Synthesis of a library of stereo- and regiochemically diverse aminoglycoside derivatives

Article abstract of DOI:10.1039/b505865a

A library of forty modified aminoglycosides was prepared in which the configuration and regiochemistry of two or three rings was widely varied. The library was based around three core ring systems: the 2-deoxystreptamine ring system found in the natural products, and both enantiomers of (1R*,2R*,4R*,5R*)-2,5-diamino-cyclohexane-1,4-diol and (1R*,3R*,4R*,6R*)-4,6-diaminocyclohexane-1,3-diol. In each case, the core was modified by glycosylation with one or two sugar rings. The absolute configuration of the sugar substituents (D or L), the configuration of the anomeric centres (α or β), and the regiochemical arrangement of the amine(s) were varied. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505865a

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A R T I C L E
Synthesis of a library of stereo- and regiochemically diverse
aminoglycoside derivatives†
Blandine Clique,a,b Alan Ironmonger,a,b Benjamin Whittaker,a,b Jacqueline Colley,a,b
James Titchmarsh,a,b Peter Stockleyb and Adam Nelson*a,b
a School of Chemistry, University of Leeds, Leeds, UK LS2 9JT
b Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, UK LS2 9JT
Received 27th April 2005, Accepted 27th May 2005
First published as an Advance Article on the web 23rd June 2005
A library of forty modified aminoglycosides was prepared in which the configuration and regiochemistry of two or
three rings was widely varied. The library was based around three core ring systems: the 2-deoxystreptamine ring
system found in the natural products, and both enantiomers of (1R*,2R*,4R*,5R*)-2,5-diamino-cyclohexane-1,4-diol
and (1R*,3R*,4R*,6R*)-4,6-diaminocyclohexane-1,3-diol. In each case, the core was modified by glycosylation with
one or two sugar rings. The absolute configuration of the sugar substituents (D or L), the configuration of the
anomeric centres (a or b), and the regiochemical arrangement of the amine(s) were varied.
orthogonally-protected sugar diamino acids have been exploited
Introduction
as building blocks in the synthesis of linear and branched
Libraries of stereo-and regioisomeric ligands can probe large
aminoglycoside derivatives.11 In addition, macrocyclic 15N-
areas of conformational space, and can be used to identify
labelled oligoaminodeoxydisaccharides have been prepared to
unnatural ligands for macromolecular targets.1 A key milestone
probe RNA binding events.12 These compounds, which have
in this area has been the synthesis of a 1,300-member library of
been derived from or have been inspired by aminoglycoside
acylated amino di- and tri-saccharides.2 This approach enabled
structure and function, are valuable tools for investigating RNA-
the identification of two compounds that exhibited a higher
mediated biological processes.
affinity for a bacterial lectin from Bauhinia purpurea than the
Here, we describe the synthesis of a library of aminoglycoside
known natural ligand. Furthermore, modifying the glycosy-
derivatives in which the configuration and regiochemistry of two
lation patterns of natural products can alter their targeting,
or three rings has been more widely varied. These compounds
biological activity and pharmacology.3 To this end, a chemoen-
retain the general structural features which enable the amino-
zymatic ‘glycorandomisation’ strategy has been developed in
glycosides to recognise specific RNA sequences. However, by
which the glycosylation patterns of natural products may be
exploring regions of conformational space that are not available
to the natural products, it was expected that novel ligands for
varied combinatorially.3
The aminoglycoside antibiotics (such as 1–5) interfere with
other important RNA sequences might be discovered. Such
protein synthesis by interacting with the 16S subunit of the
ligands could be exploited as chemical tools for investigating
bacterial ribosome, thereby inhibiting translation and causing
RNA-mediated biological processes.
