycolic acids: Absolute stereochemistry and synthesis
2020, Chemistry and Physics of LipidsCitation Excerpt :This was coupled to 15-tetrahydropyranyloxypentadecanal (18) in a Wittig reaction to provide the chain extended Z-alkene (19), which was further converted into the sulfone 20 using standard transformations (Scheme 3): Coupling of the sulfone with the protected aldehyde 21 using KHMDS (Ali et al., 2016; Muzael et al., 2010), with retention of the absolute stereochemistry of the adjacent methyl substituent, as described for the preparation of trans-alkene keto- and methoxy-MA (Ali et al., 2016), gave the diene 22. Removal of the silyl protecting group led to the methyl ester 23, which in turn was hydrolysed to the free acid 24 (Scheme 4):
A re-investigation of the mycolic acids of Mycobacterium avium
2020, Chemistry and Physics of LipidsCitation Excerpt :Again, as suggested for the α-mycolates, special biological activity or biosynthetic redundancy are possibilities. Synthetic routes to α-methyl trans-alkene mycolic acids have been established (Ali et al., 2016), so studies on the physical and biological activities of such mycolates are accessible. NMR and mass spectrometry data for purified fractions are presented in Supplementary Data (Figs. S1–S19).
Synthesis of trehalose glycolipids
2020, Organic and Biomolecular ChemistrySynthesis of Cyclopropane Fatty Acids by C(sp<sup>3</sup>)−C(sp<sup>3</sup>) Cross-Coupling Reaction and Formal Synthesis of α-Mycolic Acid
2018, Advanced Synthesis and Catalysis