Facile synthesis of novel α-methylene-pyrazole-carboxylate substituted imines and trans-β-lactams: Versatile synthons for diverse heterocyclic molecules

Article abstract of DOI:10.1080/00397911.2018.1439174

A simple, facile, and high yielding stereoselective approach for the integration of α-methylene-pyrazole-carboxylates at the β-lactam nucleus is described. These monocyclic β-lactams have been synthesized by treatment of 2-phenoxy/benzylthio/phenylthio ethanoic acids or acetoxyacetyl chloride/phthalimidoacetyl chloride 5a–e with novel α-methylene-pyrazole-carboxylate imines 4a–c using Et₃N and POCl₃ in refluxing toluene. All of the newly synthesized α-methylene-pyrazole carboxylate imines 4a–c and their β-lactam derivatives 6a–h have been fully characterized by spectroscopic techniques such as FTIR, NMR (¹H, ¹³C, and ¹³C DEPT-135), 2D-NMR (COSY and HSQC), and elemental analyses (CHN). The cycloaddition reaction was found to be highly stereoselective leading to the exclusive formation of trans-β-lactams 6a–h and trans configuration was assigned with respect to coupling constant values of C3-H and C4-H. The novel β-lactams 6a–h bearing α-methylene-pyrazole-carboxylate ring system will serve as useful synthons for highly functionalized acids, acetohydrazides/pyrazolones, alcohols, pyrazole carboxamides, peptides, and promising biologically active agents.

Full text of DOI:10.1080/00397911.2018.1439174

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Facile synthesis of novel α-methylene-pyrazole-
carboxylate substituted imines and trans-β-
lactams: Versatile synthons for diverse
heterocyclic molecules
Qudrat Hundal, Shiwani Berry, Dipika Narula, Shamsher S. Bari & Aman
Bhalla
To cite this article: Qudrat Hundal, Shiwani Berry, Dipika Narula, Shamsher S. Bari & Aman Bhalla
(2018): Facile synthesis of novel α-methylene-pyrazole-carboxylate substituted imines and trans-β-
lactams: Versatile synthons for diverse heterocyclic molecules, Synthetic Communications, DOI:
10.1080/00397911.2018.1439174
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SYNTHETIC COMMUNICATIONS®
https://doi.org/10.1080/00397911.2018.1439174
Facile synthesis of novel α-methylene-pyrazole-carboxylate
substituted imines and trans-β-lactams: Versatile synthons
for diverse heterocyclic molecules
Qudrat Hundala, Shiwani Berryb, Dipika Narulab, Shamsher S. Barib, and Aman Bhallab
b
aDepartment of Chemistry, MCM DAV College for Women, Chandigarh, India; Department of Chemistry and
Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh, India
ABSTRACT
ARTICLE HISTORY
Received 18 December 2017
A simple, facile, and high yielding stereoselective approach for the
integration of α-methylene-pyrazole-carboxylates at the β-lactam
nucleus is described. These monocyclic β-lactams have been
synthesized by treatment of 2-phenoxy/benzylthio/phenylthio etha-
noic acids or acetoxyacetyl chloride/phthalimidoacetyl chloride 5a–e
with novel α-methylene-pyrazole-carboxylate imines 4a–c using Et3N
and POCl3 in refluxing toluene. All of the newly synthesized
α-methylene-pyrazole carboxylate imines 4a–c and their β-lactam
derivatives 6a–h have been fully characterized by spectroscopic
techniques such as FTIR, NMR (1H, 13C, and 13C DEPT-135), 2D-NMR
(COSY and HSQC), and elemental analyses (CHN). The cycloaddition
reaction was found to be highly stereoselective leading to the
exclusive formation of trans-β-lactams 6a–h and trans configuration
was assigned with respect to coupling constant values of C3-H and
C4-H. The novel β-lactams 6a–h bearing α-methylene-pyrazole-
carboxylate ring system will serve as useful synthons for highly
functionalized acids, acetohydrazides/pyrazolones, alcohols, pyrazole
carboxamides, peptides, and promising biologically active agents.
KEYWORDS
β-Lactams; cycloaddition;
heterocyclic β-lactams;
imines; pyrazole-ester;
stereoselective
GRAPHICAL ABSTRACT
CONTACT Aman Bhalla
amanbhalla@pu.ac.in
Department of Chemistry and Centre of Advanced Studies in
Chemistry, Panjab University, Chandigarh 160014, India.
Color versions of one or more figures in this article can be found online at www.tandfonline.com/lsyc.
Supplemental data (full experimental details, their spectral data (4a–c and 6a–h) and copies of H NMR, 13C NMR, 13C
1
1
1
DEPT-135, H-1H COSY, and H-13C COSY of representatives) can be accessed on the publisher’s website.
© 2018 Taylor & Francis

