Armed-disarmed effect on the stability of cysteine thioglucosides

Article abstract of DOI:10.1016/j.carres.2012.06.022

Thioglucosides of cysteine show variable stability depending on the nature of the protecting groups on the glycosyl donor. Armed protecting groups (benzyl) lead to products that decompose readily while disarmed protecting groups (acetyl) lead to more stable products. Since this armed/disarmed effect of the protecting group on the stability of the thioglucosides is more pronounced for cysteine with an unprotected carboxylic group, the proposed mechanism is that decomposition is initiated by an intramolecular protonation of glycosyl sulfide and subsequent displacement of the sulfide by adventitious nucleophiles.

Full text of DOI:10.1016/j.carres.2012.06.022

Carbohydrate Research 359 (2012) 18–23
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Armed–disarmed effect on the stability of cysteine thioglucosides
Mbulelo G. Nokwequ a, , Comfort M. Nkambule a, , David W. Gammon b
⇑
⇑
a Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa
b Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2012
Received in revised form 22 June 2012
Accepted 30 June 2012
Available online 11 July 2012
Thioglucosides of cysteine show variable stability depending on the nature of the protecting groups on
the glycosyl donor. Armed protecting groups (benzyl) lead to products that decompose readily while dis-
armed protecting groups (acetyl) lead to more stable products. Since this armed/disarmed effect of the
protecting group on the stability of the thioglucosides is more pronounced for cysteine with an unpro-
tected carboxylic group, the proposed mechanism is that decomposition is initiated by an intramolecular
protonation of glycosyl sulfide and subsequent displacement of the sulfide by adventitious nucleophiles.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Cysteine
Thioglucosides
Armed/disarmed effect
Stereoselective glycosylation
1. Introduction
OR
OR
O
O
Mycothiol (MSH) is a low molecular weight thiol found in
Mycobacterium tuberculosis, and actinomycetes in general, impli-
cated in the protection of the organism against oxidative stress
and xenobiotics (Fig. 1).1–4
R1OH
RO
RO
RO
RO
S
X
O
X
OR1
NHAc
NHAc
O
As part of an ongoing study on the search for new anti-tubercu-
losis drugs we are interested in developing efficient methods for
the synthesis of mycothiol and its analogues.5,6 The enzymes in-
volved in MSH biosynthesis are reasonable drug targets since mu-
tants that are unable to synthesize MSH have lower tolerance for
oxidative stress.7–10 The target enzymes include the amidases
MshB (N-acetyl glucosaminylinositol deacetylase) and MCA (myc-
othiol S-conjugate amidase) which are involved in the detoxifica-
tion process of the capture and release of alkylating agents. Also
of interest is mycothiol reductase (Mtr) which helps in maintaining
the reduced environment by reducing mycothione (MSSM) to
MSH;11–16 mycothione and N-acetyl glucosaminylinositol thus
serve as reservoirs for MSH which is only produced when needed.
Several research groups have synthesized MSH analogues to
investigate their inhibitory effects where the glucosaminyl C-2
substituent or the myo-inositol aglycon have been modified or
replaced with different groups.6,17,18 We have identified lactone 1
as a molecule of interest in that it is an analogue of MSH, but under
the proper activation conditions it can also be a scaffold for the
stereoselective synthesis of other MSH analogues (Scheme 1).
Stereoselective glycosylation via fused bicyclic donors, whether
permanent or transient, has been demonstrated to be an effective
SH
1 X = O
3 X = NH
2 X = O
4 X = NH
Scheme 1. Proposed stereoselective
a-glycosylation via fused bicyclic cysteine
donors.
glycoside synthetic strategy, notably by the research groups of
Boons and Turnbull.19–23 Oscarson and co-workers attempted the
synthesis of MSH analogues 4 via lactam 3, but were unsuccessful.
They speculated that the unsuccessful activation of the thiogluco-
side was due to the stability of the bicyclic lactam 3.24 However,
Turnbull and co-workers have since shown that bicyclic sulfides
can be activated for glycosylation by first oxidizing them to the
sulfoxides followed by treatment with triflic anhydride and trime-
thoxybenzene.21 We envision that glycosyl donor (1) may be sim-
ilarly activated to synthesize the
a-glycosides 2; these MSH
structural mimics would then be evaluated for inhibitory activity
against MshB, MCA, and Mtr.
We imagined that lactone 1 could readily be accessed from the
glycosylation reaction between 1,2-anhydro-3,4,6-tri-O-benzyl-
a-
D
-glucopyranose (6a) and the commercially available N-acetyl-
L
-
⇑
Corresponding authors. Tel.: +27 12 382 6128; fax: +27 12 382 6286 (M.G.N.).
