Synthesis of methyl 6-deoxy-4-O -(sodium sulfonato)- α -L- talopyranoside, its C-4 epimer and both isosteric [4-C-(potassium sulfonatomethyl)] derivatives

The 4-O -sulfuric esters of methyl 6-deoxy-α -L-talo- and - α -L-mannopyranoside were prepared. The first ester is a component of the glycopeptidolipid-type cell surface antigens of M. avium . The isosteric isomers (sugar-4-CH 2 -SO 3 Na) of both sulfate esters (sugar-4-O-SO 3 Na) were synthesized using free radical addition reactions between sugar-exomethylene derivatives and either thioacetic acid or NaHSO 3 . The addition products of thioacetic acid were converted into sugar-CH 2 -sulfonic acids by oxidation with oxone. Characteristic 1 H-and 13 C-NMR data are given and discussed.


Introduction
Most species of Mycobacterium are saprophytic species found in soil and some of them are serious human health threats. 1 Besides Mycobacterium tuberculosis 2 and M. leprae 3,4 , the agents of human tuberculosis and leprosy, respectively, other 'atypical' or 'opportunistic' mycobacteria may also cause infections in humans. 5][8] Mycobacterial diseases are difficult to treat, 9 and this is directly related to the unusual complex structure of the cell-wall 9,10 of the organism.The cell-wall presents a formidable barrier 11 to the passage of antibiotics into the organism.The combination of different antibiotics can slowly destroy the integrity of the cell-wall and allow other antibiotics to pass into the organism more easily.
The mycobacterial cell-wall has five major components: 10,11 i) the plasma membrane; ii) peptidoglycan; iii) the mycolyl-arabinogalactan (AG), both sugars are in furanosyl form in the polysaccharide; iv) lipoarabinomannan (LAM) and lipomannan (LM) ; v) glycolipids noncovalently bound to the large amount of mycolate esters.Because these glycolipids are on the outermost surface of the bacteria, these outer oligosaccharide haptens are responsible for the immunological properties of the bacteria.Thus, these haptens, after conjugation with suitable proteins, might aid the serodiagnosis of mycobacterial infections. 12,13hese haptens, which are oligosaccharides, have very different, and very often exotic structures for the monosaccharide units.They are mainly deoxy sugars and contain many Omethyl substituents.Acyl groups also are present and pyruvic acid is common in acetalic form.These serospecific oligosaccharides glycosylate different "core" regions.Basically, these socalled extractable lipids, which contain the immunospecific oligosaccharides, can be classified into four groups: 12,13 glycopeptidolipids, lipooligosaccharides, phenolic glycolipids, and acylated trehalose derivatives.
The human pathogens M. tuberculosis and M. avium produce sulfated glycolipids and recently it was shown that the very conservative core region of glycopeptidolipids is also sulfated at position 4 of the 6-deoxy-L-talose of M. avium 14 and the 3,4-di-O-methyl-L-rhamnose at position 2 in the case of M. fortuitum. 15It is worth mentioning that the 4-O-sulfated 6-deoxy-Ltalose was isolated from an ethambutol-resistant M. avium strain cultured from a patient with AIDS. 14he biological role of sugar sulfate esters 16,17 and sugar C-sulfonic acids [18][19][20] cannot be overestimated.Sulfated sugars are common mediators of cell-cell and host-pathogen interactions.

MeO
In our program, we wished to replace the sugar sulfate esters by sugar sulfonic acids and by sugar-methylene-sulfonic acids.Such investigations were accomplished in the case of sialyl Lewis X 21,22 and Helicobacter pylori ligand analogs. 23We developed new methods for the preparation of sugar sulfonic acids 24,25 and we supposed that sugar methylene-sulfonic acids, which are isosteric analogs of sugar sulfates, might be better replacements for sugar O-sulfates than sugar sulfonic acids.In this paper we wish to report on the syntheses of methyl 6-deoxy-4-O-sulfate-α-Ltalopyranoside, its C-4-epimer derivative, methyl 4,6-dideoxy-4-C-methylene-sulfonic acid-α-Ltalopyranoside, and methyl 4,6-dideoxy-4-C-(sodium methylene-sulfonic acid)-α-Lmannopyranoside.

