Oligomerization of a rhamnanic trisaccharide repeating unit of O-chain polysaccharides from phytopathogenic bacteria

Article abstract of DOI:10.1016/S0040-4039(02)02237-2

An efficient synthesis of a protected trisaccharide building block α-L-Rha(1→3)-α-L-Rha(1→2)-α-L-Rha related to the structure of many lipopolysaccharide O-chains from phytopathogenic bacteria has been developed. The protecting group pattern consisting of benzoyl, benzyl and chloroacetyl groups facilitated the use of the trisaccharide building block in the synthesis of two higher oligomers, an oligorhamnosyl hexasaccharide and a nonasaccharide.

Full text of DOI:10.1016/S0040-4039(02)02237-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8879–8882
Oligomerization of a rhamnanic trisaccharide repeating unit of
O-chain polysaccharides from phytopathogenic bacteria†
Emiliano Bedini,a,b Michelangelo Parrillia,* and Carlo Unverzagtb,*
aDipartimento di Chimica Organica e Biochimica, Universita` di Napoli ‘Federico II’, Complesso Universitario Monte Santangelo,
Via Cintia 4, 80126 Napoli, Italy
bBioorganische Chemie, Geba¨ude NWI, Universita¨t Bayreuth, 95440 Bayreuth, Germany
Received 26 September 2002; revised 30 September 2002; accepted 2 October 2002
Abstract—An efficient synthesis of a protected trisaccharide building block a-
L
-Rha(1“3)-a-
L
-Rha(1“2)-a-L-Rha related to the
structure of many lipopolysaccharide O-chains from phytopathogenic bacteria has been developed. The protecting group pattern
consisting of benzoyl, benzyl and chloroacetyl groups facilitated the use of the trisaccharide building block in the synthesis of two
higher oligomers, an oligorhamnosyl hexasaccharide and a nonasaccharide. © 2002 Elsevier Science Ltd. All rights reserved.
It has been recently reviewed1 that the repeating units
of many lipopolysaccharide O-chains from phyto-
pathogenic bacteria consist of rhamnan backbones.
One of the most frequently found sequences in the
rhamnan backbones is the trisaccharide repeating unit
described in this paper aimed at the synthesis of the
trirhamnoside repeating unit E as a building block
that be easily functionalized as a glycosyl donor and a
glycosyl acceptor, thus permitting the stepwise conden-
sation of the repeating units to higher oligosaccha-
rides. In addition this method should be extendable by
modifying the building blocks to synthesize the related
linear and branched rhamnan repeating units of other
lipopolysaccharide O-chains of phytopathogenic bacte-
ria.
[“3)-a-
L
-Rhap(1“3)-a-
L
-Rhap(1“2)-a- -Rhap(1“]n
L
(Scheme 1), which was synthesized some years ago.2 In
order to obtain oligomers of this structural motif for
structure–activity relationship studies, a new synthetic
strategy was developed. The synthetic approach
Scheme 1. Retrosynthesis of the trirhamnoside repeating unit found in the lipopolysaccharide O-chains from Pseudomonas
savastanoi strains ITM 519, ITM 317 and PVF 5, Pseudomonas tabaci strain IMV 223, and Pseudomonas begoniae strain GSPB
525.
Keywords: glycosylation; oligosaccharide; lipopolysaccharide; repeating unit; phytopathogenic bacteria.
* Corresponding authors. Tel.: +39-81-674147; fax: +39-81-674393 (M.P.); Tel.: +49-921-552670; fax: +49-921-555265 (C.U.); e-mail:
parrilli@unina.it; carlo.unverzagt@uni-bayreuth.de
† This paper is dedicated to Professor Lorenzo Mangoni on the occasion of his 70th birthday.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02237-2