E-mail addresses: nokwequmg@tut.ac.za (M.G. Nokwequ), nkambulecm@tut.
cysteine, followed by a ring-closing lactonization reaction (Scheme
2).
ac.za (C.M. Nkambule).
0008-6215/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2012.06.022

M. G. Nokwequ et al. / Carbohydrate Research 359 (2012) 18–23
19
OBn
O
BnO
BnO
O
OBn
6a
OBn
O
NHAc
BnO
BnO
O
BnO
S
S
OH
+
BnO
O
OH
O
NHAc
O
NHAc
OH
HS
1a
5a
O
7
Scheme 2. Retrosynthetic approach to the synthesis of donor 1.
This approach seemed attractive because it sought to avoid the
common, but cumbersome protection of the amino acid at both the
amino and carboxyl groups which requires several steps of protect-
ing group manipulations and ultimately results in low yields.25–27
as it again underwent rapid decomposition; on TLC the spot for
compound 5b disappeared as expected, but it was replaced by
multiple new spots.
We were curious as to the source of instability of these cysteine
glucosides and wondered if it could be due to an armed/disarmed
effect emanating from the choice of protecting groups on the donor
since there are reports in the literature on the syntheses of stable
cysteine glycosides.25–27,29 The common feature of these syntheses
is that the protecting groups on the sugar unit are generally ace-
tates, but in cases where benzyl ethers are used as protecting
groups, the carboxylic acid group of the amino acid aglycon is al-
ways protected as an ester. Fraser–Reid was the first to observe
that the armed (electron donating) or disarmed (electron with-
drawing) nature of protecting groups on a glycosyl donor could
have a major influence on that compound’s reactivity.30,31 Fra-
ser–Reid further observed that the armed/disarmed effect is more
pronounced for the protecting group at C-2 of a glycosyl donor.30
To investigate this arming/disarming effect of the protecting
groups on the stability of the thioglucosides we used differently
armed glycosyl donors (6b and 6c) and monitored the stability of
their cysteinyl glycosides (Table 1). Donor 6b is more disarmed
than epoxide 6a by having an acetate at C-2, but still has three
arming benzyl ethers at C-3, C-4, and C-6; on the other hand the
pentaacetate donor 6c is fully disarmed and should yield the most
stable thioglycosides (Fig. 2).
2. Results and discussion
Tri-O-benzyl-D-glucal 8 was readily, and stereoselectively, oxi-
dized to the epoxide 6a in an excellent yield (92%) using dim-
ethyldioxirane (DMDO) generated in situ from oxone and acetone
(Scheme 3).28
The epoxide ring-opening glycosylation reaction between 6a
and N-acetyl-L-cysteine was monitored by TLC, and the reaction
was quenched with saturated NaHCO3 upon the complete disap-
pearance of the epoxide, and the emergence of a new spot of lower
Rf. Isolation of the product thioglucoside 5a proved challenging
since the compound decomposed extensively during column chro-
matographic separation on silica gel. The 1H and 13C NMR spectra
of the crude material confirmed that the epoxide was consumed,
but was inconclusive with regard to the presence of 5a. Attempts
to lactonize crude 5a failed. When epoxide 6a was reacted with
the doubly protected methyl-N-acetyl cysteine, thioglucoside 5b
was isolated in moderate yield (58%) and compound 5b was stable
to silica gel purification (Scheme 3). This confirmed that the glyco-
sylation via the epoxide ring opening is successful and the 1H NMR
spectrum confirmed that only the gluco-product is formed (H-1,
4.28 ppm, d J = 9.2 Hz). However, just like 5a compound 5b also
did not cyclize under any of the usual transesterification lactoniza-
tion conditions. Furthermore, when compound 5b was saponified
(with LiOH) to convert it to 5a, the product could not be isolated
As predicted from Fraser–Reid’s observations compound 5c pre-
pared by glycosylation of 6b with N-acetyl-L-cysteine was pro-
duced in good yield (63%), and was more stable to silica gel
purification than compound 5a. However, the disarming effect of
the C-2 OAc group proved to be insufficient to overcome the arm-
OBn
OBn
OBn
O
O
NHAc
(a)
(b)
O
BnO
BnO
BnO
BnO
S
OR
BnO
BnO
92%
OH
O
O
8
6a
5a R = H
5b R = Me (58%)
OBn
O
BnO
S
BnO
X
O
NHAc
O
1a
Scheme 3. Synthesis of lactone donor 1a. Reagents and conditions: (a) Oxone, H2O, acetone, CH2Cl2, 0 °C, 6 h; (b) (i) N-acetyl-
L
-cysteine or N-acetyl-L-cysteine methyl ester,
BF3ꢀOEt2, CH2Cl2, reflux, 2 h; (ii) NaHCO3, H2O.

