Chemoenzymatic approach to optically active 1,4-dihydropyridine derivatives

Article abstract of DOI:10.1016/j.tet.2015.04.047

A series of racemic (2-methyl)propanoyloxymethyl 4-aryl-6-chloro-5-methanoyl-2-methyl-1,4-dihydropyridine-3-carboxylates [(±)-5a-h] have been prepared by a four step sequence including a multicomponent Hantzsch process and a Vilsmeier-Haack reaction. The

Full text of DOI:10.1016/j.tet.2015.04.047

Tetrahedron xxx (2015) 1e9
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chemoenzymatic approach to optically active 1,4-dihydropyridine
derivatives
,
,
Susana Y. Torres a b, Yamila Verdecia b, Francisca Rebolledo a
*
a
ꢀ
ꢀ
Departamento de Química Organica e Inorganica, Universidad de Oviedo, 33006 Oviedo, Spain
Laboratorio de Síntesis Organica, Facultad de Química, Universidad de La Habana, 10400 La Habana, Cuba
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of racemic (2-methyl)propanoyloxymethyl 4-aryl-6-chloro-5-methanoyl-2-methyl-1,4-
Received 25 February 2015
Received in revised form 14 April 2015
Accepted 16 April 2015
Available online xxx
dihydropyridine-3-carboxylates [(ꢀ)-5aeh] have been prepared by a four step sequence including
a multicomponent Hantzsch process and a VilsmeiereHaack reaction. The subsequent resolution of
(ꢀ)-5aeh was carried out by means of lipase-catalyzed hydrolysis, the most adequate enzymes being
lipases from Candida rugosa (CRL) and Candida antarctica (CAL-B). The moderate to high enantiose-
lectivities values (E up to >200) obtained in most cases allowed us to obtain the corresponding optically
active 1,4-dihydropyridine derivatives with high enantiomeric excesses (eeꢁ94%) and yields.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
1,4-Dihydropyridine
Resolution
Hydrolysis
Lipase
2-Methylpropanoyloxymethyl esters
1. Introduction
On the other hand, 1,4-DHPs have proven to be valuable com-
pounds in organic synthesis. They are NADH mimics4 and as such
Nowadays, 1,4-dihydropyridines (1,4-DHPs) are recognized as
privileged pharmacophores widely used in clinics for the treatment
of cardiovascular diseases.1 Structural modification led to a large
family of hybrid drugs containing the 1,4-DHP moiety.2 This is the
case of the recently published BZD-DHP hybrid molecule 2, which is
obtained from the racemic 1,4-DHP ethyl ester derivative 1b (Fig.1).
Compound 2 is a potential multi-target neuroprotective agent,
since it protects mitochondria and prevents ischemic insult-
mediated neural cell death in vitro.3
they are used as the hydrogen source for the asymmetric hydro-
genation of C]C, C]N, and C]O in the presence of an optically
active organocatalyst or a metaleligand complex.5 Analogously,
optically pure C4-substituted 1,4-DHPs have shown a high enan-
tioselectivity in the reduction of activated ketones in the presence
of Mg(II) ions.6
Optically active 1,4-DHPs were prepared using chiral auxilia-
ries,7 by means of organocatalytic asymmetric synthesis proce-
dures,6a,8 or by separation of enantiomers from racemates either by
preparative enantioselective HPLC9 or by formation of di-
astereomeric salts (using the Cinchona alkaloids cinchonidine and
quinidine10 or -tartaric acid11). In addition, enzymatic methods
L
were successfully applied to the asymmetrization of prochiral 1,4-
DHP-3,5-dicarboxylates.12 In this sense, when employing lipase as
catalyst, the presence of a spacer in the carboxylic function such as
the acyloxymethyl group was required, because this kind of en-
zymes do not have the capability of acting on the carboxylic func-
tion directly joined to the 1,4-DHP. Although results obtained in the
lipase-catalyzed kinetic resolution of racemic 1,4-DHP de-
rivatives12,13 were usually poorer than those of the asymmetriza-
tion, an efficient kinetic resolution of a series of 6-alkylsulfanyl-1,4-
DHPs has recently been published.14
Fig. 1. Highly functionalized 1,4-DHP 1b and its 1H-1,5-BZD derivative 2.
Based on these results and considering that the absolute con-
figuration of the asymmetric center of chiral 1,4-DHPs has a crucial
influence on their biological activity,10,15 we thought it would be
* Corresponding author. Tel.: þ34 985 102 982; fax: þ34 985 103 446; e-mail
address: frv@uniovi.es (F. Rebolledo).
http://dx.doi.org/10.1016/j.tet.2015.04.047
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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2
S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
Table 1
interesting to investigate the potential of lipases and some ester-
ases in the resolution of the adequate precursors of 1,4-DHPs 1.
Besides compound 2, the highly functionalized optically active 1,4-
DHPs 1 could be used in the preparation of other interesting opti-
cally active hybrid drugs.
Enzymatic hydrolysis of (ꢀ)-5ca
2. Results and discussion
The first objective was the synthesis of the appropriate sub-
strates (ꢀ)-5 to be submitted to hydrolysis reaction with lipases and
esterases. As mentioned before, lipases require a spacer at the
carboxylic function. An advantage of using the acyloxymethyl
group12 to this end is that the hydrolysis of the outer ester function
of substrates (ꢀ)-5 directly affords the corresponding 1,4-DHP-3-
carboxylic acids 6 due to a spontaneous loss of formaldehyde.
Recently, we have described the synthesis of pyridones
(ꢀ)-4aeh following the strategy collected in Scheme 1.16 A multi-
component Hantzsch-type reaction led to pyridones (ꢀ)-3aeh
whose tert-butyl group was easily removed with trifluoroacetic
acid (TFA) and anisole as a scavenger. The subsequent treatment of
the resulting carboxylic acids with cesium carbonate and chlor-
omethyl isobutyrate gave pyridones (ꢀ)-4aeh, which were trans-
formed into the required racemic 1,4-DHPs (ꢀ)-5aeh by means of
a VilsmeiereHaack reaction.17 The isolated yields of (ꢀ)-5a-h after
purification by flash-chromatography were very high (84e97%).
Entry
Lipase
Solvent
t, h
3.8
26
0.22
1.3
eeSb (%)
eePc (%)
cd
Ee
1
2
3
4
CAL-B
PSL
CRL
TBME
TBME
TBME
EtOAc
49
42
74
82
67
84
88
90
42
33
46
48
8
17
34
48
CRL
a
Reactions were carried out at 28 ꢂC and 200 rpm using 10 mg of substrate, 10 mg
of lipase, and water-saturated TBME (1.0 mL) or EtOAc (0.25 mL).
b
The ee of remaining substrate 5c (eeS) was determined by enantioselective HPLC
analysis.
c
The ee of the produced acid 6c (eeP) was determined after treatment with
diazomethane, and then enantioselective HPLC analysis of the resulting methyl ester
derivative.
d
The degree of conversion (%) was calculated from eeS and eeP: c¼100eeS/
(eeSþeeP).
e
The enantioselectivity value (E) was calculated according to Ref. 18.
pyridone (ꢀ)-4c (E¼12).16 The lipase from C. rugosa (CRL) was the
most efficient and it preferably catalyzed the hydrolysis of (R)-5c
with moderate-high enantioselectivity in both TBME and ethyl
acetate (EtOAc). Although reaction in EtOAc was slower than that in
TBME, we selected EtOAc as the best solvent due to the higher
enantioselectivity exhibited by the CRL (Table 1, entry 4). It is
noteworthy that when (ꢀ)-5c was incubated with CAL-B using
EtOAc as the solvent, less than 5% of conversion was attained after
28 h of reaction.
CRL was very efficient in EtOAc, in spite of the fact that this
solvent is also susceptible to be hydrolyzed by the lipase. To dem-
onstrate to what extension EtOAc is hydrolyzed by the lipases used,
incubations of water-saturated EtOAc with CRL and CAL-B were
carried out. After 1 h of reaction at 28 ꢂC, no acetic acid was ob-
served in the reaction with CRL, less than 0.1% was present after 2 h,
and the amount was slowly increasing until 4.5% upon reaching
27 h of reaction. This shows that hydrolysis of EtOAc in the presence
of CRL is very slow, and no inhibition problems are expected by the
acetic acid released in that time range. However, CAL-B hydrolyzed
an 8.1% of EtOAc after 1 h of reaction, 9.4% after 5 h, and the amount
of acetic acid remained practically constant (10.3%) after 24 h of
reaction. The high activity of CAL-B toward EtOAc as well as the
high amount of acetic acid released to the reaction medium could
be the cause for the low activity exhibited for this lipase towards
substrate 5c. In spite of this fact, CAL-B continued showing its
enantioselective properties in this solvent towards other substrates
(vide infra).
Once CRL and EtOAc were established as the best choice for the
kinetic resolution of (ꢀ)-5c, these conditions were applied to the
hydrolysis of the other racemic 1,4-DHPs (ꢀ)-5. Results are shown
in Table 2. Processes were allowed to react until a near 50% degree
of conversion was reached, the required time being highly de-
pendent of the substitution in the aromatic ring. Thus, the presence
of substituents at the C-2 position of phenyl group had a beneficial
effect on the reaction rate. In addition, enantioselectivity was also
strongly influenced by the aryl substituent, electron-withdrawing
groups such as nitro and chlorine at the ortho- and/or meta-posi-
tions giving the best results. The highest enantioselectivity value
(E>200) was obtained in the hydrolysis of the 2-chloro-5-nitro
derivative (ꢀ)-5e. In this process, both the remaining substrate
(R)-5e and the product (S)-6e were obtained with very high ee and
yields (Table 2, entry 5). Also, the very good results achieved in the
Scheme 1. Synthesis of 1,4-DHPs (ꢀ)-5aeh. The Ar groups for 5 are shown in Table 2.
