Efficient routes toward the synthesis of the D-rhamno-trisaccharide related to the A-band polysaccharide of Pseudomonas aeruginosa

Article abstract of DOI:10.3762/bjoc.10.153

The present work describes efficient avenues for the synthesis of the trisaccharide repeating unit [α-D-Rhap-(1→3)-α-D-Rhap- (1→3)-α-D-Rhap] associated with the A-band polysaccharide of Pseudomonas aeruginosa. One of the key steps involved 6-Odeoxygenatio

Full text of DOI:10.3762/bjoc.10.153

Efficient routes toward the synthesis of the
D-rhamno-trisaccharide related to the A-band
polysaccharide of Pseudomonas aeruginosa
Aritra Chaudhury, Sajal K. Maity and Rina Ghosh*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1488–1494.
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
doi:10.3762/bjoc.10.153
Email:
Received: 27 February 2014
Accepted: 11 June 2014
Published: 01 July 2014
Rina Ghosh* - ghoshrina@yahoo.com
* Corresponding author
This article is part of the Thematic Series "Multivalent glycosystems for
nanoscience".
Keywords:
A-band polysaccharide; D-rhamno-trisaccharide; deoxygenation on
thioglycoside; multivalent glycosystems; one-pot sequential
glycosylation; Pseudomonas aeruginosa
Guest Editor: B. Turnbull
© 2014 Chaudhury et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The present work describes efficient avenues for the synthesis of the trisaccharide repeating unit [α-D-Rhap-(1→3)-α-D-Rhap-
(1→3)-α-D-Rhap] associated with the A-band polysaccharide of Pseudomonas aeruginosa. One of the key steps involved 6-O-
deoxygenation of either partially or fully acylated 4,6-O-benzylidene-1-thiomannopyranoside by radical-mediated redox rearrange-
ment in high yields and regioselectivity. The D-rhamno-thioglycosides so obtained allowed efficient access to the trisaccharide
target via stepwise glycosylation as well as a one-pot glycosylation protocol. In a different approach, a 4,6-O-benzylidene
D-manno-trisaccharide derivative was synthesized, which upon global 6-O-deoxygenation followed by deprotection generated the
target D-rhamno-trisaccharide. The application of the reported regioselective radical-mediated deoxygenation on 4,6-O-benzyli-
dene D-manno thioglycoside (hitherto unexplored) has potential for ramification in the field of synthesis of oligosaccharides based
on 6-deoxy hexoses.
Introduction
With the firm establishment of the critical roles played by oligo- cepacia complex [8], P. aeruginosa [9], Helicobacter pylori
saccharides in diverse biological processes [1-4], the field of [10], Citrobacter freundii [11], Campylobacter fetus [12],
oligosaccharide synthesis has seen a rapid development over the Stenotrophonas maltophilia [13], Xanthomonas campestri [14]
last two decades [5-7]. The D-rhamnoside motif is of particular and Brucella sp. [15].
interest with its presence established in the LPS/EPS systems of
various bacterial strains which are pathogenic towards both P. aeruginosa has long been established as an opportunistic
plants and animals. These species include the Burkholderia pathogen which infects humans having compromised immunity
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with fatal consequences in a majority of cases. Its high persis- et al. in 2007 [26], and further improvement on this method was
tence against a wide variety of antibiotics is also well docu- reported by Kiefel et al. in 2011 [27]. However, the deoxygena-
mented. It has also been observed how the colonized form of tion protocol involving halogenation under Mitsunobu condi-
this species in cystic fibrosis lungs, through non-expression of tions was found to be inefficient when applied to thioglyco-
O-antigens, offer high persistence often rendering O-antigen- sides directly. Moreover, the use of stoichiometric amounts of
based vaccines ineffective. But the conservation of the A-band the toxic tin hydride for radical-based reductive dehalogenation
polysaccharide even in the colonized form makes this repeating as required by the above method appeared undesirable espe-
unit a viable candidate for A-band polysaccharide-based cially in the preparative stages where reactions have to be set up
vaccines which can avoid the vulnerability applicable to their on a large scale. On the other hand, Kiefel’s method requires
O-antigen-based counterparts [16]. Hence, the A-band polysac- four steps for the conversion of D-mannose to D-rhamnose on
charide [16] of P. aeruginosa which has been characterized which further manipulations are required to reach the
previously as a repeating combination of [→2)-α-D-Rhap- adequately designed derivatives. This makes the starting ma-
(1→3)-α-D-Rhap-(1→3)-α-D-Rhap] (Figure 1) provides for a terial preparations rather long drawn.