2
Q. HUNDAL ET AL.
Introduction
Pyrazoles are well-known aromatic heterocycles that exhibit innumerable pharmacological
and physiological properties.[1–3] Pyrazole-carboxylates and their derivatives show
significant applications in the area of microbicides, agrochemicals, herbicides,[4–6] acts as
important intermediates for the synthesis of protectants and plant growth regulators.[7]
Pyrazolone-ester (I) demonstrates anticonvulsant effects against clonic seizures,[8] pyrazole
ester (II) inhibits phosphodiesterase,[9] 1H-pyrazole carboxylate (III) exhibits antimicro-
bial effects,[10] and pyrazole-3-carboxylate (IV) functions as cyclooxygenase-2
inhibitor[11] (Fig. 1). Molecules (IV–V) have also been utilized as effective synthons for
the further synthesis of acid/hydrazide[11] and carboxamide/carboxylate derivative,[12]
respectively (Fig. 1).
β-Lactam heterocyclic compounds are the integral part of numerous antibiotics includ-
ing monobactams, penicillins, penams, carbapenems, and cephems.[13,14] The monocyclic
β-lactams and their hetero-substituted conjugates also applied for the synthesis of many
classes of compounds which include taxol derivatives, alkaloids and amino acids.[15–17]
Arumugam et al.[18] reported the synthesis of pyrroloisoquinoline/indolizinoindole
anchored β-lactams through pictet-spengler cyclization of ester substituted β-lactams
(VI). Indole- and alkenyl-carboxylate substituted β-lactams (VII) and (VIII) further trans-
formed into their acetohydrazide/pyrazolone[19] and spiropyrrolidine/pyrrolizidine[20]
grafted β-lactams, respectively (Fig. 1). Moreover, stereoselective synthesis of novel
β-lactams is still the area of interest among organic chemists.[21]
Based on the above literature reports and in continuation of our ongoing research on
heterocyclic substituted β-lactams and their precursors,[22] it was envisaged to accommo-
date β-lactam ring system and pyrazole-ester moiety in a single molecular frame work as a
novel core structure which will be the versatile synthons for diverse heterocyclic molecules.
Figure 1. Structures of pyrazoles (I–V) and β-lactams (VI–VIII) linked with ester group.

SYNTHETIC COMMUNICATIONS®
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Results and discussion
To prepare pyrazole-carboxylate aldehyde 3 as a starting substrate, pyrazolone 1 was
initially prepared by the condensation of hydrazine hydrate with ethyl acetoacetate
in absolute ethanol. Pyrazolone 1 on heating with ethyl bromoacetate affords ethyl-1-
pyrazolacetate 2. This when subjected to Vilsmeier–Haack reaction provides desired
pyrazole-carboxylate aldehyde 3 (Scheme 1).[23]
The novel α-methylene-pyrazole-carboxylate substituted imines 4a–c were prepared
by condensation of appropriate primary amines and α-methylene-pyrazole-carboxylate
aldehyde 3 using molecular sieves (4 Å) in dichloromethane at room temperature
(Scheme 1, Table 1). The structure of α-methylene-pyrazole-carboxylate substituted imines
4a–c were confirmed on the basis of spectroscopic techniques viz., FTIR, NMR (1H, 13C),
and CHN elemental analysis.
Further, a series of novel trans-3-oxy/thio/phthalimido-4-pyrazole-carboxylate-β-
lactams 6a–h were synthesized by reacting α-methylene-pyrazole-carboxylate substituted
imines 4a–c with 2-substituted ethanoic acids or acid chlorides 5a–e (Scheme 2).
Initial studies were performed through annealation of ethyl 2-(5-chloro-3-methyl-4-
((phenylimino)methyl)-1H-pyrazol-1-yl) acetate 4a with ketenes, generated in situ from
acetoxyacetyl chloride 5a in dry CH2Cl2 at 0 °C, using triethylamine as a base and
phosphorus oxychloride as a condensing agent. However, the reaction did not afford the
anticipated product. Then the reaction was performed at higher temperature in refluxing
toluene under nitrogen atmosphere and the progress of the reaction was monitored by
thin-layer chromatography (TLC) (Scheme 2, Table 2, entry 1).
Scheme 1. Preparation of α-methylene-pyrazole-carboxylate substituted imines 4a–c.
Table 1. Synthesis of α-methylene-pyrazole-carboxylate substituted imines 4a–c.
Entry
R1
Schiff’s base 4
Yielda (%)
1
2
3
–C6H5
–C6H4CH3(p)
–C6H4OCH3(p)
4a
4b
4c
85
79
80
aYield of pure isolated product with correct analytical and spectral data.