As is shown in Scheme 1, the immediate precursors of 1,4-DHPs
(ꢀ)-5 are compounds (ꢀ)-4, which are also susceptible to be re-
solved by lipases. Our previous study with (ꢀ)-4 established the
lipase B from Candida antarctica (CAL-B)-catalyzed hydrolysis as the
best option.16 However, the obtained enantioselectivity values18
were so low (E<12) that the method only allowed us to prepare
three of the eight chosen substrates with high enantiomeric ex-
cesses and moderate yields. For this reason, we decided to explore
the enzymatic resolution in the 1,4-DHP stage.
Firstly, the 3-nitrophenyl derivative (ꢀ)-5c was used as a sub-
strate model. We tested the potential of some esterases [pig liver
(PLE-1 and PLE-5), Nocardia farcinica, Methylobacterium populi, and
Pelobacter propionicus]19 to catalyze the hydrolysis of the model
substrate, but very poor results were obtained in all the cases. For
instance, the highest enantioselectivity value (E¼4) was obtained
with esterase from M. populi. In addition, several lipase and organic
solvent combinations were also assayed, and the best results are
collected in Table 1. Lipase A from C. antarctica (CAL-A) and porcine
pancreatic lipase (PPL) did not show either catalytic or enantiose-
lective activity. CAL-B catalyzed the hydrolysis in tert-butyl methyl
ether (TBME) but with a low E value (Table 1, entry 1), which was
slightly lower than that obtained in the resolution of its precursor
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S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
3
Table 2
Enzymatic hydrolysis of racemic 1,4-DHP derivatives (ꢀ)-5a
Entry
Starting diester
Ar
Time (h)
Remaining substrate
Product
Product
c (%)
E
Substrate
Yield (%)
eeSb (%)
Yield (%)
eePc (%)
1
2
3
4
5
6
7
8
(ꢀ)-5a
(ꢀ)-5b
(ꢀ)-5c
(ꢀ)-5d
(ꢀ)-5e
(ꢀ)-5f
(ꢀ)-5g
(ꢀ)-5h
(ꢀ)-5b
(ꢀ)-5c
(ꢀ)-5h
Ph
8
(S)-5a
(S)-5b
(S)-5c
(S)-5d
(R)-5e
(S)-5f
(S)-5g
(S)-5h
(S)-5b
(S)-5c
(S)-5h
48
51
49
41
50
47
58
51
43
42
45
57
84
89
88
95
60
40
84
>99
97
(R)-6a
(R)-6b
(R)-6c
(R)-6d
(S)-6e
(R)-6f
(R)-6g
(R)-6h
(R)-6b
(R)-6c
(R)-6h
42
45
46
53
44
43
32
43
51
50
48
60
95
88d
69
98
58
62
97
90
82
94
49
47
50
56
49
51
39
46
52
54
51
7
103
46
15
>200
7
2-NO2-C6H4
3-NO2-C6H4
4-NO2-C6H4
2-Cl-5-NO2-C6H3
4-Br-C6H4
3-CH3O-C6H4
Naphthyl
2-NO2-C6H4
3-NO2-C6H4
Naphthyl
0.7
1.5
5
0.8
23
10
2.5
1.6
2.3
5.5
6
175
>100
41
9
10
11
99
170
a
Reactions were carried out at 100e1000 mg scale amount (see Experimental section) using EtOAc saturated with water as solvent, at 28 ꢂC and 200 rpm. The degree of
conversion (c, %) and the E values were calculated as in Table 1.
b
Determined by enantioselective HPLC analysis (see Section 4.6). High ee values are given in italicsface.
Determined by enantioselective HPLC analysis after treatment of products 6 with diazomethane (see Section 4.6). High ee values are given in italicsface.
A 31% yield of (R)-6c with 97% ee was isolated after submitting this sample to a selective pH-controlled crystallization (see Section 4.4.3.1).
c
d
reaction with naphthyl derivative (ꢀ)-5h were noticeable. In this
case, the optically active product (R)-6h was isolated with very high
ee (97%) and yield (Table 2, entry 8).
the hydrolysis of (ꢀ)-5f,g, and also in those of (ꢀ)-5a and (ꢀ)-5d,
for which CRL had shown a low enantioselectivity. After carrying
out the reactions in water-saturated TBME and EtOAc, no im-
provement was achieved with the 4-nitro derivative 5d, a similar E
value was reached with 5a, but satisfactory results were obtained
with 5f and 5g (see Table 3). So, product (R)-6f with 89% ee and 39%
yield was obtained from the hydrolysis of (ꢀ)-5f in EtOAc (Table 3,
entry 3). Moreover, the highest enantioselectivity value shown by
CAL-B toward 5g (E¼63) allowed us to isolate (R)-6g with 95% ee
and 33% yield (Table 3, entry 6). CAL-B preferably transformed the
same enantiomer than CRL in all the cases.
The moderate to high enantioselectivity (E>40) exhibited by the
CRL toward 1,4-DHPs 5b, 5c, and 5h enables the preparation of
these substrates with very high ee (ꢁ97%) and high isolated yields
(ꢁ42%) if the hydrolysis reactions are allowed to attain degrees of
conversion slightly higher than 50% (see Table 2, entries 9e11).
Isolation of both product and remaining substrate of each en-
zymatic hydrolysis was easily carried out by baseeacid extraction.
Once partitioned substrate 5 and product 6 between EtOAc and an
saturated aqueous solution of NaHCO3, the recovery of the car-
boxylic acid 6 from the aqueous medium was carried out by addi-
tion of aq HCl until pH¼1e2 and subsequent extraction with EtOAc.
The efficiency of this procedure was very high as judged by the very
high overall yield of both components in all the cases (90e96%).
During the acidification of the aqueous basic solution, the car-
boxylic acids usually began to precipitate at pH 4e5. Based on this
fact and considering that the racemic sample of some pyridones 4
(Scheme 1) crystallizes better than the corresponding optically
active sample,16 we tried to enhance the ee of the enzymatically
prepared carboxylic acids (R)-6 by carrying out their crystallization
at pH 4e5. This method was successfully applied to the enan-
tioenrichment of (R)-6c. Thus, the sample of (R)-6c with 88% ee
(Table 2, entry 3) was dissolved in saturated aq NaHCO3 (see Ex-
perimental section) and aq HCl was added until pH¼4e5. After
a slow crystallization, the filtered solid showed, as expected, a low
ee (26%, HPLC). Then, the pH of the filtrate was lowered to 1e2 and
extracted with EtOAc to attain (R)-6c with 97% ee and 31% yield
(calculated from the starting racemic 5c). However, by applying this
methodology the enantiomeric excesses of (R)-6a and (R)-6d were
enhanced from 60% to 92% and from 69% to 95%, respectively.
Nevertheless, the yields of both enantioenriched samples were very
poor (w10%, see Sections 4.4.1.1 and 4.4.4.1). In addition, no satis-
factory results were obtained with the acids (R)-6f and (R)-6g.
Seeing that the crystallization methodology had failed for the
acids 6f and 6g, and considering that CAL-B had shown some cat-
alytic activity in the resolution of pyridones 4f and 4g,16 we decided
to come back to the enzymatic approach. Thus, we tested CAL-B in
Table 3
CAL-B catalyzed hydrolysis of (ꢀ)-5a,f,ga
Ent. Substrate Solvent t, h Remaining (S)-5 Product (R)-6 cc
Ec
b
eeS (%)
eePb (%)
1
2
3
4
5
6
7
(ꢀ)-5a
(ꢀ)-5f
(ꢀ)-5f
(ꢀ)-5f
(ꢀ)-5g
(ꢀ)-5g
(ꢀ)-5g
TBME
TBME
EtOAc
TBME
TBME
EtOAc
TBME
24
6
25
14
9
89
75
67
97
62
49
94
60
85
89
72
88
95
80
60 11
47 27
43 34
57 24
41 29
34 63
54 31
21
19
a
Reactions conditions: 150e200 mg scale amount (see Experimental section),
L/mg of substrate) or EtOAc (50 L/mg of substrate),
CAL-B (1.0 mg/mg of substrate).
water-saturated TBME (100
m
m
b
The ee for each remaining substrate 5a,f,g (eeS) as well as the ee for each pro-
duced carboxylic acid 6a,f,g (eeP) were determined as in Table 2 (high ee values are
given in italicsface).
c
The degree of conversion (c, %) and the E values were calculated as in Table 1.