synthetic target whose efficient synthesis is worth pursuing. It is
to be noted that synthesis of a tetrasaccharide, α-D-Rhap- Keeping these limitations in mind we selected the protocol
(1→2)-α-D-Rhap-(1→3)-α-D-Rhap-(1→3)-α-D-Rhap [17] and devised by Pedersen et al. [28] and subsequently studied by
a trisaccharide, α-D-Rhap-(1→3)-α-D-Rhap-(1→2)-α-D-Rhap Dang et al. [29-32] which seemed to address all the inadequa-
[18] related to the A-band polysaccharide of P. aeruginosa were cies described above. However, the compatibility of the method
made with a view to develop glycoconjugate vaccines, but none with thioglycosides had to be verified. Upon application, and
have ultimately materialized into valid vaccine candidates.
much to our delight, it was found that thioglycosides responded
equally well compared to their O-glycosidic counterparts. It
may be mentioned here that the response of thioglycosides
towards this method was not reported in the original observa-
tion [28]. To the best of our knowledge, this method has not yet
been applied directly on thioglycosides. So, having established
an easy means to access D-rhamno-thioglycosides from their
D-manno-counterparts, we set about devising efficient routes to
reach the target trisaccharide. The following section bears elab-
oration of our efforts.
Results and Discussion
Deoxygenation at the C-6 position of D-mannose derivatives
bearing conventional or Crich’s modified 4,6-O-benzylidene
protection [19-23] has been the principal philosophy behind the
Figure 1: Repeating unit of the A-band polysaccharide of P. aerugi-
nosa.
Thus, we targeted the trisaccharide [α-D-Rhap-(1→3)-α-D- synthesis of the D-rhamnose [19-23,30,33-39] motif. This is a
Rhap-(1→3)-α-D-Rhap] as our synthetic goal toward construc- natural choice not only because it allows simultaneous selec-
tion of a probable vaccine candidate against P. aeruginosa.
tive blockage of the O-4 and O-6 positions but also sets up the
model on which various deoxygenation protocols may be tried.
The synthesis of the D-rhamnose-based oligosaccharide from The linkage pattern and stereochemistry at the glycosidic posi-
the D-mannose motif has received substantial attention over the tions on the target dictate the presence of acyl protection on the
last decade. The problem of β-D-rhamnoside synthesis has been O-2 position as shown in Scheme 1.
greatly addressed by Crich et al. [19-23]. But, Crich’s global
deoxygenating strategy, despite its ultimate efficiency, still We began our synthesis targeting two pivotal intermediates 1a
requires synthetic modification on the conventional 4,6-O- and 1b which were obtained using previously reported methods
benzylidene framework involving reagents which are rather [40,41] and then proceeding forward to the monomeric building
expensive. Moreover, additional steps are required for the blocks required according to the retrosynthetic analysis
preparation of these adequately derivatized mannose-based (Figure 2).
systems from which the D-rhamnose motif may be accessed.
Other reports on the D-rhamnose-based synthesis have also The procedures used and the results obtained to reach the inter-
surfaced in recent times [24,25]. Apart from this, the synthesis mediate targets have been summarized in Scheme 1. Accord-
of monomeric D-rhamnose has been highly streamlined by Roy ingly, compounds 2 and 3b [42] were obtained uneventfully
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Scheme 1: Preparation of the monomeric building blocks; reagents and conditions: i) Pyr., BzCl, 0 °C–rt; ii) PhC(OMe)3, CSA, MeCN, 0 °C–rt, 80%
AcOH (aq), 25 °C; iii) DTBP, TIPST, octane, reflux; iv) TCCA, acetone–H2O, rt; v) CCl3CN, DBU, DCM; vi) MeC(OEt)3, CSA, MeCN, 0 °C–rt, 80%
AcOH (aq), 25 °C.
