General and efficient transesterification of β-keto esters with various alcohols using Et3N as a br?nsted base additive

Article abstract of DOI:10.1080/00397911.2014.932812

Transesterification of β-keto esters with a wide variety of allyl, benzyl, propargyl, and alkyl alcohols using, for the first time, commercially available and inexpensive Et₃N as a Br?nsted base additive, is efficiently performed in toluene at reflux. The corresponding esters are exclusively obtained in 57-98% yields with no trace amounts of γ,δ-ketones, usually expected from the decarboxylative Carroll rearrangement.

Full text of DOI:10.1080/00397911.2014.932812

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General and Efficient Transesterification
of β-Keto Esters with Various Alcohols
Using Et3N as a Brønsted Base Additive
Olfa Mhasni a b , Imen Erray a & Farhat Rezgui a
a Faculty of Sciences de Tunis , Laboratoire de Chimie Organique
Structurale, Université of Tunis El Manar , Tunis , Tunisia
b Institute for Research and Physicochemical Analysis (INRAP), Pôle
Technologique , Sidi Thabet , Tunisia
Accepted author version posted online: 18 Jul 2014.Published
online: 26 Sep 2014.
To cite this article: Olfa Mhasni , Imen Erray & Farhat Rezgui (2014) General and Efficient
Transesterification of β-Keto Esters with Various Alcohols Using Et3N as a Brønsted Base Additive,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 44:22, 3320-3327, DOI: 10.1080/00397911.2014.932812
To link to this article: http://dx.doi.org/10.1080/00397911.2014.932812
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Synthetic Communications1, 44: 3320–3327, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.932812
GENERAL AND EFFICIENT TRANSESTERIFICATION OF
b-KETO ESTERS WITH VARIOUS ALCOHOLS USING
Et3N AS A BRØNSTED BASE ADDITIVE
Olfa Mhasni,1,2 Imen Erray,1 and Farhat Rezgui1
1Faculty of Sciences de Tunis, Laboratoire de Chimie Organique Structurale,
´
Universite of Tunis El Manar, Tunis, Tunisia
2Institute for Research and Physicochemical Analysis (INRAP),
ˆ
Pole Technologique, Sidi Thabet, Tunisia
GRAPHICAL ABSTRACT
Abstract Transesterification of b-keto esters with a wide variety of allyl, benzyl, propargyl,
and alkyl alcohols using, for the first time, commercially available and inexpensive Et3N as
a Brønsted base additive, is efficiently performed in toluene at reflux. The corresponding
esters are exclusively obtained in 57–98% yields with no trace amounts of c,d-ketones,
usually expected from the decarboxylative Carroll rearrangement.
Keywords Alcohols; Brønsted base; b-keto esters; transesterification
INTRODUCTION
Transesterification, a transformation of an ester to ester through interchange
of the alkoxy moiety, has attracted much attention from synthetic organic
chemists.[1–9] Because common ethyl and methyl esters as well as b-keto esters are
commercially available, they are usually used as starting materials for the convenient
synthesis of other useful esters.[10,11]
The preparation of b-keto esters through the conventional esterification reac-
tion between the corresponding b-keto acids and alcohols is not, in general, an
efficient method because of the equilibrium process and the low solubility of acids,
especially those having long aliphatic tails, in organic solvents,[12] as well as their
undesired facile decarboxylation by simply heating them.[13]
Because b-keto esters have been found to be useful intermediates in numerous
synthetic methods[14,15] and common moieties in natural products as well as
Received April 30, 2014.