20
M. G. Nokwequ et al. / Carbohydrate Research 359 (2012) 18–23
Table 1
Investigating the armed/disarmed effect on thioglycoside stability
Entry
Donor
Lewis acid
Temp (°C)
Time (h)
Thioglucoside
Yield (%)
63a
OBn
NHAc
OH
O
BnO
BnO
S
1
6b
BF3ꢀOEt2
25
6
OAc
O
5c
OBn
NHAc
O
BnO
BnO
S
S
S
OMe
2
3
6b
6c
BF3ꢀOEt
25
25
3
5
56
64
OAc
O
5d
OAc
NHAc
O
AcO
AcO
OH
SnCl4
OAc
O
5e
OAc
NHAc
O
AcO
AcO
OMe
4
6c
SnCl4
25
3
69
OAc
O
5f
a
Crude yield; product was identified by LC–MS.
OH
‘fully disarmed’ and very stable, while compound 5c (entry 1) is
‘partially armed’ and is less stable, and compound 5a (Scheme 3)
is the ‘most armed’ and is consequently the least stable.
O
HO
HO
NH
O
OH
OH
However, a major part of our observations from Scheme 3
(instability of 5a vs 5b) and Table 1 (entries 2 and 4), is that an es-
ter protection of the carboxylic acid group confers the most stabil-
ity on the thioglucoside, regardless of the nature of the protection
group on the donor. It is thus reasonable to conclude that the pres-
ence of the free carboxylic acid group is deleterious to the stability
of the cysteine thioglucosides when arming benzyl ethers are used
as protecting groups on the sugar unit. A plausible mechanism to
explain how the decomposition of these thioglucosides arises is
proposed in Scheme 4.
We propose that the armed thioglucosides 5a and 5c undergo
decomposition initiated by an intramolecular proton transfer from
the carboxylic acid group to the sulfide. This would correlate with
the well known armed/disarmed effect shown in a previous report
by Van Boom and co-workers that an armed ethylthio glycoside
was readily activated for glycosylation by iodonium dicollidine
perchlorate (IDCP) while a disarmed thioglycoside was unreac-
tive.32 Even though sulfur is a soft base, the combined effect of a
six-membered transition state and the ‘arming effect’ of the benzyl
groups conspire to promote the formation of sulfonium ion I. In-
deed this hypothesis has resonance with the work where it was
shown that N-iodosuccinimide (NIS) could not activate a disarmed
perbenzoylated thioglycoside in the absence of a carboxylic acid
additive.33 Therefore, the protonated S-glycosyl bond in I is scissile
and its cleavage leads to the exocyclic expulsion of the cysteinyl
carboxylate anion and the formation of the oxacarbenium interme-
diate II. This oxacarbenium intermediate (II) may react with
adventitious nucleophiles in solution to form new glycosyl prod-
ucts III. Indeed when we attempted to trap compound 5a, after
observing the consumption of 6a by TLC, via the addition of acetic
anhydride, we isolated the diacetate (6b) albeit in poor yield (40%).
This product presumably arises from the hydrolysis of 5a and sub-
sequent acetylation of 1,2-diol.
O
HO
OH
OH
AcHN
SH
MSH
Figure 1. Mycothiol (MSH).
OBn
OAc
O
O
BnO
BnO
AcO
AcO
OAc
OAc
OAc
OAc
6b
6c
Figure 2. Differently armed glucosyl donors.
ing effect of the 3,4,6-tri-O-benzyl ether groups since compound 5c
could still not be isolated as a stable product. The crude product
was characterized by 1H NMR spectroscopy which confirmed the
presence of the glycosylation product (Fig. 3) albeit some impuri-
ties were still present; further attempts to purify the compound
on silica gel chromatography led to decomposition. The key signals
in the 1H NMR spectrum of 5c are: d 6.68 (d, J = 6.8 Hz) for N–H;
4.92 (dd, J = 9.6 Hz, 10 Hz) for H-2, 4.34 (d, J = 10 Hz) for H-1,
3.42 (dd, J = 4.4 Hz, 15.2 Hz) and 3.05 (dd, J = 3.2 Hz, 14.8 Hz) for
the cysteinyl H-30a and H-30b. The identity of 5c was also con-
firmed by LC–MS high resolution mass spectroscopy which showed
the protonated parent ion at 638.2446 (calculated 638.2424).