miscoding.4 The structural basis of the recognition of amino-
The library is based around three core ring systems: the 2-
glycosides by the prokaryotic A site has been determined.5 In
deoxystreptamine ring system found in the natural products (see
addition, the aminoglycosides are recognised by a range of other
structures 6‡ and 9 / 9ꢀ,§ Fig. 1), and two alternative cores (see
RNA sequences: aminoglycosides inhibit the splicing of group I
introns,6 and can disrupt key protein–RNA recognition events;
for example, the formation of the RRE–Rev7 and TAR–Tat8
‡ The 2-deoxystreptamine core is prochiral, so its symmetrically substi-
tuted derivatives may be designated as 6XY; the order of the substituents
denotes how the enantiotopic 4- and 6-hydoxy groups have been
substituted (first letter and second letter respectively). Throughout the
complexes required in the life cycle of the HIV virus.
A wide range of modified aminoglycosides, in which
substitution9 of one or more carbohydrates10 was varied
systematically, have been prepared previously. Furthermore,
paper, the descriptors X and Y refer to generic glycosyl substituents.
§ The 4,5- and 5,6-disubstituted derivatives of 2-deoxystreptamine are
designated 9XY and 9ꢀXY respectively; the second letter denotes the
† Electronic supplementary information (ESI) available: characterisa-
tion data. See http://dx.doi.org/10.1039/b505865a
5-substituent of the core.
2 7 7 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Fig. 1 Core ring systems in the aminoglycoside derivatives. Sug1 and Sug2 denote the generic substituents: either hydrogen or the sugars A–H/Aꢀ–Hꢀ.
Fig. 2 Variations in the sugar substituents.
Fig. 3 Examples of possible aminoglycoside derivatives.
structures 7 / 7ꢀ and 8 / 8ꢀ,¶ Fig. 1). In each case, the core has
been further modified by glycosylation with one or two of the
substituents A–H/Aꢀ–Hꢀ (see Fig. 2). The absolute configuration
of the sugar substituents (D or L), the configuration of the
anomeric centres (a or b), and the regiochemical arrangement
of amine(s) have been varied widely.* Examples of possible
aminoglycoside derivatives include those shown in Fig. 3.
Synthesis of the cores
The 2-deoxystreptamine core, prepared by degradation of
neomycin,13 was desymmetrised by enantioselective ester hydrol-
ysis. Using this approach, the meso-triacetate 10 was converted
into the chiral derivative 11 with >95% ee (Scheme 1).10b
Scheme 1
Both enantiomers of the C2-symmetrical diols 13 and 15
were prepared by desymmetrisation of the corresponding
bisepoxides14 12 and 14 by double enantioselective epoxide
opening with Me3SiN3 (Scheme 2).15 The enantiomeric diols 13
and 13ꢀ were prepared by desymmetrisation of the bisepoxide 12
using 2 mol% of the catalysts (R,R)- and (S,S)-16; the products
13 / 13ꢀ were shown to have 70% ee by chiral HPLC. The
desymmetrisation of the bisepoxide 14 using (R,R)- and (S,S)-16
Scheme 2
¶ The cores of 7 and 8 are C2-symmetrical, and two enantiomeric cores
are possible in each case (7 / 7ꢀ and 8 / 8ꢀ). Here, the order of the
substituents is irrelevant (7ꢀAB and 7ꢀBA would be the same compound);
however, by convention, we have listed the substituents in alphabetical
order.
* All D sugars are designated A–H and L sugars Aꢀ-Hꢀ; the substituents
A, C, E and G are a-configured and B, D, F and H are b-configured.
gave the required diols, 15 and 15ꢀ, in good yield, together with
the centrosymmetric diol 17 (5% yield). The major products
15 and 15ꢀ were shown by chiral HPLC to have >96% ee.
