Synthesis of C-spiro-glycoconjugates from sugar lactones via zinc mediated Barbier reaction

Article abstract of DOI:10.1039/c3ra46796a

Anomeric gem-diallylation, mono-β-crotylation and mono-β- propargylation of sugar 1,5 and 1,4 lactones have been achieved under Barbier reaction conditions using Zn powder and a catalytic amount of TMSCl as an activator. Ring closing olefin metathesis of the synthesized gem-diallyl derivatives furnished C-spiro cyclopentene glycosides. Finally, the cyclopentene rings were converted into carbohydrate based tricyclic morpholine fused triazole glycoconjugates as potential SGLT2 inhibitors.

Full text of DOI:10.1039/c3ra46796a

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Synthesis of C-spiro-glycoconjugates from sugar
lactones via zinc mediated Barbier reaction†
Cite this: RSC Adv., 2014, 4, 11023
Mallikharjuna Rao Lambu,ab Altaf Hussain,ab Deepak K. Sharma,ab
c
b
b
ab
*
*
Syed Khalid Yousuf, Baldev Singh, Anil. K. Tripathi and Debaraj Mukherjee
Anomeric gem-diallylation, mono-b-crotylation and mono-b-propargylation of sugar 1,5 and 1,4 lactones
have been achieved under Barbier reaction conditions using Zn powder and a catalytic amount of TMSCl as
an activator. Ring closing olefin metathesis of the synthesized gem-diallyl derivatives furnished C-spiro
cyclopentene glycosides. Finally, the cyclopentene rings were converted into carbohydrate based
tricyclic morpholine fused triazole glycoconjugates as potential SGLT2 inhibitors.
Received 18th November 2013
Accepted 7th January 2014
DOI: 10.1039/c3ra46796a
www.rsc.org/advances
reaction yields, side product formation, stringent conditions
and moisture sensitivity of some of the reagents limit their
use and urge the development of new reagents that are more
efficient and user friendly, leading to synthetically useful
procedures with improved yields.
Introduction
C-Spiro-glycosides1 represent structural subunits of biologi-
cally signicant and structurally demanding natural prod-
ucts.2 Because of their intrinsic stability these compounds
can be used to synthesise specic biological targets.3 Despite
their widespread distribution and usefulness, there are only a
few scattered reports for accessing these privileged scaffolds.4
Synthetically these compounds are the downstream products
of C,C-disubstituted glycosyl diene derivatives, which are
appropriate predecessors for unsaturated spiro bicyclic sugar
offshoots owing to their structural make up which allows
them to undergo ring closing olen metathesis.5 However,
there are no general methods for the synthesis of C,C-diallyl
glycosides. Among the reported methods, application of
Keck’s one electron C–C-bond forming reaction to generate a
diallyl system by quenching the allylic radical generated
in situ with allyltri-n-butylstannane,6 the use of an allyl
Grignard reagent for the synthesis of a gem-diallyl sugar from
a gluconolactone in two steps, namely the opening of the
sugar to obtain the gem-allyl derivative with an open chain
diol and thereaer an oxidative cyclisation,7 the radical
reaction of sugar dihalides5b and allyltributyltin under UV
irradiation forming a mixture of products, and the Lewis acid
mediated nucleophilic substitution of sugar hemiacetals8 with
allyltrimethylsilane are noteworthy (Fig. 1). However the
excess use of reagents, multi-step reaction procedures, low
C–C bond formation using the Barbier reaction has been
well known for many decades. It has been successfully
applied recently in the preparation of new b-lactam antibi-
otics,9 the propargylation of cyclic imides,10 the asymmetric
allenylation of aliphatic aldehydes catalyzed by a chiral
formamide,11 and the synthesis of propargylic and allenic
alcohols.12 Although bis-allylation reactions of nitriles, anhy-
drides, carbonyl compounds and acid chlorides under Barb-
ier conditions using Zn13 and Sm14 are well documented,
their application in carbohydrate chemistry has not been
reported so far. In continuation of our research interest on
glycosylation,15 here we report zinc mediated gem-dia-
llylation, mono-crotylation and mono-propargylation reac-
tions of sugar 1,5 and 1,4 lactones and their application
in the synthesis of spiro-C-glycosides as potential SGLT2
inhibitors.
