Indian Journal of Chemistry Vol. 52B, July 2013, pp 915-921

An efficient L-proline catalyzed four-component synthesis of

β-acetamido ketones and esters

Neetu Singh, Satish Kumar Singh & Krishna Nand Singh*

Department of Chemistry [Centre of Advanced Study], Faculty of Science

Banaras Hindu University, Varanasi 221 005, India

E-mail: knsinghbhu@yahoo.co.in

Received 16 August 2012; accepted (revised) 26 February 2013

An environmentally benign synthesis of β-acetamido carbonyl compounds has been achieved in high yields by one-pot multicomponent condensation of aryl aldehyde, acetyl chloride, acetonitrile/benzonitrile and enolisable ketone/ester in the catalytic presence of L-proline.

Keywords: Multicomponent reaction, β-acetamido ketone/ester, L-proline, one-pot synthesis, aromatic aldehydes

β-Acetamido ketones and esters are useful inter-

mediates in organic syntheses because of their presence

in several bioactive compounds and also due to their

polyfunctional nature1,2

. These compounds are also employed in the synthesis of many important organic

molecules like 1,3-amino alcohols or β-aminoacids3,4

,

as well as for the production of antibiotics such as nikkomycins and neopolyoxines

5,6. The most

fascinating and studied reaction, for the synthesis of β-

acetamido carbonyl compounds, involves multi-

component coupling of aldehyde, enolizable ketone, acetyl chloride and acetonitrile, reported by Iqbal and

co-workers using catalysts such as cobalt(II) chloride,

polyaniline supported cobalt catalyst and Montmorillonite K10 (Ref 7-11). Subsequently, a

number of other catalysts such as Cu(OTf)2 (Ref 12),

silica sulfuric acid13

, Zeolite14

, BiOCl (Ref 15), ZrOCl2.8H2O (Ref 16), CeCl3.7H2O (Ref 17),

Amberlyst-15 (Ref 18), heteropolyacid

19, I2 (Ref 20),

K5CoW12O40.3H2O (Ref 21), FeCl3.6H2O (Ref 22),

Nafion-H (Ref 23), ZnO (Ref 24), Fe(HSO4)3 (Ref 25), sulfated zirconia

26, SnCl2.2H2O (Ref 27), SiCl4-ZnCl2

(Ref 28), HClO4-SiO2 (Ref 29), NaHSO4.H2O (Ref 30),

[HO3S(CH2)4MIM][HSO4] (Ref 31), polyaniline-supported acid

32, Zr(HSO4)4/Mg(HSO4)2 (Ref 33),

Fe(ClO4)3.6H2O (Ref 34), and Selectfluor

TM (Ref 35),

have been exploited for the synthesis of such systems. Although these methods are generally useful, many of

them have limitations such as moisture sensitivity of

the catalyst, use of expensive metal salt as catalyst,

longer reaction times, harsh reaction conditions and

tedious workup procedure. For that reason, the

development of simple, efficient, and clean approach

using organo-catalysts for synthesis of these

compounds is highly desired. Now a days, there is immense interest in catalytic

transformations using small organic molecules as they

offer potential reduction in environmental contamination and thus make the process green

36. L-

Proline is regarded as the simplest ‘enzyme’ and has

been elegantly used as a versatile organocatalyst in

various synthetic processes37

. The rigid ring structure, easy availability, nontoxic nature, simple handling

and economical viability make this tiny molecule a

remarkable organocatalyst for synthesizing molecules of biological interest. Multicomponent reactions

(MCRs) have emerged as a powerful tool in

combinatorial chemistry for the generation of small-molecule libraries

38-41. Considering the modern

“paradigm shift” toward green synthesis,

multicomponent reactions utilizing organocatalysis

are of vital significance.