Address correspondence to Farhat Rezgui, University of Tunis El Manar, Faculty of Science,
Campus, 2092 Tunis, Tunisia. E-mail: rez_far@yahoo.fr
3320

Et3N-MEDIATED TRANSESTERIFICATION OF b-KETO ESTERS
3321
pharmaceutical compounds,[16] their synthesis has attracted great interest. In this
regard, in the 1980s, two common methods were mainly employed for the transester-
ification of b-keto esters, i.e., the reaction of diketene with primary or secondary
alcohols[17] and dimethylaminopyridine (DMAP)-catalyzed ester exchange reactions
in the presence of molecular sieves in refluxing toluene.[18] As diketene was found to
be corrosive and DMAP is both toxic and expensive, the two previous methods
became less attractive and therefore a large variety of catalysts have been proposed
for the transesterification protocols such as zinc(II) oxide,[19] niobium(V) oxide,[20]
borate zirconia,[21] diode,[22] and Zn=I2.[23] Moreover, recent reports dealing with this
topic have demonstrated that noncorrosive and environmentally benign catalysts
including boric acid,[24] montmorillonite K-10,[25] and natural clays[26,27] can be used
as efficient and reusable catalysts.
Although triethylamine is one of the most common amines, it is well known
that it only catalyzes the esterification of highly reactive substrates such as diketenes
and oxazolinones.[28] Moreover, with the exception of the DMAP-catalyzed ester
exchange reactions,[8] it is notable that none of these methods reported that tertiary
bases, such as Et3N, could efficiently catalyze transesterification of b-keto esters.
In the course of our study on the BH chemistry, we have reported that the
DMAP-promoted direct C-allylation of b-ketoesters with BH alcohols,[29] whereas
Et3N promoted their transesterification.[30] In continuation of our investigation on the
use of amines in these transesterification protocols, we report herein our results on this
topic involving a variety of allyl, benzyl, propargyl, and alkyl alcohols, using commer-
cially available and inexpensive Et3N for the first time as a Brønsted base additive.
RESULTS AND DISCUSSION
Encouraged by our successful results dealing with the transesterification of
b-ketoesters with functionalized allyl alcohols, we were curious to determine whether
our previous protocol still worked with various alcohols that are devoid of
electron-withdrawing groups, in contrast to those of Baylis–Hillman.[30] For this
purpose, we have investigated and optimized the reaction between cinnamyl alcohol
1a (5 mmol) and ethyl acetoacetate (10 mmol) in toluene at reflux with azeotropic
removal of EtOH, using Et3N (10 mmol) as one of the most common amines. The
transesterification reaction was completed within 5 h, affording the sole ester 2a in
95% yield, with no trace amounts of the Carroll decarboxylative rearrangement
ketone (Scheme 1, Table 1, entry 1).[31,32]
Next, to explore the scope of this transesterification methodology, we have
selected a variety of alcohols. As shown in Table 1, the present protocol worked well
with various classes of allyl, benzyl, and propargyl alcohols (Table 1, entries 1–7), as
Scheme 1. Et3N-mediated transesterification of ethyl acetoacetate with various alcohols.

3322
O. MHASNI, I. ERRAY, AND F. REZGUI
Table 1. Et3N-mediated transesterification of ethyl acetoacetate with various alcohols
Entry
1
Alcohol
Time (h)
Ester 2
Yield (%)
Keto=enol (%)
1a
5
95
93=7
2
3
1b
1c
10
5
91
87
91=9
91=9
4
5
6
1d
1e
1f
6
24
3
82
57
72
91=9
92=8
91=9
7
8
1g
1h
7
78
74
93=7
92=8
25
9
1i
1j
27
22
80
97
92=8
91=9
10
11
12
1k
8
84
98
92=8
1l
21
90=10

Et3N-MEDIATED TRANSESTERIFICATION OF b-KETO ESTERS
3323
Scheme 2. Proposed mechanism for Et3N-mediated transesterification of ethyl acetoacetate with alcohols
ROH.
well as alkyl alcohols (Table 1, entries 8 ꢀ 10) and cyclic ones (Table 1, entries 11 and
12), in 57–98% yields.