In order to obtain the remaining substrates with high ee, hy-
drolysis of (ꢀ)-5f and (ꢀ)-5g has to be carried out to a high degree
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4
S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
of conversion. This means that reaction times have to be longer
than 25 h if EtOAc is chosen as the solvent. As the activity of CAL-B
in EtOAc is significantly lowered after 24 h of reaction (as a conse-
quence of the acetic acid released to the reaction medium, see
above), we decided to use TBME to this end. Finally, remaining
substrates (S)-5f (Table 3, entry 4) and (S)-5g (Table 3, entry 7) were
obtained with high ee and moderate-high yields (37% and 43%,
respectively).
between the asymmetric carbon and the reactive carbonyl function
of the substrates used, enantioselectivity values obtained in most
enzymatic processes were moderate to very high, especially when
CRL was the catalyst. The involvement of very cheap catalysts and
the possibility to work to a gram scale are interesting characteris-
tics of this chemoenzymatic method. Transformation of optically
active 1,4-DHP derivatives 1 into hybrids with potential biological
activity is currently under study.
In order to establish the enantiopreference of both CRL and CAL-
B in the hydrolysis reactions, both the retention time of the HPLC-
analysis and the optical rotation of the remaining substrate 5f
[isolated from the enzymatic hydrolysis of (ꢀ)-5f (Table 2, entry 6
and Table 3, entry 4)] were compared with those values obtained
for a sample of (S)-5f, which was prepared from the configura-
tionally known pyridone (S)-4f16 (Scheme 2). From this compari-
son, the (S) configuration was assigned to the remaining substrate
5f. That means that both lipases preferentially catalyze the hydro-
lysis of the (R) enantiomer of 5f. Based on the structural re-
semblance among all the substrates reported here, we have
tentatively assigned the (R) configuration to the other produced
acids 6, except to the acid 6e, for which the absolute configuration
is (S) due to a change of priority of the substituents of its chiral
center. In addition, we have also assigned the (S) configuration for
all the remaining optically active substrates, except newly for 5e, for
which it was (R).
4. Experimental section
4.1. General
Pig liver esterases (isoenzymes 1 and 5) as well as esterases from
Nocardia farcinica, M. populi, and P. propionicus were purchased
from Enzymicals. Lipase B from C. antarctica (CAL-B, Novozyme 435,
available immobilized on polyacrylamide, 7300 PLU/g) was gifted
by Novo Nordisk Co. Immobilized lipase A from C. antarctica (CAL-A,
NZL-101, 6.2 U/g) was purchased from Codexis. Immobilized lipase
from Burkholderia cepacia (PSL-IM, 783 U/g), which previously was
classified as Pseudomonas cepacia, was purchased from Amano
Pharmaceutical Co. C. rugosa lipase was purchased from Sigma-
eAldrich (CRL type VII, ꢁ700 U/mg). Melting points were taken on
samples in open capillary tubes and are uncorrected. For the en-
zymatic hydrolysis reaction TBME or EtOAc saturated with water
was used. IR spectra were recorded using KBr pellets. 1H NMR and
proton-decoupled 13C NMR spectra (CDCl3 solutions) were ob-
tained using AC-300 or DPX300 (1H, 300.13 MHz and 13C, 75.5 MHz)
spectrometers using the
d scale (ppm) for chemical shifts. Calibra-
tion was made on the signal of the solvent (13C: CDCl3, 77.16;
DMSO-d6, 39.52 ppm) or the residual solvent partially or non-
deuterated (1H: CHCl3, 7.26; DMSO-d5, 2.50 ppm).
In Fig. 2 we show the 4-aryl-1,4-DHP unit with the numbering
used in the assignation of the NMR signals.
Scheme 2. Synthesis of (S)-5f from the configurationally known (S)-4f (see Ref. 16).
Finally, optically active esters 5 were smoothly converted into
the acids 6 by conventional basic hydrolysis (aq 3 N NaOH, acetone,
rt). No racemization took place in these reactions as was proven by
enantioselective HPLC-analysis. In addition, most of the enan-
tioenriched carboxylic acids 6, particularly those with high ee
(ꢁ88%), were easily transformed into the ethyl esters 1 by reaction
with cesium carbonate and ethyl iodide in N,N-dimethylformamide
(DMF) as the solvent (Scheme 3). These DHPs 1 are the starting
materials for the synthesis, among other derivatives,17a,20 of the
hybrids BZP-DHP 2 (see Fig. 1).
Fig. 2. 1,4-DHP derivatives.
4.2. General procedure for the synthesis of ( )-5
An analogous procedure as that described by Martín, Seoane
et al.21 was followed. Anhydrous DMF (2.45 mL, 31.6 mmol) was
cooled at 0 ꢂC, and POCl3 (2.90 mL, 31.6 mmol) was slowly added for
10 min. After 30 min, a solution of (ꢀ)-4 (7.89 mmol) in anhydrous
dichloromethane (32 mL) was added, and the resulting solution
was stirred at room temperature for 20e25 h. Then, an aqueous
6.5 M sodium acetate solution (59 mL) was added. After 1 h, the
mixture was partitioned between water and CH2Cl2, and the
aqueous phase was extracted with EtOAc (3ꢃ15 mL). The organic
phases were mixed and successively washed with water and brine,
dried with Na2SO4, and concentrated in vacuo. Flash chromatog-
raphy of the crude material (hexane/EtOAc 3:1) gave compounds
(ꢀ)-5, which were isolated with 84e97% yield.
Scheme 3. Synthesis of optically active 1,4-DHP ethyl esters derivatives 1. The Ar
groups for 6 and 1 are the same shown in Table 2.
3. Conclusion
4.2.1. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-
2-methyl-4-phenyl-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5a]. Yellow solid; mp 127e128 ꢂC; yield 92%; nmax (KBr) 3269;
We have developed an efficient chemoenzymatic procedure to
obtain a series of highly functionalized 1,4-DHP derivatives with
very high yields and enantiomeric excesses, the key step being
a lipase-catalyzed hydrolysis reaction. In spite of the remoteness
1722; 1637; 1603; 1489 cmꢄ1
; dH (300.13 MHz, CDCl3) 1.14e1.00
[6H, two overlapped d centered to 1.07 (3J 7.0 Hz) and 1.09 (3J
Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047

S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
5
7.0 Hz), CH(CH3)2], 2.54e2.33 [mþs, 4H, CH(CH3)2 and singlet
centered to 2.43 corresponding to CH3], 5.11 (s, 1H, H-4), 5.73 (s, 2H,
OeCH2eO), 6.44 (br s, 1H, NH), 7.33e7.09 (m, 5H, Ph), 9.78 (s, 1H,
HC]O); dC (75.5 MHz, CDCl3) 18.7 (2ꢃCH3), 19.4 (CH3), 33.8 (CH),
38.4 (C-4), 79.0 (CH2), 105.4 (C-5), 113.5 (C-3), 127.0 (C-40), 128.0 (C-
20 and C-60), 128.4 (C-30 and C-50), 141.6 (C-2), 144.9 (C-10), 145.8 (C-
6), 165.2 (CO), 175.8 (CO), 187.6 (CHO); HRMS (ESIþ): MNaþ, found:
400.0922. C19H20ClNNaO5 requires 400.0888.
(C), 144.1 (C), 146.6 (C), 146.8 (C), 164.7 (CO), 175.6 (CO), 187.2
(CHO); HRMS (ESIþ): MNaþ, found: 479.0383. C19H18Cl2N2NaO7
requires 479.0371.
4.2.6. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 4-(4-bromophenyl)-6-
chloro-5-methanoyl-2-methyl-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5f]. Yellow solid; mp 77e78 ꢂC; yield 94%; nmax (KBr) 3467;
1757; 1718; 1637; 1597; 1483 cmꢄ1
; dH (300.13 MHz, CDCl3)
1.13e1.01 [6H, two overlapped d centered to 1.07 (3J 6.9 Hz) and
1.09 (3J 6.9 Hz), CH(CH3)2], 2.54e2.36 [mþs, 4H, CH(CH3)2 and
singlet centered to 2.43 corresponding to CH3], 5.06 (s, 1H, H-4),
5.72 (AB system, j2Jj 5.6 Hz, OeCH2eO), 6.42 (br s, 1H, NH), 7.15 (d,
4.2.2. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-
2-methyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5b]. Yellow solid, mp 134e135 ꢂC; yield 84%; nmax (KBr) 3269;
3191; 1728; 1714; 1658; 1535; 1491 cmꢄ1
;
dH (300.13 MHz, CDCl3)
2H, 3J 8.5 Hz, H-20 and H-60), 7.34 (d, 2H, J 8.5 Hz, H-30 and H-50),
3
1.13e0.99 [6H, two overlapped d centered to 1.06 (3J 7.0 Hz) and
1.07 (3J 7.0 Hz), CH(CH3)2], 2.53e2.33 [mþs, 4H, CH(CH3)2 and
singlet centered to 2.41 corresponding to CH3], 5.67 (AB system, j2Jj
5.5 Hz, OeCH2eO), 5.93 (s, 1H, H-4), 6.68 (br s, 1H, NH), 7.34e7.27
(m, 1H, H-60), 7.54e7.44 (m, 2H, H-40 and H-50), 7.75 (d, 1H, 3J 7.8 Hz,
H-30), 9.69 (s, 1H, CHO); dC (75.5 MHz, CDCl3) 18.6 (CH3), 18.7 (CH3),
19.2 (CH3), 33.7 (CH), 34.5 (C-4), 79.4 (CH2), 104.6 (C-5), 112.7 (C-3),
124.5 (C-30), 127.7 (C-40), 131.3 (C-60), 133.0 (C-50), 139.5 (C-2), 143.0
(C-6), 147.2 (C-20), 148.5 (C-10), 165.0 (CO), 175.7 (CO), 187.7 (CHO);
HRMS (ESIþ): MHþ, found: 423.0954. C19H20ClN2O7 requires
423.0968.
