Multicomponent one pot synthesis of C-glucosides of 1-azaindolizines

Article abstract of DOI:10.1080/00397911.2020.1786586

A protocol based on Groebke-Blackburn-Bienayame (GBB) multicomponent reaction has been developed for efficient and atom economical synthesis of C-glucosides of 1-azaindolizine, i.e. 2-(β-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizine. Thus, a series of fourteen novel 2-(β-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines have been synthesized in moderate to good yields by reaction of a perbenzylated β-C-glucopyranosyl aldehyde with differently substituted 2-aminopyridines and alkyl isocyanides using InCl₃ as acid catalyst. All synthesized β-C-glucosides were unambiguously characterized with the help of spectroscopic (IR, ¹H-NMR, ¹³C-NMR and mass spectra) data analysis.

Full text of DOI:10.1080/00397911.2020.1786586

Synthetic Communications
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Multicomponent one pot synthesis of C-glucosides
of 1-azaindolizines
Banty Kumar, Bhawani Shankar, Sandeep Kumar, Jyotirmoy Maity & Ashok
K. Prasad
To cite this article: Banty Kumar, Bhawani Shankar, Sandeep Kumar, Jyotirmoy Maity & Ashok
K. Prasad (2020): Multicomponent one pot synthesis of C-glucosides of 1-azaindolizines, Synthetic
Communications, DOI: 10.1080/00397911.2020.1786586
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https://doi.org/10.1080/00397911.2020.1786586
Multicomponent one pot synthesis of C-glucosides of
1-azaindolizines
Banty Kumara,b, Bhawani Shankara, Sandeep Kumara, Jyotirmoy Maityc, and
Ashok K. Prasada
b
aDepartment of Chemistry, University of Delhi, Delhi, India; Department of Chemistry, Rajdhani
c
College, University of Delhi, Delhi, India; Department of Chemistry, St. Stephen’s College, University of
Delhi, Delhi, India
ABSTRACT
ARTICLE HISTORY
Received 17 February 2020
A protocol based on Groebke-Blackburn-Bienayame (GBB) multicom-
ponent reaction has been developed for efficient and atom econom-
ical synthesis of C-glucosides of 1-azaindolizine, i.e. 2-(b-D-
glucopyranosyl)-3-N-alkylamino-1-azaindolizine. Thus, a series of four-
teen novel 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines
have been synthesized in moderate to good yields by reaction of a
perbenzylated b-C-glucopyranosyl aldehyde with differently substi-
tuted 2-aminopyridines and alkyl isocyanides using InCl3 as acid
catalyst. All synthesized b-C-glucosides were unambiguously charac-
terized with the help of spectroscopic (IR, 1H-NMR, 13C-NMR and
mass spectra) data analysis.
KEYWORDS
b-C-glucopyranosyl alde-
hyde; C-glucosides of 1-
azaindolidines; 2-(b-D-
glucopyranosyl)-3-N-alkyla-
mino-1-azaindolizines; mul-
ticomponent reaction;
perbenzylated
GRAPHICAL ABSTRACT
Introduction
Heterocyclic amines are essential molecules of life. 1-Azaindolizine is one of those important
heterocyclic structural moieties, which has drawn the attention of synthetic chemists in recent
past due to its wide range of biological activities, such as antibacterial,[1] anti-inflammatory,[2]
antimicrobial,[3] anticancer,[4] anti-HIV,[5] anti-protozoal[6] and anti-osteoporotic[7] effects.
1-Azaindolizine is structural motif of many drug molecules such as saripidem (anxiolytic),
alpidem (anxiolytic) and zolpidem (hypnotic) (Figure 1). [8,9]
This structural moiety, i.e. 1-azaindolizine can be synthesized through tandem
Michael addition reaction,[10] microwave assisted Domino’s reaction,[11] oxidative cou-
pling reaction[12] and electrocyclization reaction.[13] However, an elegant multicompo-
nent reaction (MCR) always remains an excellent route to get this molecular scaffold.
CONTACT Ashok K. Prasad
ashokenzyme@gmail.com
Department of Chemistry, University of Delhi, Delhi, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
B. KUMAR ET AL.
Figure 1. Some 1-azaindolizine based drug candidates.
Literature revealed that MCR has been used prominently to achieve 1-azaindoli-
zine[14–16] by utilizing a variety of aliphatic/aromatic aldehydes, 2-amidine derivatives
and isocyanides. Further, GBB reaction has generated different bioconjugates, such as
imidazo[1,2-a]pyridine bioconjugates with chalcone,[17] chromone,[18] 2-julolidine,[19]
admantane[20] and boron-dipyrromethene (BODIPY). Herein, we report the utility of
GBB reaction for the synthesis of C-glucosides of 1-azaindolizine by the reaction of per-
benzylated b-C-glucopyranosyl aldehyde with substituted 2-amino-pyridines and alkyl
isocyanides in the presence of InCl3.
Results and discussion
Synthesis of 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines started with the
preparation of perbenzylated b-C-glucopyranosyl aldehyde 1 following the literature
procedure from D-glucose in 85% overall yield.[21] Thus, D-glucose was converted into
peracetylated C-1-styrylglucose by its reaction with dibenzoylmethane in ethanol-water
in presence of NaHCO3, acetylation of C-1-benzoylmethylglucose followed by reduction
of benzoylketonic group of the resulted peracetals to diastereomeric alcohols which on
dehydration in presence of P2O5 afforded targeted alkene. Further, peracetylated C-1-
styrylglucose was converted to perbenzylated C-1-styrylglucose, which on oxidative
cleavage of the double bond of styrene moiety formed b-D-glucopyranosyl aldehyde
1.[21] Synthesis of C-glucoside of 1-azaindolizines were explored by multicomponent
reaction of b-C-glucopyranosyl aldehyde 1 with differently substituted 2-aminopyridines
and alkyl isocyanides in the presence of acedic catalyst as per the procedure developed
by Manvar et al.[22] Thus, the reaction of aldehyde 1 with 2-aminopyridine 2a and
cyclohexyl isocyanide 3a in the presence of catalytic amount of acetic acid afforded 2-
(b-D-glucopyranosyl)-3-N-cyclohexylamino-1-azaindolizine
in
moderate
yield.
Encouraged by the formation of C-glucosides of 1-azaindolizines, we studied optimiza-
tion of the reaction to improve the yield by using catalytic amount of different acids
and reaction solvents (Table 1).
Thus, the reaction of aldehyde 1 with 2-aminopyridine (2a) and cyclohexyl isocyanide
(3a) in inorganic acids, such as HCl, H2SO4, HNO3, H3PO4 and HClO4 (20 mol%) does
not lead to the formation of desired product, except with HClO4 that too in very poor
yield (entries 1-5, Table 1). The result of condensation reaction with organic acids, such
as CH3SO3H, CF3COOH, p-toluenesulfonic acid (PTSA), formic acid and acetic acid or
solid supported acid such as seralite resin Hþ was also not encouraging as the product