When all the benzyl ether groups were replaced by disarming
acetates, the product (5e) was easily isolated in moderate yield
(64%) as a stable solid that survives purification by silica gel col-
umn chromatography. The variable stability of the products 5a,
5c, and 5e confirms that indeed an armed/disarmed effect is influ-
encing the stability of these glucosides. Compound 5e (entry 3) is
We continue to investigate the details of the mechanism of this
decomposition to determine the scope and its limitations by prop-
erly trapping and isolating the intermediates and products and will

M. G. Nokwequ et al. / Carbohydrate Research 359 (2012) 18–23
21
Figure 3. 1H NMR zoom of compound 6b and 5c showing the linkage of the glucose and cysteine fragments.
OBn
OBn
O
O
OBn
O
O
O
O
OBn
O
H
S
O
BnO
BnO
Nuc
H
S
BnO
BnO
BnO
BnO
BnO
BnO
NHAc
Nuc
OR
NHAc
OR
OR
OR
III
II
I
5a:
R = H
5c: R = Ac
+
O
O
HS
NHAc
Scheme 4. Mechanism to explain the decomposition of armed thioglucosides.
report these results in due time. However, we believe that the
observation that armed protecting groups on the donor preferen-
tially activate 3-mercaptopropionic acid thioglucosides compared
to the carboxylic acid protected thioglucosides is significant and
may be exploited to open new pathways for glycosylations. This
might include the possible glycosylation of suitably protected bicy-
clic donors 1 and 3.
out further purification. Reactions were monitored by TLC using
Silica Gel 60 UV254 (Alugram) pre-coated silica gel plates; detec-
tion was by means of a UV lamp and by heating the plate after
spraying with a solution of CAS [Preparation: 63 g Ceric ammo-
nium sulfate dissolved in 500 mL of 6% H2SO4 and diluted to 1 L
mark with distilled H2O]. Organic layers were dried over anhy-
drous MgSO4 prior to evaporation on a Buchi rotary evaporator
B-490 with a bath temperature of 40 °C. Column chromatography
was carried out on Machery Nagel Silica Gel 60. IR spectra were re-
corded on a Nicolet Impact 400 FT-IR spectrophotometer as thin
films on NaCl windows for oils or nujol mull for solids. 1H and
13C NMR spectra were recorded on Varian Gemini 400 at ambient
temperature in CDCl3. The splitting patterns are reported as fol-
lows: singlet (s), doublet (d), triplet (t), doublet of doublets (dd),
multiplet (m) and broad singlet (br s). Mass spectra were obtained
on a Waters Synapt G2 mass spectrometer. Optical rotations were
measured on an Atago AP100 polarimeter.
3. Experimental
3.1. General methods
All reactions were carried out under an inert N2 atmosphere.
Dichloromethane and acetic anhydride were dried by distilling
from P2O5 and acetonitrile was dried by distilling from CaH2.
BF3ꢀOEt2 was distilled and stored over activated molecular sieves
prior to use. All commercially available reagents were used with-

22
M. G. Nokwequ et al. / Carbohydrate Research 359 (2012) 18–23
3.2. N-Acetyl-S-(3,4,6-tri-O-benzyl-b-
cysteine methyl ester (5b)
D
-glucopyranosyl)-
L
-
of saturated NaHCO3 solution (20 mL). This mixture was stirred un-
til the fizzing stopped. The solution was extracted with ethyl ace-
tate (3 ꢁ 30 mL). The organic layers were combined and dried over
anhydrous MgSO4, filtered, and concentrated on the rotary evapo-
rator. The crude was purified by silica gel column chromatography
(3:2 EtOAc–hexane) to afford a brownish oil (1.80 g, 63%). This
compound was characterized by MS and NMR, even though the
NMR spectrum was not clean. The compound decomposed on at-
tempts to purify it further.