9.76 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 18.7 (2ꢃCH3), 19.5 (CH3),
33.8 (CH), 38.2 (C-4), 78.9 (CH2), 105.1 (C-5), 113.3 (C-3), 121.0 (C),
129.9 (2ꢃCH), 131.5 (2ꢃCH), 141.2 (C), 143.9 (C), 145.8 (C), 165.0
(CO),175.8 (CO),187.4 (CHO); HRMS (ESIþ): MNaþ, found: 478.0027.
C19H19BrClNNaO5 requires 478.0014.
4.2.7. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-
4-(3-methoxyphenyl)-2-methyl-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5g]. Yellow solid; mp 114e115 ꢂC; yield 94%; nmax (KBr) 3467;
3269; 1766; 1687; 1631; 1483 cmꢄ1
; dH (300.13 MHz, CDCl3)
1.14e0.98 [6H, two overlapped d centered to 1.07 (3J 6.9 Hz) and
1.09 (3J 6.9 Hz), CH(CH3)2], 2.53e2.35 [mþs, 4H, CH(CH3)2 and
singlet centered to 2.43 corresponding to CH3], 3.78 (s, 3H, OCH3),
5.10 (s, 1H, H-4), 5.74 (s, 2H, OeCH2eO), 6.40 (br s, 1H, NH),
6.75e6.65 (m, 1H, H-20), 6.90e6.77 (m, 2H, H-40 and H-60), 7.15 (t,
4.2.3. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-
2-methyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5c]. Yellow solid; mp 149e150 ꢂC; yield 96%; nmax (KBr) 3304;
1729; 1720; 1657; 1645; 1529; 1486 cmꢄ1
;
dH (300.13 MHz, CDCl3)
1H, J 7.9 Hz, H-50), 9.79 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 18.6
3
1.07 (t, 6H, 3J 6.8 Hz, CH(CH3)2), 2.55e2.38 [mþs, 4H, CH(CH3)2 and
singlet centered to 2.47 corresponding to CH3], 5.21 (s, 1H, H-4),
5.72 (s, 2H, OeCH2eO), 6.63 (br s, 1H, NH), 7.41 (t, 1H, 3J 7.8 Hz, H-
(2ꢃCH3), 19.2 (CH3), 33.8 (CH), 38.3 (C-4), 55.2 (OCH3), 79.0 (CH2),
105.1 (C-5), 111.8 (C-40), 113.2 (C-3), 114.2 (C-20), 120.4 (C-60), 129.3
(C-50), 142.1 (C-2), 146.2 (C), 146.5 (C), 159.6 (C-30), 165.3 (CO), 175.8
(CO), 187.8 (CHO); HRMS (ESIþ): MNaþ, found: 430.1028.
3
50), 7.72 (d, 1H, J 7.7 Hz, H-60), 8.08e7.99 (m, 2H, H-20 and H-40),
9.77 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 18.1 (CH3), 18.17 (CH3),
18.22 (CH3), 32.8 (CH), 37.8 (C-4), 78.7 (CH2), 102.3 (C-5), 110.9 (C-
3), 121.7 (C-40), 121.9 (C-20), 129.9 (C-50), 134.1 (C-60), 143.0 (C-2),
147.3 (C), 147.6 (C), 148.7 (C-10), 164.3 (CO), 174.8 (CO), 186.7 (CHO);
HRMS (ESIþ): MNaþ, found: 445.0773. C19H19ClN2NaO7 requires
445.0779.
C20H22ClNNaO6 requires 430.1026.
4.2.8. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-2-
methyl-4-(1-naphthyl)-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5h]. Yellow solid; mp 89e91 ꢂC; yield 97%; nmax (KBr) 3465;
1716; 1637; 1599; 1464 cmꢄ1
; dH (300.13 MHz, CDCl3) 1.04e0.90 [6H,
two overlapped d centered to 0.97 (3J 7.0 Hz) and 0.98 (3J 7.0 Hz),
CH(CH3)2], 2.36e2.21 [m, 1H, CH(CH3)2], 2.41 (s, 3H, CH3), 5.53 (AB
system, j2Jj 5.6 Hz, OeCH2eO), 5.89 (s, 1H, H-4), 6.57 (br s, 1H, NH),
7.51e7.30 (m, 3H, H-30, H-60and H-70), 7.65e7.56 (m, 1H, H-20), 7.67
(dd,1H, 4J 1.7, 3J 7.4 Hz, H-40), 7.74 (d,1H, 3J 7.6 Hz, H-50), 8.72 (d,1H, 3J
8.6 Hz, H-80), 9.74 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 18.5 (CH3),
18.6 (CH3), 19.2 (CH3), 33.65 (CH), 33.68 (C-4), 79.0 (CH2), 107.1 (C-5),
114.7 (C-3), 125.2 (CH), 125.5 (CH), 125.7 (CH), 126.2 (CH), 127.0 (CH),
127.9 (CH), 128.1 (CH), 130.9 (C), 133.5 (C), 141.4 (C), 143.8 (C-2), 145.0
(C-6), 165.4 (CO), 175.6 (CO), 188.0 (CHO); HRMS (ESIþ): MNaþ,
found: 450.1079. C23H22ClNNaO5 requires 450.1051.
4.2.4. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-5-methanoyl-
2-methyl-4-(4-nitrophenyl)-1,4-dihydropyridine-3-carboxylate
[(ꢀ)-5d]. Yellow solid; mp 150e152 ꢂC; yield 94%; nmax (KBr) 3462;
3265; 1757; 1724; 1631; 1477 cmꢄ1
; dH (300.13 MHz, CDCl3) 1.06 [t,
6H, 3J 7.0 Hz, CH(CH3)2], 2.58e2.33 [mþs, 4H, CH(CH3)2 and singlet
centered to 2.47 corresponding to CH3], 5.21 (s, 1H, H-4), 5.80e5.65
3
(m, 2H, OeCH2eO), 6.58 (br s, 1H, NH), 7.46 (d, 2H, J 7.9 Hz, H-20
and H-60), 8.10 (d, 2H, 3J 7.9 Hz, H-30 and H-50), 9.77 (s, 1H, HC]O);
dC (75.5 MHz, CDCl3) 18.7 (2ꢃCH3), 19.7 (CH3), 33.8 (CH), 38.8 (C-4),
79.1 (CH2), 104.5 (C-5), 112.7 (C-3), 123.8 (C-30 and C-50), 129.2 (C-20
and C-60), 141.5 (C-2), 146.4 (C), 146.9 (C), 151.8 (C-10), 164.6 (CO),
175.7 (CO), 187.1 (CHO); HRMS (ESIþ): MNaþ, found: 445.0773.
4.3. General procedure for the preparation of racemic 1,4-
DHP-3-carboxylic acids ( )-6
C
19H19ClN2NaO7 requires 445.0746.
4.2.5. (ꢀ)-[2-(Methyl)propanoyloxy]methyl 6-chloro-4-(2-chloro-5-
nitrophenyl)-5-methanoyl-2-methyl-1,4-dihydropyridine-3-
carboxylate [(ꢀ)-5e]. Yellow solid; mp 185e186 ꢂC; yield 90%; nmax
To a solution of racemic 1,4-DHP (ꢀ)-5 (0.438 mmol) in acetone
(7.7 mL), aq 3 M NaOH (1.75 mmol) and water (15.4 mL) were
added. The solution was stirred at room temperature until disap-
pearance of the starting material (TLC control). Then, acetone was
eliminated under reduced pressure and the basic aqueous phase
extracted with CH2Cl2 (2ꢃ7 mL). After, aq 3 M HCl was added to the
aqueous phase until pH was 1e2, and the precipitation took place.
The resulting solid was filtered and washed with water to give the
corresponding carboxylic acid (ꢀ)-6 in a pure form.
(KBr) 3464; 3305; 1753; 1724; 1649; 1604; 1491 cmꢄ1
; dH
(300.13 MHz, CDCl3) 1.14e1.10 [6H, two overlapped d centered to
1.07 (3J 7.0 Hz) and 1.08 (3J 7.0 Hz), CH(CH3)2], 2.55e2.35 [mþs, 4H,
CH(CH3)2 and singlet centered to 2.42 corresponding to CH3], 5.52
(s, 1H, H-4), 5.70 (AB system, j2Jj 5.6 Hz, OeCH2eO), 6.76 (br s, 1H,
NH), 7.45 (d, 1H, 3J 8.8 Hz, H-30), 7.95 (dd, 1H, 4J 2.7, 3J 8.8 Hz, H-40),
8.22 (d,1H, 4J 2.7 Hz, H-60), 9.74 (s,1H, HC]O); dC (75.5 MHz, CDCl3)
18.7 (2ꢃCH3), 19.7 (CH3), 33.8 (CH), 38.1 (C-4), 79.4 (CH2), 104.0 (C-
5), 112.2 (C-3), 122.8 (CH), 126.8 (CH), 130.8 (CH), 140.5 (C), 142.2
4.3.1. (ꢀ)-6-Chloro-5-methanoyl-2-methyl-4-phenyl-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6a]. Reaction time: 4 h.
Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047

6
S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
Yellow solid; mp 254e255 ꢂC; yield 95%; nmax (KBr) 3467; 1682;
1628; 1599; 1491 cmꢄ1
dH (300.13 MHz, DMSO-d6) 2.33 (s, 3H,
(C), 144.9 (C), 145.0 (C), 167.8 (CO2H), 186.5 (CHO); HRMS (ESIþ):
MNaþ, found: 377.9503. C14H11BrClNNaO3 requires 377.9509.
