"Click chemistry" en route to pseudo-starch

Article abstract of DOI:10.1039/b504293c

The application of click chemistry based on cycloaddition of substituted azide and alkynes to the synthesis of well-defined branched oligosaccharide related to starch was discussed. This modular approach allowed the construction of a number of [1,2,3]-tri

Full text of DOI:10.1039/b504293c

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C O M M U N I C A T I O N
“Click chemistry” en route to pseudo-starch
Laurence Marmuse,a Sergey A. Nepogodiev*a and Robert A. Field*a,b
a Centre for Carbohydrate Chemistry, School of Chemical Sciences and Pharmacy, University of
East Anglia, Norwich, UK NR4 7TJ. E-mail: s.nepogodiev@uea.ac.uk
b Department of Biological Chemistry, John Innes Centre, Norwich, UK NR4 7UH.
E-mail: r.a.field@uea.ac.uk
Received 29th March 2005, Accepted 3rd May 2005
First published as an Advance Article on the web 11th May 2005
Rapid assembly of starch fragment analogues was achieved
using “click chemistry”. Specifically, two hexadecasaccha-
ride mimics containing two parallel maltoheptaosyl chains
linked via [1,2,3]-triazoles to a maltose core were synthe-
sized using Cu(I)-catalyzed [3 + 2] dipolar cycloaddition
of azido saccharides and 6,6ꢀ- and 4ꢀ,6ꢀ-dipropargylated p-
methoxyphenyl maltoside.
chains attached to a maltose template through heterocyclic
bridges. Two types of molecules, with attachment points at the
4ꢀ,6ꢀ and 6,6ꢀ positions of a maltose template, were selected as
targets. The strategy for the introduction of a matching pair of
reactive groups, suitable for 1,3-dipolar cycloaddition, requires
the simple and efficient introduction of azide and alkyne groups
into suitable building blocks. The branching template chosen
was a dipropargylated maltose derivative, whereas linear chains
containing an azido group in the reducing terminal anomeric
positions comprised the cycloaddition partner.
Isomeric di-O-propargyl derivatives 6 and 10 were synthesized
starting from readily available13 maltose peracetate 1 (Scheme 1).
Glycosidation of 1 with p-methoxyphenol in the presence of
BF3·OEt2 gave an a,b mixture of aryl glycosides from which
pure b-anomer 2 was isolated by crystallisation in 41% yield.
Deacetylation of 2 followed by selective protection of primary
OH groups via alkylation with TrCl in pyridine afforded 6,6ꢀ-di-
O-trityl derivative 3 in 42% overall yield. For the synthesis of
4ꢀ,6ꢀ di-O-propargyl maltoside 10, glycoside 2 was deacetylated
and selectively benzylidenated to produce acetal 7 in 65% overall
yield. After benzylation of the remaining hydroxy groups in 3
and 7, acid-labile temporary triphenylmethyl and benzylidene
groups in 4 and 8 were removed to give diols 5 and 9 in 90%
and 77% yield, respectively. Reactions of dialkoxides prepared
in situ from diols 5 and 9 with propargyl bromide led to target
Detailed knowledge of structural features is essential for un-
derstanding the biosynthesis of starch components and their
assembly into the starch granule. Through this understanding,
rational alteration of the pathway of starch biosynthesis using
genetic manipulation promises to deliver a new generation of
novel raw materials.1 Starch is composed of two polysaccha-
rides: an essentially linear a-(1→4)-D-glucan amylose and the
highly branched macromolecule amylopectin, consisting of rel-
atively short (1→4)-D-glucan chains attached through a-(1→6)-
linkages to an a-(1→4)-D-glucan backbone.2 It is believed that
local organization of these branched chains of amylopectin
is responsible for the ordered semi-crystalline structure of
the starch granule in which crystalline regions alternate with
amorphous zones. The short branches of amylopectin form
double helices that are stacked in the crystalline regions, whereas
amorphous zones are occupied by amylopectin fragments in-
corporating (1→6)-branch points. To gain an insight into the
role of such branch points in the propagation of double helices,
computer modelling3,4 and NMR spectroscopic studies4 have
been undertaken on amylopectin fragments. However, obtaining
experimental information about these fragments is hampered
by the fact that they represent only a small fragment of the
total polysaccharide material. Therefore synthetic, well-defined
fragments of amylopectin incorporating a-(1→6)-branch points
would be useful tools for physicochemical and biochemical
studies.
Synthesis of oligosaccharides related to starch have been
reported in the literature,5 but poor stereoselectivity in the 1,2-
cis-glucosylation reaction is known6 to be a serious obstacle
in the assembly of the large branched fragments. To enforce
interactions of two parallel oligosaccharide chains, they need
to be attached to a template, as in the case of a cellulose II
mimic developed by Vasella and co-workers.7 Application of
the template concept to the construction of amylopectin frag-
ment analogues requires development of a simple and efficient
strategy for conjugation of long-chain maltooligosaccharides
to a template. One of the reactions that can satisfy these
requirements is Huisgen’s 1,3-dipolar cycloaddition8 of azides
and terminal acetylenes, yielding triazoles. The potential of this
reaction has been recently enhanced by the discovery9 that Cu(I)
catalyzes formation of a single regioisomer of substituted 1,2,3-
triazoles, making this reaction one of the most powerful “click
chemistry”10 transformations. In carbohydrate chemistry this
methodology has been successfully applied for the synthesis of
multivalent saccharides11 and cyclodextrin analogues.12
Scheme 1 Reagents and conditions: (a) p-methoxyphenol, CH2Cl2,
BF3·OEt2; (b) 1. MeOH, NaOMe 2. TrCl, pyridine; (c) BnBr, NaH;
(d) TsOH, MeOH–CH2Cl2; (e) CH≡CCH2Br, NaH, THF–DMPU;
(f) 1. NaOMe, MeOH, 2. PhCH(OMe)2, TsOH, DMF; (g) 90% AcOH.
Continuing our efforts on generating synthetic amylopectin
fragments6 we describe herein an approach to the construction of
amylopectin analogues composed of two linear maltoheptaose
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
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 5 – 2 2 2 7
2 2 2 5
©