BF3ꢀOEt2 (1.00 mL, 7.29 mmol) was added dropwise to a solu-
tion of 1,2-anhydro-3,4,6-tri-O-benzyl- -glucopyranose (1.05 g,
2.43 mmol) and N-acetyl- -cysteine methyl ester (0.85 g,
a
-D
L
4.80 mmol) in dry CH2Cl2 (15 mL) at 0 °C. The reaction mixture
was stirred at 0 °C for 30 min and for a further 24 h at room tem-
perature. The reaction was quenched by diluting with CH2Cl2
(20 mL) and adding saturated NaHCO3 solution (20 mL). The aque-
ous phase was extracted with CH2Cl2 (3 ꢁ 20 mL) and the com-
bined organic layers were dried over anhydrous MgSO4, filtered,
and concentrated. The crude product was purified by silica gel col-
umn chromatography (1:9 hexane–EtOAc) to afford 5b as a clear
1H NMR (400 MHz, CDCl3): d 7.33–7.11 (m, 15H), 6.68 (d, 1H,
J = 6.8 Hz); 4.92 (dd, 1H, NH, J = 9.6 Hz, 10 Hz), 4.78 (dd, 1H,
J = 10 Hz, 10.4 Hz), 4.67–4.62 (m, 3H), 4.51–4.42 (m, 5H), 4.34 (d,
1H, H-1, J = 10 Hz), 3.70–3.59 (m, 3H), 3.50–3.40 (m, 2H) 4.34 (d,
J = 10 Hz), 3.42 (dd, 1H, J = 4.4 Hz, 15.2 Hz), 3.05 (dd, 1H,
J = 3.2 Hz, 14.8 Hz), 1.96 (s, 3H), 1.94 (s, 3H);13C NMR (100 MHz,
CDCl3): d 171.42, 170.28, 169.61, 137.72, 137.14, 135.73, 128.62,
128.57, 128.52, 128.50, 128.47, 128.45, 128.13, 128.04, 127.96,
127.89, 127.79, 127.72, 87.33, 84.29, 78.49, 77.60, 75.39, 73.68,
70.01, 52.44, 36.92, 22.90, 20.89; HR-ESIMS (m/z) calcd for
oil (0.53 g, 56%). ½a D26
ꢂ
+62.0 (c 1.0, MeOH), IR (neat, cmꢃ1): 3344
(b), 3057, 1746, 1669, 1601, 1010; 1H NMR (400 MHz, CDCl3): d
7.34–7.27 (m, 15H), 4.92–4.80 (m, 5H,), 4.60–4.47 (m, 4H); 4.28
(d, 1H, J = 9.2); 3.73 (s, 3H); 3.63–3.44 (m, 5H); 3.20 (dd, 1H,
J = 3.6 Hz, 14.8); 3.08 (dd, 1H, J = 6.4 Hz, 14.8 Hz); 1.89 (s, 3H);
13C NMR (100 MHz, CDCl3): d 170.69, 170.40, 138.36, 137.77,
137.42, 128.49, 128.44, 128.41, 128.19, 127.95, 127.91, 127.85,
127.82, 85.85, 75.22, 75.04, 73.59, 72.00, 68.00, 52.59, 32.98,
22.65; HR-ESIMS (m/z) calcd for C33H40NO8S (M+H+): 610.2475;
found: 610.2474.
C
34H40NO9S (M+H+): 638.2424; found: 638.2446.
3.5. N-Acetyl-S-(2-O-acetyl-3,4,6-tri-O-benzyl-b-
D-
glucopyranosyl)- -cysteine methyl ester (5d)
L
1,2-Di-O-acetyl-3,4,6-tri-O-benzyl-
D-glucopyranoside (0.37 g,
3.3. 1,2-Di-O-acetyl-3,4,6-tri-O-benzyl-D-glucopyranoside (6b)
0.69 mmol) and N-acetyl- -cysteine methyl ester (0.20 g,
L
1.04 mmol) were dissolved in dry CH2Cl2 (5 mL) under N2 flow.
This mixture was cooled in an ice bath followed by slow addition
of BF3ꢀOEt2 (0.2 mL, 1.50 mmol). This reaction mixture was stirred
at 0 °C for 30 min and then at room temperature for 12 h. The reac-
tion was quenched by addition of water (5 mL) followed by addi-
tion of saturated NaHCO3 solution (5 mL). This mixture was
stirred until the fizzing stopped. The solution was extracted with
ethyl acetate (3 ꢁ 20 mL). The organic layers were combined and
dried over anhydrous MgSO4, filtered, and concentrated. The crude
was purified by silica gel chromatography (1:9 hexane–EtOAc) to
afford 5d as a clear oil which solidified when left in the freezer
1,2-Anhydro-3,4,6-tri-O-benzyl- -glucopyranose
a
-D
(1.21 g,
2.80 mmol) was dissolved in 10 mL CH3CN followed by the addi-
tion of HCl (5 mL, 5 M). This mixture was stirred at room temper-
ature overnight. This mixture was then diluted with ethyl acetate
(20 mL) and the organic layer was separated from the aqueous
layer. The aqueous layer was extracted with ethyl acetate
(3 ꢁ 20 mL). The combined organic layers were dried over anhy-
drous MgSO4, filtered, and concentrated to dryness. The crude
3,4,6-tri-O-benzyl-D-glucopyranoside (1.25 g, 2.78 mmol) was ta-
ken-up in 5 mL dry pyridine followed by the addition of DMAP
(0.03 g, 0.27 mmol). This mixture was cooled to 0 °C followed by
the dropwise addition of Ac2O (9 mL, 0.1 mol). This reaction mix-
ture was stirred at 0 °C for 30 min and then at room temperature
overnight. The reaction mixture was then diluted with CH2Cl2
(50 mL) and then dumped on crushed ice. This mixture was al-
lowed to stand at room temperature until all the ice had dissolved.