;
CH3), 4.93 (s, 1H, H-4), 7.32e7.05 (m, 5H, Ph), 9.69 (s, 1H, HC]O),
10.25 (br s, 1H, NH), 12.14 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6)
17.7 (CH3), 37.8 (C-4), 105.5 (C-5), 111.0 (C-3), 126.4 (C-40), 127.3
(2ꢃCH),128.2 (2ꢃCH),142.8 (C-2),144.6 (C),145.7 (C),168.0 (CO2H),
186.6 (CHO); HRMS (ESIþ): MNaþ, found: 300.0398. C14H12ClNNaO3
requires 300.0355.
4.3.7. (ꢀ)-6-Chloro-5-methanoyl-4-(3-methoxyphenyl)-2-methyl-
1,4-dihydropyridine-3-carboxylic acid [(ꢀ)-6g]. Reaction time: 5 h.
Yellow solid; mp 241e242 ꢂC; yield 93%; nmax (KBr) 3234; 3159;
1682; 1624; 1593; 1487 cmꢄ1
; dH (300.13 MHz, DMSO-d6) 2.33 (s,
3H, CH3), 3.68 (s, 3H, OCH3), 4.91 (s, 1H, H-4), 6.76e6.63 (m, 3H, H-
3
20, H-40 and H-60), 7.16 (t, 1H, J 7.9 Hz, H-50), 9.69 (s, 1H, HC]O),
4.3.2. (ꢀ)-6-Chloro-5-methanoyl-2-methyl-4-(2-nitrophenyl)-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6b]. Reaction time: 6 h.
Yellow solid; mp 187e190 ꢂC; yield 90%; nmax (KBr) 3467; 1672;
10.32 (br s, 1H, NH), 12.15 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6)
17.7 (CH3), 37.6 (C-4), 54.9 (OCH3), 105.3 (C-5), 110.8 (C-3), 111.0 (C-
40), 113.5 (C-20), 119.4 (C-60), 129.3 (C-50), 142.9 (C-2), 144.7 (C-10),
147.1 (C-6), 159.1 (C-30), 167.9 (CO2H), 186.6 (CHO); HRMS (ESIþ):
MHþ, found: 308.0684. C15H15ClNO4 requires 308.0655.
1606; 1525; 1460 cmꢄ1
; dH (300.13 MHz, DMSO-d6) 2.28 (s, 3H,
CH3), 5.67 (s, 1H, H-4), 7.52e7.23 (m, 2H, H-40 and H-60), 7.62 (t, 1H,
3J 7.4 Hz, H-50), 7.74 (d, 1H, 3J 7.9 Hz, H-30), 9.60 (s, 1H, HC]O), 10.31
(br s, 1H, NH), 12.16 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6) 17.8
(CH3), 34.1 (C-4), 105.4 (C-5), 110.6 (C-3), 123.9 (C-50), 127.6 (C-30),
130.8 (C-40), 133.2 (C-60), 139.9 (C-2), 143.6 (C-6), 144.7 (C-20), 148.0
(C-10), 167.6 (CO2H), 186.5 (CHO); HRMS (ESIþ): MHþ, found:
323.0429. C14H12ClN2O5 requires 323.0417.
4.3.8. (ꢀ)-6-Chloro-5-methanoyl-2-methyl-4-(1-naphthyl)-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6h]. Reaction time: 5 h.
Yellow solid; mp 172e174 ꢂC; yield 94%; nmax (KBr) 3425; 1684;
1628; 1597; 1491 cmꢄ1
; dH (300.13 MHz, DMSO-d6) 2.34 (s, 3H,
CH3), 5.69 (s,1H, H-4), 7.38e7.29 (m,1H, H-20), 7.62e7.38 (m, 3H, H-
30, H-60 and H-70), 7.73 (d, 1H, 3J 8.0 Hz, H-40), 7.82 (d, 1H, 3J 7.4 Hz,
H-50), 8.64 (d,1H, 3J 8.6 Hz, H-80), 9.65 (s,1H, HC]O),10.32 (br s,1H,
NH),12.01 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6) 17.7 (CH3), 33.5
(C-4), 107.6 (C-5), 112.3 (C-3), 125.36 (2ꢃCH), 125.40 (CH), 125.9
(CH),126.4 (CH),127.1 (CH),127.8 (CH),130.3 (C),132.9 (C),142.8 (C),
143.3 (C-2), 144.6 (C-6), 168.0 (CO2H), 186.8 (CHO); HRMS (ESIþ):
MNaþ, found: 350.0554. C18H14ClNNaO3 requires 350.0555.
4.3.3. (ꢀ)-6-Chloro-5-methanoyl-2-methyl-4-(3-nitrophenyl)-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6c]. Reaction time: 2 h.
Yellow solid; mp 228e229 ꢂC; yield 96%; nmax (KBr) 3491; 3292;
1684; 1630; 1591; 1473 cmꢄ1
; dH (300.13 MHz, DMSO-d6) 2.36 (s,
3H, CH3), 5.05 (s, 1H, H-4), 7.66e7.50 (m, 2H, H-50 and H-60), 7.96 (s,
1H, H-20), 8.03 [dt, 1H, 4J 2.0 (d), 3J 7.6 (t) Hz, H-40], 9.69 (s, 1H, HC]
O), 10.47 (br s, 1H, NH), 12.30 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-
d6) 17.8 (CH3), 38.1 (C-4), 104.6 (C-5), 110.2 (C-3), 121.6 (C-40), 121.7
(C-20), 129.9 (C-50), 134.1 (C-60), 143.4 (C-2), 145.6 (C), 147.67 (C),
147.68 (C), 167.6 (CO2H), 186.6 (CHO); HRMS (ESIþ): MNaþ, found:
345.0249. C14H11ClN2NaO5 requires 345.0239.
4.4. General procedure for the lipase-catalyzed hydrolysis
reactions
Compound (ꢀ)-5 was dissolved in the organic solvent (EtOAc or
TBME) previously saturated with water. Then, lipase (CRL or CAL-B)
was added to this solution, and the mixture was shaken at 28 ꢂC and
200 rpm. After the time collected in Table 2 or 3, the enzyme was
filtered and thoroughly washed with methanol. Solvents were
eliminated under reduced pressure and the residue was dissolved
with EtOAc. The organic solution was extracted three times with aq
saturated NaHCO3. The organic phase was successively washed
with water and brine, dried with Na2SO4 and concentrated in vacuo
to give the optically active remaining substrate 5. On the other
hand, the basic aqueous phase containing the produced optically
active carboxylic acid 6 was acidified (pH¼1e2) with aq 3 M HCl
and subsequently extracted with EtOAc (three times). The normal
work-up of this new organic phase gave the corresponding acid 6.
The best approach for each substrate is described below.
4.3.4. (ꢀ)-6-Chloro-5-methanoyl-2-methyl-4-(4-nitrophenyl)-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6d]. Reaction time: 4 h.
Yellow solid; mp 224e226 ꢂC (dec); yield 98%; nmax (KBr) 3462;
1680; 1630; 1600; 1522; 1487 cmꢄ1
; dH (300.13 MHz, DMSO-d6)
2.35 (s, 3H, CH3), 5.04 (s, 1H, H-4), 7.42 (d, 2H, 3J 8.8 Hz, H-20 and H-
60), 8.13 (d, 2H, 3J 8.8 Hz, H-30 and H-50), 9.67 (s, 1H, HC]O), 10.40
(br s, 1H, NH), 12.30 (br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6) 17.9
(CH3), 38.3 (C-4),104.5 (C-5),110.0 (C-3),123.6 (C-30 and C-50),128.6
(C-20 and C-60), 143.4 (C-2), 145.6 (C-6), 146.1 (C-40), 152.9 (C-10),
167.6 (CO2H), 186.5 (CHO); HRMS (ESIþ): MHþ, found: 323.0429.
C
14H12ClN2O5 requires 323.0398.
4.3.5. (ꢀ)-6-Chloro-4-(2-chloro-5-nitrophenyl)-5-methanoyl-2-
methyl-1,4-dihydropyridine-3-carboxylic acid [(ꢀ)-6e]. Reaction
time: 2 h. Yellow solid; mp 190e192 ꢂC; yield 97%; nmax (KBr) 3467;
4.4.1. Enzymatic hydrolysis of (ꢀ)-5a (Table 3, entry 1). To a solution
of (ꢀ)-5a (200 mg, 0.530 mmol) in TBME (20 mL), CAL-B (200 mg)
was added and the mixture was shaken at 28 ꢂC and 200 rpm
during 24 h. After, the general procedure was followed. Thus, the
remaining substrate (S)-5a was isolated with 39% yield (78 mg) and
1682; 1637; 1601; 1522; 1464 cmꢄ1
; dH (300.13 MHz, DMSO-d6)
2.29 (s, 3H, CH3), 5.32 (s, 1H, H-4), 7.61 (d, 1H, 3J 8.8 Hz, H-30), 8.00
(dd,1H, 4J 2.8, 3J 8.8 Hz, H-40), 8.08 (d,1H, 4J 2.8 Hz, H-60), 9.63 (s,1H,
HC]O), 10.45 (br s, 1H, NH), 12.24 (br s, 1H, CO2H); dC (75.5 MHz,
DMSO-d6) 17.8 (CH3), 38.4 (C-4), 104.4 (C-5), 109.7 (C-3), 122.8 (CH),
126.1 (CH), 130.9 (CH), 139.2 (C), 144.0 (C), 144.7 (C), 145.2 (C), 146.2
(C), 167.4 (CO2H), 186.6 (CHO); HRMS (ESIþ): MNaþ, found:
378.9859. C14H10Cl2N2NaO5 requires 378.9846.
89% ee; [
a
]
20 ꢄ19.6 (c 1.0, CHCl3).