The absolute configurations of 13 / 13ꢀ and 15 / 15ꢀ have
been assigned by analogy with many previous examples of
enantioselective epoxide openings.15
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5
2 7 7 7

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Desymmetrisation of a centrosymmetric diacetate
The centrosymmetric diol 17 was converted into the correspond-
ing diacetate 18, and desymmetrised16 by enantioselective ester
hydrolysis using pig liver esterase (Scheme 3). The corresponding
hydroxy acetate 19 was obtained in 45% yield, and was shown
to have >90% ee by conversion into the diastereomeric esters
20a and 20b derived from (+)- and (−)-camphanic acid. The
absolute configuration of 17 has been assigned by analogy with
the desymmetrisation of a similar centrosymmetric diacetate
using the same enzyme.16f The hydroxy acetate 19 is a protected
version of a diastereoisomer of 13, and could be used as an
additional core for the synthesis of aminoglycoside derivatives.
Scheme 3
Preparation of the glycosyl donors
The glycosyl donors 21 were prepared using literature
procedures,10b,17 or using the synthetic approaches described in
Scheme 4. The donor 21B was prepared by benzoylation of
23. Global deacetylation of 24, diazo transfer17 and then p-
tosylation gave the p-tosylate 25; displacement using sodium
azide (→26) and benzylation gave the donor 21G. The donors
21Aꢀ, 21Bꢀ and 21Hꢀ were prepared from L-glucose pentaacetate
using methods which were analogous to those used to prepare
the enantiomeric donors.10b,16,18
Glycosylation of the cores
The cores 11, 13, 13ꢀ, 15 and 15ꢀ were exploited in the preparation
of the monoglycosylated aminoglycoside derivatives 27X–29X
(see Scheme 5 and Table 1). In order to minimise the number
of individual intermediates to be prepared, it was decided to
introduce greater structural diversity in the second glycosylation
step (see below). Accordingly, the cores were glycosylated with
around three of the appropriate donors 21X under the reaction
Scheme 4
2 7 7 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5

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conditions which are summarized in Table 1; the products were
purified, using preparative HPLC where necessary.
Under our preferred reaction conditions (NIS, 10 mol%,
AgOTf, 4 A molecular sieves, CH2Cl2, 0 ◦C), the stereoselec-
˚
tivity of the glycosylations with 21A / 21Aꢀ and 21B were
complementary and highly stereoselective (entries 1a–c, Table 1):
glycosylation of the core 11 with 21A / 21Aꢀ and 21B gave the
required products 27A / 27Aꢀ and 27B as the only detectable
diastereoisomers (a and b, respectively). The protected amino-
glycoside derivatives 27A and 27Aꢀ were deacetylated under
standard conditions to give 31A and 31Aꢀ (Scheme 6).
Scheme 6
The C2 symmetry of 13 / 13ꢀ and 15 / 15ꢀ, and the hence
homotopicity of their hydroxyl groups, led us to glycosylate these
cores without first protecting one of the alcohol groups. How-
ever, the functionalisations of 15 / 15ꢀ were still complicated by
the possibility of a second glycosylation and by their insolubility
in dichloromethane (entries 3a–g, Table 1). With these cores, the
glycosylation reactions were performed using a mixed solvent
system, ether–CH2Cl2. Under these conditions, glycosylation
was less stereoselective.19 For example, 15ꢀ was glycosylated with
21A to give the expected products 29ꢀA and 29ꢀAA, as well as
the side product†† 29ꢀAb in which one of the sugar substituents
is b-linked (compare entries 1a–b with entry 3b).
Scheme 5
Table 1 Synthesis of the protected aminoglycosides 27, 28 and 29
The ratio of mono-and di-glycosylated products could be
varied by changing the equivalents of donor used. For example,
with 1.2 equivalents of 21F relative to the acceptor 15ꢀ, the
monoglycosylated product, 29ꢀF, was obtained in 45% yield.
However, with 2.0 equivalents of the donor, an optimized 33%
yield of the diglycosylated product, 29ꢀFF, was obtained.
The reactions of the cores 13 and 13ꢀ were further complicated
by the poor enantiomeric excesses (70% ee) of these compounds.
