Stereoselective synthesis of optically active 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one through lipase-catalyzed esterification and transesterification processes

Article abstract of DOI:10.1016/j.molcatb.2014.10.015

The enantioselective synthesis of 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate (4) and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5), which are important intermediates in pharmaceutical industry, was carried ou

Full text of DOI:10.1016/j.molcatb.2014.10.015

Accepted Manuscript
Title: Stereoselective synthesis of optically active 1-benzyl-
4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate
and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-
4(5H)-one through lipase-catalyzed esterification and
transesterification processes 71042
Author: Zerrin Zerenler Caliskan Mediha Suleymanoglu
Ersez
PII:
S1381-1177(14)00290-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcatb.2014.10.015
MOLCAB 3053
To appear in:
Journal of Molecular Catalysis B: Enzymatic
Received date:
Revised date:
Accepted date:
18-7-2014
27-10-2014
28-10-2014
Please cite this article as: Z.Z. Caliskan, M.S. Ersez, Stereoselective
synthesis of optically active 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-
1H-indol-7-yl acetate and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-
indol-4(5H)-one through lipase-catalyzed esterification and transesterification
processes 71042, Journal of Molecular Catalysis B: Enzymatic (2014),
http://dx.doi.org/10.1016/j.molcatb.2014.10.015
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Page 1 of 14

1
Stereoselective synthesis of optically active 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-
oxo-1H-indol-7-yl acetate and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-
4(5H)-one through lipase-catalyzed esterification and transesterification processes 71042
Zerrin Zerenler Caliskana,* and Mediha Suleymanoglu Erseza
aDepartment of Molecular Biology and Genetics, Yildiz Technical University, 34210
Davutpasa, Istanbul, Turkey
Abstract
The enantioselective synthesis of 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-
yl acetate (4) and 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5),
which are important intermediates in pharmaceutical industry, was carried out for the first
time, both by enzyme-mediated hydrolysis and transesterification reactions with high
enantiomeric excesses in the presence of various lipases. In either case S enantiomer of (5)
was obtained with high enantiomeric excesses at low rate of conversion and E value.
However, R enantiomer of (5) was also obtained by transesterification reaction with high
optical purity. In the transesterification reaction of (rac-5a) with several lipases in different
solvent systems in the peresence of DMAP as an additive and vinyl acetate, E value of the
reaction were raised for some enzyme and solvent combination (THF-MJL with >99 %ee and
E value:41; for acetonitrile–MJL with 91 % ee and E value:51; for acetonitrile-Amano with
99 %ee, E value:68) showed R-(5) selectivity. Furthermore the conversion value was also
increased. The best conversion of the transesterification reaction was 39% with DMSO-HPL
showed 73% ee and E value:15 for R-(5) selectivity and 47% for S-(4) selectivity. The two
procedures can therefore be considered as complementary with respect to the final
composition.
Keywords: indole; enzyme-mediated hydrolysis; transesterification
1. Introduction
Enzymatic catalysis has recently been successfully used for the optical resolution of different
extremely functionalized chiral molecules such as amino acids, diols, diesters, and hydroxy
acids. Surprisingly very little notice has been paid to the enzymatic resolution of chiral
tetrahydro indol derivatives in spite of their importance as products have become important
targets in the pharmaceutical industry[1, 2]. The indol derivatives are useful not only as akey
intermediate for synthesis of Pindolol, which is an excellent drug prevention and treatment of
arrhythmia, but also as an intermediate for preparing various kinds of drugs, such as
antibiotics, antipsycotic agents and blood platelet aggregation inhibitors. The biological
activity of 4-oxo-tetrahydroindol derivatives and their structural relationship to indoles make
these compounds important targets in drug industry[3, 4].
In our previous investigations, we reported the preparation of enantiopure α׳-acetoxy and α׳-
hydroxy-4-oxo-tetrahydroindole derivatives using lipase enzyme-mediated hydrolysis
Page 2 of 14