Results and Discussion
The reagents most frequently employed for sulfation are complexes of sulfur trioxide with a tertiary amine (e.g.trimethylamine or pyridine) or an amide (formamide or dimethylformamide) in pyridine, dimethyl sulfoxide or N,N-dimethylformamide as the solvent.Other sulfation reagents that can be used are chlorosulfonic acid or piperidine-N-sulfonic acid in a polar, aprotic solvent.py, DMF, 1h; NaHCO 3 ; 77% for 2, 79% for 5; b) 96% AcOH, 30 min., quant.
Pyridine complex in DMF for 1h at room temperature to give methyl 2,3-O-isopropylidene-4-O-(sodium sulfonato)-α-L-rhamnopyranoside (2) in 77% yield.Its 13 C-NMR spectrum showed the presence of a strong deshielding effect, which led to a downfield shift for C-4 of 6.9 ppm.In this case we could not detect any upfield shift in the adjacent carbon signals. 27Hydrolysis of the isopropylidene group was achieved with acetic acid at room temperature for 30 min and the yield was quantitative.In the 13 C-NMR spectrum of methyl 4-O-(sodium sulfonato)-α-Lrhamnopyranoside (3), C-4 resonates at 81.1 ppm.This α-shift is 8.2 ppm.The 13 C-NMR data for methyl α-L-rhamnopyranoside and for compound 1 were published previously by Argentinean authors. 28imilar treatment of methyl 6-deoxy-2,3-O-isopropylidene-α-L-talopyranoside (4) 29 with the SO 3 .
The AIBN or ABCN catalysed free radical addition reaction between compound 7 and thioacetic acid in toluene resulted in a 1:1 mixture of methyl 4,6-dideoxy-2,3-O-isopropylidene-4-C-(acetylthiomethyl)-α-L-mannopyranoside (8) and -α-L-talopyranoside (9).The yield was very low (22%) even at high temperature (80ºC) and after a long reaction time (8h).The C-4 epimers could be separated, and their chirality also could be determined by measuring the values of the 3 J H4,H5 coupling constants (10.1 Hz for 8 and 6.9 Hz for 9).The COSY and HETCOR 1 Hand 13 C-NMR spectra and Mw measurements for compounds 8 and 9 using ESI-TOF verified the postulated structures.Some very characteristic 13 C-NMR data are worthy of mention: the C-4 resonance of the manno-isomer (8) is at 44.1 ppm, whereas, in the case of the talo-isomer (9), it is at higher field (36.4 ppm).The CH 2 -groups in each compound resonate nearly at the same fields (27.6 and 27.8 ppm).
Oxidation of compound 8 with oxone 38 resulted in the methylene-sulfonic acid potassium salt (10).Analogous treatment of compound 9 gave the talo-isomer 11.The carbon NMR spectra for both isomers (10 and 11) showed the same C-4 chemical shift values and the differences which were observed for the C-4 epimers of the thioacetates 8 and 9.
Hydrolysis of the isopropylidene group in compound 10 using acetic acid resulted in a quantitative yield of 12, and similarly compound 11 yielded 13.Interestingly, the chemical shift values of the epimeric C-4's are nearly the same (42.3 and 41.9 ppm), but a considerable difference was observed for the CH 2 -group-values (51. 5  It was mentioned earlier that the thioacetic acid addition to the perbenzylated exocyclic glycal resulted only in the equatorial methylene-thioacetate derivative.To clear up the role of the isopropylidene acetal in compound 7, we hydrolysed the isopropylidene group and the resulting methyl 4,6-dideoxy-4-C-exomethylene-α-L-lyxo-hexopyranoside ( 14) was treated with thioacetic acid in the presence of AIBN catalyst.Surprisingly, only one isomer was formed and it proved to be methyl 4,6-dideoxy-4-C-(acetylthiomethyl)-α-L-mannopyranoside (15) which is the equatorial isomer.Unfortunately, the yield was again very low (17%) and it could not be increased.It seems very propable that the uloso-glycosides also react in a radical addition reaction, and the photoaddition leads mainly to the equatorial product.The flexible conformation of the unsaturated compound is an important prediction.This assumption was also supplied by the fact that hydrogensulfite addition catalysed by tert-butyl peroxybenzoic acid resulted exclusively in compound 16.This observation is in a good accordance with earlier results where the authors [39][40][41] allowed methyl 6-deoxy-α-and ß-D-xylo-hex-5-enopyranoside, methyl 6-deoxyß-and -α-L-arabino-hex-5-enopyranoside and methyl 6-deoxy-α-D-lyxo-hex-5-enopyranoside to react with hydrogen sulfite in aqueous solution.The main or exclusive products were the Dsugars in which the 6-sulfonate moieties were in an equatorial position.
In conclusion, we have demonstrated that the isosteric analogs of compounds 3 and 6, which are sugar-O-sulfates, could be obtained from sugar exomethylene derivatives by radical addition reactions using either thioacetic acid or sodium hydrogen sulfite.