Stable Alkynyl Glycosyl Carbonates: Catalytic Anomeric Activation and Synthesis of a Tridecasaccharide Reminiscent of Mycobacterium tuberculosis Cell Wall Lipoarabinomannan

Article abstract of DOI:10.1002/anie.201511695

Oligosaccharide synthesis is still a challenging task despite the advent of modern glycosidation techniques. Herein, alkynyl glycosyl carbonates are shown to be stable glycosyl donors that can be activated catalytically by gold and silver salts at 25 °C in just 15 min to produce glycosides in excellent yields. Benzoyl glycosyl carbonate donors are solid compounds with a long shelf life. This operationally simple protocol was found to be highly efficient for the synthesis of nucleosides, amino acids, and phenolic and azido glycoconjugates. Repeated use of the carbonate glycosidation method enabled the highly convergent synthesis of tridecaarabinomannan in a rapid manner.

Full text of DOI:10.1002/anie.201511695

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International Edition: DOI: 10.1002/anie.201511695
German Edition: DOI: 10.1002/ange.201511695
Glycosidation
Stable Alkynyl Glycosyl Carbonates: Catalytic Anomeric Activation
and Synthesis of a Tridecasaccharide Reminiscent of Mycobacterium
tuberculosis Cell Wall Lipoarabinomannan
Bijoyananda Mishra, Mahesh Neralkar, and Srinivas Hotha*
Abstract: Oligosaccharide synthesis is still a challenging task
despite the advent of modern glycosidation techniques. Herein,
alkynyl glycosyl carbonates are shown to be stable glycosyl
donors that can be activated catalytically by gold and silver
salts at 258C in just 15 min to produce glycosides in excellent
yields. Benzoyl glycosyl carbonate donors are solid com-
pounds with a long shelf life. This operationally simple
protocol was found to be highly efficient for the synthesis of
nucleosides, amino acids, and phenolic and azido glycoconju-
gates. Repeated use of the carbonate glycosidation method
enabled the highly convergent synthesis of tridecaarabino-
mannan in a rapid manner.
noglycosides,[3h] thioglycosides,[3i–k] n-pentenyl glycosides,[3l]
alkynyl glycosides,[3m] alkyl 1,2-O-orthoesters,[3n–p] glycosyl
phosphates,[3q] and hemiacetals.[3r] The identification of alkyl
glycosyl and thioglycosyl donors has been a transformative
advance in the glycosciences, as the alkyl and thio groups
serve as stable appendages at the anomeric position, and the
compounds can be triggered to become glycosyl donors with
an appropriate promoter.
Our own research efforts identified propargyl glycosides
as glycosyl donors in the presence of a catalytic amount of
AuCl3.[3m] Subsequently, Yu and co-workers reported o-
alkynyl esters[3s] and Zhu and co-workers reported S-but-3-
ynyl glycosides[3t] as glycosyl donors with gold catalysts. Gold-
catalyzed transglycosidation[3m] has proven to be a robust
reaction for the synthesis of glycosides, but has limitations
including I) the suitability of only an ether functional group at
the C2 position,[3m] II) the lack of stereocontrol through
anchimeric assistance,[4a] and III) the hydrolysis of the inter-
glycosidic bond in some instances.[4b,c] Propargyl 1,2-orthoest-
ers were utilized to enable 1,2-trans diastereoselectivity.[3p] In
the search for a versatile and stable glycosyl donor that can be
activated in a catalytic fashion, our attention was drawn to the
most popular trichloroacetamidates. However, some glycosyl
trichloroacetamidates have a short life time. The hemiacetal
precursor to imidates is readily accessible and highly stable;
hence, we hypothesized that the conversion of the hemiacetal
into a stable, versatile, and reactive glycosyl donor could be
highly rewarding.