2
reactions [5]. To the best of our knowledge, there has been no research carried out on the
kinetic resolution of γ-acetoxy-4-oxo-tetrahydroindole and γ-hydroxy-4-oxo-tetrahydroindole
derivatives. It is therefore of notable interest to improve enabled methods for the preperation
of these indole derivatives. In this paper, we report the synthesis of the enantiomers of γ-
acetoxy and γ-hydroxy-4-oxo-tetrahydroindole derivatives, using enantioselective
esterification and transesterification reactions.
2. Experimental
2.1. Materials and methods
NMR spectra were obtained on a Bruker Avance III spectrometer at 500 MHz. Chemical
shifts, δ, are reported in parts per million relative to CDCl3 (1H:δ=7.27), CDCl3 (13C:δ=77.0)
and CCl4 (13C:δ=96.4) as internal standards. Column chromatography was performed on silica
gel 60 (40-63 µm). TLC was carried out on silica gel, 60F254 (Merck), and the spots were
observed with UV light (λ=254 nm). IR spectra were recorded on a Perkin Elmer Spectrum
100 FT-IR Spectrometer. Enantiomeric excesses were identified by HPLC analysis using an
Agilent 1100 Series supplied with a suitable chiral phase column. The lipases CCL (lipase
from Candida cyclindracea) BioChemika (62316), PFL (lipase from Pseudomonas
fluorecens) BioChemika (95608), CAL (lipase from Candida antarctica) BioChemika
(62299), HPL (Hog pancreas lipase) BioChemika (62300), MJL (lipase from Mucor
javanicus) BioChemika (62304), PCL (lipase from Pseudomonas cepacia) (62309), RAL
(Rhizopus arrhizus lipase) BioChemika (62305) were taken from a Fluka lipase basic kit
(62327). Amano PS, from Burkholderia cepacia (Pseudomonas cepacia) was obtained from
Aldrich (534641). Optical rotations were determined with a Dr.Kernchen elektronic-
automation Sucromatdigital automatic saccharimeter. Mass spectra were recorded with an
Agilent 7890A GC system using an Agilent 5975C VLMSD having a triple-axis detector with
an HP-5 capillary GC column (30 m length, 0.32 mmID, 0.25 μm film thickness).
2.2. General procedure for Mn(OAc)3 oxidation
7.5 mmol Mn(OAc)3 in 100 ml benzene-acetic acid (10:1) were refluxed. To this solution, 1.8
mmol of benzofuranone was added and reflux was continued for 37-40 h. After all of the
starting material was consumed, the reaction mixture was extracted with ether and the organic
layer was washed with brine. The resulting organic phase was dried over MgSO4 and
concentrated under vacuum. The crude product was purified by column chromatography (1:5
EtOAc:hexane) to yield acetoxy-benzofuranone.
2.3. General procedure for the synthesis of indole derivatives
Alkylamine (18,7 mmol) and benzofuranone (6,25 mmol) in 20% aqueous ethanol (5 ml)
was heated at 145-150°C in a sealed tube for 12 hours [14]. The reaction mixture was poured
into water, extracted with CH2Cl2, dried over MgSO4 and concentrated under vacuum. The
crude products were purified by column chromatography (1:2 EtOAc:hexane) elution to yield
the indole derivatives.
2.4. Procedure for the synthesis of 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-
7-yl-acetate
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3
A solution of 3 mmol of KMnO4 in 100 ml benzene-acetic acid (10:1) was stirred under reflux