The organic layer was separated and washed with HCl (2 ꢁ 20 mL,
2 M). The combined organic layers were dried over anhydrous
MgSO4, filtered, and concentrated. The crude was purified by silica
gel column chromatography (ethyl acetate) to isolate the diacetate
as a yellow oil (1.39 g, 94%).1H NMR (400 MHz, CDCl3): d7.28–7.07
(m, 15H), 6.28 (d, 1H, J = 10 Hz), 5.13 (dd, 1H, J = 9.2 Hz, 9.6 Hz),
4.89–4.63 (m, 5H), 4.50 (m, 2H), 3.75 (m, 4H), 3.57 (m, 1H), 1.99
(s, 3H), 1.97 (s, 3H); 13C NMR (100 MHz, CDCl3) d 171.35, 170.43,
138.13, 137.83, 137.71, 128.45, 128.40, 128.39, 127.99, 127.92,
127.84, 127.79, 127.76, 127.65, 83.11, 78.26, 77.45, 76.39, 75.44,
75.06, 73.56, 73.13, 67.93, 23.46, 20.82.
(0.25 g, 56%). ½a D26
ꢂ
+19.5 (c 5.0, CHCl3), IR (cmꢃ1): 3302, 3040,
1739, 1653, 1407, 1546, 1407, 1379; 1H NMR (400 MHz, CDCl3):
d 7.28–7.07 (m, 15H); 6.78 (d, 1H, J = 8 Hz); 4.92 (m, 1H); 4.76–
4.70 (m, 3H); 4.62–4.42 (m, 4H); 4.27 (d, 1H, J = 9.6 Hz); 3.72–
3.60 (m, 7H); 3.43 (m, 1H); 3.17 (dd, 1H, J = 3.6 Hz, 14.4); 2.96
(dd, 1H, J = 6.4 Hz, 14.8 Hz); 1.90 (s, 3H); 1.85 (s, 3H); 13C NMR
(100 MHz, CDCl3):
d 169.83, 169.06, 168.64, 136.98, 136.64,
136.442, 127.49, 127.45, 127.42, 127.13, 127.06, 127.01, 126.96,
126.82, 126.75, 83.11, 81.76, 78.24, 76.46, 74.27, 74.12, 70.01,
67.57, 51.44, 51.12, 30.38, 21.63, 19.89; HR-ESIMS (m/z) calcd for
C
35H42NO9S (M+H+): 652.2560; found: 652.2588.
3.6. N-Acetyl-S-(2,3,4,6-tetra-O-acetyl-b-
D
-glucopyranosyl)-
L
-
cysteine (5e)
1,2,3,4,6-Penta-O-acetyl-b-
mmol) and N-acetyl-
D
-glucopyranoside
(2.14 g,
5.48
L
-cysteine (1.34 g, 8.22 mmol) were dissolved
in dry CH2Cl2 (20 mL) under N2 flow. This was followed by drop-
wise addition of SnCl4 (1.3 mL, 10.96 mmol). The mixture was stir-
red at room temperature for 3 h, then diluted with CH2Cl2 (20 mL),
and washed with HCl solution (2 ꢁ 20 mL, 1 M). The organic phase
was dried over anhydrous MgSO4, filtered, and concentrated. The
crude was purified by column chromatography (1:9 MeOH–
3.4. N-Acetyl-S-(2-O-acetyl-3,4,6-tri-O-benzyl-b-
glucopyranosyl)- -cysteine (5c)
D-
L
1,2-Di-O-acetyl-3,4,6-tri-O-benzyl-D-glucopyranoside (2.40 g,
4.50 mmol) and N-acetyl- -cysteine (1.47 g, 9.0 mmol) were dis-
L
solved in dry CH2Cl2 (20 mL) under N2 flow. This mixture was
cooled in an ice bath followed by slow addition of BF3ꢀOEt2
(1.5 mL, 11.25 mmol). This reaction mixture was stirred at 0 °C
for 30 min and then at room temperature for 12 h. The reaction
was quenched by addition of water (10 mL) followed by addition
CH2Cl2) to afford a white foam. (1.76 g, 64%). ½a D21
ꢂ
28.30 (c 10.0,
MeOH), IR (cmꢃ1): 3289, 2957, 2731, 1745, 1678, 1543, 1457,