All b-keto esters 2 exist as a mixture of two tautomeric forms in CDCl3
solution, whose relative ratios were determined by means of H NMR spectroscopy
1
(Table 1). For example, the singlet at dH 3.46 of 1a was assigned to the methylene
proton (ꢀCOꢀCH2ꢀCOꢀ) of the corresponding keto tautomer whereas the two
singlets at dH 5.02 and dH 12.06 were respectively assigned to the vinyl proton
(ꢀC(ꢀOH) ¼ CHꢀ) and the hydroxyl one (OH) of the corresponding enol tautomer.
Mechanistically, we assumed that ethyl acetoacetate, for instance, can undergo,
in the presence of Et3N, as a nitrogen Brønsted base, a b-elimination of EtOH to give
an a-keto ketene intermediate I, highly reactive, on which further addition of alco-
hols ROH generates esters 2a–l. This reaction mechanism, involving a-keto ketene,
was previously proposed by other authors[33–37] and supported by Witzeman studies
(Scheme 2).[38,39]
CONCLUSION
We have successfully developed a simple and efficient Et3N-mediated transes-
terification of b-keto esters with various allyl, benzyl, propargyl, and aliphatic alco-
hols. The desired esters were obtained in moderate to excellent yields with no trace
amounts of the Carroll decarboxylative rearrangement ketones.
EXPERIMENTAL
All chemicals were purchased from specialized suppliers with analytical purity
1
and without further purification. H NMR and 13C NMR spectra were recorded
1
either on a Bruker AC-300 spectrometer (300 MHz for H and 75 MHz for 13C) in
CDCl3, using tetramethylsilane (TMS) as an internal standard (chemical shifts in d
values, J in hertz). High-resolution mass spectra (HRMS) were recorded as
EI-HRMS on an Autospec Ultima=micromass mass spectrometer. Analytical
thin-layer chromatography (TLC) was performed using Fluka Kieselgel 60 F254
precoated silica-gel plates. Visualization was achieved by ultraviolet (UV) light
(254 nm). Column chromatography was performed using Fluka silica gel 60 (70–
230 mesh ASTM) and a gradient solvent system (petroleum ether=ether as eluent).
General Procedure for the Transesterification of b-Keto Esters Using
Various Alcohols
A mixture of alcohol 1a (0.67 g, 5 mmol), ethyl acetoacetate (1.30 g, 10 mmol),
and Et3N (1.01 g, 10 mmol) in toluene (50 mL) was heated to 110 ꢁC in a two-necked,

3324
O. MHASNI, I. ERRAY, AND F. REZGUI
round-bottomed flask provided with a distillation condenser to remove ethanol.
After completion (TLC), the reaction mixture was cooled, washed with brine, and
dried. The toluene was removed and the residue was purified by column chromato-
graphy on silica gel (40% ether=petroleum ether) to give the pure allylic ester 2a.
Cinnamyl 3-Oxobutanoate (2a)[27]
Yellow oil (1.03 g, 95% yield); keto (93%)–enol (7%); dH (keto, CDCl3)
7.38–7.23 (m, 5H), 6.64 (d, J 15.0 Hz, 1H), 6.25 (dt, J 15.0 and 6.0 Hz, 1H), 4.77
(d, J 6.0 Hz, 2H), 3.46 (s, 2H), 2.23 (s, 3H); dH (enol, CDCl3) 12.06 (s, 1H),
7.38–7.23 (m, 5H), 6.64 (d, J 15.0 Hz, 1H), 6.25 (dt, J 15.0 and 6.0 Hz, 1H), 5.02
(s, 1H), 4.77 (d, J 6.0 Hz, 2H), 1.93 (s, 3H); dC (keto, CDCl3) 200.4, 166.8, 136.0,
134.7, 128.6, 128.1, 127.0, 122.5, 65.8, 49.9, 30.0; dC (enol, CDCl3) 175.8, 172.2,
136.1, 134.0, 128.6, 128.1, 126.9, 123.1, 89.5, 64.2, 21.1. HRMS (EI): [M þ Na]þ,
found 241.0835. C13H14O3Na requires 241.0841.