(Dean-Stark apparatus). When the purple color of KMnO4 turns brown, 1 mmol of indole
derivative was added and reflux was continued [16]. The reaction was monitored by TLC.
After all the starting material was consumed, the reaction mixture was extracted with ether
and the organic layer was neutralized with NaHCO3. The resulting organic phase was dried
over MgSO4, concentrated and purified by column chromatography (1:6 EtOAc:hexane) to
yield (25 %) of the γ-acetoxy-indole derivative.
2.5. General procedure for the lipase-catalyzed kinetic resolution
Lipase (200-300 mg) was dissolved in a phosphate buffer (pH=7,30 ml) and added to a
solution of the pure substrate (0.5 mmol) in solvent (3 mL) and the reaction mixture left
agitating at 37°C. Conversion (up to 50%) was monitored by TLC and HPLC. After this, the
filtrate was extracted with chloroform, dried over MgSO4, concentrated, and purified by
column chromatography (1:2 EtOAc:hexane).
2.6. General procedure for the lipase-catalyzed transesterification in the presence of additives
A solution of rac-5 (0.5 mmol) in solvent (1 ml) or without solvent was stirred at RT with
vinyl acetate (10 mmol) and 2.5 mmol DMAP. To this solution, lipase (200-300 mg) was
added and the reaction mixture left agitating at 37oC. The reaction was monitored by TLC and
HPLC. When 50% conversion was attained, the reaction was terminated. After filtration, the
filtrate was extracted with chloroform, dried over MgSO4, concentrated, and separated by
column chromatography (1:2 EtOAc:hexane).
2.7. 1-Benzyl-6,7-dihydro-6,6-dimethyl-1H-indol-4(5H)-one (3)
1
Yield: 888 mg, 56%, colourless crystals, (mp: 81°C). IR (CHCl3): ν=1647, 2926 cm-1. H
NMR (500 MHz, CDCl3): δ 1.00 (s, 6H, CH3), 2.29 (s, 2H, CH2), 2.43 (s, 2H, CH2), 4.98 (s,
2H, CH2Ph), 6.52 (d, J=3.00 Hz, 1H, H-3), 6.57 (d, J=3.00 Hz, 1H, H-2), 6.94-7.29 (m, 5H,
Ph). 13C NMR (125 MHz, CDCl3): δ 28.64; 35.57; 35.75; 50.45; 51.77; 105.64; 120.04;
123.24; 126.35; 127.91; 128.07; 128.78; 128.98; 136.70; 142.70; 193.76
2.8. 1-Benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl-acetate (4)
Yield: 99.50 mg, 64%. IR (CHCl3): ν=1657, 1737, 2967 cm-1.1H-NMR (500 MHz, CDCl3): δ
0.98 (s, 3H, CH3), 1.07 (s, 3H, CH3), 1.83 (s, 3H, COCH3), 2.20 (d, J=16.65 Hz, 1H, CH2),
2.81 (d, J=16.65 Hz, 1H, CH2), 5.13 (d, J=16.254 Hz, 1H, CH2-Ph), 5.24 (d, J=16.254 Hz,
1H, CH2-Ph), 5.83 (s, 1H, CHO), 6.65 (d, J=3.001 Hz, 1H, H-3), 6.75 (d, J=2.951 Hz, 1H,
H-2), 6.95-7.35 (m, 5H, Ph). 13C-NMR (125 MHz, CDCl3): δ 20.44; 25.64; 25.77; 38.78;
47.46; 50.57; 68.48; 105.99; 121.74; 125.20; 126.15; 127.78; 128.83; 137.03; 138.29; 170.54;
193.38. Anal.Calcd. for C19H21NO3 (311.37): C, 73.29; H, 6.80; N, 4.50. Found: C, 72.15; H,
7.02; N, 4.42. GC/MS (m/z) 311.2 (M+), 252.1, 213.1, 178.1, 168.1, 91.1
2.9. 1-Benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5)
Yield: 872 mg, 52%, colourless crystals (mp:156.6°C). IR (CHCl3): ν=1644, 2961, 3312 cm-1;
1H-NMR (500 MHz, CDCl3) δ 0.90 (s, 3H, -CH3), 1.10 (s, 3H, -CH3), 2.08 (d, J=16.51 Hz,
1H, CH2), 2.69 (d, J=16.55 Hz, 1H, CH2), 4.33 (s, 1H, H-7), 5.14 (d, J=15.90 Hz, 1H,
Page 4 of 14

