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Chemical modification of the AMG class of molecules
has been well documented;4 however, systematic
replacement of the sugars substituents has not been well
explored until recently due to the inherit problem that
carbohydrate-based medicinal chemistry is laborious.
Complementary work by Chang and co-workers at
Utah has shown that the 5-linked disaccharide (ring III
and IV) in neomycin B and recently the 6-linked
monosaccharide in kanamycin (ring III) could be
replaced by novel amino sugars while retaining a sig-
nificant degree of antimicrobial activity.5 Our strategy
was to use a tobramycin-derived pseudo-disaccharide
because of tobramycin’s clinical importance and then
modify the 6-linked sugar using our glyco-optimization
technology to gain insight into the sugar moiety’s con-
tribution to antimicrobial activity.
with a series of p-tolylthioglycoside derivatives gener-
ated a library of novel pseudo-trisaccharides. These
thioglycoside donors 6a–t, with high to low reactivity
values, were activated by a combination of NIS–TfOH
affording pseudo-trisaccharides 7a–y in good to excel-
lent yield (42–92%) followed by deprotection to afford
8a–y (Scheme 2 and Table 1, respectively).7 For glyco-
sylation with donor 6r, we employed NIS–AgOTf as the
activating reagent due to the presence of the N,N-di-
methyl group. The a-isomers were favored as expected
for thioglycosides with nonparticipating groups (i.e., H,
OBn, and N3 in the present study) at the 2-position,
while 6n, possessing the b-directing 2-O-Bz, gave the b-
isomer as expected.
3.2. Pseudo-tetrasaccharide preparation via OPopSTM
OPopSTM is Optimer’s proprietary chemistry technology
for glyco-optimization and the creation of oligosacchar-
ide diversity. In the process, a reaction is started with a
highly reactive thioglycoside sugar donor with no free
OH group (donor A) and a less reactive thioglycoside
acceptor with one free OH group (donor B). The reac-
tivity of donor A is normally 10 times greater than that
of donor B, which serves as an acceptor during the first
reaction. The in situ formed disaccharide A–B, conve-
niently monitored by TLC, is subsequently activated
with an additional equivalent of activator in the pres-
ence of the final acceptor, in this case nebramine
derivative 4, to produce a diverse series of pseudo-tet-
rasaccharides in good yields (Scheme 3). Because of the
difference in the reactivity of thioglycoside derivatives
and the ability to monitor each step of the process by
TLC, the sequence of the final products has been shown
to be highly predictable.8 Thus, using this powerful
glyco-optimization strategy we have prepared a first set
of diverse 4,6-linked pseudo-tetrasaccharide AMG
derivatives from a selection of aminosugars in an
attempt to identify a unique AMG antibiotic.
2. Preparation of a nebramine aglycone
The suitably protected nebramine aglycone 4 was
obtained by the degradation of tobramycin (1) as out-
lined in Scheme 1. Treatment of 1 with trifluoro-
methanesulfonyl azide6 gave per-azido tobramycin 2,
quantitatively, which was benzylated to afford a fully
protected tobramycin 3. Regioselective, acidic hydroly-
sis of 3 gave the desired protected nebramine 4 with the
free 6-OH group in good yield.
3. Glyco-optimization (OPopSTM
)
3.1. Pseudo-trisaccharide preparation
We first performed glycosylation of the nebramine
derivative 4 with a series of monosaccharides with var-
ious functionalities 6a–t. Glycosylation of acceptor 4
N3
O
NH2
O
3.3. Deprotection
RO
HO
N3
H2N
N3
H2N
O
O
N3
OR
NH2
OH
a
Global deprotection of the fully protected pseudo-tri-
and pseudo-tetrasaccharides was carried out in three
steps: (1) O-debenzoylation with NaOMe, (2) Stau-
dinger reduction for the transformation of azido group
to amino group, and (3) O-debenzylation by hydro-
genolysis over 20% Pd(OH)2/carbon without any major
problems. The final AMG derivatives (8a–y and 10a–n)
were purified using preparative-LCMS on C-18 and a
modified mobile phase.9 The compounds were obtained
as hygroscopic pentafluoropropionic acid (PFPA) salts.
The salts were converted to free-base on Dowexâ
50WX4-400 and eluted with aqueous ammonia
hydroxide and freeze-dried to obtain a white powder.
NMRs were obtained to assess the anomeric ratios. The
spectra of free-bases could not be interpreted due to
broad signals and multiple protonation states. Addition
of a drop of 0.1 M DCl in D2O produced sharp signals
and the anomeric ratios were reported in Table 1. In
general, the deprotection and purification process yield-
RO
HO
O
O
O
O
N3
NH2
OR
OH
RO
HO
1
2 R = H
3 R = Bn
b
N3
O
c
BnO
N3
N3
O
N3
BnO
OH
4
Scheme 1. Preparation of 40,5-O-dibenzyl-per-azidonebramine 4.
Reagents and conditions: (a) N3Tf, CuSO4 (cat.), TEA, DCM–
MeOH–H2O; (b) NaH, BnBr, DMF, 0 °C to rt; (c) H2SO4, MeOH,
reflux.
Yao, Sulan