D
The product (R)-6a was isolated with 57% yield (82 mg) and 60%
ee.
4.4.1.1. Enantioenrichment of the enzymatically produced (R)-
4.3.6. (ꢀ)-4-(4-Bromophenyl)-6-chloro-5-methanoyl-2-methyl-1,4-
dihydropyridine-3-carboxylic acid [(ꢀ)-6f]. Reaction time: 5 h. Yel-
low solid; mp 215e217 ꢂC; yield 98%; nmax (KBr) 3745; 1682; 1633;
6a. A partially enantioenriched sample of acid (R)-6a (80 mg, 60%
ee) was dissolved in aq saturated NaHCO3 (40 mL). Then, aq 3 M HCl
was added until pH¼4e5. After 12 h at 5 ꢂC, the solid formed was
filtered off and the pH of the resulting aqueous solution adjusted to 1.
Extraction with EtOAc allowed us to isolate a small amount of highly
1589; 1489 cmꢄ1
; dH (300.13 MHz, DMSO-d6) 2.32 (s, 3H, CH3), 4.89
3
(s, 1H, H-4), 7.09 (d, 2H, J 8.4 Hz, H-20 and H-60), 7.44 (d, 2H, 3J
8.5 Hz, H-30 and H-50), 9.67 (s,1H, HC]O),10.30 (br s,1H, NH),12.20
(br s, 1H, CO2H); dC (75.5 MHz, DMSO-d6) 17.8 (CH3), 37.6 (C-4),
105.0 (C-5), 110.6 (C-3), 119.5 (C), 129.5 (2ꢃCH), 131.1 (2ꢃCH), 143.0
enantioenriched (R)-6a (18 mg); [
a
]
20 ꢄ24.6 (c 0.40, DMSO), 92% ee.
D
4.4.2. Enzymatic hydrolysis of (ꢀ)-5b (Table 2, entries 2 and 9). To
a solution of (ꢀ)-5b (1.00 g, 2.37 mmol) in EtOAc (25 mL), CRL
Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047

S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
7
(1.00 g) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 43 min (Table 2, entry 2). After following the general
procedure, the remaining substrate (S)-5b was isolated with 51%
yield (512 mg) and 84% ee. The product (R)-6b was isolated with
(200 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm during 21 h (Table 3, entry 6). After following the general
procedure, the remaining substrate (S)-5g was isolated with 62%
yield (124 mg) and 49% ee. The product (R)-6g was isolated with
45% yield (346 mg); [
a]
20 þ104.6 (c 0.38, DMSO), 95% ee.
33% yield (50 mg) and 95% ee; [
a
]
20 ꢄ79.7 (c 0.82, MeOH).
D
D
When the enzymatic hydrolysis of (ꢀ)-5b (500 mg) was main-
When the enzymatic hydrolysis of (ꢀ)-5g (200 mg) was carried
tained for 95 min (Table 2, entry 9), the remaining substrate (S)-5b
out with CAL-B (200 mg) in TBME (20 mL), (S)-5g (86 mg, 43%) was
20
(217 mg, 43% yield) was isolated with ee >99%; [
a
]
20 ꢄ201.9 (c 0.52,
isolated with 94% ee after 19 h of reaction (Table 3, entry 7); [a]
D
D
CHCl3); mp 155e157 ꢂC.
ꢄ25.3 (c 0.95, CHCl3).
4.4.3. Enzymatic hydrolysis of (ꢀ)-5c (Table 2, entries 3 and 10). To
a solution of (ꢀ)-5c (650 mg, 1.54 mmol) in EtOAc (16 mL), CRL
(650 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 90 min (Table 2, entry 3). After following the general
procedure, the remaining substrate (S)-5c was isolated with 49%
yield (317 mg) and 89% ee. The product (R)-6c was isolated with
46% yield (228 mg) and 88% ee.
4.4.8. Enzymatic hydrolysis of (ꢀ)-5h (Table 2, entries 8 and 11). To
a solution of (ꢀ)-5h (300 mg, 0.701 mmol) in EtOAc (7.5 mL), CRL
(300 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 2.5 h (Table 2, entry 8). After following the general
procedure, the remaining substrate (S)-5h was isolated with 51%
yield (153 mg) and 84% ee. The product (R)-6h was isolated with
20
43% yield (98 mg) and 97% ee; [
a]
þ90.5 (c 0.63, MeOH); mp
D
When the enzymatic hydrolysis of (ꢀ)-5c (700 mg) was main-
160e161 ꢂC (dec).
tained for 140 min (Table 2, entry 10), the remaining substrate (S)-
When the enzymatic hydrolysis of (ꢀ)-5h (300 mg) was allowed
5c (294 mg, 42% yield) was isolated with 97% ee; [
a
]
20 ꢄ57.1 (c 1.02,
to react for 5.5 h (Table 2, entry 11), the substrate (S)-5h (135 mg,
D
CHCl3); mp 122e123 ꢂC.
45% yield) was isolated with 99% ee; [
a
]
20 ꢄ12.7 (c 1.0, CHCl3); mp
D
93e94 ꢂC.
4.4.3.1. Enantioenrichment of the enzymatically produced (R)-
6c. To a sample of acid (R)-6c (227 mg, 0.703 mmol, 88% ee) was
applied the method described for the enantioenrichment of (R)-6a
4.5. Typical procedure for the synthesis of optically active 1,4-
DHPs ethyl ester 1
(see Section 4.4.1.1). Thus, (R)-6c (154 mg, 0.477 mmol) with 97% ee
20
was obtained [31% yield calculated from the starting (ꢀ)-5c]; [
a
]
To a solution of the corresponding optically active carboxylic
acid 622 (0.30 mmol) in DMF (1.6 mL), cesium carbonate
(0.39 mmol) was added. After stirring the suspension for 2e3 min,
D
þ12.7 (c 0.52, DMSO), mp 205e206 ꢂC.
4.4.4. Enzymatic hydrolysis of (ꢀ)-5d (Table 2, entry 4). To a solu-
tion of (ꢀ)-5d (200 mg, 0.473 mmol) in EtOAc (5.0 mL), CRL
(200 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 5 h. After following the general procedure, the
ethyl iodide (96 mL, 1.2 mmol) was added and the mixture was
maintained at room temperature for 1e3 h. After this time, CH2Cl2
(20 mL) was added and the resulting organic phase was succes-
sively washed with water (3ꢃ10 mL) and brine (3ꢃ10 mL). Organic
solvent was eliminated and the residue was submitted to flash
chromatography. Spectroscopic data for optically active 1aed and
1g are in good agreement with those previously published for the
corresponding racemic compound [(ꢀ)-1a,b23 and (ꢀ)-1c,d,g24].
remaining substrate (S)-5d was isolated with 41% yield (82 mg) and
20
88% ee; [
a]
ꢄ19.5 (c 1.01, CHCl3).
D
The product (R)-6d was isolated with 53% yield (81 mg) and 69%
ee.
4.4.4.1. Enantioenrichment of the enzymatically produced (R)-
6d. Applying the method described in Section 4.4.1.1 to a sample of
4.5.1. Ethyl (S)-6-chloro-5-methanoyl-2-methyl-4-phenyl-1,4-
dihydropyridine-3-carboxylate [(S)-1a]. It was obtained from (S)-
6a (47 mg, 0.17 mmol, 88% ee). Reaction time: 2 h. Purification of
the crude material by flash chromatography (CH2Cl2 as the eluant)
acid (R)-6d (80 mg, 69% ee), 21 mg of enantioenriched (R)-6d (95%
20
ee) was isolated; [
a
]
þ12.8 (c 1.01, DMSO).
D
gave (S)-1a (42 mg) as a yellow solid; 81% yield; [
a
]
20 þ16.3 (c 0.75,
D
4.4.5. Enzymatic hydrolysis of (ꢀ)-5e (Table 2, entry 5). To a solu-
tion of (ꢀ)-5e (200 mg, 0.438 mmol) in EtOAc (5.0 mL), CRL
(200 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 50 min. After following the general procedure, the
CHCl3), 88% ee.
4.5.2. Ethyl 6-chloro-5-methanoyl-2-methyl-4-(2-nitrophenyl)-1,4-
dihydropyridine-3-carboxylate (1b). Reaction time with both enan-
tiomers: 1 h. After purification by flash chromatography (hexane/
EtOAc 5:1), both enantiomers of 1b were obtained as yellow solids.
Compound (S)-1b was obtained from (S)-6b (153 mg,
0.474 mmol, >99% ee) with 78% yield (130 mg); mp 155e157 ꢂC;
remaining substrate (R)-5e was isolated with 50% yield (100 mg)
20
and 95% ee; [
a
]
D
ꢄ86.6 (c 0.52, CHCl3).
