Modular stereoselective synthesis of (1→2)-C-glycosides based on the sp2-sp3 Suzuki-Miyaura reaction

Article abstract of DOI:10.1002/chem.201406591

This work reports a modular and rapid approach to the stereoselective synthesis of a variety of α- and β-(1→2)-linked C-disaccharides. The key step is a Ni-catalyzed cross-coupling reaction of D-glucal pinacol boronate with alkyl halide glycoside easily prepared from commercially available D-glucal. The products of this sp²-sp³ cross-coupling reaction can be converted to glucopyranosyl, mannopyranosyl, or 2-deoxy-glucopyranosyl C-mannopyranosides by one- or two-step stereoselective oxidative-reductive transformations. To the best of our knowledge, we demonstrated the first synthetic application of a challenging sp²-sp³ Suzuki-Miyaura cross-coupling reaction in carbohydrate chemistry.

Full text of DOI:10.1002/chem.201406591

DOI: 10.1002/chem.201406591
Communication
&
Carbohydrate Chemistry
Modular Stereoselective Synthesis of (1!2)-C-Glycosides based
on the sp2–sp3 Suzuki–Miyaura Reaction
Beata Oroszova,[a] Jan Choutka,[a] Radek Pohl,[b] and Kamil Parkan*[a]
rangement chemistry, and methods based on organometallic
Abstract: This work reports a modular and rapid approach
chemistry.[4] Unfortunately, each of these approaches suffers
to the stereoselective synthesis of a variety of a- and b-
from specific limitations. These methods are, however, devel-
(1!2)-linked C-disaccharides. The key step is a Ni-cata-
oped only for specific linkages (1!6, 1!4, 1!3, or 1!2) and
lyzed cross-coupling reaction of d-glucal pinacol boronate
require multistep reaction sequences and in some cases com-
with alkyl halide glycoside easily prepared from commer-
plex starting materials that are not readily available. Recently,
cially available d-glucal. The products of this sp2–sp3
the Werz group introduced two modern Sonogashira[4 h] and
cross-coupling reaction can be converted to glucopyrano-
Stille[4e] cross-coupling reactions into carbohydrate chemistry,
syl, mannopyranosyl, or 2-deoxy-glucopyranosyl C-manno-
allowing for preparation of different (1!2, 1!3, 1!4, 1!6)-
pyranosides by one- or two-step stereoselective oxida-
linked C-disaccharides from relatively simple saccharide subu-
tive–reductive transformations. To the best of our knowl-
nits.
edge, we demonstrated the first synthetic application of
Our alternative strategy is based on the use of the popular
a challenging sp2–sp3 Suzuki-Miyaura cross-coupling reac-
Suzuki–Miyaura reaction.[5] The cross-coupling reactions of bor-
tion in carbohydrate chemistry.
onic acids and related derivatives with aryl-, alkenyl-, or the
more challenging alkyl electrophiles are one of the most pow-
C-glycosides are compounds in which the interglyco-
sidic oxygen atom is replaced with a methylene
group to form glycoside analogues that are stable to
enzyme or chemical hydrolysis. C-glycosides belong
to an important class of carbohydrate mimics that
often exhibit a range of interesting biological proper-
ties.[1] For example, the C-analogue of a blood group
antigen (H-antigen) exhibited interesting activity
against lectin I of Ulex europaeus (UEA-I).[2] Another
Figure 1. C-analogues of H antigen and KRN7000.
potent C-analogue of a-d-galactosyl ceramide
(KRN7000) revealed antitumor activity in tumor-bear-
ing B16 mice by activation of the immune system[3]
erful carbon–carbon bond-formation reactions in organic syn-
thesis. The electron-rich sp2–sp2 coupling reaction works well,
but apart from a few specific examples, the coupling with sp3
electrophiles fails, which limits the application of this reaction
to relatively simple molecules. This lack of success is primarily
caused by unwanted side reactions associated with the much
faster b-hydride elimination that competes with the much
slower oxidative addition and reductive elimination steps.[5a]
Such inherent problems have created a demand for alternative
strategies. The most successful strategy was created by Fu and
co-workers, who developed a series of palladium- and nickel-
catalyzed alkenylations of simple primary and secondary alkyl
halides with alkenylzinc,[6] zirconium,[7] tin,[8] and boron[9] re-
agents. However, thus far, these methods have not been ap-
plied to the coupling of polyfunctional compounds.