Results and Discussion

In view of the above and as a part of our interest on

multicomponent synthesis42

, we describe herein L-proline-catalyzed one-pot procedure for the efficient

synthesis of β-amino carbonyl compounds in excellent

yields via four-component reaction of aldehyde, enolizable ketone/ester, acetyl chloride and

acetonitrile/benzonitrile at room temperature

(Scheme I).

INDIAN J. CHEM., SEC B, JULY 2013

916

To reveal the catalytic potential of L-proline for the

above reaction, a model reaction using benzaldehyde

1a, acetophenone 2a, acetyl chloride and, acetonitrile was carried out at room temperature using 5, 10, 12,

15 and 20 mol% of L-proline, resulting in the

formation of 57, 74, 78, 87 and 86% of the product yields respectively. Thus, the maximum yield (87%)

of the product was obtained with 15 mol% of L-

proline in 2.5 hr at RT, and the control experiment in the absence of catalyst could not bring about any

conversion to the product even after 15 hr.

Under the optimized reaction conditions, various

aromatic aldehydes viz. benzaldehyde 1a, m-chloro-benzaldehyde 1b, p-chlorobenzaldehyde 1c, p-nitro-

benzaldehyde 1d, p-tolualdehyde 1e, p-anisaldehyde

1f, p-bromobenzaldehyde 1g, m-bromobenzaldehyde 1h, o-nitrobenzaldehyde 1i, m-nitrobenzaldehyde 1j,

cinnamaldehyde 1k were made to react with

actophenone 2a or ethyl acetoacetate 2b, acetyl chloride, and acetonitrile or benzonitrile at room

temperature to afford β-acetamido ketones 3a-l or

esters 4a-e. The outcome is given in Table I.

All sorts of the unsubstituted 1a, electron-rich and electron-deficient aromatic aldehydes 1b-j, and α,β-

unsaturated aldehyde 1k worked well in the reaction.

After completion of the reaction, the reaction mixture was poured into ice water. The resulting solid product

was filtered, washed with cold water and

recrystallized from ethyl acetate/n-hexane to give the

pure product. All the products were crystalline compounds and were fully characterized by their

melting points and spectral data (IR and 1H NMR).

Based on product isolation and existing literature, a plausible mechanism is outlined in Scheme II.

Experimental Section

All the reagents were purchased from Aldrich USA

and Merck India, and were used as supplied. IR spectra were recorded on a JASCO FT/IR-5300

spectrophotometer. NMR spectra were run on a JEOL

AL300 FTNMR spectrometer; chemical shift are given in δ ppm, relative to TMS as internal reference.

General Procedure for the preparation of β-ace-

tamido ketones/esters

A mixture of aromatic aldehyde (1, 2 mmol),

acetophenone or ethyl acetoacetate (2, 2 mmol),

acetyl chloride (0.5 mL), acetonitrile/benzonitrile (1

mL) and L-proline (15 mol%) was taken in a 25 mL round bottom flask and was stirred at RT. The

progress of the reaction was monitored by TLC. After

the completion of reaction, mixture was poured into 20 mL of ice cold water. The solid product obtained

was filtered, washed with cold water and

recrystallized using ethyl acetate/n-hexane to give pure β-acetamido ketones (3a-l)/esters (4a-e). All the

products were characterized by their physical and

spectral data. Spectral data of the products are

provided below:

β-Acetamido-β-(phenyl) propiophenone, 3a

IR (KBr): 3272, 3093, 1690, 1643, 1557, 1451, 1347, 1295 cm

-1;

1H NMR (300 MHz, CDCl3): δ 2.03

(s, 3H), 3.40 (dd, J = 6 & 16.8 Hz, 1H), 3.73 (dd, J =

4.8 & 16.8 Hz, 1H), 5.53-5.60 (m, 1H), 6.65 (d, J = 7.2 Hz, 1H), 7.22-7.59 (m, 8H), 7.89 (d, J = 7.8 Hz,

2H).

