D. Giunta et al. / Tetrahedron Letters 54 (2013) 5122–5125
5123
involving the use of carbodiimides and catalytic DMAP, is a com-
mon and efficient method used in laboratory practice.5 Mitsunobu
reaction, the use of diazomethane or Staab’s reagent, just to men-
tion a few, are also effective methods although the reagents used
are toxic, shock and light sensitive or corrosive.6,7 Finally, when
used in organic solvents, also lipases catalyse the esterification of
carboxylic acids with no production of side products and without
the need of particularly harsh or complex reaction conditions.8
Generally speaking, lipases show a wide-ranging substrate
scope.9 However, while the application to the synthesis of aliphatic
esters is feasible, the lipase catalysed reaction with aromatic acids
(i.e., benzoic acids derivatives) proceeds rather ineffectively.10 Les-
zczak and Tran-Minh attained reasonable yields in the synthesis of
methyl benzoate after 3 days at 37 °C using Candida rugosa in a
hexane/toluene mixture with added water.11 Schmid and co-work-
ers reported a study based on a molecular dynamics simulation
explaining why benzoic and gallic acid esters cannot be obtained
by Candida antarctica lipase B (CALB) catalysis: they assume that
the presence of at least a methylene spacer is mandatory to obtain
appreciable conversion values.12 This statement is confirmed by
the good yields achieved with some cinnamic acid derivatives de-
scribed elsewhere.10b,c,e Nonetheless, good conversions have been
obtained by Vosmann et al. with the esterification of 3- and 4-
phenoxybenzoic acid with cis-9-octadecen-1-ol under vacuum
and in a solventless manner.13 However, this report lacks a wider
substrate scope and a rather long purification procedure is neces-
sary to isolate the reaction products. Krause reported about the
esterification of benzoic acid and n-hexanol in a solventless system
using several lipases, including immobilised CALB (Novozym 435),
although all the tested enzymes gave negligible chemical yields.3
plementary data for details). Unarguably, for a specific chemical
reaction, the best solvent would be no solvent at all.16 However,
benzoic acid esterification resulted impracticable if carried out
with a 10% excess of 1-heptanol and no other solvent, even when
vacuum is applied to remove water (Table 1, entry 1). As shown
in Table 1, appreciable conversion values are obtained only in
few solvents, apolar hydrophobic ones being clearly preferred (Ta-
ble 1, entries 3, 8 and 10). On the contrary, esterification in acetone,
acetonitrile, THF, MeTHF, MTBE and Et2O lead to very modest
chemical yields (Table 1 entries 2, 4–7 and 9). These hydrophilic
media may subtract bound water which is essential for catalytic
activity, thus yields do not exceed 15% in all cases.17 Moderate con-
version is obtained in hexane at 40 °C (Table 1, entry 3). Finally,
reaction can be easily performed in cyclohexane and toluene with
very high chemical yields (Table 1, entries 8 and 10).18 Although
cyclohexane is not commonly used in organic synthesis, the com-
parison between the Health parameter values (Table 1) prompted
us to select this solvent as the greenest alternative in our hands
to be used for the rest of the present study.19,20
We next investigated the substrate scope in the CALB catalysed
esterification of substituted benzoic acids (Scheme 1, Table 2).
According to the ping–pong mechanism, the acyl-enzyme interme-
diate is first formed by reaction with a serine residue in the active
Table 1
Solvent screening in the CALB catalysed benzoic acid esterification reaction with 1-
heptanol
Entry Solvent
Health
Reaction temp. (°C) Yieldb,d (%)
parametera
1
2
3
4
5
6
7
8
9
None
10
8
4
6
6
4
5
7
5
4
45
RT
40
80
40
80
40
60
RT
80
<1c,d
—
45
ˇ
More recently, Kren and co-workers failed to react gallic acid with
Acetone
Hexane
ACN
THF
MeTHF
MTBE
Cyclohexane
Et2O
Toluene
the primary 23-OH of the silybin A using Novozym 435 as the cat-
alyst.14 In summary, although lipase catalysed esterification of car-
boxylic acids is a process known from decades, reaction with
substituted benzoic acids is still challenging and not broadly ap-
plied in organic synthesis on a laboratory scale.
The aim of the present work was to extend the substrate scope
of lipase catalysed esterification of substituted benzoic acids, in or-
der to find new and more sustainable ways to the synthesis of
some local anaesthetic drugs.
It is well known that immobilisation onto solid supports facili-
tates recycling and improves the stability and selectivity of the en-
zyme. Particularly, CALB immobilised onto a macroporous DVB-
crosslinked polymer (Novozym 435) is an extraordinary catalyst
for both small organic molecules syntheses and for polymerization
reactions.15 We thought that the use of this biocatalyst for the syn-
thesis of the mentioned anaesthetic drugs can be highly advanta-
geous in terms of practical laboratory scale. Clean reaction
products can be easily isolated by simple filtration therefore reduc-
ing work-up procedures.
Initially, we approach the study of this catalytic system by car-
rying out the esterification of benzoic acid 1a and 1-heptanol
(Scheme 1). This test reaction was carried out for 24 h in several or-
ganic solvents. All reactions have been performed in stoppered test
tubes on a 0.4 mmol substrate scale, an equimolar amount of alco-
hol and 50 mg of Novozym 435 at different temperatures (see Sup-
—
<1c
3c
15
100
<1c
90
10
a
b
c
Health effects on humans according to GSK solvent selection guide.19
Isolated yield after filtration on silica.
Yield determined by 1H NMR analysis of the crude reaction mixture after sol-
vent evaporation.
d
Reaction performed with applied vacuum (120 mm Hg).
Table 2
Substrate scope in the CALB catalysed esterification reaction of benzoic acid
derivativesa
Entry
R
Heptyl substituted benzoate
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
100
100
38
p-CH3
p-OCH3
p-iPr
63
p-Cl
100
100
100
80
p-CF3
p-NO2
p-nBu
o-CH3
o-OCH3
o-Cl
m-CH3
m-OCH3
m-Cl
<1c
31
4c
100
65
100
60
O
R
CALB
O
C7H15OH
+
+ H2O
O(CH2)6CH3
R
m-NO2
OH
1a−1o
a
All reactions were carried out on a 0.4 mmol scale with 1 equiv of 1-heptanol,
5 mL cyclohexane, 80 °C, 24 h.
2
3a−3o
b
c
R = H, CH3, OCH3, iPr, Cl, CF3, NO2, nBu
Isolated yield after filtration on silica.
Yield determined by 1H NMR analysis of the crude reaction mixture after sol-
vent evaporation.
Scheme 1. CALB catalysed esterification of benzoic acid derivatives.