Intramolecular α-glucosaminidation: Synthesis of mycothiol

Article abstract of DOI:10.1021/ol1008334

A protected cyclitol aglycon was tethered to an (N-arylsulfonyl)glucosamine donor by a methylene linker; the exclusively α-selective intramolecular glycosyation reaction was then initiated by electrophilic activation of the thioglycoside donor portion. Further transformations of the glycosylation product to give the M. tuberculosis detoxifier mycothiol and its oxidized congener, the disulfide mycothione, are detailed.

Full text of DOI:10.1021/ol1008334

ORGANIC
LETTERS
2010
Vol. 12, No. 11
2630-2633
Intramolecular r-Glucosaminidation:
Synthesis of Mycothiol
Kehinde Ajayi, Vinay V. Thakur, Robert C. Lapo, and Spencer Knapp*
Department of Chemistry and Chemical Biology, Rutgers-The State UniVersity of New
Jersey, 610 Taylor Road, Piscataway, New Jersey 08854
spencer.knapp@rutgers.edu
Received April 12, 2010
ABSTRACT
A protected cyclitol aglycon was tethered to an (N-arylsulfonyl)glucosamine donor by a methylene linker; the exclusively r-selective intramolecular
glycosyation reaction was then initiated by electrophilic activation of the thioglycoside donor portion. Further transformations of the glycosylation
product to give the M. tuberculosis detoxifier mycothiol and its oxidized congener, the disulfide mycothione, are detailed.
Two million people die annually from tuberculosis (TB), and
an estimated 8 million develop the disease each year. Because
Mycobacterium tuberculosis, the TB causative agent, has
increasingly developed resistance to many treatments, most
of which are largely ineffective against the dormant state, a
better understanding of the organism’s defense mechanisms
is critical to battling infections.1,2 Mycothiol (1, Figure 1)3
is the low molecular weight thiol used by mycobacteria as
their main line of defense against foreign electrophilic agents
such as radicals, oxidants, and drugs;4,5 glutathione plays
an analogous role in eukaryotes, which lack 1 altogether.
Disruption of mycothiol metabolic pathways6,7 is fatal to M.
(1) Barry, C. E., III; Duncan, K. Drug DiscoVery Today 2004, 1, 491–
496
.
(2) Xie, Z.; Siddiqi, N.; Rubin, E. J. Antimicrob. Agents Chemother.
2005, 49, 4778–4780
.
(3) Isolation: Newton, G. L.; Fahey, R. C.; Cohen, G.; Aharonowitz,
Y. J. Bacteriol. 1993, 175, 2734–2742. Sakuda, S.; Zhou, Z.-Y.; Yamada,
Y. Biosci. Biotechnol. Biochem. 1994, 58, 1347–1348. Spies, H. S. C.;
Steenkamp, D. J. Eur. Biochem. 1994, 224, 203–213.
Figure 1. Structures of mycothiol and its disulfide.
(4) Newton, G. L.; Arnold, K.; Prince, M. S.; Sherrill, C.; delCardayre´,
S. B.; Aharonowitz, Y.; Cohen, G.; Davies, J.; Fahey, R. C.; Davis, C. J.
tuberculosis, so this is a promising avenue for the develop-
ment of new treatments.8 The efficient synthesis of 1 and
its congener disulfide, mycothione 2, as well as simpler
analogues, could contribute significantly to these efforts.9-12
Bacteriol. 1996, 178, 1990–1995
(5) Recent reviews: Jothivasan, V. K.; Hamilton, C. J. Nat. Prod. Rep.
2008, 25, 1091–1117. Newton, G. L.; Buchmeier, N.; Fahey, R. C.
.
Microbiol. Mol. Biol. ReV. 2008, 72, 471–494
.
10.1021/ol1008334  2010 American Chemical Society
Published on Web 05/05/2010

Forging the (R) cis-1′,2′ glycosidic linkage of 1 is the
principal synthetic challenge.13 Where complex glycosyl
donors and acceptors are required, an intramolecular gly-
cosylation process14 (Figure 2) offers several advantages:
activated under mild conditions ought to direct the desired
glycosidic bond formation. We are pleased to report the
realization of these objectives, and additionally, the efficient
transformation of the glycosylation product to 1 and 2.