Scheme I — Synthesis of β-amino carbonyl compounds

SINGH et al.: SYNTHESIS OF β-ACETAMIDO KETONES AND ESTERS

917

\

Table I — L-Proline catalyzed multicomponent synthesis of β-amino carbonyls

m.p. (ºC) Entry Aldehyde

1

Ketone/Ester

2

Product 3/4

Yield (%)a

Obs. Lit.22,23, 31,43,44

1

1a

2a

3a

87 101-02 102-04

2

1b

2a

3b

91 103-05 105-08

3

1c

2a

ONH

O

Cl

3c

96 148-49 147-48

4

1d

2a

3d

91 148-50 150-51

5

1e

2a

3e

90 111-12 108-10

6

1f

2a

3f

92 105-06 106-08

7

1g

2a

3g

94 147-49 148-50

—Contd

INDIAN J. CHEM., SEC B, JULY 2013

918

Table I ― L-Proline catalyzed multicomponent synthesis of β-amino carbonyls.--Contd

m.p. (ºC) Entry Aldehyde

1

Ketone/Ester

2

Product 3/4

Yield (%)a

Obs. Lit.22,23, 31,43,44

8

1h

2a

3h

92 102-04 100-03

9

1i

2a

ONH

O

NO2

3i

92 188-89 190-91

10

1j

2a

3j

92 140-41 139-40

11

1k

2a

3k

75 120-21 123-25

12

1a

2a

3l

79 153-54 150-52

13

1a

2b

4a

86 107-08 106-09

14

1c

2b

O

O

NH

O

O Cl 4b

92 104-05 106-08

— Contd

SINGH et al.: SYNTHESIS OF β-ACETAMIDO KETONES AND ESTERS

919

Table IL-Proline catalyzed multicomponent synthesis of β-amino carbonyls --Contd

m.p. (ºC) Entry Aldehyde

1

Ketone/Ester

2

Product 3/4

Yield (%)a

Obs. Lit.22,23, 31,43,44

15

1e

2b

4c

86 100-02 98-01

16

1f

2b

O

O

NH

O

O OCH3 4d

87 102-03 103-05

17

1g

2b

90 119-20 120-22

aIsolated mass yield based on 1.

Scheme II ― Plausible reaction mechanism

INDIAN J. CHEM., SEC B, JULY 2013

920

β-Acetamido-β-(3-chlorophenyl)propiophenone, 3b

IR (KBr): 3290, 3079, 1692, 1653, 1547, 1441,

1356, 1296, 1229 cm-1

; 1

H NMR (CDCl3, 300 MHz): δ 2.05 (s, 3H), 3.40 (dd, 1H, J = 5.7 and 17.1 Hz),

3.70 (dd, J = 4.8 and 17.1 Hz, 1H), 5.53 (m, 1H), 6.78

(d, J = 7.5 Hz, 1H), 7.21-7.58 (m, 7H), 7.88 (d, J =

7.5 Hz, 2H).

β-Acetamido-β-(4-chlorophenyl)propiophenone, 3c

IR (KBr): 3291, 2329, 1687, 1647, 1547, 1445,

1349, 1229, 1009, 754 cm-1

; 1H (CDCl3, 300 MHz,): δ

2.01 (s, 3H), 3.41 (dd, 1H, J = 6.0 and 16.8 Hz), 3.73

(dd, 1H, J = 5.2 and 17.2 Hz), 5.50-5.55 (m, 1H), 6.74

(d, 1H, J = 7.2 Hz), 7.25-7.56 (m, 7H), 7.87 (d, 2H, J

= 8.4 Hz).