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Table 1. Optimization of reaction conditions for the synthesis of 2-(b-D-glucopyranosyl)-3-N-cyclo-
hexylamino-1-azaindolizine (4a).a
Entry
Acid catalyst (Mol%)
Temp (oC )
Solvent
% Yieldb
1
2
3
4
5
6
7
8
HCl (20)
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
70
30
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
Water
THF
NRc
NRc
NRc
NRc
11
H2SO4 (20)
HNO3 (20)
H3PO4 (20)
HClO4 (20)
CH3SO3H (20)
CF3COOH (20)
PTSA (20)
HCOOH (20)
CH3COOH (20)
Seralite resin Hþ (20)
AlCl3 (20)
SnCl4 (20)
BF3.Et2O (20)
FeCl3 (20)
ZnCl2 (20)
BCl3 (20)
InCl3 (20)
InCl3 (30)
InCl3 (40)
InCl3 (50)
InCl3 (20)
InCl3 (20)
InCl3 (20)
InCl3 (20)
NRc
NRc
34
9
34
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
50
NRc
NRc
NRc
11
23
25
50
72
70
65
60
NRc
10
DMF
Toluene
MeOH
NRc
NRc
65
InCl3 (20)
aThe reaction was conducted in methanol (5 mL) at 70 ꢀC using 1a (1 mmol), 2a (1 mmol), 3 (1 mmol) and catalyst
c
(20 mol%); bIsolated yields; NR: no reactions.
formation was observed only in case of PTSA, HCOOH and acetic acid in 34, 34, and
50% yields, respectively (entries 6–11, Table 1). Condensation reaction with some of the
Lewis acids (20 mol%) in methanol was better although there was no reaction with
AlCl3 and SnCl4 (entries 12–18, Table 1). The best result was obtained with the catalytic
use of InCl3 (20 mol%) in methanol, which led to the formation of 2-(b-D-glucopyrano-
syl)-3-N-cyclohexylamino-1-azaindolizine in 72% yield (entry 18, Table 1). The increase
of mol% of InCl3 (30–50 mol%) or change of solvent from methanol to water, THF,
DMF or toluene, or decrease of reaction temperature from 70 ꢀC to the ambient (30 ꢀC)
led to either decrease in yield or reaction did not take place at all (entries 19–26,
Table 1).
After optimization with 2-aminopyridine (2a) and cyclohexyl isocyanide (3a),
sugar aldehyde 1 was reacted with six differently substituted 2-aminopyridines, i.e. 4-
methyl-2-aminopyridine (2b), 5-methyl-2-aminopyridine (2c), 6-methyl-2-aminopyri-
dine (2d), 4-ethyl-2-aminopyridine (2e), 6-methoxy-2-aminopyridine (2f) and