But-2-enyl 3-Oxobutanoate (2b)[18]
Yellow oil (0.71 g, 91% yield); keto (91%)–enol (9%); dH (keto, CDCl3) 5.85–5.77
(m, 1H), 5.63–5.55 (m, 1H), 4.57–4.54 (m, 2H), 3.46 (s, 2H), 2.26 (s, 3H), 1.72 (m, 3H);
dH (enol, CDCl3) 12.04 (s, 1H), 5.85–5.77 (m, 1H), 5.63–5.55 (m, 1H), 4.99 (s, 1H),
4.71–4.69 (m, 2H), 1.94 (s, 3H), 1.72 (m, 3H); dC (keto, CDCl3) 200.5, 167.0, 131.9,
124.7, 65.9, 50.0, 30.0, 17.5; dC (enol, CDCl3) 175.6, 172.4, 131.3, 125.2, 89.6, 64.6,
21.1, 17.7. HRMS (EI): Mþ, found 156.0786. C8H12O3 requires 156.0786.
2-Methylprop-2-enyl 3-Oxobutanoate (2c)
Yellow oil (0.68 g, 87% yield); keto (91%)–enol (9%); dH (keto, CDCl3) 4.99 (s,
1H), 4.95 (s, 1H), 4.56 (s, 2H), 3.50 (s, 2H), 2.27 (s, 3H), 1.76 (s, 3H); dH (enol,
CDCl3) 12.02 (s, 1H), 5.04 (s, 1H), 4.99 (s, 1H), 4.95 (s, 1H), 4.56 (s, 2H), 1.96
(s, 3H), 1.76 (s, 3H); dC (keto, CDCl3) 200.4, 166.8, 139.4, 113.5, 68.5, 49.9, 30.1,
19.4; dC (enol, CDCl3) 175.9, 172.2, 140.0, 112.8, 89.5, 67.0, 21.1, 19.4. HRMS
(EI): Mþ, found 156.0786. C8H12O3 requires 156.0786.
1-Methylprop-2-enyl 3-Oxobutanoate (2d)[18]
Yellow oil (0.64 g, 82% yield); keto (91%)–enol (9%); dH (keto, CDCl3) 5.90–5.79
(m, 1H), 5.44–5.35 (m, 1H), 5.30–5.14 (m, 2H), 3.46 (s, 2H), 2.26 (s, 3H), 1.34 (d, J 6.0Hz,
3H); dH (enol, CDCl3) 12.08 (s, 1H), 5.90–5.79 (m, 1H), 5.44–5.35 (m, 1H), 5.30–5.14 (m,
2H), 4.99 (s, 1H), 1.95 (s, 3H), 1.34 (d, J 6.0 Hz, 3H); dC (keto, CDCl3) 200.5, 166.3,
137.1, 116.3, 72.1, 50.3, 30.0, 19.8; dC (enol, CDCl3) 175.6, 171.9, 137.7, 115.6, 89.9,
70.5, 21.1, 20.0. HRMS (EI): Mþ, found 156.0786. C8H12O3 requires 156.0786.
1,1-Dimethylprop-2-enyl 3-Oxobutanoate (2e)
Yellow oil (0.49 g, 57% yield); keto (92%)–enol (8%); dH (keto, CDCl3)
6.09–6.05 (m, 1H), 5.13–5.09 (m, 2H), 3.38 (s, 2H), 2.24 (s, 3H), 1.54 (s, 6H);

Et3N-MEDIATED TRANSESTERIFICATION OF b-KETO ESTERS
3325
dH (enol, CDCl3) 12.1 (s, 1H), 6.09–6.05 (m, 1H), 5.13–5.09 (m, 2H), 4.93 (s, 1H),
1.91 (s, 3H), 1.54 (s, 6H); dC (keto, CDCl3) 200.8, 166.3, 141.9, 113.1, 82.1, 51.1,
30.0, 26.3; dC (enol, CDCl3) 175.1, 172.1, 142.8, 112.4, 90.7, 80.9, 26.7, 21.1. HRMS
(EI): Mþ, found 170.0945. C9H14O3 requires 170.0943.