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di-O-propargyl maltosides 6 and 10 in 88% and 93% yield,
respectively.
The results of 1,3-dipolar cycloaddition of dipropargylated
maltosides and azidoglucosides are shown in Scheme 3. All
reactions were carried out using (Ph3P)3·CuBr as a catalyst in
the presence of DIPEA as a base as described previously,11a
except that, instead of microwave irradiation, a longer re-
action time (12 h) at room temperature was applied. The
yields of cycloaddition reactions varied between 65 and 27%,
decreasing when increasing length of the azidooligosaccharide
chain.
A series of peracetylated b-glycosyl azides was synthesized by
reaction14 of Me3SiN3 with glucosyl bromide 11, maltotriosyl
bromide 12,15 and maltoheptaosyl bromide 1315 in the presence
of Bu4NF. Thus glycosyl azides 14,14 15, and 16 were prepared
in 65–83% yield (Scheme 2).
The structure of cycloaddition products was confirmed by
1
NMR spectroscopy and mass spectrometry. Both H and 13C
NMR spectra of 17–20 revealed very close but distinguishable
resonances (dH ca. 5.7–5.9 and dC ca. 85–86) corresponding to the
anomeric center of the glucopyranose residues attached to N-1
of the triazole unit. From the rest of the considerably overlapping
resonances of acetylated glucoparanose residues, only clusters
corresponding to signals of a-anomeric (dC ca. 95–96) and C-6
(dC ca. 61.5–62) carbon atoms were reliably assignable. Charac-
ꢀ
teristic signals of anomeric carbon atoms (dC-1 ca. 102 and dC-1
ca. 97), as well as resonances corresponding to p-methoxyphenyl
group (dOMe ca. 55.5 and aromatics d ca. 115 and dC ca. 118)
were observed in the 13C NMR spectra of compounds 17–20.
1
The [1,2,3]-triazole unit was evident from H NMR spectra by
the presence of two separate resonances (dH ca. 7.7). Therefore,
NMR data clearly indicated formation of a single isomer in each
case, which for the copper(I)-catalyzed cycloaddition reaction is
known to be the 1,4-substituted [1,2,3]-triazole.9b Regioselectiv-
ity of cycloaddition in the synthesis of 6,6ꢀ-di-substituted deriva-
tives 17–19 also followed from the observation of only one pair of
Scheme 2 Reagents and conditions: Me3SiN3, TBAF, THF.
Scheme 3 Synthesis of templated bis-glucopyranoside (17), bis-maltotrioside (18), and bis-maltoheptaosides (19 and 20). Reagents and conditions:
(a) (Ph3P)3CuBr, DIPEA, toluene, 12 h, room temperature.
2 2 2 6
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 5 – 2 2 2 7