4
B. KUMAR ET AL.
Scheme 1. Synthesis of 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines (4a–4n).
Scheme 2. Possible reaction mechanism of formation of azaindolizine with sugar aldehyde 1, 2-ami-
nopyridines and alkyl isocyanides.
3-carboxy-2-aminopyridine (2 g) and two alkyl isocyanides, i.e. cyclohexyl isocyanide
(3a) and tert-butyl isocyanide (3b) under optimized conditions in the presence of
20 mol% of InCl3 in methanol at 70 ꢀC to afford 4b–4n with 56–88% yields. It was
observed that the reaction of sugar aldehyde 1 with alkyl isocyanides and 2-aminopyri-
dine having electron withdrawing substitutions, such as chloro, cyano, nitro or trifluoro-
methyl under optimized conditions did not yield any product (Scheme 1).
A possible mechanism for multicomponent reaction of sugar aldehyde 1 with substi-
tuted 2-aminopyridines and alkyl isocyanides to synthesize sugar-based 1-azaindolizine
is given in Scheme 2. The reaction starts with nucleophilic attach by amino group of 2-
aminopyridine(s) at aldehydic carbon of the sugar aldehyde to form an intermediate of
carbinolamine, which follows a dehydration step to form Schiff base intermediate.
Coordination of indium chloride with the nitrogen atom of Schiff base allows alkyl iso-
R
N
C
)
cyanide
carbon to attack on sugar carbon of Schiff base. Further, rearrangement
(

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SYNTHETIC COMMUNICATIONSV
5
of electrons in the pyridine ring facilitates the ring closure between nitrogen of pyridine
and carbon from alkyl isocyanide followed by elimination of the catalyst. Finally, iso-
merization of the resulted compound produces 1-azaindolizine. 2-Aminopyridine with
electron withdrawing groups as substituent, such as –Cl, –CN, –NO2, –CF3 on the C-5/
C-6 position of the 2-aminopyridine reduced the nucleophilicity of the –NH2 group and
this could be the reason of no formation of azaindolizines with the use of
such substrates.
Conclusion
We have demonstrated the GBB multicomponent reaction’s synthetic scope on sugars
by synthesizing fourteen new 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines
with good to excellent yields. In this methodology, InCl3 was proved to be the most
effective acid catalyst to bring about this transformation. Evaluation of biological activity
of the synthesized compounds is in progress.
Experimental section
All solvents, reagents and adsorbents used were purchased from Sigma-Aldrich
Chemicals Pvt. Ltd. New Delhi (India) and SD Fine Chemicals Ltd. New Delhi (India).
Solvents were distilled before use. Melting points were determined on Buchi M-560
instrument and are uncorrected. The IR spectra were recorded on a Perkin-Elmer
model 2000 FT-IR spectrometer by making KBr disk for solid samples and thin film for
oils. The optical rotation was measured on Rudolph autopol II automatic polarimeter
1
using light of 589 nm wavelength. H NMR and 13C NMR spectra were recorded on
Jeol Delta 400 and 100.6 MHz spectrometer, respectively using tetramethylsilane (TMS)
as internal standard. The chemical shift values are on d scale and the coupling constant
1
(J) are in Hz. Signals from NH group(s) in H NMR spectra recorded in CDCl3 were
verified by D2O exchange method. HRMS analysis was carried out using Agilent
G6530AA LC Q-TOF mass spectrometer. Analytical TLCs were performed on precoated
Merck silica-gel 60 F254 plates; Visualization of the developed plates was performed by
UV light and charring with 5% alcoholic sulfuric acid. Silica gel (100–200 mesh) was
used for column chromatography.
General procedure for synthesis of 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-
azaindolizines
A mixture of perbenzylated b-C-glucopyranosyl aldehyde (1, 1 mmol), 2-aminopyridine/
substituted 2-aminopyridine (2a–2g, 1 mmol) and substituted isocyanide (3a–3b,
1 mmol) in methanol (5 mL) in a round bottomed flask was added InCl3 (20 mol%) and
the reaction was heated at 70 ꢀC for 2–3 hours. Progress of the reaction was monitor by
TLC. After completion of the reaction, reaction mixture was brought to room tempera-
ture and solvent was evaporated under reduced pressure. Crude reaction mixture was
extracted with ethyl acetate (50 mL ꢁ 3) and combined organic solvent was