1,3,4,6-Tetra-O-acetyl-ꢀ-D-glucosamine16 (3, Scheme 1)
was converted17 to the 2-naphthalenesulfonamide 4 and then
Scheme 1. Model Intramolecular R-Glucosaminidation
Figure 2. Intramolecular glucosamidation featuring a tether Y, an
N-protecting group Z, and an activatable leaving group X.
the donor-acceptor stoichiometry is 1:1, the tether enforces
syn delivery of the aglycon (OR), and the bond formation
can potentially be accomplished quite efficiently and to the
near exclusion of nonproductive donor fates such as elimina-
tion or hydrolysis. While no intramolecular R-glucosamini-
dation has yet been demonstrated,15 careful selection of a
short and temporary tether Y, an easily removable amine
protecting group Z, and a leaving group X that can be
(6) Mycothiol biosynthesis: Bornemann, C.; Jardine, M. A.; Spies,
H. S. C.; Steenkamp, D. J. Biochem. J. 1997, 325, 623–629. Newton, G. L.;
Koledin, T.; Gorovitz, B.; Rawat, M.; Fahey, R. C.; Av-Gay, Y. J. Bacteriol.
2003, 185, 3476–3479. Newton, G. L.; Ta, P.; Bzymek, K. P.; Fahey, R. C.
J. Biol. Chem. 2006, 281, 33910–33920. Newton, G. L.; Av-Gay, Y.; Fahey,
R. C. J. Bacteriol. 2000, 182, 6958–6963. Koledin, T.; Newton, G. L.;
Fahey, R. C. Arch. Microbiol. 2002, 178, 331–337. Sareen, D.; Steffak,
M.; Newton, G. L.; Fahey, R. C. Biochemistry 2002, 41, 6885–6890.
(7) Mycothiol mediated detoxification: Newton, G. L.; Av-Gay, Y.;
Fahey, R. C. Biochemistry 2000, 39, 10739–10746. Park, J.-H.; Roe, J.-H.
Mol. Microbiol. 2008, 68, 861–870. Steffak, M.; Newton, G. L.; Av-Gay,
Y.; Fahey, R. C. Biochemistry 2003, 42, 12067–12076.
further18 to the p-tolyl thioglycoside 5. Menthol was selected
as a model aglycon; coupling of 5 with (+)-menthyl
chloroformate (6) was particularly efficient in the presence
of the phosphazene base 2-tert-butylimino-2-diethylamino-
1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP).19 The
intramolecular glycosylation of 7 was carried out by activating
the thioglycoside with dimethyl(methylthio)sulfonium tetrafluo-
roborate.20 Upon quench, the temporary methylene tether was
lost, presumably as formaldehyde, and the (exclusively R)
glucosaminide 8 was isolated in good yield. Acetate metha-
nolysis followed by reductive removal21 of the 2-naphthalene-
sulfonyl protecting group provided the amino triol 9.
(8) Payne, D. J.; Gwynn, M. N.; Holmes, D. J.; Pompliano, D. L. Nat.
ReV. Drug DiscoVery 2007, 6, 29–40.
(9) MSH synthesis: (a) Lee, S.; Rosazza, J. P. N. Org. Lett. 2004, 6,
365–368.
(10) Bimane conjugate of MSH: Jardine, M. A.; Spies, H. S. C.;
Nkambule, C. M.; Gammon, D. W.; Steenkamp, D. J. J. Bioorg. Med. Chem.
2002, 10, 875–881. Nichols, G. M.; Kovac, P.; Bewley, C. A. J. Am. Chem.
Soc. 2002, 124, 3492–3493.
(11) Knapp, S.; Gonzalez, S.; Myers, D. S.; Eckman, L. L.; Bewley,
C. A. Org. Lett. 2002, 4, 4337–4339.
An analogous approach to the synthesis of 1 requires a
protected cyclitol aglycon component, such as the penta-O-
benzylinositol 10 (Scheme 2). Preparation of 10 followed
(12) MSH analogues: (a) Gammon, D. W.; Hunter, R.; Steenkamp, D. J.;
Mudzunga, T. T. Bioorg. Med. Chem. Lett. 2003, 13, 2045–2049. Knapp,
S.; Amorelli, B.; Darout, E.; Ventocilla, C. C.; Goldman, L. M.; Huhn,
R. A.; Minnihan, E. C. J. Carbohydr. Chem. 2005, 24, 103–130. (c)
Metaferia, B. B.; Ray, S.; Smith, J. A.; Bewley, C. A. Bioorg. Med. Chem.