Figure 2: Retrosynthetic analysis.
from 1a in 92% and 75%, respectively (Scheme 1). All attempts with 80% aq AcOH. Compound 3a which is the phenyl thiogly-
to place an acetyl protection at the O-2 position of 1b with high coside analogue of 4 was also accessed similarly. Both the com-
yield failed. The poor yield was presumed to be due to the pounds were next converted to their rhamnoside counterparts 6
migration of the 2-O-acetyl group to the O-3 position leading to and 7 [44], respectively by treatment with di-tert-butyl peroxide
a mixture which was hard to separate. Hence we switched to (DTBP) and triisopropylsilanethiol (TIPST) under reflux in
O-benzoyl protection which was found to be less susceptible to octane over 2–3 h [30] in overall 74% and 64% yields, respect-
migration [25]. The 2-O-benzoylated compound 4 was prepared ively, over two steps. It is worth noting here that during column
by conversion of 1b into its corresponding methyl 2,3-orthoben- chromatographic purification after AcOH-mediated cleavage of
zoate derivative followed by the selective cleavage of the same the orthobenzoate derivative some losses were incurred due to
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Beilstein J. Org. Chem. 2014, 10, 1488–1494.
the migration of the 2-O-benzoyl group to the O-3 position. by carrying out the whole glycosylation process in one-pot
However the susceptibility of the benzoyl group to migration leading to the target trisaccharide 9 in 79% yield (Scheme 3). In
was much lower than that of its acetate analogue.
this case the disaccharide synthesis was set up as described
previously, and then the second acceptor 7 was introduced into
The same problem persisted even in the rhamnosides 6 and 7. the reaction mixture at −50 °C after which NIS and TMSOTf
Hence, to minimize the loss during purification, the chromato- were added successively at the same temperature. The tempera-
graphic purification was reserved until the end of the deoxy- ture was then raised upto −10 °C, and the reaction mixture was
genation step. Thus, the intermediate compounds 3a and 4 were stirred for another 1 h at that temperature to give the target tri-
used without purification by column chromatography. Com- saccharide as summarized in Scheme 3.
pound 2 was deoxygenated to its rhamnosyl counterpart 2a in a
high yield (88%). Next, the thioglycoside was hydrolyzed to the At this point, our search for overall synthetic efficiency was
hemiacetal 2b with trichloroisocyanuric acid (TCCA) in greatly augmented by a change of approach. Since, multiple
acetone–H2O [43] in 85% yield and was further converted deoxygenations had been observed by Dang et al. [30] on
almost quantitatively to the corresponding trichloroacetimidate trehalose derivatives previously, we figured that we could
5 [44]. Having arrived at the monomeric building blocks 5, 6, access the rhamnose-based trisaccharide directly from a suit-
and 7 we carried forward to the rhamnose-based disaccharide 8 ably derivatized manno-trisaccharide bearing the three 4,6-O-
which being a thioglycoside would subsequently serve as the benzylidene protecting groups by a single global deoxygena-
glycosyl donor in the next step. Accordingly, 5 and 6 were tion step. Accordingly, we coupled the monomeric units 2, 3b
coupled almost quantitatively to give 8. The disaccharide so [45] and 4 to reach the mannose-based trisaccharide 12
obtained was then coupled with 7 to give the protected tri- (Scheme 4). The deoxygenation protocol, being incompatible
saccharide 9 in 90% yield using a N-iodosuccinimide- with the O-benzyl protecting group, required an acyl protection
trimethylsilyl trifluoromethanesulfonate (NIS-TMSOTf) combi- profile on O-2 and O-3 positions. Such an arrangement was also
nation as the activating reagent. The target trisaccharide was in agreement with the stereochemical requirements at the
then obtained via deprotection under the Zémplen conditions to anomeric positions of our target trisaccharide.
give 10 quantitatively, as is summarized in Scheme 2.
Since compounds 2 and 3b bear structural similarity with each
However, in the interest of time and economy of resources other it was expected that they would have similar reactivity
consumed, a one-pot synthesis is always desirable. Accordingly, towards glycosylative activation. Hence, a preactivation-based
further optimization of the glycosylation protocol was achieved glycosylation protocol was devised for the first step leading to
Scheme 2: Sequential stepwise synthesis of the trisaccharide; reagents and conditions: i) TMSOTf, DCM, molecular sieves 4 Å, N2, −50 °C; ii) NIS,
TMSOTf, DCM, molecular sieves 4 Å, N2, −10 °C; iii) NaOMe, MeOH, rt, 12 h.