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doublets of aromatic protons (dH ca. 6.9 and dH ca. 7.0) belonging
to the anomeric p-methoxyphenyl group in the 1H NMR spectra.
We noted previously6 that the chemical shifts of these signals are
highly sensitive to the stereochemistry of a substituent at the 6
position of a p-methoxyphenyl b-maltoside unit. All triazole-
bridged products were analyzed by MALDI-TOF MS, giving
the expected sodium adducts of molecular ions: 1744.7 (17),
2897.1 (18), 5202.7 (19), and 5207.6 (20).
In summary, we have described the first application of “click
chemistry” based on cycloaddition of substituted azide and
alkynes to the synthesis of well-defined branched oligosac-
charide mimics. Starting from dipropargylated maltoside and
azido maltooligosaccharides this modular approach allowed the
construction of a number of [1,2,3]-triazole-based analogues
of amylopectin fragments in one simple coupling step. These
analogues include two isomeric hexadecasaccharide analogues
which have potential for templating formation of double helixes
between two parallel maltoheptaosyl chains attached to a core
maltose unit. Studies to investigate such assembly processes are
ongoing.
3 (a) A. Imberty and S. Pe´rez, Int. J. Biol. Macromol., 1989, 11, 177–
185; (b) A. Buleon and V. Tran, Int. J. Biol. Macromol., 1990, 12,
345–352; (c) A. Neszmelyi and J. Hollo, Starch/Staerke, 1989, 41,
1–3.
4 F. Corzana, M. S. Motawia, C. Herve´ du Penhoat, F. van den Berg,
A. Blennow, S. Pe´rez and S. B. Engelsen, J. Am. Chem. Soc., 2004,
126, 13144–13155.
5 (a) M. S. Motawia, C. E. Olsen, K. Enevoldsen, J. Marcussen and
B. L. Møller, Carbohydr. Res., 1995, 277, 109–123; (b) I. Damager,
C. E. Olsen, B. L. Møller and M. S. Motawia, Carbohydr. Res., 1999,
320, 19–30; (c) M. S. Motawia, K. Larsen, C. E. Olsen and B. L.
Møller, Synthesis, 2000, 11, 1547–1556; (d) I. Damager, C. E. Olsen,
B. L. Møller and M. S. Motawia, Synthesis, 2002, 418–426; (e) I.
Damager, C. E. Olsen, A. Blennow, K. Denyer, B. L. Møller and
M. S. Motawia, Carbohydr. Res., 2003, 338, 189–197.
6 L. Marmuse, S. A. Nepogodiev and R. A. Field, Tetrahedron:
Asymmetry, 2005, 16, 477–485.
7 B. Bernet, J. W. Xu and A. Vasella, Helv. Chim. Acta, 2000, 83,
2072–2114.
8 R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, vol. 1, ed.
A. Padwa, Wiley, 1984, pp. 1–176.
9 (a) V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless,
Angew. Chem., Int. Ed., 2002, 41, 2596–2599; (b) C. W. Tørnoe, C.
Christensen and M. Meldal, J. Org. Chem., 2002, 67, 3057–3064;
(c) V. O. Rodionov, V. V. Fokin and M. G. Finn, Angew. Chem., Int.
Ed., 2005, 44, 2210–2215.
Acknowledgements
10 C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed.,
2001, 40, 2004–2021.
11 (a) F. Perez-Balderas, M. Ortega-Munoz, J. Morales-Sanfrutos, F.
Herna´ndez-Mateo, F. G. Calvo-Flores, J. A. Calvo-Asin, J. Isac-
Garc´ıa and F. Santoyo-Gonza´lez, Org. Lett., 2003, 5, 1951–1954;
(b) S. Chittaboina, F. Xie and Q. Wang, Tetrahedron Lett., 2005, 46,
2331–2336.
We thank the EPSRC and the Weston Foundation for finan-
cial support. The EPSRC Mass Spectrometry Service Centre,
Swansea are acknowledged for invaluable support.
References
1 (a) A. M. Smith, Curr. Opin. Plant Biol., 1999, 2, 223–229; (b) C. J.
Slattery, I. H. Kavakli and T. W. Okita, Trends Plant Sci., 2000, 5,
291–298; (c) A. G. Heyer, J. R. Lloyd and J. Kossmann, Curr. Opin.
Biotechnol., 1999, 10, 169–174.
2 (a) D. J. Gallant, B. Bouchet and P. M. Baldwin, Carbohydr. Polym.,
1997, 32, 177–191; (b) R. F. Tester, J. Karkalas, in Biopolymers:
Polysaccharides II, vol. 6, Wiley-VCH, Weinheim, 2002, pp. 383–
438; (c) A. Buleon, P. Colonna, V. Planchot and S. Ball, Int. J. Biol.
Macromol., 1998, 23, 85–112; (d) A. Imberty, A. Buleon, V. Tran and
S. Pe´rez, Starch/Staerke, 1991, 43, 375–384.
12 (a) K. D. Bodine, Y. Gin and M. S. Gin, J. Am. Chem. Soc., 2004,
126, 1638–1639; (b) B. Hoffmann, B. Bernet and A. Vasella, Helv.
Chim. Acta, 2002, 85, 265–287.
13 K. P. R. Kartha, M. Aloui and R. A. Field, Tetrahedron Lett., 1996,
37, 8807–8810.
14 E. D. Soli, A. S. Manoso, M. C. Patterson, P. DeShong, D. A. Favor,
R. Hirschmann and A. B. Smith, III, J. Org. Chem., 1999, 64, 3171–
3177.
15 E. Farkas, L. Ja´nossy, J. Harangi, L. Kandra and A. Lipta´k,
Carbohydr. Res., 1997, 303, 407–415.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 5 – 2 2 2 7
2 2 2 7