Lett. 2007, 17, 444–447. (d) Metaferia, B. B.; Fetterolf, B. J.; Shazad-ul-
Hussan, S.; Moravec, M.; Smith, J. A.; Ray, S.; Gutierrz-Lugo, M.-T.;
Bewley, C. A. J. Med. Chem. 2007, 50, 6326–6336. (e) Stewart, M. J. G.;
Jothivasan, V. K.; Rowan, A. S.; Wagg, J.; Hamilton, C. J. Org. Biomol.
Chem. 2008, 6, 385–390.
(15) Portions of this work have appeared: Ajayi, K. Ph.D. Dissertation,
Rutgers University, Piscataway, NJ, 2010. Lapo, R. C.; Knapp, S. Abstracts
of Papers, 225th Meeting of the American Chemical Society; American
Chemical Society: Washington, DC, 2003; p U260, Meeting Abstract 56-
CARB.
(13) Recent glucosaminidation methods: Li, Y.; Yang, Y.; Yu, B.
Tetrahedron Lett. 2008, 49, 3604–3608. Mensah, E. A.; Nguyen, H. M.
J. Am. Chem. Soc. 2009, 131, 8778–8780. Yang, Y.; Li, Y.; Yu, B. J. Am.
Chem. Soc. 2009, 131, 12076–12077.
(16) Bergmann, M.; Zervas, L. Chem. Ber. 1931, 64, 973–980.
(17) Nyasse, B.; Grehn, L.; Maia, H. L. S.; Monteiro, L. S.; Ragnarsson,
U. J. Org. Chem. 1999, 64, 7135–7139.
(18) Pozsgay, V.; Jennings, H. J. Tetrahedron Lett. 1987, 28, 1375–
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(14) Intramolecular glycosylation: Barresi, F.; Hindsgaul, O. J. Am.
Chem. Soc. 1991, 113, 9376–9377. Stork, G.; Kim, G. J. Am. Chem. Soc.
1992, 114, 1087–1088. Bols, M. J. Chem. Soc., Chem. Commun. 1992,
913–914. Ito, Y.; Ogawa, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 1765–
1767. (e) Fairbanks, A. J. Synlett 2003, 1945–1958. (f) Cumpstey, I.
Carbohydr. Res. 2008, 343, 1553–1573.
(19) Schwesinger, R.; Willaredt, J.; Schlemper, H.; Keller, M.; Schmitt,
D.; Fritz, H. Chem. Ber. 1994, 127, 2435–2454.
(20) Trost, B. M.; Murayama, E. J. Am. Chem. Soc. 1981, 103, 6529–
6530.
(21) Chavez, F.; Sherry, A. D. J. Org. Chem. 1989, 54, 2990–2992.
Org. Lett., Vol. 12, No. 11, 2010
2631

The glycosylation reaction conditions were adjusted to
include a good nucleophile (chloride ion) capable of inter-
cepting 17 and blocking the hydride shift, particularly at
lower temperatures. Although treatment of 13 with 1.5 equiv
each of phenylsulfenyl choride and silver(I) trifluoromethane-
sulfonate26 below -20 °C gave 15 only (hydride shift is rapid
even at low temperature), the use of PhSeCl with catalytic
AgOTf gave the desired glycoside 14 in high yield after the
quench, with barely a trace of 15.
Scheme 2. Mycothiol Core and a 1 f 9 Hydride Shift
Deprotection of the acetates and the 2-naphthylsulfonamide
as before led smoothly to the amino triol 20 (Scheme 3).