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Scheme 3: Synthesis of the trisaccharide by sequential one-pot glycosylation reactions; reagents and conditions: i) (a) TMSOTf, DCM, molecular
sieves 4 Å, N2, −50 °C; (b) NIS, TMSOTf, −50 °C, DCM, molecular sieves 4 Å, N2, −10 °C.
Scheme 4: Synthesis of the target trisaccharide via global deoxygenation strategy; reagents and conditions: i) BSP, Tf2O, –60 °C, DCM, molecular
sieves 4 Å, N2, –78 °C; ii) NIS, TMSOTf, DCM, molecular sieves 4 Å, N2, −10 °C; iii) DTBP, TIPST, octane, reflux, 3 h; iv) NaOMe, MeOH, rt, 12 h.
the disaccharide 11 using 1-benzenesulfinyl piperidine–triflic rhamnose units from the non-reducing end coincided at δ
anhydride (BSP–Tf2O) [46]. The subsequent step was carried 102.3 ppm (as also evidenced from HSQC data) with the corres-
out using NIS–TMSOTf to give the mannose-based tri- ponding 1JC1-H1 of 170.4 Hz and that of the anomeric carbon of
saccharide 12. This trisaccharide was next deoxygenated glob- the reducing end monomer unit appeared at 98.9 ppm with the
ally to give the rhamnose-based trisaccharide 13 with high yield corresponding 1JC1-H1 of 170.2 Hz. The value at δ 102.3 ppm
(80%). This was then deprotected under the Zémplen condi- can be corroborated with the reported [16] C1-chemical shifts at
tions to yield 10 quantitatively as is summarized in Scheme 4. δ 103.19 and 103.43 with 1JC1-H1 of 173 Hz, exhibited in D2O
corresponding to the two [→3)-α-D-Rhap-(1→] units in the
1H NMR in MeOH-d4 of the target trisaccharide 10 showed the A-band polysaccharide of P. aeruginosa. The small difference
anomeric protons of the consecutive rhamnose residues from in the chemical shifts and the corresponding 1JC1-H1 values may
the non-reducing end appearing at δ 5.09, 5.06 and 5.30 ppm, be attributed to the difference in the solvents (MeOH-d4 and
respectively. 13C NMR in the same solvent revealed that the D2O), chosen in these two cases for recording NMR.
chemical shifts of the anomeric carbons of the two consecutive Comparing and considering the NMR data in these two cases
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7. Hsu, C.-H.; Hung, S.-C.; Wu, C.-Y.; Wong, C.-H.
we surmised that all the three anomeric carbons are α-config-
Angew. Chem., Int. Ed. 2011, 50, 11872–11923.
doi:10.1002/anie.201100125
ured [47].
8. Herasimenka, Y.; Cescutti, P.; Impallomeni, G.; Campana, S.;
Taccetti, G.; Ravenni, N.; Zanetti, F.; Rizzo, R. J. Cystic Fibrosis 2007,
6, 145–152. doi:10.1016/j.jcf.2006.06.004
Conclusion
In short, we have described three different efficient routes to
access the A-band trisaccharide associated with P. aeruginosa.
The radical-based 6-deoxygenation protocol on 4,6-O-benzyli-
dene (hitherto unexplored on thioglycosides) was utilized to
arrive at the D-rhamno-thioglycoside derivatives from their
D-manno-counterparts. This, along with a similar global
6-deoxygenation strategy on D-manno-trisaccharide derivative
(bearing 4,6-O-benzylidene protection on each mannose unit)
offer great potential in future oligosaccharide syntheses based
on 6-deoxy hexoses.
9. Yokota, S.-i.; Kaya, S.; Sawada, S.; Kawamura, T.; Araki, Y.; Ito, E.
Eur. J. Biochem. 1987, 167, 203–209.
doi:10.1111/j.1432-1033.1987.tb13324.x
10.Kocharova, N. A.; Knirel, Y. A.; Widmalm, G.; Jansson, P. E.;
Moran, A. P. Biochemistry 2000, 39, 4755–4760.
doi:10.1021/bi992635k
11.Kocharova, N. A.; Borisova, S. A.; Zatonsky, G. V.; Shashkov, A. S.;
Knirel, Y. A.; Kholodkova, E. V.; Staniskavsky, E. S. Carbohydr. Res.