1375, 1228, 1043; 1H NMR (400 MHz, CDCl3): d 6.91 (d, 1H,
J = 7.6 Hz); 5.20 (t, 1H, J = 9.2 Hz); 5.05 (t, 1H, J = 9.6 Hz); 4.94
(dd, 1H, J = 9.2, 10 Hz); 4.75 (m, 1H); 4.56 (d, 1H, J = 10 Hz);

M. G. Nokwequ et al. / Carbohydrate Research 359 (2012) 18–23
23
4.21–4.13 (m, 2H); 3.73–3.68 (m, 1H), 3.21 (dd, 1H, J = 4.8 Hz,
References
14 Hz); 3.07 (dd, 1H, J = 6 Hz, 14 Hz); 2.04 (s, 3H), 2.02 (s, 3H),
1.99 (s, 3H), 1.97 (s, 3H), 1.94 (s, 3H); 13C (100 MHz, CDCl3): d
170.9, 170.6, 160.0, 169.9, 169.5, 169.4, 83.3, 76.2, 73.5, 69.7,
1. Bornemann, C.; Jardine, M. A.; Spies, H. S.; Steenkamp, D. J. Biochem. J. 1997,
325, 623–629.
2. Buchmeier, N. A.; Newton, G. L.; Koledin, T.; Fahey, R. C. Mol. Microbiol. 2003,
47, 1723–1732.
3. Jardine, M. A.; Spies, H. S. C.; Nkambule, C. M.; Gammon, D. W.; Steenkamp, D. J.
Bioorg. Med. Chem. 2002, 10, 875–881.
68.0, 61.8, 31.7, 20.7, 20.5; HR-ESIMS (m/z) calcd for C19H28NO12
S
(M+H+): 494.1332; found: 494.1335.
4. Miller, C. C.; Rawat, M.; Johnson, T.; Av-Gay, Y. Antimicrob. Agents Chemother.
2007, 51, 3364–3366.
3.7. N-Acetyl-S-(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosyl)-L-
5. Nkambule, C. M.; Kwezi, N. W.; Kinfe, H. H.; Nokwequ, M. G.; Gammon, D. W.;
Oscarson, S.; Karlsson, E. Tetrahedron 2010, 67, 618–623.
6. Gammon, D. W.; Hunter, R.; Steenkamp, D. J.; Mudzunga, T. T. Bioorg. Med.
Chem. Lett. 2003, 13, 2045–2049.
cysteine methyl ester (5f)
1,2,3,4,6-Penta-O-acetyl-b-
D
-glucopyranoside
(1.06 g,
7. Newton, G. L.; Av-Gay, Y.; Fahey, R. C. Biochemistry 2000, 39, 10739–10746.
8. Buchmeier, N.; Fahey, R. C. FEMS Microbiol. Lett. 2006, 264, 74–79.
9. Rawat, M.; Kovacevic, S.; Billman-Jacobe, H.; Av-Gay, Y. Microbiology 2003, 149,
1341–1349.
10. Sareen, D.; Steffek, M.; Newton, G. L.; Fahey, R. C. Biochemistry 2002, 41, 6885–
6890.
11. Huang, X.; Hernick, M. Anal. Biochem. 2011, 414, 278–281.
12. Hamilton, C. J.; Finlay, R. M. J.; Stewart, M. J. G.; Bonner, A. Anal. Biochem. 2009,
388, 91–96.
13. Stewart, M. J. G.; Jothivasan, V. K.; Rowan, A. S.; Wagg, J.; Hamilton, C. J. Org.
Biomol. Chem. 2008, 6, 385–390.
14. Metaferia, B. B.; Ray, S.; Smith, J. A.; Bewley, C. A. Bioorg. Med. Chem. Lett. 2007,
17, 444–447.
15. Fetterolf, B.; Bewley, C. A. Bioorg. Med. Chem. Lett. 2004, 14, 3785–3788.
16. Nicholas, G. M.; Eckman, L. L.; Newton, G. L.; Fahey, R. C.; Ray, S.; Bewley, C. A.
Bioorg. Med. Chem. 2003, 11, 601–608.
17. 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.
18. Gammon, D. W.; Steenkamp, D. J.; Mavumengwana, V.; Marakalala, M. J.;
Mudzunga, T. T.; Hunter, R.; Munyololo, M. Bioorg. Med. Chem. 2010, 18, 2501–
2514.