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E. Bedini et al. / Tetrahedron Letters 43 (2002) 8879–8882
The oligomerization of repeating unit building blocks
has been used previously in the synthesis of oligosac-
charide chains.3,4 However, it was not clear from the
beginning in which way the envisioned oligorhamnoside
sequence could be assembled. Therefore the two rham-
noside building blocks A and B were deducted allowing
maximum flexibility in the sequence of glycosylation.
The assembly of the trisaccharide was initiated by
coupling of the glycosyl acceptor A and the donor B
(Scheme 3). Following the typical conditions for the
activation of the disarmed thioglycosides with N-iodo-
succinimide and triflic acid (NIS/TfOH),9 the disaccha-
ride
6 was obtained in high yield (83%). The
a-configuration of the newly formed glycosidic linkage
in 6 was ascertained by the heteronuclear C-1%,H-1%
coupling constant of 173 Hz measured in a coupled
HMQC-COSY experiment.10 Selective removal of the
chloroacetyl moiety was achieved by treating the disac-
charide 6 with thiourea in ethanol.11 The disaccharide
acceptor 7 was obtained in 80% yield without any
detectable benzoyl migration. Subsequent coupling with
glycosyl donor B using the NIS/TfOH activation
method afforded the a-configurated trisaccharide 8 in
high yield (96%, JC-1¦,H-1¦=174 Hz). Trisaccharide 8
was readily converted to the acceptor trisaccharide D
(83%) by dechloroacetylation with thiourea in ethanol.
The glycosyl acceptor A was synthesized as outlined in
Scheme 2.
L-Rhamnose was first protected at the
anomeric position by a Fischer glycosylation furnishing
benzyl a-
L
-rhamnopyranoside 25 in 54% yield.
Regioselective isopropylidenation of the triol 2 with
2,2-dimethoxypropane/TsOH gave the alcohol 3 (98%),
which was subsequently benzoylated followed by acid
hydrolysis of the acetonide. The resulting diol 4 (90%
yield) was selectively benzoylated at the more reactive
equatorial OH-36 to afford the glycosyl acceptor A
(57%). Glycosyl donor B was obtained by treating ethyl
2,4-di-O-benzoyl-1-thio-a-
L
-rhamnopyranoside7 5 with
In order to obtain a glycosyl donor from trisaccharide
8, the deprotection the benzylated anomeric position
was required, thus allowing further activation of the
chloroacetyl chloride in a 1:1 mixture of pyridine and
DMF8 (71%).
Scheme 2. Reagents and conditions: (a) Amberlyst 15-H+, BnOH, 45°C, 16 h, 54%; (b) pTsOH, 2,2-dimethoxypropane, rt, 1 h,
98%; (c) i. BzCl, pyridine, 0°C, 1 h; ii. TFA/water 4:1, rt, 45 min, 90%; (d) BzCl, pyridine/CH2Cl2 1:2, −25°C, 100 min, 57%; (e)
lit.6; (f) ClCH2COCl, pyridine/DMF 1:1, −45°C, 3 h, 71%.
,
Scheme 3. Reagents and conditions: (a) NIS, TfOH, molecular sieves 4 A, CH2Cl2, −20°C, 90 min, 82%; (b) NH2CSNH2, EtOH,
,
rt, 16 h, 80%; (c) NIS, TfOH, molecular sieves 4 A, −20°C, 15 min, 96%; (d) NH2CSNH2, EtOH, rt, 2 days, 83%; (e) anhydrous
FeCl3, CH2Cl2, rt, 45 min, 58%; (f) Cl3CCN, DBU, CH2Cl2, 0°C, 80 min, 76%.

E. Bedini et al. / Tetrahedron Letters 43 (2002) 8879–8882
8881
,
Scheme 4. Reagents and conditions: (a) BF3·OEt2, molecular sieves 4 A, CH2Cl2, −30°C, 1 h, 71%; (b) NH2CSNH2, EtOH/DMF
,
1:1, rt, 2 days, 78%; (c) BF3·OEt2, molecular sieves 4 A, CH2Cl2, −30°C, 90 min, 53%.
trichloroacetimidate building block E in an overall yield
resulting hemiacetal as
a glycosyl donor of the
trichloroacetimidate type.12 At first trisaccharide 8 was
treated with H2/Pd–C in 10:1 MeOH/AcOH to remove
the benzyl group by catalytic hydrogenation. This how-
ever, resulted in a 1:1 mixture of the desired product 9
and a homologous hemiacetal where the chloroacetyl
group had been reduced to an acetate. To avoid reduc-
tion of the chloroacetyl group, the benzyl group was
alternatively replaced by an acetate using acetolysis
conditions (acetic anhydride with 1% H2SO4, 84%
yield). The following cleavage of the anomeric acetate
by hydrazinium acetate afforded, both in DMF and
ethyl acetate, the simultaneous loss of the chloroacetate
at OH-3¦. This reduced chemoselectivity at the
anomeric center can be explained considering the rela-
tive rates for alkaline hydrolysis of acetates and
chloroacetates (1:760)3,13 and the generally high reactiv-
ity of the equatorial position 3 of rhamnose as shown
before.
of 44% over two steps.
With the two key building blocks D and E in hands a
sequence of oligomerization reactions was investigated.
When activated with BF3·OEt2 the two trisaccharides D
and E gave the fully protected hexasaccharide 10 (71%)
(Scheme 4). The a-configuration of the newly estab-
lished glycosidic bond was confirmed by the C-1D,H-1D
coupling constant of 174 Hz. The hexasaccharide 10
was converted to glycosyl acceptor F (78%) by cleavage
of the chloroacetyl group with thiourea in a 1:1 mixture
of EtOH–DMF to allow the complete solubility of 10.
Finally, hexasaccharide acceptor F was elongated with
the trisaccharide imidate E to afford the nonasaccha-
ride 1115 (53%). Deprotection of the hexasaccharide 10
using basic conditions (NaOMe in MeOH at T=40°C)
assured the complete removal of all protective functions
except of the anomeric benzyl group.
Finally, a method for the conversion of 8 to hemiacetal
9 in a single step was found in a procedure employing
anhydrous FeCl3 in CH2Cl2.14 It is noteworthy that the
stability of the chloroacetyl protective group under
these reaction conditions was not reported previously.
In conclusion we have developed an efficient strategy
for the preparation of oligomeric repeating units of the
rhamnan type found on many O-chain polysaccharides
from phytopathogenic bacteria thus providing sufficient
quantities for future structure–activity studies of the
deprotected oligomers.
The hemiacetal
9
could be converted to the

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E. Bedini et al. / Tetrahedron Letters 43 (2002) 8879–8882
15. Compound 11: [a]2D2=+156 (c 0.5, dichloromethane);
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