Glycosylation of the acceptor 13 with the donor 21A (entry 2,
Table 1) gave mixtures of mono-glycosylated products (32%)
and di-glycosylated products (32%) (which were then separated
by preparative HPLC for characterisation). A summary of the
purified products of the reaction is provided in Scheme 7. Not
only was the reaction poorly diastereoselective, leading to 28b
and 28Ab, but significant yields of the products 28ꢀA and 28ꢀAA,
stemming from the minor enantiomer of the acceptor, were also
obtained.
Starting
Entry material
Donor
Conditionsa Product
Yieldb (%)
1a
1b
1c
2
11
11
11
13c
21A
21Aꢀ
21B
21A
A
A
A
B
27A
65
55
75
13
3
4
15
2
27Aꢀ
27B
28Ad
28ꢀAd
28bd
28AAe
28ꢀAAe
28ꢀAbe
29A
5
3a
3b
15
21A
21A
A
B
29
15
2
1
8
45
33
14
26
15ꢀ
29ꢀA
29ꢀAA
29ꢀAb
29ꢀB
3c
3d
3e
3f
15ꢀ
15ꢀ
15ꢀ
15
21B
21F
21Ff
21G
21G
A
A
A
B
B
29ꢀF
29ꢀFF
29G
The monoglycosylated products, 28X, 29X and 31X, together
with 30G, prepared by the controlled degradation of a derivative
of neomycin,20 were further modified by glycosylation (see
Scheme 8 and Table 2). The glycosylations of 31A and 31Aꢀ
were, on the whole, rather straightforward (entries 1–16). Some
3g
15ꢀ
29ꢀG
A: NIS, 10 mol% AgOTf, 4 A molecular sieves, CH2Cl2, 0 ◦C; B: NIS,
a
˚
10 mol% AgOTf, 4 A molecular sieves, Et2O–CH2Cl2, 0 ◦C. b Yield
of purified product. c The starting material had 70% ee. d 32% of a
diastereomeric mixture of 28A, 28ꢀA and 28b was obtained; an aliquot
of the product was purified by preparative HPLC to give the yields of
purified products shown. e 32% of a diastereomeric mixture of 28AA,
28ꢀAA and 28Ab was obtained; an aliquot of the product was purified
by preparative HPLC to give the yields of purified products shown. f 2.0
equivalents of the donor was used.
˚
†† Lower case letters are used to describe the side-products of unselective
glycosylation reactions. Here, in 29ꢀAb, the second sugar ring is b-linked.
The use of the lower case descriptor signifies that the protecting group
pattern is not the same as that found in the donor 21B. Hence,29ꢀAB
and 29ꢀAb bear different protecting groups, but deprotection would give
the same product, 8ꢀAB.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5
2 7 7 9

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Scheme 7
2 7 8 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5

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Table 2 Synthesis of the protected aminoglycosides 27, 28, 29 and 30
Starting
Entry material
Donor Conditionsa Product Yieldb (%)
1
2
3
4
5
6
7
8
9
31A
31A
31Aꢀ
31Aꢀ
31A
31A
31Aꢀ
31Aꢀ
31A
21A
21Aꢀ
21A
21Aꢀ
21B
21Bꢀ
21B
21Bꢀ
21C
A
A
A
A
A
A
A
A
A
27AA
27AAꢀ
27AꢀA
27AꢀAꢀ
27AB
27ABꢀ
27AꢀB
27AꢀBꢀ
27AC
27Ad
62
50
21
26
51
28
32
26
30
11
40
12
54
37
24
24
19
26
51
42
21
5
10
45
3
5
42
3
9
30
4
11
8
8
10
31Aꢀ
21C
A
27AꢀC
27Aꢀd
27AD
27AꢀD
27AE
27Af
11
12
13
31A
31Aꢀ
31A
21D
21D
21E
A
A
A
14
31Aꢀ
21E
A
27AꢀE
27Aꢀf
15
16
17
31A
31Aꢀ
28Ac
21F
21F
21Aꢀ
A
A
A
27AF
27AꢀF
28AAꢀ
28ꢀAAꢀ
28Aꢀb
28AB
28ꢀAB
28Bb
18
19
20
28Ac
28Ac
28Ac
21B
21Bꢀ
21C
A
A
A
28ABꢀ
28ꢀABꢀ
28bBꢀ
28AC
28ꢀAC
28Ad
28bC
21
22
23
24
25
26
27
28
29
30
31
28Ac
29A
29ꢀA
29ꢀA
29ꢀA
29ꢀA
29ꢀF
29ꢀF
30G
30G
30G
21D
21A
21Aꢀ
21B
21Bꢀ
21C
21B
21F
21F
21H
21Hꢀ
A
B
B
A
A
B
A
A
A
B
B
28AD
29AA
29ꢀAAꢀ
29ꢀAB
29ꢀABꢀ
29ꢀAC
29ꢀBF
29ꢀFF
30GF
30GH
30GHꢀ
18
28
37
34
48
11
33
25
22
38
A: NIS, 10 mol% AgO◦Tf, 4 A molecular sieves, CH2Cl2, 0 ◦C; B:
a
˚
˚
Scheme 8
BF3·Et2O, CH2Cl2, −60 C, 4 A molecular sieves. b Yield of purified
product. c The starting material was a ca. 70 : 15 : 15 diastereomeric