T
he chemical synthesis of oligosaccharides has emerged as
a viable approach offering advantages including homogeneity,
scalability, and the ability to synthesize unnatural glycocon-
jugates, which can have great ramifications in modern
medicine and materials science.[1] Two saccharides are chemi-
cally coupled by a glycosidation reaction that involves
a glycosyl donor 1, a fully protected saccharide with a leaving
group at the anomeric position, and a glycosyl acceptor
(R1OH), usually containing a single hydroxy group.[2] Pro-
moters activate the leaving group to give a highly reactive
oxocarbenium ion intermediate 2 that will be susceptible to
the attack of the acceptor, thus resulting in a glycoside 3
(Scheme 1).[2]
Methods for the decarboxylative glycosidation of carbon-
ate donors is known; however, they have not been widely
utilized owing to forcing reaction conditions and poor yields.[5]
Yu and co-workers reported gold-catalysis conditions that can
activate o-alkynyl esters but not 2-butynyl carbonates even at
an elevated temperature (Scheme 2).[6a] The failure to acti-
Scheme 1. General glycosidation reaction. LG=leaving group, P=pro-
tecting group.
Glycosidation methods that are reliable and scalable and
involve stable glycosyl donors are still scarce even after
several decades since the first glycoside synthesis. Well-
studied glycosyl donors[3] include glycosyl halides,[3a–d] glyco-
syl esters,[3e] glycosyl trichloroacetamidates,[3f] glycals,[3g] sele-
[*] B. Mishra, M. Neralkar, Prof. S. Hotha
Department of Chemistry
Scheme 2. Hypothesis for the use of alkynyl carbonate glycosyl donors.
Indian Institute of Science Education and Research
Dr. Homi Bhabha Road, Pune (India)
E-mail: s.hotha@iiserpune.ac.in
vate 2-butynyl carbonates can be attributed to the possible
higher degree of freedom of the leaving group, thereby
diminishing the chances of gold–alkyne coordination. In our
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201511695.
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laboratory, acceleration of glycosidation was noticed upon
philic AuCl3 (30 mol%) produced the disaccharide 9 in 60%
shifting from propargyl to ethynylcyclohexyl glycosides owing
to the well-understood Thorpe–Ingold effect.[4a] Furthermore,
cyclohexyl alkynyl carbonates were demonstrated to cyclize
into spirocyclic alkylidenes under gold-catalysis by Buzas and
Gagosz.[6b]
Therefore, the conformationally rigid glucosyl ethynyl-
cyclohexyl carbonate 4a containing a strategically positioned
alkyne was designed to bring the Thorpe–Ingold effect into
the alkynyl carbonate donor. This key alkynyl glucosyl
carbonate 4a was synthesized in two simple steps. Commer-
cially available 1-ethynylcyclohexanol (5) was first converted
into the carbonate 6, which was further treated with readily
accessible per-O-benzoyl glucopyranose (7)[7] to give 4a. We
began our glycosidation studies by treating carbonate 4a and
glucosyl acceptor 8 with several Lewis and Brønsted acids
known to be either carbonate activators or alkynophilic
(Scheme 3).
yield along with formation of the by-product 10.[8]
The yield dropped when the amount of AuX3 (X = Cl, Br)
was reduced to 15 mol%; however, the introduction of
HAuCl4 (15 mol%) raised the yield to 72%. Discouraging
results were observed with a range of Au complexes, such as
15 mol% of AuCl, [Ph3PAuCl], and [(p-CF3C6H4)3AuCl]. A
highly alkynophilic gold phosphite complex[9] and AgOTf also
failed to yield the desired disaccharide. However, the addition
of 15 mol% each of the gold phosphite 11 and AgOTf
dramatically improved the performance of the reaction to
near-quantitative yields within 15 min. Individually, AgOTf
and the gold phosphite complex were not reactive at all;
however, a combination of the two was observed to be an
effective catalytic system. Thus, the glycosidation by complex
11 and AgOTf falls under the recently propounded category
of a type II Au–Ag bimetallic reaction.[9a]
Complex 11 in combination with a silver salt, such as
AgSbF6 and AgNTf2, gave the disaccharide 9 in 80–90%
yield. A reduction in the amount of the gold phosphite
complex 11 and AgOTf to 8 mol% each did not compromise
the yield, but further reduction was not encouraging. There-
Glycosyl donor 4a did not react at all with 30 mol% of the
known alkyne activators, such as RuCl3, I2, RhCl3, PdCl2,
CuCl, CuCl2, Cu(OTf)2, or BF3·Et2O, and reacted poorly with
Sc(OTf)3, InBr3, and TfOH at 25–608C. However, alkyno-
Scheme 3. Screening of reagents for the coupling of alkynyl carbonate glycoside 4a with glucosyl acceptor 8. Bz=benzoyl, DBU=1,8-
diazabicyclo[5.4.0]undec-7-ene, MS=molecular sieves, Tf =trifluoromethanesulfonyl.