4
Q. HUNDAL ET AL.
Scheme 2. Synthetic route for trans-4-α-methylene-pyrazole-carboxylate β-lactam derivatives 6a–h.
After completion of the reaction, solvent was evaporated under vacuum and crude
product was purified by silica gel column chromatography using 10% EtOAc/hexane as
eluant. The product was identified as trans-ethyl-2-(4-(1-phenyl-3-acetoxy-4-oxo-azetidin-
2-yl)-5-chloro-3-methyl-1H-pyrazol-1-yl)acetate 6a on the basis of various spectroscopic
techniques viz., FTIR, 1H NMR, 13C NMR, and elemental analysis. The reaction was found
to be highly stereoselective and it resulted in the exclusive formation of only trans-β-lactam
6a in excellent yield.
Further, the protocol was extended to synthesize other 4-α-methylene-pyrazole-
carboxylate β-lactam derivatives 6b–h by altering R and R1 substituents (R CH3COO,
=
C6H5O, C6H5S, C6H5CH2S, phthamlimido; R1 ¼ C6H5, (p)CH3OC6H4, (p)CH3C6H4)
(Table 2, entries 2–8). The highly pure novel trans monocyclic β-lactams 6b–h were
isolated by workup of the reaction, purified by silica gel column chromatography using
10% EtOAc/hexane followed by crystallization in dichloromethane-hexane (1:3). The
structures of all these trans-4-α-methylene-pyrazole-carboxylate β-lactams 6a–h were estab-
lished on the basis of various spectroscopic techniques viz., FTIR, NMR (1H and 13C), 13C
1
1
DEPT-135, H-1H COSY, and H-13C COSY in representative cases and their elemental
1
analysis. The H NMR spectra of 6a–h exhibited two doublets for vicinal methine protons
between δ ¼ 4.05–5.64 ppm (J ¼ 1.8–2.7 Hz; C3-H and C4-H) which confirms the formation
Table 2. trans-4-α-Methylene-pyrazole-carboxylate β-lactams 6a–h.
Entry
R
R1
trans-β-Lactams 6
Yielda (%)
1
2
3
4
5
6
7
8
CH3COO–
CH3COO–
PhO–
–C6H5
6a
6b
6c
6d
6e
6f
60
64
70
58
72
69
62
55
–C6H4OCH3(p)
–C6H4OCH3(p)
–C6H5
PhS–
PhS–
PhS–
PhCH2S–
–C6H4OCH3(p)
–C6H4CH3(p)
–C6H4OCH3(p)
–C6H4OCH3(p)
6g
6h
aYield of pure isolated product after chromatographic purification with correct analytical and spectral data.