(Figure 1).
Stereoselective synthesis of C-disaccharides is of a great im-
portance in carbohydrate chemistry, and therefore, continual
efforts have been put forth for over thirty years to develop
more efficient synthetic methods. These approaches include
anionic methods, cationic chemistry, free-radical approaches,
cyclization strategies, methods based on sigmatropic rear-
[a] B. Oroszova,+ J. Choutka,+ Dr. K. Parkan
Department of Chemistry of Natural Compounds
University of Chemistry and Technology, Prague
Technickꢀ 5, 166 28 Prague 6 (Czech Republic)
E-mail: parkank@vscht.cz
[b] Dr. R. Pohl
Institute of Organic Chemistry and Biochemistry
Academy of Sciences of the Czech Republic, v. v. i.
Flemingovo nꢀm. 2, 166 10 Prague 6 (Czech Republic)
Recently, we reported a new approach to the synthesis of
different C-glycosides based on a Pd-catalyzed cross-coupling
reaction of saccharide-based alkenyl boronic derivatives with
electron-rich sp2 electrophiles (aryl, heteroaryl and alkenyl hal-
ides).[10]
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406591.
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In this work, we present a novel and modular nickel-cata-
lyzed cross-coupling reaction between d-glucal pinacol boro-
nate and unactivated saccharide-based alkyl halides (sp3 elec-
trophiles) that promote C-disaccharide of d-Glucalp-(1!2)-a-d-
ManpOMe containing one endocyclic double bond. Subse-
quent oxidative–reductive transformations of this double bond
enabled one- or two-step stereoselective preparation of gluco-
pyranosyl, 2-deoxyglucopyranosyl, or mannopyranosyl C-glyco-
sides. These straightforward transformations produced the
most challenging a- and b-linked (1!2)-C-disaccharides in
a stereoselective manner under mild conditions.
ring, and only methyl a-glycoside 7 was isolated in a good
yield (73%). When the three-membered ring was opened with
N-iodosuccinimide (NIS) in a mixture of THF and MeOH 2:1,
only 50% conversion was achieved after two days, and the
iodo derivative
(Scheme 2).
8 was acquired in 45% isolated yield
Initially, the developed conditions were tested for coupling
of saccharide subunits 1 and prepared 7–8. Unfortunately, the
Pd-catalyzed Suzuki–Miyaura reaction provided C-disaccharide
9 in negligible yield (see the Supporting Information, Table S3,
entries 1–2). We assumed that this low yield could be caused
by chelation[14] of the organopalladium intermediate, which
prevents the coupling reaction. Thus, we focused our attention
on the cross-coupling reaction with several nickel cata-
lysts.[5a,9a–c]
First, we optimized the coupling of d-glucal pinacol boro-
nate 1 (prepared[10] from commercially available d-glucal) with
methyl
6-deoxy-6-iodo-2,3,4-tri-O-benzyl-a-d-glucopyrano-
side[11] 2 under various conditions (see the Supporting Informa-
tion, Table S2). After exploring a wide array of conditions, we
determined that the Pd-catalyzed cross-coupling reaction with
[HP(tBu)2Me]BF4 furnished the target (1!6)-pseudo-C-disac-
charide 3 with a satisfactory 72% yield. The reaction with
[Ni(cod)2] (cod=1,5-cyclooctadiene) in tert-amyl alcohol led to
The cross-coupling reaction of 7 and 8 catalyzed with
[Ni(cod)2], afforded the corresponding C-analogue of d-Glu-
calp-(1!2)-a-d-ManpOMe 9 in moderate isolated yields of 50
and 57%, respectively. Subsequent screening of various reac-
tion conditions revealed that the best result could be achieved
with NiBr2·diglyme in the presence of 4,4’-di-tert-butyl-2,2’-di-
pyridyl ligand in tert-amyl alcohol at 608C (see the Supporting
Information, Table S3). The reac-
the expected cross-coupling product
(Scheme 1).
3 with 57% yield
tion with alkyl bromide 7 pro-
duced the C-disaccharide 9 in
a good 72% yield. The reaction
with 8 to produce 9 proceeded
with a better yield of 78%, and
the reaction rate was considera-
bly faster than that with
(Scheme 3).