Benzyl 3-Oxobutanoate (2f)[21–23,25]
Viscous yellow liquid (0.69 g, 72% yield); keto (91%)–enol (9%); dH (keto,
CDCl3) 7.35–7.30 (m, 5H), 5.15 (s, 2H), 3.47 (s, 2H), 2.21 (s, 3H); dH (enol, CDCl3)
12.05 (s, 1H), 7.35–7.30 (m, 5H), 5.15 (s, 2H), 5.04 (s, 1H), 1.93 (s, 3H); dC (keto,
CDCl3) 200.4, 166.9, 135.3, 128.5, 128.4, 128.1, 67.0, 49.9, 30.0; dC (enol, CDCl3)
175.9, 172.3, 135.9, 128.5, 128.4, 128.1, 89.6, 65.6, 21.1. HRMS (EI): Mþ, found
192.0784. C11H12O3 requires 192.0786.
Propargyl 3-Oxobutanoate (2g)[13,14]
Yellow oil (0.55 g, 78% yield); keto (93%)–enol (7%); dH (keto, CDCl3) 4.74 (s,
2H), 3.52 (s, 2H), 2.59 (s, 1H), 2.28 (s, 3H); dH (enol, CDCl3) 11.8 (s, 1H), 5.05
(s, 1H), 4.74 (s, 2H), 2.59 (s, 1H), 1.95 (s, 3H); dC (keto, CDCl3) 200.2, 166.4,
77.2, 75.5, 52.7, 49.6, 30.1; dC (enol, CDCl3) 176.6, 171.5, 89.1, 76.7, 75.0, 50.1,
21.2. HRMS (EI): Mþ, found 140.0471. C7H8O3 requires 140.0473.
2-Methylpropyl 3-Oxobutanoate (2h)[23,24]
Yellow oil (0.59 g, 74% yield); keto (92%)–enol (8%); dH (keto, CDCl3) 3.92 (d,
J 6.0 Hz, 2H), 3.48 (s, 2H), 2.27 (s, 3H), 1.98–1.93 (m, 1H), 0.94 (d, J 6.0 Hz, 6H); dH
(enol, CDCl3) 12.08 (s, 1H), 5.01 (s, 1H), 3.92 (d, J 6.0 Hz, 2H), 1.98–1.93 (m, 4H),
0.94 (d, J 6.0 Hz, 6H); dC (keto, CDCl3) 200.5, 167.2, 71.3, 50.0, 30.0, 27.5, 19.2; dC
(enol, CDCl3) 175.5, 172.7, 89.7, 69.9, 27.7, 21.1, 19.2. HRMS (EI): Mþ, found
158.0931. C8H14O3 requires 158.0943.
Butyl 3-Oxobutanoate (2i)[23–25,27]
Spectroscopic data of compound 2i are in agreement with those reported in
the literature.[40,41] Yellow oil (0.63 g, 80% yield); keto (92%)–enol (8%); dH (keto,
CDCl3) 4.14 (t, J 6.0 Hz, 2H), 3.46 (s, 2H), 2.27 (s, 3H), 1.68–1.59 (m, 2H),
1.45–1.35 (m, 2H), 0.94 (t, J 6.0 Hz, 3H); dH (enol, CDCl3) 12.11 (s, 1H), 4.99
(s, 1H), 4.14 (t, J 6.0 Hz, 2H), 1.95 (s, 3H), 1.68–1.59 (m, 2H), 1.45–1.35 (m, 2H),
0.94 (t, J 6.0 Hz, 3H); dC (keto, CDCl3) 200.6, 167.3, 65.1, 49.8, 30.6, 30.0, 19.1,
13.9; dC (enol, CDCl3) 175.5, 172.8, 89.7, 62.4, 30.8, 21.1, 19.1, 13.9.