SYNTHETIC COMMUNICATIONS®
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Scheme 3. Plausible mechanism for the formation of trans-4-α-methylene-pyrazole-carboxylate
β-lactams 6.
of trans-β-lactams as the only product.[21] CHN elemental analysis data of all the synthesized
molecules 6a–h were also in full support with their depicted structures. These α-methylene
pyrazole-carboxylate-β-lactams 6a–h are air- and moisture-stable, soluble in solvents such
as dichloromethane, chloroform, acetone, toluene, ethyl acetate and obtained as stable solids.
A plausible mechanism for 6a–h is depicted below (Scheme 3) and is in accordance with
the literature reports.[22,24] The reaction of 2-substituted ethanoic acid 5 with phosphorus
oxychloride or 2-substituted acetylchloride 5 in the presence of triethylamine generate the
corresponding ketene IX (Scheme 3). Then the nucleophilic attack of imine nitrogen
of novel pyrazole-carboxylate imines 4 to the carbonyl carbon of the ketene, which is
generated in situ, leading to a zwitterionic intermediate X. This zwitterionic intermediate
X upon direct conrotatory electrocyclic ring closure yields thermodynamically less stable
cis-β-lactams which is highly unfavorable due to steric factors associated with bulky group
at C-4. As α-methylene pyrazole-carboxylate is a large group, steric hindrance between
Ar-group of the ketene and the α-methylene pyrazole-ester chain of imine in the
intermediate X causes isomerization to generate intermediate XI. Further, electrocyclic
ring closure reaction of zwitterionic intermediate XI through conrotatory mode leads to
the formation of trans isomer of β-lactams 6.
Conclusion
In conclusion, we have synthesized stereoselectively novel trans-4-α-methylene-pyrazole-
carboxylate β-lactams 6a–h from the reaction of various 2-substituted-ethanoic

6
Q. HUNDAL ET AL.
acids/acetylchloride and novel hetero-ester substituted imines 4a–c using Et3N. The
structures and stereochemistry of all the novel compounds were established on the basis
of various spectroscopic techniques such as FTIR, NMR (1H and 13C NMR), 13C
1
1
DEPT-135, H–1H COSY, H–13C COSY, and elemental analysis. In addition, efforts are
in progress to synthesize corresponding acid, hydrazide, pyrazolone, alcohol, pyrazole
carboxamide, and peptide substituted β-lactams through modifications of these novel
pyrazole-ester substituted β-lactam analogues
6 in accordance with literature
reports[11,19,25–27] (Fig. 2). Further, detailed results of these synthetic applications with
complete spectroscopic analysis and structure activity relationship studies of newly
synthesized imines 4a–c and β-lactams 6a–h will be reported in due course of time.
Figure 2. Synthetic utility of synthesized trans-4-α-methylene-pyrazole-carboxylate β-lactams 6.