β-Acetamido-β-(4-nitrophenyl)propiophenone, 3d

IR (KBr): 3306, 1696, 1646, 1595, 1537, 1350 cm-1;

1H NMR (CDCl3, 300 MHz): δ 2.10 (s, 3H), 3.51 (dd,

1H, J = 5.6 and 17.6 Hz), 3.81 (dd, 1H, J = 5.2 and

17.6 Hz), 5.65-5.67 (m, 1H), 6.96 (d, 1H, J = 8.0 Hz),

7.47 (t, 2H, J = 8.0 Hz), 7.51 (d, 2H, J = 8.8 Hz, 2H), 7.60 (t, 1H, J = 7.2 Hz), 7.89 (d, 2H, J = 7.2 Hz), 8.17

(d, 2H, J = 8.8 Hz).

β-Acetamido-β-(4-methylphenyl)propiophenone, 3e

IR (KBr): 3290, 1675, 1645 cm-1

; 1H NMR

(CDCl3, 300 MHz): δ 2.17 (s, 3H), 2.23 (s, 3H), 3.55

(dd, 1H, J = 6.2 and16.7 Hz), 3.82 (dd, 1H, J = 5.1

and 16.7 Hz), 5.56 (m, 1H), 7.05 (d, 2H, J = 7.7 Hz), 7.28-7.58 (m, 5H), 7.88 (d, 2H, J = 7.6 Hz), 8.70 (d,

1H, J = 7.5 Hz).

β-Acetamido-β-(4-methoxyphenyl)propiophenone, 3f

IR (KBr): 3310, 1690, 1650, 1550, 760, 690 cm-1;

1H NMR (CDCl3, 300 MHz): δ 2.01 (s, 3H), 3.37 (dd,

1H, J = 6.3 and 16.8 Hz), 3.73 (dd, 1H, J = 5.1 and

16.2 Hz), 3.78 (s, 3H), 5.49 (m, 1H), 6.56 (d, 1H, J =

7.5 Hz), 6.81 (d, 2H, J = 8.4 Hz), 7.23-7.58 (m, 5H), 7.89 (d, 2H, J = 7.5 Hz).

β-Acetamido-β-(4-bromophenyl)propiophenone, 3g

IR (KBr): 3291, 2329, 1687, 1647, 1547, 1445,

1349, 1229, 1009, 754 cm-1

; 1H NMR (CDCl3, 300

MHz): δ 2.03 (s, 3H), 3.38 (dd, 1H, J = 5.7 and 17.1

Hz), 3.70 (dd, 1H, J = 4.5 and 17.1 Hz), 5.53 (m, 1H), 6.73 (d, 1H, J = 7.5 Hz), 7.19-7.47 (m, 6H), 7.55 (t,

1H, J = 6.9 Hz), 7.87 (d, 2H, J = 7.5 Hz).

β-Acetamido-β-(2-nitrophenyl)propiophenone, 3i

IR (KBr): 3326, 2379, 1686, 1651, 1544, 1517,

1357, 1337, 1059, 682 cm-1;

1H NMR (CDCl3, 300

MHz): δ 2.00 (3H, s), 3.63 (dd, 1H, J = 5.6 and 16.8

Hz), 3.71 (dd, 1H, J = 6.4 and 17.2 Hz), 5.93–5.97

(m, 1H), 7.07 (d, 1H, J = 5.6 Hz), 7.39 (t, 1H, J = 8.0

Hz), 7.46 (t, 2H, J = 8.0 Hz), 7.57 (t, 2H, J = 7.6 Hz), 7.71 (d, 1H, J = 8.0 Hz), 7.92 (d, 2H, J = 7.2 Hz),

7.94 (d, 1H, J = 6.8).

β-Acetamido-β-(3-nitrophenyl)propiophenone, 3j

IR (KBr): 3291, 1689, 1653; 1H NMR (CDCl3, 300

MHz): δ 2.11 (s, 3H), 3.54 (dd, 1H, J = 5.5 and 17.6

Hz,), 3.83 (dd, 1H, J = 5.0 and 17.5 Hz,), 5.68 (m,

1H), 7.18 (d, 1H, J = 7.8 Hz), 7.45-7.53 (m, 3H), 7.61 (t, 1H, J = 7.5 Hz), 7.74 (d, 1H, J = 7.7 Hz) 7.91 (d,

2H, J = 8.2 Hz), 8.10 (d, 1H, J = 8.2 Hz), 8.24 (s,

1H).