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fore, future reactions were car-
ried out with 8 mol% each of
catalyst 11 and AgOTf in
CH2Cl2 at 258C for 15 min as
the optimal conditions. The 1,2-
trans selectivity, the reaction
time, and the yield of the gly-
cosidation was found to be
independent of the anomeric
configuration of the initial gly-
cosyl donor 4a, thus signifying
the intermediacy of an oxocar-
benium ion.[8]
Scheme 4. Performance of carbonate glycosyl donors 4b–i in the glycosidation of 8 in the presence of 11
(8 mol%) and AgOTf (8 mol%) in CH2Cl2 at 258C.
In contrast to the propargyl
glycosides, substituted alkynes
were tolerated in the carbonate
glycosyl donors, as in 4b
(Scheme 4). The significance
of the Thorpe–Ingold effect is
apparent, since the perfor-
mance of the reaction did not
drastically change with other
dialkyl substituents (glycosyl
donors 4c,d). However, the
reaction yield dropped to 10%
when there was only one CH3
group (in 4e), and no reaction
was observed with the prop-
argyl carbonate donor 4 f or
homopropargyl
carbonate
donor 4g. The vinyl carbonate
4h could not be prepared
owing to its unstable nature.[10]
An alkyne is required for this
glycosidation, as further evi-
dent from the total lack of
reactivity of carbonate 4i
under the Au–Ag catalytic con-
ditions.
Scheme 5. Plausible mechanism for the alkynyl carbonate glycosidation.
The “silver effect” can be
attributed to I) the chloride-
ion-scavenging ability of silver
to form [LAuOTf] (path a)
and/or II) the prevention of the formation of unreactive
chloride-bridged dinuclear species [LAuClAuL]+ (path b;
Scheme 5).[9] Although the detailed mechanism requires
further investigation, a simple plausible mechanism can be
put forward. The reaction of catalyst 11 with AgOTf can lead
to the formation of the complex [LAuOTf] (path a), which
can coordinate with the alkyne group of the donor 4a to
afford a gold–alkyne complex A1 if Ag scavenges the chloride
ion. Alternatively, both [Au] and [Ag] can arrest the
formation of undesired dinuclear species [LAuClAuL]+ by
forming an Au/Ag–alkyne complex A2 (path b). In either
case, a lone pair of electrons from the endocyclic oxygen atom
can trigger an electron-flow cascade to release the vinylgold
cyclic carbonate B and the oxocarbenium ion C. The cationic
charge on the intermediate C can get delocalized as shown in
trioxolenium ion D through neighboring-group participation.
Subsequently, intermediate D can react with glucosyl
acceptor 8 to give the disaccharide 9 with the release of
a proton, which causes intermediate B to undergo proto-
deauration to extrude alkene 10 and the catalytic species.
We explored the generality of the reaction with various
glycosyl carbonates (compounds 4a,j–p) and acceptors (com-
pounds 8 and 12–16) under these optimized glycosidation
conditions. Glucosyl carbonate 4a reacted with glycosyl
acceptors containing secondary OH groups (compounds 12
and 13) to smoothly provide disaccharides 17 and 18 in high
yields (Scheme 6). Carbonate 4a proved to be an excellent
glycosyl donor: Alicyclic, benzylic, and steroidal acceptors
14–16 were all transformed into the expected glycosides 19–
21.[8] The generality of this new glycosidation protocol with
respect to other glycosyl carbonates was also investigated. A
number of glycosyl carbonates, 4j–p, were observed to be
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exemplary donors and afforded var-
ious glycosides, disaccharides, and
oligosaccharides (products 22–37).