1998, 306, 331–333. doi:10.1016/S0008-6215(97)10049-0
12.Senchenkova, S. N.; Shashkov, A. S.; Knirel, Y. A.; Mcgovern, J. J.;
Moran, A. P. Eur. J. Biochem. 1996, 239, 434–438.
doi:10.1111/j.1432-1033.1996.0434u.x
13.Winn, A. M.; Wilkinson, S. G. Carbohydr. Res. 1996, 294, 109–115.
doi:10.1016/S0008-6215(96)90623-0
Supporting Information
14.Molinaro, A.; Silipo, A.; Lanzetta, R.; Newman, M.-A.; Dow, M. J.;
Parrilli, M. Carbohydr. Res. 2003, 338, 277–281.
doi:10.1016/S0008-6215(02)00433-0
Supporting Information File 1
Experimental details for the preparation of compounds 2a,
2b, 3a, 4, 6–13 and the corresponding characterization data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-153-S1.pdf]
15.Kihlberg, J.; Bundle, D. R. Carbohydr. Res. 1992, 216, 67–78.
doi:10.1016/0008-6215(92)84151-H
16.Arsenault, T. L.; Hughes, D. W.; MacLean, D. B.; Szarek, W. A.;
Kropinski, A. M. B.; Lam, J. S. Can. J. Chem. 1991, 69, 1273–1280.
doi:10.1139/v91-190
Supporting Information File 2
17.Tsvetkov, Y. E.; Backinowsky, L. V.; Kochetkov, N. K. Carbohydr. Res.
1989, 193, 75–90. doi:10.1016/0008-6215(89)85108-0
18.Zou, W.; Sen, A. K.; Szarek, W. A.; MacLean, D. B. Can. J. Chem.
1993, 71, 2194–2200. doi:10.1139/v93-275
1H and 13C NMR of compounds 2a, 4, 6, 8–13 and 2D
NMR (COSY, HSQC and HMBC) of compound 10.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-153-S2.pdf]
19.Picard, S.; Crich, D. Chimia 2011, 65, 59–64.
doi:10.2533/chimia.2011.59
20.Crich, D.; Bowers, A. A. Org. Lett. 2006, 8, 4327–4330.
doi:10.1021/ol061706m
Acknowledgements
The authors are grateful to CSIR, New Delhi, India for
21.Crich, D.; Bowers, A. A. J. Org. Chem. 2006, 71, 3452–3463.
doi:10.1021/jo0526688
providing a fellowship (SRF) to AC and funding of project (No. 22.Crich, D.; Yao, Q. J. Am. Chem. Soc. 2004, 126, 8232–8236.
doi:10.1021/ja048070j
2382/01/10/2010 EMR II) to RG. Financial assistance from
23.Crich, D.; Li, L. J. Org. Chem. 2009, 74, 773–781.
DST-SERB, India to RG and supports from CAS-UGC, India
doi:10.1021/jo8022439
and FIST-DST, India to the Department of Chemistry, Jadavpur
24.Bedini, E.; Carabellese, A.; Barone, G.; Parrilli, M. J. Org. Chem. 2005,
University, Kolkata are also acknowledged.
70, 8064–8070. doi:10.1021/jo051153d
25.Bedini, E.; Carabellese, A.; Corsaro, M. M.; De Castro, C.; Parrilli, M.
Carbohydr. Res. 2004, 339, 1907–1915.
References
1. Varki, A. Glycobiology 1993, 3, 97–130. doi:10.1093/glycob/3.2.97
doi:10.1016/j.carres.2004.06.010
26.Fauré, R.; Shiao, T. C.; Damerval, S.; Roy, R. Tetrahedron Lett. 2007,
2. Varki, A.; Cummings, D. C.; Esko, J. D.; Freeze, H. H.; Stanley, P.;
48, 2385–2388. doi:10.1016/j.tetlet.2007.01.122
Bertozzi, C. R.; Hart, G. W.; Etzler, M. E. Essentials of Glycobiology;
27.Zunk, M.; Kiefel, M. J. Tetrahedron Lett. 2011, 52, 1296–1299.