7
Scheme 1. Cross-coupling of pinacol boronate 1 and alkyl halide 2. Conditions A) 5% Pd(OAc)2, 10%
[HP(tBu)2Me]BF4, Na2CO3 (2 equiv), 4:1 DME/H2O, 808C, 72%; B) 4% [Ni(cod)2], 8% bathophenanthroline, tBuOK
(1.6 equiv), tert-amyl alcohol, 508C, 57%.
The cross-coupling product 9
with one endocyclic double
bond offers several possibilities
Second, it was necessary to prepare corresponding saccha-
ride subunits 7–8, which were subsequently tested under opti-
mized cross-coupling conditions. The perbenzylated d-glucal 5,
prepared by benzylation of commercially available d-glucal 4
with BnBr and NaH in the pres-
for further stereoselective transformations. To obtain high over-
all yields of final C-disaccharides, we converted silyl protected
derivative 9 to perbenzylated compound 10. This offered clean
and high yielding one step deprotection in very last step of
ence of tetrabutylammonium
iodide (TBAI), was converted into
1,2-cyclopropanated derivative 6
by using the Friedrich-modified
version of the Simmons–Smith
reaction[12] (Scheme 2).
In the next step, N-bromosuc-
cinimide (NBS) in methanol was
Scheme 3. Cross-coupling of pinacol boronate 1 and alkyl halides 7–8. Conditions B) 4% [Ni(cod)2], 8% bathophe-
nanthroline, tBuOK (1.6 equiv), tert-amyl alcohol, 508C, 50% from 7 or 57% from 8; C) 10% NiBr2·diglyme, 20%
4,4’-di-tert-butyl-2,2’-dipyridyl, tBuOK (2.2 equiv), tert-amyl alcohol, 608C, 72% from 7 or 78% from 8.
used as a source of Br+ for
opening[13] of the cyclopropane
Scheme 2. Synthesis of saccharide based alkyl halides 7 and 8.
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Scheme 4. Synthesis of b-(1!2)-C-disaccharides 12 and 15.
the synthetic protocol, which was found to be more advanta-
geous over two-step deprotection of silyl and benzyl protect-
ing groups.[15] The silyl-protecting group was removed by tetra-
butylammonium fluoride (TBAF) treatment, and subsequent
benzylation under standard conditions in THF provided per-
benzylated C-disaccharide d-Glucalp-(1!2)-a-d-ManpOMe 10
in a 74% yield over two steps. Our stereoselective synthetic
route to C-mimetics of b-d-Glcp-(1!2)-a-d-ManpOMe 12 and
b-d-Manp-(1!2)-a-d-ManpOMe 15 is outlined in Scheme 4. It
was found that hydroboration[10,16] of the double bond in 10
with a BH3·Me2S complex followed by oxidation of the CꢀB
bond with alkaline hydrogen peroxide produced 11 as a single
stereoisomer. The formation of C-disaccharide 12 with native
configuration of hydroxyl groups was achieved by reductive
debenzylation in 53% overall yield from compound 9.
ides a-d-Glcp-(1!2)-a-d-ManpOMe 17 and a-d-Manp-(1!2)-
a-d-ManpOMe 20 were obtained by epoxidation of the double
bond with dimethyldioxirane[18] (DMDO), followed by subse-
quent cleavage of the formed epoxide ring with lithium trie-
thylborohydride (Super-hydride).[4e,h,15,19] The successful isola-
tion of derivative 16 (69%) strongly depended on the oxidative
decomposition of excess of Super-hydride with alkaline hydro-
gen peroxide (see the Supporting Information). Thus, the
double bond of d-Glucalp-(1!2)-a-d-ManpOMe 10 was stereo-
selectively oxidized with more concentrated 3,3-dimethyldioxir-
ane (DMDO)[20] in dichloromethane, and the formed a-epoxide
was subsequently opened by a highly nucleophilic hydride
source (SN2-type ring opening). As mentioned previously, the
benzyl-protecting groups were removed by catalytic hydroge-
nation, and the C-analogue of a-d-Glcp-(1!2)-a-d-ManpOMe
17 was obtained in 89% yield (Scheme 5).