Noticeably, the use of per-O-
benzyl-protected carbonate donors
4o,p resulted in trans/cis glycosides
34–37.[8] Furthermore, carbonate
glucoside 4a was observed to be
a very good donor for the synthesis
of azido glucoside 38 in the presence
of azidotrimethylsilane (Scheme 7).
Nucleoside 39 could be conveniently
synthesized from carbonate 4a in
15 min at 258C by using BSA and
8 mol% each of catalyst 11 and
AgOTf, and the serinyl glucoside
40 and phenolic glucoside 41 were
also synthesized in excellent yield
under the standard reaction condi-
tions.[8]
An enticing perspective for
probing the nuances of this glycosyl
carbonate chemistry was the synthe-
sis of a mannose-capped arabinan
reminiscent of the lipoarabino-
mannan complex of the Mycobacte-
rium tuberculosis cell wall.[11]
Assembly of the tridecasaccharide
required two monosaccharide build-
ing blocks 45 and 47, which could be
readily accessed from the known 3,5-
di-O-benzoyl-1,2-O-orthoester 42 of
arabinofuranose
(Scheme 8).[12]
Saponification of orthoester 42
under ZemplØn conditions afforded
the diol 43, which was converted into
disilyl ether 44. Gold-catalyzed gly-
cosidation with 4-penten-1-ol, fol-
lowed by fluoride-mediated desily-
lation, afforded the n-pentenyl fur-
anoside 45. Treatment with one
equivalent of tert-butyldiphenylsilyl
chloride (TBDPSCl) and subse-
quent benzoylation gave the
orthoester 46, which upon hydrolysis
under gold-catalysis conditions
afforded the required hemiacetal
47. Finally, the treatment of 47 with
carbonate 6 in DBU afforded the
desired furanosyl donor 48.
Synthesis of the tridecafurano-
side commenced with an Au/Ag-
catalyzed reaction between arabino-
furanosyl donor 48 and acceptor 45.
Gratifyingly, the reaction between
acceptor 45 and donor 48 (2.5 equiv)
gave the trisaccharide 49 in 95%
within 15 min in the presence of
8 mol% each of the gold phosphite
Scheme 6. Substrate-compatibility studies. Bn=benzyl.
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loading, this transformation was shown to be
applicable to the synthesis of a biologically
significant tridecasaccharide segment remi-
niscent of the mycobacterial cell surface.
Acknowledgements
B.M. and M.N. acknowledge a fellowship from
CSIR-UGC-NET. S.H. thanks the DST, New
Delhi for financial support and a Swarnajayan-
thi Fellowship.
Scheme 7. Further glycosylation reactions of 4a: a) 11 (8 mol%), AgOTf (8 mol%), 4 Š
MS powder, CH2Cl2, 258C, 15 min; b) bis(trimethylsilyl)acetamide (BSA), 11 (8 mol%),
AgOTf (8 mol%), 4 Š MS powder, CH2Cl2, 258C, 15 min. Fmoc=9-fluorenylmethoxy-
carbonyl.
Keywords: carbonates · glycoconjugates ·
glycosidation · gold catalysis · oligosaccharides
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7786–7791
Angew. Chem. 2016, 128, 7917–7922
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a) NaOCH3, CH3OH, 258C, 30 min, 94%; b) TBDPSCl (2 equiv for 44
and 1 equiv for 46), imidazole, DMF, 258C, 2 h, 90% for 44, 76% for
46; c) 4-penten-1-ol, AuBr3 (6 mol%), CH2Cl2, 4 Š MS powder, 258C,
2 h, 87%; d) HF·py, 0–258C, 3 h, 95%; e) BzCl, pyridine, 258C, 2 h,
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(8 mol%), 4 Š MS powder, CH2Cl2, 258C, 15 min; b) N-iodosuccin-
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86%; d) 6, DBU, CH2Cl2, 0–258C, 3 h, 92%.
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Received: December 17, 2015
Published online: February 16, 2016
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