Cold Spring Harbor Press: New York, 2009.
doi:10.1016/j.tetlet.2011.01.064
3. Seeberger, P. H.; Werz, D. B. Nature 2007, 446, 1046–1051.
28.Jeppesen, L. M.; Lundt, I.; Pedersen, C. Acta Chem. Scand. 1973, 27,
doi:10.1038/nature05819
3579–3585. doi:10.3891/acta.chem.scand.27-3579
4. Fuster, M. M.; Esko, J. D. Nat. Rev. Cancer 2005, 5, 526–542.
29.Roberts, B. P.; Smits, T. M. Tetrahedron Lett. 2001, 42, 3663–3666.
doi:10.1038/nrc1649
doi:10.1016/S0040-4039(01)00540-8
5. Zhu, X.; Schmidt, R. R. Angew. Chem., Int. Ed. 2009, 48, 1900–1934.
30.Dang, H.-S.; Roberts, B. P.; Sekhon, J.; Smits, T. M.
doi:10.1002/anie.200802036
Org. Biomol. Chem. 2003, 1, 1330–1341. doi:10.1039/b212303g
6. Nicolaou, K. C.; Mitchell, H. J. Angew. Chem., Int. Ed. 2001, 40,
31.Fielding, A. J.; Franchi, P.; Roberts, B. P.; Smits, T. M.
1576–1624.
J. Chem. Soc., Perkin Trans. 2 2002, 155–163. doi:10.1039/B106140M
doi:10.1002/1521-3773(20010504)40:9<1576::AID-ANIE15760>3.0.CO
;2-G
1493

Beilstein J. Org. Chem. 2014, 10, 1488–1494.
32.Cai, Y.; Dang, H.-S.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 1
2002, 2449–2458. doi:10.1039/b208200b
33.Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1035–1044.
doi:10.1021/jo01256a059
34.Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1045–1053.
doi:10.1021/jo01256a060
35.Hanessian, S.; Plessas, N. R. J. Org. Chem. 1969, 34, 1053–1058.
doi:10.1021/jo01256a061
36.Chana, J. S.; Collins, P. M.; Farnia, F.; Peacock, D. J.
J. Chem. Soc., Chem. Commun. 1988, 94–96.
doi:10.1039/c39880000094
37.Binkley, R. W.; Goewey, G. S.; Johnston, J. C. J. Org. Chem. 1984, 49,
992–996. doi:10.1021/jo00180a008
38.Hanessian, S. Org. Synth. 1987, 65, 243–250.
39.Hullar, T. L.; Siskin, S. B. J. Org. Chem. 1970, 35, 225–228.
doi:10.1021/jo00826a046
40.Niu, Y.; Wang, N.; Cao, X.; Ye, X.-S. Synlett 2007, 2116–2120.
doi:10.1055/s-2007-984903
41.Shivatare, S. S.; Chang, S.-H.; Tsai, T.-I.; Ren, C.-T.; Chuang, H.-Y.;
Hsu, L.; Lin, C.-W.; Li, S.-T.; Wu, C.-Y.; Wong, C.-H.
J. Am. Chem. Soc. 2013, 135, 15382–15391. doi:10.1021/ja409097c
42.Franzyk, H.; Meldal, M.; Paulsen, H.; Bock, K.
J. Chem. Soc., Perkin Trans. 1 1995, 2883–2898.
doi:10.1039/p19950002883
43.Basu, N.; Maity, S. K.; Chaudhury, A.; Ghosh, R. Carbohydr. Res.
2013, 369, 10–13. doi:10.1016/j.carres.2013.01.001
44.Fauré, R.; Shiao, T. C.; Lagnoux, D.; Giguère, D.; Roy, R.
Org. Biomol. Chem. 2007, 5, 2704–2708. doi:10.1039/b708365c
45.Chevalier, R.; Esnault, J.; Vandewalle, P.; Sendid, B.; Colombel, J.-F.;
Poulain, D.; Mallet, J.-M. Tetrahedron 2005, 61, 7669–7677.
doi:10.1016/j.tet.2005.05.098
46.Crich, D.; Smith, M. J. Am. Chem. Soc. 2001, 123, 9015–9020.
doi:10.1021/ja0111481
47.Kasai, R.; Okihara, M.; Asakawa, J.; Mizutani, K.; Tanaka, O.
Tetrahedron 1979, 35, 1427–1432. doi:10.1016/0040-4020(79)85038-3
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