SYNTHETIC COMMUNICATIONS®
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Experimental
Melting points were determined in an open capillary on melting point apparatus (Perfit
GSI-MP-3) and are uncorrected. IR spectra were recorded using Thermo scientific Nicolet
iS50 (FTIR) spectrophotometer (υmax in cm−1). 1H and 13C NMR spectra were recorded on
JEOL AL 300 MHz and BRUKER AVANCE II 400 MHz spectrometer using TMS as an
internal standard. The elemental analysis (C, H, N) were recorded on Flash 2000 Organic
elemental analyzer. All the reactions were monitored by TLC using precoated silica
60 F254, 0.25 mm aluminum plates (Merck) with visualization under UV light. Column
chromatography was performed using Merck Silica Gel (60–120 mesh) using ethyl
acetate-hexanes (10:90) as an eluant system.
Preparation of novel pyrazole-carboxylate linked trans-3-oxy/thio/phth-β-lactams was
performed under dry and deoxygenated nitrogen atmosphere. Phosphorus oxychloride
(Merck), triethylamine (Qualigen), hydrazine hydrate (Qualigen) and all other commer-
cially available compounds/reagents/solvents were of reagent grade quality and used
without any further purification. Dimethylformamide and dichloromethane were dried
and distilled over anhydrous calcium chloride (CaCl2) and phosphorus pentoxide (P2O5)
respectively. Toluene was distilled under N2 from sodium-benzophenone immediately
before use.
General procedure for the preparation of imines 4a–c
Solution of aromatic amine (1 mmol) and pyrazole-carboxylate aldehyde (1 mmol) in the
presence of molecular sieves (4 Å) in dry methylene chloride (15 mL) was stirred at room
temperature. Progress of the reaction was monitored by TLC. After completion, the reac-
tion mixture was filtered and solvent was evaporated under vacuum to yield crude product
as colorless oil.
Ethyl 2-(5-chloro-3-methyl-4-((phenylimino)methyl)-1H-pyrazol-1-yl) acetate 4a
1
Colorless oil; yield 85%; FTIR (νmax, cm−1): 1740, 1624; H NMR (300 MHz, CDCl3): 1.18
(3H, t, J ¼ 7.2 Hz, CH2CH3), 2.46 (3H, s, CH3), 4.09 (2H, q, J ¼ 7.2 Hz, CH2CH3), 4.73
(2H, s, NCH2), 6.46–7.24 (5H, m, ArH), 8.20 (1H, s, -N CH); 13C NMR (75 MHz, CDCl3):
=
δ 14.00, 14.47, 49.71, 61.76, 114.66, 114.73, 116.53, 118.10, 120.57, 125.33, 128.84, 128.99,
130.76, 134.09, 146.29, 149.88, 150.81, 151.03, 152.65, 165.56, 166.11; Anal. calcd for
C15H16ClN3 O2: C 58.92, H 5.27, N 13.74%. Found: C 58.84, H 5.19, N 13.61%.
General procedure for synthesis of trans-4-α-methylene-pyrazole-carboxylate
β-lactams 6a–h
A solution of phosphorus oxychloride (POCl3, 0.69 mmol, 1.5 equiv.) in dry toluene
(10 mL) was added dropwise to a stirred solution of 2-substituted ethanoic acid
(0.55 mmol, 1.2 equiv.), imine (0.46 mmol, 1 equiv.), and distilled triethyl amine
(1.38 mmol, 3 equiv.) in dry toluene (20 mL) under nitrogen atmosphere. The reaction
mixture was refluxed for 3–4 h. The solvent was evaporated and crude product was
extracted with CH2Cl2. The organic layer was washed with water (3 � 10 mL), 1N HCl
(3 � 10 mL), 5% NaHCO3 (3 � 10 mL), and brine (3 � 10 mL), then dried (Na2SO4) and

8
Q. HUNDAL ET AL.
concentrated. The crude product was then isolated by column chromatography over silica
gel using hexane/EtOAc (90:10) as eluent to afford pure products.
Trans-ethyl-2-(4-(1-phenyl-3-acetoxy-4-oxo-azetidin-2-yl)-5-chloro-3-methyl-1H-
pyrazol-1-yl)acetate 6a
1
Yellowish white solid; yield: 60%; mp 64–66 °C; FTIR (νmax, cm−1): 1730, 1725, 1700; H
NMR (400 MHz, CDCl3): δ 1.28 (3H, t, J ¼ 7.2 Hz, CH2CH3), 2.02 (3H, s, CH3), 2.12
(3H, s, CH3COO), 4.23 (1H, d, J ¼ 1.8 Hz, C4-H), 4.25 (2H, q, J ¼ 7.1 Hz, CH2CH3),
4.85 (2H, s, NCH2), 5.6 (1H, d, j = 1.8 Hz, C3-H), 7.15–7.35 (m, 5H, ArH); 13C NMR (100
MHz, CDCl3): δ 12.47, 14.13, 20.51, 44.71, 50.16, 54.17, 62.15, 79.71, 109.32, 127.82, 128.06,
128.87, 128.92, 134.07, 148.55, 163.70, 166.76, 169.61; Anal. calcd for C20H22ClN3O5: C
57.21, H 5.28, N 10.01%. Found: C 57.11, H 5.20, N 9.86%.
Funding
AB gratefully acknowledges the financial support for this work from Department of Science and
Technology (DST), New Delhi, Government of India, Project No. SR/FT/CS-037/2010 dated
28-10-2010, FIST-II/PURSE-II (DST) and UGC-CAS, Panjab University, Chandigarh. Qudrat
Hundal and Shiwani Berry acknowledges the financial support from Council of Scientific and
Industrial Research (CSIR) and University Grants Commission (UGC), New Delhi vides Award
No. F.No.10-2(5)/2007(ii)-vE.U.II and F1-17.1/2011-12/RGNF-ST-HIM-9284, respectively.
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