The product (S)-6e was isolated with 44% yield (69 mg) and 98%
20
ee; [
a]
þ80.3 (c 0.51, DMSO); mp 176e178 ꢂC.
D
[
a
]
20 ꢄ242.4 (c 0.51, CHCl3), 99% ee.
D
4.4.6. Enzymatic hydrolysis of (ꢀ)-5f (Table 3, entries 3 and 4). To
a solution of (ꢀ)-5f (150 mg, 0.328 mmol) in EtOAc (7.5 mL), CAL-B
(150 mg) was added and the mixture was shaken at 28 ꢂC and
200 rpm for 25 h (Table 3, entry 3). After following the general
procedure, the remaining substrate (S)-5f was isolated with 53%
yield (80 mg) and 67% ee. The product (R)-6f was isolated with 39%
Compound (R)-1b was obtained from (R)-6b (220 mg,
0.682 mmol, 95% ee) with 75% yield (179 mg); [
CHCl3), 98% ee.
a]
20 þ254.7 (c 0.52,
D
4.5.3. Ethyl 6-chloro-5-methanoyl-2-methyl-4-(3-nitrophenyl)-1,4-
dihydropyridine-3-carboxylate (1c). Reaction time: 2 h. After puri-
fication by flash chromatography (hexane/EtOAc 5:1), both enan-
tiomers of 1c were obtained as yellow solids.
yield (46 mg) and 89% ee; [
a
]
20 ꢄ59.0 (c 0.50, MeOH).
D
When the enzymatic hydrolysis of (ꢀ)-5f (150 mg) was carried
out in TBME (15 mL) and CAL-B (150 mg) for 14 h (Table 3, entry 4),
Compound (S)-1c was obtained from (S)-6c (100 mg,
20
the substrate (S)-5f (56 mg, 37% yield) was isolated with 97% ee;
0.310 mmol, 97% ee) with 88% yield (96 mg); [
CHCl3), 97% ee.
a
]
ꢄ36.8 (c 0.49,
D
20
[
a]
ꢄ19.9 (c 0.70, CHCl3).
D
Compound (R)-1c was obtained from (R)-6c (130 mg,
20
4.4.7. Enzymatic hydrolysis of (ꢀ)-5g (Table 3, entries 6 and 7). To
a solution of (ꢀ)-5g (200 mg, 0.490 mmol) in EtOAc (10 mL), CAL-B
0.403 mmol, 97% ee) with 92% yield (130 mg); mp 156e157 ꢂC; [
þ37.0 (c 0.50, CHCl3), 99% ee.
a]
D
Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047

8
S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
4.5.4. Ethyl (S)-6-chloro-5-methanoyl-2-methyl-4-(4-nitrophenyl)-
1,4-dihydropyridine-3-carboxylate [(S)-1d]. It was obtained from
(S)-6d (56 mg, 0.174 mmol, 88% ee). Reaction time: 1 h. Purification
of the crude material by flash chromatography (hexane/EtOAc 5:1)
127.8 (CH), 128.1 (CH), 130.9 (C), 133.4 (C), 141.5 (C), 142.8 (C-2),
144.2 (C-6), 166.8 (CO), 188.0 (CHO); HRMS (ESIþ): MHþ, found:
356.1048. C20H19ClNO3 requires 356.1036.
Compound (S)-1h was obtained from (S)-6h (81 mg, 0.25 mmol,
gave (S)-1d (43 mg) as a yellow solid; 70% yield; [
a]
20 ꢄ28.2 (c 0.73,
95% ee) with 75% yield (66 mg); [
Compound (R)-1h was obtained from (R)-6h (90 mg, 0.27 mmol,
95% ee) with 80% yield (78 mg); [
a
]
20 ꢄ85.3 (c 0.90, CHCl3), 95% ee.
D
D
CHCl3), 87% ee.
a
]
20 þ87.1 (c 1.0, CHCl3), 95% ee.
D
4.5.5. Ethyl 6-chloro-4-(2-chloro-5-nitrophenyl)-5-methanoyl-2-
methyl-1,4-dihydropyridine-3-carboxylate (1e). Reaction time for
both enantiomers: 4 h. Purification of the corresponding crude
material was carried out by flash chromatography (hexane/EtOAc
4:1). Yellow solid; nmax (KBr) 3289; 1709; 1684; 1643; 1600; 1487;
4.6. Determination of the enantiomeric excesses
Enantioselective HPLC was used for the determination of the
enantiomeric excesses of compounds 5 and 6. Previously to the
measurement, carboxylic acids 6 were transformed into the cor-
responding methyl ester by treatment with diazomethane. HPLC-
conditions and chromatograms are included in Supplementary
data.
1348 cmꢄ1
; dH (300.13 MHz, CDCl3) 1.18 (t, 3H, CH3CH2), 2.41 (s, 3H,
CH3), 4.18e3.96 (m, 2H, CH3CH2), 5.53 (s, 1H, H-4), 6.57 (br s, 1H,
NH), 7.45 (d, 1H, 3J 8.8 Hz, H-30), 7.96 (dd, 1H, 4J 2.7, 3J 8.8 Hz, H-40),
8.23 (d,1H, 4J 2.7 Hz, H-60), 9.74 (s,1H, HC]O); dC (75.5 MHz, CDCl3)
14.2 (CH3CH2), 19.1 (CH3), 38.4 (C-4), 60.7 (CH3CH2), 105.3 (C-5),
111.6 (C-3), 122.8 (CH), 126.9 (CH), 130.8 (CH), 140.4 (C), 143.2 (C),
144.4 (C), 144.9 (C), 146.7 (C), 166.2 (CO), 187.6 (CHO); HRMS (ESIþ):
MNaþ, found: 407.0172. C16H14Cl2N2NaO5 requires 407.0145.
Compound (R)-1e was obtained from (R)-6e (71 mg, 0.20 mmol,
Acknowledgements
Financial support from the Spanish MICINN (CTQ2011-24237) is
gratefully acknowledged. S.Y.T. thanks to the University of Oviedo
for a grant.
95% ee) with 86% yield (66 mg); mp 182e184 ꢂC; [
CHCl3), 99% ee.
a
]
20 ꢄ52.8 (c 0.50,
D
Compound (S)-1e was obtained from (S)-6e (58 mg, 0.16 mmol,
20 þ60.4 (c 1.0, CHCl3), 97% ee.
Supplementary data
98% ee) with 80% yield (50 mg); [
a]
D
Copy of the HPLC chromatograms of the racemic and optically
active compounds. Supplementary data associated with this article
can be found in the online version, at http://dx.doi.org/10.1016/
j.tet.2015.04.047.
4.5.6. Ethyl 4-(4-bromophenyl)-6-chloro-5-methanoyl-2-methyl-1,4-
dihydropyridine-3-carboxylate (1f). Reaction time for both enan-
tiomers: 2 h. Purification of the corresponding crude material was
carried out by flash chromatography (hexane/EtOAc 5:1). Yellow
solid; nmax (KBr) 3282; 1699; 1635; 1590; 1479 cmꢄ1
; dH
(300.13 MHz, CDCl3) 1.18 (t, 3H, 3J 7.1 Hz, CH3CH2), 2.42 (s, 3H, CH3),
4.07 (q, 2H, 3J 7.1 Hz, CH3CH2), 5.08 (s, 1H, H-4), 6.26 (br s, 1H, NH),
7.17 (d, 2H, 3J 8.4 Hz, H-20 and H-60), 7.35 (d, 2H, 3J 8.4 Hz, H-30 and
H-50), 9.76 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 14.3 (CH3CH2), 19.1
(CH3), 38.4 (C-4), 60.5 (CH3CH2), 106.5 (C-5), 112.8 (C-3), 120.8 (C),
129.9 (2ꢃCH), 131.4 (2ꢃCH), 142.0 (C), 143.8 (C), 144.3 (C), 166.5
(CO), 187.6 (CHO); HRMS (ESIþ): MHþ, found: 383.9997.
References and notes
1. (a) Ioan, P.; Carosati, E.; Micucci, M.; Cruciani, G.; Broccatelli, F.; Zhorov, B. S.;
Chiarini, A.; Budriesi, R. Curr. Med. Chem. 2011, 18, 4901e4922; (b) Carosati, E.;
Ioan, P.; Micucci, M.; Broccatelli, F.; Cruciani, G.; Zhorov, B. S.; Chiarini, A.;
Budriesi, R. Curr. Med. Chem. 2012, 19, 4306e4323; For a recent contribution,
ꢀ
see: (c) Tenti, G.; Parada, E.; Leon, R.; Egea, J.; Martínez-Revelles, S.; Briones, A.
ꢀ
ꢀ
M.; Sridharan, V.; Lopez, M. G.; Ramos, M. T.; Menendez, J. C. J. Med. Chem. 2014,
57, 4313e4323.
C
16H16BrClNO3 requires 383.9987.
ꢀ
2. For some reviews and recent examples of 1,4-DHP hybrids, see: (a) Leon, R.;
Compound (S)-1f was obtained from (S)-6f (42 mg, 0.12 mmol,
Marco-Contelles, J. Curr. Med. Chem. 2011, 18, 552e576; (b) Christiaans, J. A. M.;
Timmerman, H. Eur. J. Pharm. Sci. 1996, 4, 1e22; (c) Sashidhara, K. V.; Kumar, M.;
Khedgikar, V.; Kushwaha, P.; Modukuri, R. K.; Kumar, A.; Gautam, J.; Singh, D.;
Sridhar, B.; Trivedi, R. J. Med. Chem. 2013, 56, 109e122; (d) Locatelli, A.;
Cosconati, S.; Micucci, M.; Leoni, A.; Marinelli, L.; Bedini, A.; Ioan, P.; Spampi-
nato, S. M.; Novellino, E.; Chiarini, A.; Budriesi, R. J. Med. Chem. 2013, 56,
3866e3877.