aAcademy of Scientic and Innovative Research, New Delhi, India. E-mail:
debaraj@iiim.ac.in; khalidiiim@gmail.com; Fax: +91-191-2569111; Tel: +91-191-
2569000
bNPC(Microbes), CSIR-Indian Institute of Integrative Medicine, Canal Road, Jammu (J
& K), India-180001
cMedicinal Chemistry Division, CSIR-Indian Institute of Integrative Medicine, Sanat
Nagar, Srinagar, J & K, India-190005
† Electronic supplementary information (ESI) available: Experimental section and
copies of 1H and 13C NMR of all compounds. See DOI: 10.1039/c3ra46796a
Fig. 1 Synthetic routes to C,C-diallyl glycosides available in the
literature.
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Results and discussion
A typical procedure for the synthesis of sugar 1,5 lactone starts
with an easily available free sugar such as D-glucose (1.1), D-
galactose (2.1), D-mannose (3.1) or L-rhamnose (4.1), which was
peracetylated and reacted with thiophenol in the presence of a
Lewis acid in a one-pot procedure to give phenyl 2,3,4,6-tetra-O-
acetyl-1-thio-b-D-glucopyranoside (1.2, 85%), phenyl 2,3,4,6-tetra-
O-acetyl-1-thio-b-D-galactopyranoside (2.2, 82%), phenyl 2,3,
4,6-tetra-O-acetyl-1-thio-b-D-mannopyranoside (3.2, 80%), or
phenyl 2,3,4-tri-O-acetyl-1-thio-b-L-rhamnopyranoside (4.2, 78%).
Deacetylation under Zemplen conditions16 followed by a subse-
´
quent perbenzylation reaction afforded phenyl 2,3,4,6-tetra-O-
benzyl-1-thio-b-D-glucopyranoside (1.3, 96%), phenyl 2,3,4,6-
tetra-O-benzyl-1-thio-b-D-galactopyranoside (2.3, 98%), phenyl
2,3,4,6-tetra-O-benzyl-1-thio-b-D-mannopyranoside (3.3, 95%) or
phenyl 2,3,4-tri-O-benzyl-1-thio-b-L-rhamnopyranoside (5.3,
93%). The thioglycosides 1.3, 2.3, 3.3 and 4.3 thus obtained were
hydrolysed using NIS in water/acetone to give the hemiacetals
2,3,4,6-tetra-O-benzyl-D-glucopyranose (1.4, 97%), 2,3,4,6-tetra-
O-benzyl-D-galactopyranose (2.4, 94%), 2,3,4,6-tetra-O-benzyl-
D-mannopyranose (3.4, 96%) and 2,3,4-tri-O-benzyl-1-thio-b-
L-rhamnopyranoside (4.4, 95%).17
Aerwards these compounds were oxidized to the corre-
sponding lactones 2,3,4,6-tetra-O-benzyl-D-glucono-1,5-lactone
(1, 92%), 2,3,4,6-tetra-O-benzyl-D-galactno-1,5-lactone (2, 87%),
2,3,4,6-tetra-O-benzyl-D-mannono-1,5-lactone (3, 96%) and
2,3,4-tri-O-benzyl-1-thio-b-L-rhamno-1,5-lactone (4, 95%)
(Scheme 1).18 A PCC mediated oxidation reaction19 of tri-O-
benzyl-D-glucal afforded 3,4,6-tri-O-benzyl-2-deoxy-D-glucono-
1,5-lactone (5) and 4,6-di-O-benzyl-2,3-dideoxy-D-erythro-hex-
2-enono-l,5-lactone (6) in 60% and 15% yield, respectively
(Scheme 2).
The synthesis of 2,3,5-tri-O-benzyl-D-ribono-1,4-lactone (7)
starts with D-ribose, which was reacted with methanol under
Fischer glycosylation conditions. A subsequent perbenzylation
reaction formed methyl-2,3,5-tri-O-benzyl-a-D-riboside (7.2,
82%), and then hydrolysis by 3 N HCl yielded 2,3,5-tri-O-benzyl-
D-ribofuranose (7.3, 75%). Finally, oxidation of 7.3 with DMSO/
Ac2O18 afforded 7 in 86% yield. Compound 8 was synthesised by
Scheme 1 Synthetic route to sugar 1,5 lactones (1–4).