mixture of 28A, 28ꢀA and 28b.
of the reactions were complicated by poor stereoselectivity: in
particular, glycosylations involving the donors 21C and 21E gave
ca. 3 : 1 mixtures of aand banomers (see entries 9–10 and 13–14).
For the second glycosylation reaction, of the core 28, we found
it easiest to use directly the ca. 70 : 15 : 15 diastereomeric
mixture of 28A, 28ꢀA and 28b which had been prepared by
glycosylation of acceptor 13 (see Scheme 7). Accordingly,
the mixture of acceptors was glycosylated with four different
donors, and the products were purified by preparative HPLC
(entries 17 and 21). Here, the glycosylations were performed
in neat dichloromethane; under these conditions,19 high levels
of diastereoselectivity were observed with donor 21Aꢀ, and the
corresponding a-linked products 28AAꢀ, 28ꢀAAꢀ and 28Aꢀb were
obtained (compare entry 17, Table 2 with entry 2, Table 1). With
donor 21C, a lower level of diastereoselectivity was observed,
resulting in a significant (11%) yield of the b-linked product 28Ad
(entry 20, Table 2). The glycosylations of the acceptors 29X were
straightforward, and the corresponding diglycosylated products
were easily obtained in each case (see entries 22–28, Table 2).
The acceptor 30G, prepared by controlled degradation of
a neomycin derivative,20 was rather more hindered than those
previously used. A reasonable (25%) yield of 30GF was obtained
with donor 21F under our standard glycosylation conditions. In
order the install the H and Hꢀ rings, however, it proved necessary
to use the imidate21, 22 donors 21H and 21Hꢀ in conjunction
with a Lewis acid (boron trifluoride etherate):10c under these
conditions, reasonable yields of the required products 30GH
and 30GHꢀ were obtained.
Deprotection of the aminoglycoside derivatives
The protected aminoglycosides 27–30 were deprotected under
standard conditions to give the aminoglycoside derivatives 6–9
(Table 3) . First, acyl and/or phthalimido protecting groups
were removed, if necessary. Two alternative procedures were
used for the reduction of the azides: either Staudinger10b reaction
with PMe3, followed by purification through a short silica–celite
column, or hydrogenolysis10c in 1 : 1 : 1 EtOAc–MeOH–
H2O. Finally, the remaining benzyl groups were removed by
hydrogenation in 1 : 1 H2O–AcOH to give the unprotected
aminoglycoside derivatives as their peracetate salts. In practical
terms, our preferred method for azide reduction and benzyl
deprotection involved the two hydrogenolysis reactions: first in
1 : 1 : 1 EtOAc–MeOH–H2O and then in 1 : 1 H2O–AcOH.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5
2 7 8 1

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2 7 8 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5

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Table 3 Deprotection reactions to yield the aminoglycosides 6, 7, 8 and 9
Summary
A total of 40 aminoglycoside derivatives were prepared that
might be expected to probe large regions of conformational
space. The most dramatic variation was the central core of the
derivatives: although most of the derivatives (6 and 9) had the 2-
deoxystreptamine core found in the aminoglycoside antibiotics,
C2-symmetrical cores were also incorporated in the derivatives
7 and 8. The configuration of the aminoglycosides was varied
widely: both D- and L-sugars were attached to the cores through
both a- and b-glycosidic linkages. Furthermore, the number and
regiochemistry of the amino substituents in the sugar rings were
varied.