20
96% ee) with 74% yield (34 mg); [
a
]
þ10.6 (c 1.0, CHCl3), 95% ee.
D
Compound (R)-1f was obtained from (R)-6f (40 mg, 0.11 mmol,
89% ee) with 77% yield (93 mg); [
a
]
20 ꢄ9.4 (c 0.95, CHCl3), 89% ee.
D
4.5.7. Ethyl 6-chloro-5-methanoyl-2-methyl-4-(3-methoxyphenyl)-
1,4-dihydropyridine-3-carboxylate (1g). Reaction time: 3 h. After
purification by flash chromatography (hexane/EtOAc 3:1), both
enantiomers of 1g were obtained as yellow solids.
~
ꢀ
ꢀ
3. Nunez-Figueredo, Y.; Ramírez-Sanchez, J.; Delgado-Hernandez, R.; Porto-Ver-
ꢀ
decia, M.; Ochoa-Rodríguez, E.; Verdecia-Reyes, Y.; Marin-Prida, J.; Gonzalez-
Durruthy, M.; Uyemura, S. A.; Rodrigues, F. P.; Curti, C.; Souza, D. O.; Pardo-
Andreu, G. L. Eur. J. Pharmacol. 2014, 726, 57e65.
4. Paul, C. E.; Arends, I. W. C. E.; Hollmann, F. ACS Catal. 2014, 4, 788e797.
5. For some recent examples, see: (a) Chen, M.-W.; Cai, X.-F.; Chen, Z.-P.; Shi, L.;
Zhou, Y.-G. Chem. Commun. 2014, 12526e12529; (b) Wang, Z.; Ai, F.; Wang, Z.;
Zhao, W.; Zhu, G.; Lin, Z.; Sun, J. J. Am. Chem. Soc. 2015, 137, 383e389; For some
selected reviews, see: (c) You, S.-L. Chem. Asian J. 2007, 2, 820e827; (d)
Rueping, M.; Sugiono, E.; Schoepke, F. R. Synlett 2010, 852e865; (e) Rueping,
M.; Dufour, J.; Schoepke, F. R. Green Chem. 2011, 13, 1084e1105; (f) Zheng, C.;
You, S.-L. Chem. Soc. Rev. 2012, 41, 2498e2518.
Compound (S)-1g was obtained from (S)-6g (60 mg, 0.19 mmol,
96% ee) with 76% yield (50 mg); mp 148e149 ꢂC; [
CHCl3), 98% ee.
a]
20 þ12.8 (c 0.90,
D
Compound (R)-1g was obtained from (R)-6g (47 mg, 0.15 mmol,
95% ee) with 78% yield (40 mg); [
a
]
20 ꢄ12.1 (c 0.85, CHCl3), 95% ee.
D
6. (a) Franke, P. T.; Johansen, R. L.; Bertelsen, S.; Jørgensen, K. A. Chem. Asian J.
2008, 3, 216e224; (b) Murakami, Y.; Kikuchi, J.-i.; Hisaeda, Y.; Hayashida, O.
Chem. Rev. 1996, 96, 721e758.
7. (a) Fustero, S.; Catalan, S.; Sanchez-Rosello, M.; Simon-Fuentes, A.; del Pozo, C.
Org. Lett. 2010, 12, 3484e3487; (b) Kwon, K.; Shin, J. A.; Lee, H.-Y. Tetrahedron
2011, 67, 10222e10228; (c) Bull, J. A.; Mousseau, J. J.; Pelletier, G.; Charette, A. B.
Chem. Rev. 2012, 112, 2642e2713.
4.5.8. Ethyl 6-chloro-5-methanoyl-2-methyl-4-(1-naphthyl)-1,4-
dihydropyridine-3-carboxylate (1h). Reaction time for both enan-
tiomers: 3 h. Purification of the corresponding crude material was
carried out by flash chromatography using CH2Cl2 as eluant. Yellow
ꢀ
ꢀ
ꢀ
ꢀ
solid; nmax (KBr) 3429; 3261; 1703; 1637; 1593; 1489 cmꢄ1
; dH
(300.13 MHz, CDCl3) 0.89 (t, 3H, 3J 7.1 Hz, CH3CH2), 2.41 (s, 3H, CH3),
4.03e3.70 (m, 2H, CH3CH2), 5.89 (s, 1H, H-4), 6.38 (br s, 1H, NH),
7.50e7.30 (m, 3H, H-30, H-60 and H-70), 7.63e7.53 (m, 1H, H-20), 7.68
8. By using (S)-diarylprolinol-TMS ether: (a) Noole, A.; Borissova, M.; Lopp, M.;
Kanger, T. J. Org. Chem. 2011, 76, 1538e1545; See also Ref. 6a. Employing op-
tically active phosphoric acids: (b) Yu, J.; Shi, F.; Gong, L.-Z. Acc. Chem. Res. 2011,
44, 1156e1171; Using ureas derived from cinchona alkaloids: (c) Molleti, N.;
Allu, S.; Ray, S. K.; Singh, V. K. Tetrahedron Lett. 2013, 54, 3241e3244.
3
3
(d, 1H, J 7.6 Hz, H-40), 7.76 (d, 1H, J 8.2 Hz, H-50), 8.75 (d, 1H, 3J
8.6 Hz, H-80), 9.76 (s, 1H, HC]O); dC (75.5 MHz, CDCl3) 14.0
(CH3CH2), 18.9 (CH3), 33.8 (C-4), 60.3 (CH3CH2), 108.7 (C-5), 114.5
(C-3), 125.5 (CH), 125.58 (CH), 125.63 (CH), 125.8 (CH), 127.0 (CH),
ꢀ
ꢀ
9. Dolle, F.; Hinnen, F.; Valette, H.; Fuseau, C.; Duval, R.; Peglion, J.-L.; Crouzel, C.
Bioorg. Med. Chem. 1997, 5, 749e764.
10. Zhang, B.-l.; He, W.; Shi, X.; Huan, M.-l.; Huang, Q.-j.; Zhou, S.-y. Bioorg. Med.
Chem. Lett. 2010, 20, 805e808.
Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047

S.Y. Torres et al. / Tetrahedron xxx (2015) 1e9
9
11. Gotrane, D. M.; Deshmukh, R. D.; Ranade, P. V.; Sonawane, S. P.; Bhawal, B. M.;
Gharpure, M. M.; Gurjar, M. K. Org. Process Res. Dev. 2010, 14, 640e643.
12. (a) Sobolev, A.; Franssen, M. C. R.; Duburs, G.; de Groot, A. E. Biocatal. Bio-
transform. 2004, 22, 231e252; (b) Marchalín, S.; Chudík, M.; Mastihuba, V.;
Decroix, B. Heterocycles 1998, 48, 1943e1958.
18. (a) Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J. Am. Chem. Soc. 1982, 104,
7294e7299; (b) Faber, K.; Hoenig, H. Program “Selectivity”. http://www.cis.
TUGraz.at/orgc/.
19. Esterase-catalyzed hydrolyses of (ꢀ)-5c were carried out at 28 ꢂC in phosphate
buffer pH 7.5 (100 mM, 100 mL/mg of substrate) using DMSO as a co-solvent
13. Sobolev, A.; Zhalubovskis, R.; Franssen, M. C. R.; Vigante, B.; Chekavichus, B.;
Duburs, G.; de Groot, A. E. Chem. Heterocycl. Compd. 2004, 40, 931e937.
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Krauze, A. Heterocycles 2014, 89, 43e58.
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E.; Knaus, E. E. J. Med. Chem. 1995, 38, 2851e2859; (b) Shan, R.; Howlett, S. E.;
Knaus, E. E. J. Med. Chem. 2002, 45, 955e961; (c) Kobayashi, T.; Sada, T. U.S.
Patent US 20,050,272,715 A1, 2005; CAN 2005:1291987.
(10
mL/mg of substrate). The amount of esterase was 1.0e0.25 mg/mg of sub-
strate, depending of the activity provided by the supplier.
ꢀ
ꢀ
20. Suarez, M.; Ochoa, E.; Pita, B.; Espinosa, R.; Gonzalez, L.; Martín, N.; Quinteiro,
M.; Seoane, C.; Soto, J. L. J. Heterocycl. Chem. 1997, 34, 931e935.
ꢀ
ꢀ
21. Verdecia, Y.; Suarez, M.; Morales, A.; Rodríguez, E.; Ochoa, E.; Gonzalez, L.; Martín,
N.; Quinteiro, M.; Seoane, C.; Soto, J. L. J. Chem. Soc., Perkin Trans. 11996, 947e951.
22. Carboxylic acids (R)- and/or (S)-6 were used, the last being previously prepared
from the optically active remaining substrates (S)-5 such as was described for
the synthesis of (ꢀ)-6 (Section 4.3).
16. Torres, S. Y.; Ochoa, E.; Verdecia, Y.; Rebolledo, F. Tetrahedron 2014, 70,
4675e4684.
ꢀ
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Please cite this article in press as: Torres, S. Y.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.047