decreased the yield of 1b (30%). When the molar ratio of
Zn : allyl bromide : TMSCl increased to 6 : 3 : 0.3, formation of
1b : 1a was observed in the ratio of 1.5 : 1.0. Aer a careful
optimisation of the reaction conditions, it was found that the
best result for the formation of the C,C-diallyl 1a was obtained
when the relative molar ratio of zinc powder : allyl bromi-
de : TMSCl was increased to 6 : 4 : 0.3 (entry 8). The formation
of 1a was conrmed by spectroscopic analysis. The appearance
of characteristic bis-allylic peaks in the 1H NMR spectrum of 1a
and also the anomeric quaternary carbon at d 75.8 ppm in the
13C NMR spectrum were in full agreement with the literature
data.8 Subjecting the reaction to reuxing conditions or soni-
cation at 40 ꢀC did not improve the reaction yield (entries 9
and 10).
20
the direct oxidation of 2-deoxy-D-ribose with Br2/K2CO3 in
water with 70% yield (Scheme 3).
The synthesis of 2,3:5,6-di-O-isopropylidene-D-manno-1,4-
lactone (9) starts with D-mannose (3.1), which was easily con-
verted into the diisopropylidene derivative 9.2 (75%) in the
presence of dry acetone and conc. H2SO4.21 Anomeric oxidation
of 8.2 with DMSO/Ac2O17 yielded 9 (85%) (Scheme 3).
2,3,4,6-tetra-O-benzyl-D-glucono-1,5-lactone (1) was chosen
as a model substrate for an allylation reaction under Barbier
conditions. The optimisation results are summarised in Table
1. Initially, treatment of 1 with 1 equiv. of allyl bromide, 4.0
equiv. of zinc powder and 0.3 equiv. of TMSCl resulted in the
formation of the monoallyl sugar derivative 1b (20%) as a
mixture of a : b ¼ 1 : 1. With 2.0 equiv. of allyl bromide, the
yield of 1b increased (37%) without the formation of 1a.
However, increasing the molar ratio of TMSCl to 0.5 equiv.
Scheme 2 Synthesis of sugar 1,5 lactones (5 and 6).
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diallylation. The results are presented in Table 2. It can be
seen that in all cases the reaction proceeded smoothly, leading
to the formation of the expected gem-allyl sugar derivatives in
good to excellent yields. It was observed that the nature of the
sugar hardly affected the reaction time and yield. Aer getting
encouraging results for the bis-allylation reaction, we thought
about the bis-crotylation reaction of sugar lactones using crotyl
bromide. As such, compound 1 was treated with crotyl
bromide under the standard reaction conditions. In this case
we ended up with the monosubstituted derivative 10 (b : a ¼
1 : 1) in almost quantitative yield. Increasing the molar
proportion of the reagents did not give successful results. The
same result, i.e. monopropargylation, was obtained when we
attempted a bis-propargylation reaction of 1 with propargyl
bromide using the developed Barbier reaction conditions
1
affording 11 (b : a ¼ 9 : 1) in 85% yield. The H NMR peak at
2.05 ppm (t, J ¼ 2.5 Hz, 1H) and the 13C NMR anomeric carbon
peak at 96.9 ppm indicated the formation of the b isomer
predominantly.
Scheme 3 Synthetic routes to sugar 1,4 lactones.
As discussed earlier, bis-allyl sugar derivatives are important
starting materials for synthesising C-spiro cyclopentene
compounds. Thus, we turned our attention to some of the
applications of the bisallyl sugar derivatives.
Performing the reaction without TMSCl using the same
molar ratio of 6 : 4 (Zn : allyl bromide) decreased the yield of the
desired product. However, a further increase in the molar ratio
of zinc and allyl bromide to 6 : 6 in the absence of TMSCl led to
the formation of 1a as the sole product (entry 13). It is note-
worthy that the reaction of gluconolactone with an excess of
allyl magnesium bromide leads to an opening of the sugar ring
without formation of the bis-allyl product.7
With these optimised reaction conditions in hand, a series
of sugar lactones, including pyrano-lactones (2, 3), deoxy-
lactones (4, 5), 2,3-a,b-unsaturated gluconolactone (6) and
furano-lactones (7, 8, 9), were subjected to C,C-gem-
Initially, 1a was subjected to RCM using a Grubbs 2nd
generation catalyst to afford the C-spiro cyclopentenyl
compound 12 in 85% yield (Scheme 4). The formation of 12 was
conrmed by the disappearance of the proton signals at d 5.90–
5.80 ppm and 5.19–4.98 ppm, and the appearance of proton
1
signals at d 5.66–5.62 ppm in the H NMR spectrum. Further,
the disappearance of signals at 118.5 and 118.4 ppm in the 13
C
NMR spectrum of compound 12 was in accordance with our
predicted structure. Similarly, compounds 2a and 4a were also
Table 1 Optimization of the reaction conditionsa
Entry
Zn powder (equiv.)