The library might be exploited as a refined tool for the
analysis of many RNA-mediated biological processes. Since the
library is based on structures which are largely isomeric, the
specificity of interactions with RNA may be probed critically
by in vitro study. Furthermore, the library may be a valuable
resource in chemical genetic studies, with isomeric structures
providing useful controls in cell-based assays. The details of
these experiments will be reported in due course.
Entry
Starting material
Conditionsa
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
31A
a,b
a,b
e,b
e,b
e,b
e,b
a,b
a,b
a,b
a,b
c,a,b
c,a,b
a,b
a,b
a,b
a,b
a,b
a,b
a,b
d,a,b
d,a,b
e,b
e,b
e,b
c,e,b
c,e,b
c,e,b
c,e,b
e,b
e,b
e,b
e,b
e,b
c,e,b
e,b
c,e,b
e,b
d,a,b
c,a,b
c,a,b
6A
86
51
42
100
67
100
77
25
46
41
37
58
48
33
88
56
67
63
6
31Aꢀ
6Aꢀ
29A
8A
29ꢀA
8ꢀA
29G
7G
29ꢀG
7ꢀG
27AA
27AAꢀ
27AꢀA
27AꢀAꢀ
27AB
27ABꢀ
27AꢀB
27AꢀBꢀ
27AꢀC
27AD
27AꢀD
27AE
27AꢀE
27AF
27AꢀF
28AA
28ꢀAA
28ꢀAAꢀ
28AB
28ꢀAB
28ABꢀ
28ꢀABꢀ
28AC
28ꢀAC
29ꢀAA
29AA
29ꢀAAꢀ
29ꢀAB
29ꢀAb
29ꢀABꢀ
29ꢀAC
30GF
30GH
30GHꢀ
6AA
6AAꢀ
6AꢀA
6AꢀAꢀ
6AB
6ABꢀ
6AꢀB
6AꢀBꢀ
6AꢀC
6AD
6AꢀD
6AE
6AꢀE
6AF
6AꢀF
7AA
7ꢀAA
7ꢀAAꢀ
7AB
7ꢀAB
7ABꢀ
7ꢀABꢀ
7AC
7ꢀAC
8ꢀAA
8AA
8ꢀAAꢀ
8ꢀAB
8ꢀAB
8ꢀABꢀ
8ꢀAC
9GF
9GH
9GHꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
4
4
85
84
68
77
98
83
98
79
73
58
98
77
59
82
97
59
42
48
61
Experimental
Characterisation data for all of the compounds described in this
paper is provided in the electronic supplementary information.
General procedures for the synthesis of the aminoglycoside
library are provided below.