Decanyl 3-Oxobutanoate (2j)[25]
Yellow oil (1.17 g, 97% yield); keto (91%)–enol (9%); dH (keto, CDCl3) 4.05
(t, J ¼ 6.0 Hz, 2H), 3.37 (s, 2H), 2.19 (s, 3H), 1.59–1.52 (m, 2H), 1.23–1.19 (m, 14H),
0.80 (t, J ¼ 6.0 Hz, 3H); dH (enol, CDCl3) 12.03 (s, 1H), 4.90 (s, 1H), 4.05 (t, J
6.0Hz, 2H), 1.87 (s, 3H), 1.59–1.52 (m, 2H), 1.23–1.19 (m, 14H), 0.80 (t, J 6.0 Hz,

3326
O. MHASNI, I. ERRAY, AND F. REZGUI
3H); dC (keto, CDCl3) 200.2, 167.0, 65.3, 49.9, 31.5, 29.8, 29.4, 29.3, 29.1, 29.0, 28.4,
25.7, 22.5, 13.9; dC (enol, CDCl3) 175.2, 172.5, 89.6, 63.9, 31.7, 29.4, 29.3, 29.1, 29.0,
28.4, 25.7, 22.5, 20.9, 13.9. HRMS (EI): Mþ, found 242.1883. C14H26O3 requires
242.1882.
Cyclohexyl 3-Oxobutanoate (2k)[21,23–25,27]
Yellow oil (0.77 g, 84% yield); keto (92%)–enol (8%); dH (keto, CDCl3)
4.85–4.77 (m, 1H), 3.43 (s, 2H), 2.26 (s, 3H), 1.88–1.83 (m, 2H), 1.75–1.69 (m,
2H), 1.53–1.25 (m, 6H); dH (enol, CDCl3) 12.1 (s, 1H), 4.96 (s, 1H), 4.85–4.77
(m, 1H), 1.94 (s, 3H), 1.88–1.83 (m, 2H), 1.75–1.69 (m, 2H), 1.53–1.25 (m, 6H); dC
(keto, CDCl3) 200.7, 166.6, 73.7, 50.4, 31.4, 30.0, 25.4, 23.6; dC (enol, CDCl3)
175.2, 172.2, 90.2, 72.2, 31.7, 25.4, 23.6, 21.1. HRMS (EI): Mþ, found 184.1022.
C10H16O3 requires 184.1021.
Menthyl 3-Oxobutanoate (2l)[21,23,24,27]
Yellow oil (1.18 g, 98% yield); keto (90%)–enol (10%); dH (keto, CDCl3)
4.70–4.61 (m, 1H), 3.36 (s, 2H), 2.17 (s, 3H), 1.97–1.93 (m, 1H), 1.84–1.79 (m,
1H), 1.65–1.58 (m, 2H), 1.43–1.27 (m, 2H), 1.02–0.90 (m, 2H), 0.86–0.78 (m, 7H),
0.69 (d, J 9.0 Hz, 3H); dH (enol, CDCl3) 12.1 (s, 1H), 4.88 (s, 1H), 4.70–4.61
(m, 1H), 1.97–1.93 (m, 1H), 1.91 (s, 3H), 1.84–1.79 (m, 1H), 1.65–1.58 (m, 2H),
1.43–1.27 (m, 2H), 1.02–0.90 (m, 2H), 0.86–0.78 (m, 7H), 0.69 (d, J 9.0 Hz, 3H);
dC (keto, CDCl3) 200.1, 166.4, 75.1, 50.2, 46.7, 40.5, 34.0, 31.2, 29.7, 26.0, 23.2,
21.8, 20.5, 16.0; dC (enol, CDCl3) 175.1, 172.1, 89.8, 73.4, 46.9, 40.9, 34.1, 31.2,
26.2, 23.4, 21.8, 20.9, 20.5, 16.3. HRMS (EI): Mþ, found 240.1724. C14H24O3
requires 240.1725.
FUNDING
We are grateful to the DGRST, Ministry of Higher Education, Tunisia, for
financial support of this work.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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