An oxidative–reductive approach was used for the inversion
of C-analogue of b-d-Glcp-(1!2)-a-d-ManpOMe 11 to perben-
zylated
b-d-Manp-(1!2)-a-d-
ManpOMe 15. The newly formed
2-hydroxyl group of 11 was first
oxidized with Dess–Martin peri-
odinane (DMP) to ketone 13,
which was reduced with sodium
borohydride[17] to obtain a mix-
ture of b-C-disaccharides 14 and
11 in a ratio of 95:5. Selective re-
duction of the keto group by
using bulky lithium tri-sec-butyl-
borohydride (l-Selectride) led ex-
clusively to the formation of dia-
stereoisomer 14 in excellent
84% yield. The subsequent de-
benzylation of 14 using palladi-
um on charcoal in ethanol af-
forded C-linked b-d-Manp-(1!
2)-a-d-ManpOMe
disaccharide
15 in 91% yield (Scheme 4).
The corresponding a-C-linked
mimetics of natural disacchar- Scheme 5. Synthesis of a-(1!2)-C-disaccharides 17 and 20.
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Communication
In view of the aforementioned
results, the synthesis of C-disac-
charide
a-d-Manp-(1!2)-a-d-
ManpOMe 20 was executed
using the reaction sequence
shown in Scheme 5. The litera-
ture shows that reduction of the
corresponding ketones arising
from similar a-derivatives with
gluco-configuration leads back
to the starting material.[4 h,21]
Scheme 6. Synthesis of C-analogue 22.
However, we applied the previously discussed oxidative–reduc-
tive approach for inversion of the equatorial 2-hydroxyl group
to an axial position (see the Supporting Information, Table S4).
Oxidation of the free-formed 2-hydroxyl group of C-glycoside
a-d-Glcp-(1!2)-a-d-ManpOMe 16 with DMP led to ketone 18,
which was subsequently reduced with NaBH4, and the mixture
of epimers 19 and 16 was obtained in a 2:7 ratio. When we
tested bulky l-Selectride or Super-hydride for the reduction of
ketone 18, corresponding mixtures of the two C-disaccharides
19 and 16 were obtained in ratios of 1.3:1 and 1:1, respective-
ly. The advantage of this oxidative–reductive procedure was
that the respective C-glycosides a-d-Glcp-(1!2)-a-d-ManpOMe
19 and a-d-Manp-(1!2)-a-d-ManpOMe 16 could be easily sep-
arated by column chromatography on silica gel (Scheme 5).
To invert the configuration of carbon bearing sterically hin-
dered secondary hydroxyl more effectively, the Mitsunobu re-
action with 4-nitrobenzoic acid (4-NO2-BnCOOH) was tested.[22]
The excess of PPh3, diethyl azodicarboxylate (DEAD), and 4-
NO2-BnCOOH after Zemplꢁn deprotection formed C-glycoside
a-d-Manp-(1!2)-a-d-ManpOMe 19 directly from derivative 16
in an excellent 85% overall yield (Scheme 5). The correspond-
ing C-mimetic of natural a-d-Manp-(1!2)-a-d-ManpOMe disac-
charide 20 was obtained after catalytic hydrogenation in
a nearly quantitative yield.
further transformation of d-Glucalp-(1!2)-a-d-ManpOMe 10
was explored and resulted in the diastereoselective synthesis
of a- and b-C-linked disaccharides with 2-deoxy-gluco, gluco,
and manno configurations in good overall yields starting from
coupling product 9. This methodology applied a challenging
sp2–sp3 cross-coupling for the first time in carbohydrate
chemistry and offered high synthetic potential that could be
exploited in the synthesis of complex oligosaccharides or gly-
coconjugates.
Acknowledgements
The research leading to these results was supported by the
Grant Agency of the Czech Republic (grant No. P207/12/P713
and 15-17572S).
Keywords: C-disaccharides
·
C-glycosides
·
diastereoselectivity · Mitsunobu reaction · sp2–sp3 coupling
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Received: December 21, 2014
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Published online on && &&, 0000
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COMMUNICATION
&
Carbohydrate Chemistry
B. Oroszova, J. Choutka, R. Pohl,
K. Parkan*
&& – &&
Synthetic application of a challenging
sp2–sp3 Suzuki–Miyaura cross-coupling
reaction in carbohydrate chemistry is re-
ported. Subsequent oxidative–reductive
transformations provide glucopyranosyl,
mannopyranosyl, or 2-deoxy-glucopyra-
nosyl C-mannopyranosides stereoselec-
tively in excellent overall yields.
Modular Stereoselective Synthesis of
(1!2)-C-Glycosides based on the sp2–
sp3 Suzuki–Miyaura Reaction
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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