Allyl bromide (equiv.)
TMSCl (equiv.)
Solvent
Yieldb (%) (1a : 1b)
1
2
3
4
5
6
7
8
4
4
4
6
6
6
6
6
6
6
6
6
6
6
6
1
2
2
2
2
3
3
4
4
4
4
5
6
4
4
0.3
0.3
0.5
0.3
0.5
0.3
0.5
0.3
0.3
0.3
—
THF
THF
THF
THF
THF
THF
THF
THF
THFc
THFd
THF
THF
THF
DCE
ACN
20 (0 : 1)
37 (0 : 1)
30 (0 : 1)
48 (0 : 1)
42 (0 : 1)
60 (1.5 : 1)
53 (1.5 : 1)
95 (1 : 0)
82 (1 : 0)
93 (1 : 0)
57 (2 : 1)
73 (1 : 0)
86 (1 : 0)
0
9
10
11
12
13
14
15
—
—
0.3
0.3
0
a
General reaction conditions: 1 equiv. of compound 1 was used in THF at rt for 4 h. b Isolated yield aer column chromatography. c Reaction mixture
was reuxed at 60 ꢀC. d Reaction mixture was sonicated at 40 ꢀC.
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Table 2 Gem-diallylation, mono-crotylation and mono-propargylation reactions of various sugar 1,5 and 1,4 lactones
Entry
Substrate
Activated halides
Producta
Yieldb (%)
95
1
Allyl bromide
2
Allyl bromide
89
3
4
Allyl bromide
Allyl bromide
92
84
5
6
Allyl bromide
Allyl bromide
87
67
7
8
Allyl bromide
Allyl bromide
65
86
9
Allyl bromide
81
10
1
1
Crotyl bromide
80c
85d
11
Propargyl bromide
Identied using spectroscopic analysis. b Isolated yield aer column chromatography. b : a ¼ 1 : 1. d b : a ¼ 9 : 1.
a
c
subjected to RCM using the above conditions to afford wide variety of different bioactive compounds. Compound 12
compounds 13 (81%) and 14 (86%). was subjected to a mCPBA mediated epoxidation reaction.
Fused triazole compounds are of interest due to their various Gratifyingly, we observed the formation of a single diastereomer
1
biological properties22 and also their clinical applications.23 15 containing a b epoxide ring. This was conrmed by the H
Keeping in mind the pharmaceutical importance of morpho- NMR spectrum, which showed a coupling constant of J ¼ 3.5 Hz
line,24 we became interested in synthesising new scaffolds between the two protons of the epoxy ring which is character-
containing the morpholine fused 1,2,3-triazole based on a istic of b epoxide formation. Energy minimisation data gener-
carbohydrate core by modifying the spiro cyclopentene part of ated from ChemDraw soware was in agreement with the
compound 12. This could pave the way for the preparation of a observed result. Treatment of epoxide 15 with a NaN3/NH4Cl
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temperature. These reactions avoid the necessary maintenance
of cryogenic temperatures, have shorter reaction times and
result in good yields. The application of this methodology for
the synthesis of C-spiro-cyclopentenyl glycosides, C-spiro-mor-
pholine fused triazole based glycoconjugates, b-C-crotyl and b-
C-propargyl glycosides has also been highlighted.
Acknowledgements
The authors are thankful to Dr Ram A. Vishwakarma, Director
IIIM Jammu and DST (IFCH-18, GAP 1145) for their generous
funding. ML, AH and DKS are thankful to CSIR-UGC New Delhi
for the junior research fellowship. IIIM publication no. IIIM/
1641/2014.
Scheme 4 Synthesis of the C-spiro cyclopentenyl glycosides.
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