6
B. KUMAR ET AL.
concentrated and purified by column chromatography using ethyl acetate-hexane system
as an eluent to afford pure 2-(b-D-glucopyranosyl)-3-N-alkylamino-1-azaindolizines.
2-(2’,3’,4’,6’-Tetra-O-benzyl-b-D-glucopyranosyl)-3-N-cyclohexylamino-1-
azaindolizine (4a)
Purified compound 4a was obtained as yellow orange sticky liquid (0.531 g) in 72%
32
yield. Rf¼0.33 (2% MeOH/chloroform); [a]D ¼ ꢂ1.78 (c 0.1, methanol); IR (CHCl3,
1
cmꢂ1): 3250, 3030, 2925, 2854, 1354, 1062, 739, 697; H NMR (400 MHz, CDCl3): d
8.02 (1H, d, J ¼ 6.4 Hz), 7.53 (1H, d, J ¼ 8.8 Hz), 7.33–7.27 (13H, m), 7.17–7.04 (6H,
m), 6.83 (2H, d, J ¼ 6.3 Hz), 6.77 (1H, t, J ¼ 6.4 Hz), 4.99–4.85 (3H, m), 4.58–4.46
(5H, m), 4.17 (2H, dd, J ¼ 9.8 Hz, 20.6 Hz), 3.86–3.66 (5H, m), 2.78 (2H, brs),
1.94–1.83 (2H, m), 1.64 (2H, brs), 1.28–1.12 (6H, m); 13C NMR (100 MHz, CDCl3):
d 141.5, 139.0, 138.4, 138.3, 138.2, 134.6, 128.4, 128.0, 127.9, 127.7, 127.5, 127.4,
123.8, 122.8, 117.9, 111.6, 87.3, 81.9, 79.5, 78.3, 75.5, 75.1, 74.8, 74.7, 73.5, 69.6,
57.3, 34.4, 25.8, 25.1, 24.9; HRMS (ESI): m/z calcd for C47H52N3O5 [M þ H]þ
738.3901; found: 738.3926.
Debenzylation of 2-(2’,3’,4’,6’-tetra-O-benzyl-b-D-glucopyranosyl)-3-N-tert-
butylamino-1-azaindolizine (4h) to 2-(b-D-glucopyranosyl)-3-N-tert-butylamino-1-
azaindolizine (4h’)
Compound 4h (1 mmol) was dissolved in methanol (3 mL), followed by the addition
of Pd/C (0.1 mmol, 10 mol%). Hydrogen gas was fluxed into the reaction mixture
under high pressure with the continue stirring of reaction mixture at room tempera-
ture till completion of reaction. Next, the reaction mixture was filtered and the fil-
trate was evaporated to get the crude product. The crude product was purified with
column chromatography (with 10% MeOH in chloroform) to get the pure 2-(b-D-
glucopyranosyl)-3-N-tert-butylamino-1-azaindolizine (4h0, 0.323 g) as off-white solid in
92% yield. It was obtained as off white color solid (0.323 g) in 92% yield; M.pt:
32
67–72 ꢀC; Rf ¼ 0.42 (20% MeOH/chloroform); [a]D ¼ þ5.56 (c 0.1, methanol); IR
1
(Neat, cmꢂ1): 3464, 3246, 2924, 1354, 1085; H NMR (400 MHz, D2O): d 8.57 (1H,
d, J ¼ 6.8 Hz), 7.81–7.74 (1H, m), 7.66 (1H, d, J ¼ 9.0 Hz), 7.29 (1H, t, J ¼ 6.8 Hz),
3.77 (1H, d, J ¼ 12.3 Hz), 3.65 (2H, dd, J ¼ 16.4, 7.1 Hz), 3.58–3.44 (3H, m), 1.09
(9H, s); 13C NMR (100 MHz, D2O): d 138.0, 134.3, 128.5, 126.0, 125.9, 116.8, 111.9,
80.4, 76.9, 72.6, 72.0, 69.5, 60.8, 55.7, 28.8; HRMS (ESI): m/z calcd for C17H26N3O5
[M þ H]þ 352.1867; found: 352.1888.
Acknowledgments
We are thankful to CIF-USIC, University of Delhi, for providing the NMR spectral and HRMS
recording facility.

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