General procedure for glycosylation (Method A)
The glycosyl donor (0.43 mmol) and the acceptor (0.34 mmol),
both freshly dried azeotropically by removal of toluene, were
dissolved in dry dichloromethane (2.6 mL) and transferred
via syringe into a flame dried round bottom flask containing
˚
activated 4 A molecular sieves. The reaction mixture was cooled
◦
to 0 C, N-iodosuccinimide (104 mg, 0.46 mmol) and silver(I)
trifluoromethanesulfonate (9 mg, 0.03 mmol) were added simul-
taneously, the reaction was stirred for 2 h and then quenched
with Et3N (1 mL). The reaction mixture was filtered through
celite, eluting with dichloromethane (15 mL), then washed with
10% aqueous Na2S2O3 solution (2 × 10 mL) and brine (2 ×
10 mL). The combined organic extracts were dried (Na2SO4) and
concentrated under reduced pressure to give the crude product.
a (a) PMe3, THF–0.1 M NaOH solution; (b) H2, Pd(OH)2/C (20%
Degussa type), 1 : 1AcOH–H2O; (c) NaOMe, MeOH; (d) N2H4, toluene–
EtOH, 110 ◦C; (e) H2, Pd(OH)2/C (20% Degussa type), 1 : 1 : 1 EtOAc–
MeOH–H2O. b Yield of purified products.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 7 7 6 – 2 7 8 5
2 7 8 3

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General procedure for glycosylation (Method B)
removed under reduced pressure to give the crude product which
was purified by column chromatography (elution: 7 : 3 petrol–
EtOAc) to yield a solution of the crude product which was
evaporated under reduced pressure.
The glycosyl donor (5.57 mmol) and the acceptor (7.06 mmol),
both freshly dried azeotropically by removal of toluene, were
dissolved in dry dichloromethane (13 mL) and diethyl ether
(40 mL), respectively, and transferred via syringe into a
Alternative procedure for the deprotection of benzylated
aminoglycosides (Method G)
˚
flame dried round bottom flask containing activated 4 A
molecular sieves. The reaction mixture was cooled to 0 ◦C,
N-iodosuccinimide (1.63 g, 7.23 mmol) and silver(I) trifluo-
romethanesulfonate (286 mg, 1.11 mmol) were added simul-
taneously, the reaction was stirred for 3 h and then quenched
with Et3N (5 mL). The reaction mixture was filtered through
celite, eluting with dichloromethane (50 mL), then washed with
10% aqueous Na2S2O3 solution (2 × 50 mL) and brine (2 ×
50 mL). The combined organic extracts were dried (Na2SO4) and
concentrated under reduced pressure to give the crude product.
The perbenzylated azidoaminoglycoside (142.4 mg, 0.12 mmol)
was dissolved in 1 : 1 : 1 EtOAc–MeOH–H2O (6 mL),
Pd(OH)2/C (150 mg) was added and the reaction was stirred
under an atmospheric pressure of hydrogen. After 2 d, the
reaction mixture was filtered through a short pad of celite,
eluting sequentially with ethyl acetate, methanol and water. The
filtrate was concentrated under reduced pressure, redissolved in a
degassed solution of 1 : 1 AcOH–H2O (4 mL), Pd(OH)2/C (20%
Degussa type) added and the reaction mixture stirred under an
atmospheric of hydrogen. After 2 d, the reaction mixture was
filtered under a short pad of celite, eluting with water. The
filtrate was concentrated under reduced pressure to give the
crude product.
General procedure for glycosylation with a trichloroacetimidate
donor (Method C)
A solution of the trichloroacetimidate donor (1 equivalent),
the acceptor (1.2 equivalents) and powdered 4 A molecular
˚
sieves (80 mg) in dichloromethane (5 mL) were stirred at
room temperature for 1 h. The reaction mixture was cooled to
−60 ◦C and boron trifluoride diethyl etherate (0.1 equivalents)
was added dropwise. The reaction was stirred at −60 ◦C for
2 h and then quenched with solid NaHCO3 and stirred for
15 min. The mixture was filtered through celite, washing with
dichloromethane and EtOAc, and the solvent was removed
under reduced pressure to yield the crude product.
Acknowledgements
We thank BBSRC and the Wellcome Trust for funding, and the
EPSRC Mass Spectrometry Service, Swansea, for accurate mass
determinations.
References
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General procedure for the deprotection of benzylated
aminoglycoside derivatives (Method D)
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