(E)-N-(5-oxo-1, 5-diphenylpent-1-en-3-yl)acetami-

de, 3k

IR (KBr): 3291, 3065, 2928, 1687, 1648, 1635, 1547, 1445, 1366, 1083, 751 cm

-1;

1H NMR (CDCl3,

300 MHz,): δ 2.04 (s, 3H), 3.35 (dd, 1H, J = 5.6 and

17.6 Hz), 3.53 (dd, 1H, J = 4.4 and 17.6 Hz), 5.09-

5.14 (m, 1H), 6.35 (dd, 1H, J = 6.8 & 16.0 Hz), 6.54 (d, 1H, J = 15.6 Hz), 7.20 (d, 1H, J = 6.8 Hz), 7.26 (t,

2H, J = 6.8 Hz), 7.29 (d, 3H, J = 7.2 Hz), 7.46 (t, 2H,

J = 7.6 Hz), 7.57 (t, 1H, J = 7.6 Hz), 7.91 (d, 2H, J = 7.2 Hz).

N-(3-oxo-1,3-diphenylpropyl)benzamide, 3l

IR (KBr): 3306, 3062, 1681, 1634, 1599, 1488, 1357, 981, 754 cm

-1;

1H NMR (CDCl3, 300 MHz,): δ

3.50 (dd, 1H, J = 6.0 and 16.4 Hz), 3.86 (dd, 1H, J =

4.8 and 16.8 Hz), 5.73-5.78 (m, 1H), 7.24 (t, 1H, J =

7.2 Hz), 7.31 (t, 2H, J = 7.6 Hz), 7.37-7.45 (m, 5H), 7.49 (t, 2H, J = 7.2 Hz), 7.55 (t, 1H, J = 7.2 Hz),7.60

(d, 1H, J = 8.0 Hz), 7.82 (d, 2H, J = 8.0 Hz), 7.90 (d,

2H, J = 8.0 Hz).

Ethyl 2-(acetamido(phenyl)methyl)-3-oxobutano-

ate, 4a

IR (KBr): 3329, 3049, 2961, 1747, 1717, 1643,

1528, 1451, 1371 cm-1;

1H NMR (300 MHz, CDCl3):

δ 1.18 (t, 3H, J = 6.9 Hz,), 2.01 (s, 3H), 2.15 (s, 3H),

4.07 (d, 1H, J = 5.4 Hz,), 4.16 (q, 2H, J = 6.9 Hz),

5.76 (m, 1H), 6.83 (d, 1H, J = 9.3 Hz,), 7.09-7.17 (m, 5H).

SINGH et al.: SYNTHESIS OF β-ACETAMIDO KETONES AND ESTERS

921

Ethyl 2-(acetamido-(4-methylphenyl)methyl)-3-oxo-

butanoate, 4c

IR (KBr): 3336, 3080, 1747, 1720, 1649, 1531 cm-1;

1H NMR (300 MHz, CDCl3): δ 1.21 (t, 3H, J = 6.9

Hz,), 1.99 (s, 3H), 2.15 (s, 3H), 2.30 (s, 3H), 4.03 (d,

1H, J = 6.0 Hz,), 4.11(q, 2H, J = 6.9 Hz,), 5.69 (m,

1H), 6.83 (d, 1H, J = 8.7 Hz,), 7.09-7.17 (m, 4H).

Conclusion

In conclusion, a mild and efficient method for the

synthesis of β-acetamido carbonyl compounds using L-proline as an organocatalyst has been developed.

Excellent yields of the products, simplicity of

operations involved, and easy work-up procedure are

some of the advantages of this methodology.

Acknowledgement

The authors are thankful to the Department of

Biotechnology, New Delhi for financial support.

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