19. Boons, G.-J.; Geurtsen, R.; Holmes, D. Tetrahedron Lett. 1995, 36, 6325–6328.
20. Kim, J.-H.; Yang, H.; Park, J.; Boons, G.-J. J. Am. Chem. Soc. 2005, 127, 12090–
12097.
2.72 mmol) and N-acetyl-
L
-cysteine methyl ester (0.72 g,
4.07 mmol) were dissolved in dry CH2Cl2 (20 mL) under N2 flow.
This was followed by addition of SnCl4 (0.6 mL, 5.44 mmol). The
mixture was stirred at room temperature for 3 h, then diluted with
CH2Cl2 (20 mL) and washed with HCl solution (2 ꢁ 20 mL, 1 M).
The organic phase was dried over anhydrous MgSO4, filtered, and
concentrated. The crude was purified by column chromatography
(1:9 hexane–EtOAc) to afford a white foam (0.94 g, 69%). ½a D25
ꢂ
8.2
(c 6.2, CHCl3), IR (cmꢃ1): 3384, 3001, 2098, 1744, 1674, 1539,
1437, 1371, 1207, 1027, 920; 1H NMR (400 MHz, CDCl3): d 6.40
(d, 1H, J = 7.2 Hz); 5.15 (m, 1H); 5.00 (m, 1H); 4.91 (dd, 1H,
J = 9.2, 10.4 Hz); 4.77–4.73 (m, 1H); 4.48 (d. 1H, J = 10.4 Hz); 4.19
(dd, 1H, J = 2.8 Hz, 4.8 Hz); 4.15 (dd, 1H, J = 2.8 Hz, 4.8 Hz); 3.70
(s, 3H); 3.66 (m, 1H); 3.15 (dd, 1H, J = 4.8 Hz, 14 Hz); 2.99 (dd,
1H, J = 5.6 Hz, 14 Hz); 2.04 (s, 3H), 2.01 (s, 3H), 2.00 (s, 3H), 1.99
(s, 3H), 1.98 (s, 3H); 13C (100 MHz, CDCl3): d 170.9, 170.6, 160.0,
169.9, 169.5, 169.4, 83.3, 76.2, 73.5, 69.7, 68.0, 61.8, 31.7, 20.7,
20.5; HR-ESIMS (m/z) calcd for C20H30NO12S (M+H+): 508.1469;
found: 508.1495.
21. Fascione, M. A.; Adshead, S. J.; Stalford, S. A.; Kilner, C. A.; Leach, A. G.; Turnbull,
W. B. Chem. Commun. 2009, 5841–5843.
22. Cox, D. J.; Fairbanks, A. J. Tetrahedron: Asymmetry 2009, 20, 773–780.
23. Boltje, T. J.; Kim, J.-H.; Park, J.; Boons, G.-J. Org. Lett. 2011, 13, 284–287.
24. Slattegard, R.; Gammon, D. W.; Oscarson, S. Carbohydr. Res. 2007, 342, 1943–
1946.
25. Monsigny, M. L. P.; Delay, D.; Vaculik, M. Carbohydr. Res. 1977, 59, 589–593.
26. Käsbeck, L.; Kessler, H. Liebigs Ann. 1997, 1997, 165–167.
27. Salvador, L. A.; Elofsson, M.; Kihlberg, J. Tetrahedron 1995, 51, 5643–5656.
28. Cheshev, P.; Marra, A.; Dondoni, A. Carbohydr. Res. 2006, 341, 2714–2716.
29. Bernardes, G. J. L.; Grayson, E. J.; Thompson, S.; Chalker, J. M.; Errey, J. C.; El
Oualid, F.; Claridge, T. D. W.; Davis, B. G. Angew. Chem., Int. Ed. 2008, 47, 2244–
2247.
Acknowledgements
We acknowledge the institutional and financial support of the
Tshwane University of Technology and the financial support from
the National Research Foundation (NRF-South Africa) for the Thu-
thuka Grant for Researchers in Training (GUN 66173) to M.G.N.
and Humans and Institutional Capacity Development Programmes
grants (IRDP GUN 62464) to C.M.N.
30. Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Reid, B. J. Am. Chem. Soc.
1988, 110, 5583–5584.
31. Fraser-Reid, B.; Wu, Z.; Udodong, U. E.; Ottosson, H. J. Org. Chem. 1990, 55,
6068–6070.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.carres.
2012.06.022.
32. Veeneman, G. H.; van Boom, J. H. Tetrahedron Lett. 1990, 31, 275–278.
33. Veeneman, G. H.; van Leeuwen, S. H.; van Boom, J. H. Tetrahedron Lett. 1990, 31,
1331–1334.