Scheme 3. Completion of the Synthesis of Mycothiol
the known route,22 which includes a resolution based on
purification of its (+)-menthol mixed carbonate. Crystal-
lographic analysis (see Supporting Information) confirmed
the absolute configuration of the resolved material. The
required chloromethyl derivative 12 was generated in situ23
from methylthiomethyl ether 11.24 BEMP-mediated coupling
with 5 was again successful and produced the glycosylation
substrate 13.
Use of dimethyl(methylthio)sulfonium tetrafluoroborate as
before to trigger the intramolecular glycosylation led not to
the desired glucosaminide 14, but instead to the N-methyl-
sulfonamide carbinol 15 (96%) in which one benzyl ether
has been selectively cleaved and the methylene linker has
been reduced. The structure of 15 was secured by NMR
analysis of its acetate derivative 16. In particular, the methine
adjacent to -OH (H-6, δ ) 4.09, td, J ) 9.5, 3.0 Hz) in 15
can be identified by COSY analysis and moves downfield
(δ ) 5.75, t, J ) 10 Hz) upon acetylation. An unusually
facile 1 f 9 hydride shift25 within the intermediate iminium
ion 17, followed by hydrolysis of the oxonium species 18,
accounts for this result.
The selective reductive cleavage of the N-sulfonyl protecting
group in the presence of five O-benzyl ethers is notable.
Hydrogenolysis of these benzyls gave the amino octa-ol as
its hydrochloride salt 21. The amino group, upon liberation
by base treatment, was coupled with N-Boc-S-acetylcysteine
22,11 and the resulting amide was deprotected on nitrogen
by treatment with trifluoroacetic acid. Neutralization of the
resulting ammonium trifluoroacetate salt induced S f N
vicinal migration of acetyl, and upon simple concentration
the natural product mycothiol (1) was obtained essentially
quantitatively. When stored under an argon atmosphere, the
mercaptan 1 is stable for weeks.
(22) Massy, D. J. R.; Wyss, P. HelV. Chim. Acta 1990, 73, 1037–1057.
Baker, G. R.; Billington, D. C.; Gani, D. Tetrahedron 1991, 47, 3895–
3908. Koto, S.; Hirooka, M.; Yoshida, T.; Takenaka, K.; Asai, C.;
Nagamitsu, T.; Sakuma, H.; Sakurai, M.; Masuzawa, S.; Komiya, M.; Sato,
T.; Zen, S.; Yago, K.; Tomonaga, F. Bull. Chem. Soc. Jpn. 2000, 73, 2521–
2529.
1
Analysis of 1 by H, 13C NMR, and exact mass methods
and comparison of the values with those in the literature9
(see Supporting Information) leave no doubt about its
identity. Oxidative conversion of mycothiol to its disulfide
mycothione (2, Scheme 4) was also quantitative, and similar
analysis of 2 confirmed its identity as well. Finally, the amide
(23) Jones, J. H.; Thomas, D. W.; Wood, M. E. Synth. Commun. 1986,
16, 1607–1610.
(24) Corey, E. J.; Bock, M. G. Tetrahedron Lett. 1975, 16, 3269–3270.
(25) A 1 f 5 iminium-induced hydride shift: Wolfling, J.; Frank, E.;
Schneider, G.; Tietze, L. F. Angew. Chem., Int. Ed. Engl. 1999, 38, 200–
201. (b) Wolfling, J.; Frank, E.; Schneider, G.; Tietze, L. F. Eur. J. Org.
Chem. 2004, 90–100.
(26) Martichonok, V.; Whitesides, G. M. J. Org. Chem. 1996, 61, 1702–
1706.
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useful in the development of inhibitors of mycothiol process-
ing enzymes.
Scheme 4. Conversion of Mycothiol to Mycothione
Acknowledgment. The authors are grateful to NIH
(AI055760) for partial financial support, to Thomas J. Emge
for the crystallographic analysis, and to the GAANN program
for a graduate fellowship to K.A.
Supporting Information Available: Experimental details
and spectroscopic characterization for 1, 2, and all new
compounds, and CIF file for the menthol carbonate of 10.
This material is available free of charge via the Internet at
http://pubs.acs.org.
coupling method in Scheme 3 was applied to D-glucosamine
(72% overall yield), resulting in the simplified mycothiol
glycon, GlcN-Cys-NAc (24). Derivatives of 24 may prove
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