4
CH2Ph), 5.28 (d, J=15.95 Hz, 1H, CH2Ph), 6.50 (d, J=3.00 Hz, 1H, H-3), 6.62 (d, J=3.05 Hz,
1H, H-2), 7.05-7.34 (m, 5H, Ph). 13C-NMR (125 MHz, CDCl3): δ 25.25; 26.15; 39.64; 47.19;
50.56; 69.06; 69.09; 105.55; 119.83; 124.48; 126.66; 127.96; 128.95; 129.27; 137.06; 142.43;
194.52. Anal.Calcd for C17H19NO2 (269.34): C, 75.81; H, 7.11; N, 5.20. Found: C, 75.11; H,
7.27; N, 5.16. GC/MS (m/z) 269.0 (M+), 253.2, 197.1, 168.0, 106.0, 91.1
2.10. (S)1-Benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5)
(65.9 mg, 49%); [α]20D= -2.68 (c 0.01, CHCl3); HPLC: Chiralcell OD-H column, UV
detection at 254 nm, eluent: hexane/2-propanol=98:2, flow 1.0 ml min-1 20oC retention time:
4.1 min.
2.11. (R)1-Benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (5)
(68.9 mg, 51%); [α]20D= +23.08 (c0.0013, EtOAc); HPLC: Chiralcell OD-H column, UV
detection at 254 nm, eluent: hexane/2-propanol=98:2, flow 1.0 ml min-1 20oC retention time:
4.5 min.
2.12. X-ray crystal structure analysis of 3 and (rac-5a)
For the crystal structure determination, a single crystal of compounds (3) and (rac-5a) was
used for data collection on a four-circle Rigaku R-AXIS RAPID-S diffractometer (equipped
with a two-dimensional area IP detector). Graphite-monochromated Mo-Kα radiation ( =
0.71073 Å) and oscillation scans technique with w = 5º for one image were used for data
collection. The lattice parameters were determined by the least-squares methods on the basis
of all reflections with F2> 2(F2). Integration of the intensities, correction for Lorentz and
polarization effects and cell refinement was performed using CrystalClear (Rigaku/MSC Inc.,
2005) software [6]. The structures were solved by direct methods using SHELXS-97 [7] and
refined by a full-matrix least-squares procedure using the program SHELXL-9721. H atoms
were positioned geometrically and refined using a riding model. The final difference Fourier
maps showed no peaks of chemical significance. Crystal data for (3): C17H19NO, crystal
system, space group: monoclinic, P21/n; (no:14); unit cell dimensions: a = 6.3605(2), b =
24.8456(2), c = 9.0033(3) Å, α= 90, β = 93.088(2), γ = 90 Å; volume: 1420.73(8) Å3; Z = 4;
calculated density: 1.184 g/cm3; absorption coefficient: 0.073 mm-1 ; F(000): 544; θ-range for
data collection 2.4–26.4º ; refinement method: full matrix least-square on F2; data/parameters:
1387/175; goodness-of-fit on F2: 1.014; final R-indices [I > 2(I)]: R1 = 0.056, wR2 = 0.133;
largest diff. peak and hole: 0.135 and -0.130 e Å-3. CCDC: 895984. Crystal data for (rac-5):
C17H19NO2, crystal system, space group: monoclinic, P21/n; (no:14); unit cell dimensions: a =
12.2872(4), b = 7.3242(2), c =16.5399(5) Å, α= 90, β = 93.049(4), γ = 90 Å; volume:
1486.38(8) Å3; Z = 4; calculated density: 1.204 g/cm3; absorption coefficient: 0.079 mm-1 ;
F(000): 576; θ-range for data collection 2.1–30.5º ; refinement method: full matrix least-
square on F2; data/parameters: 1857/184; goodness-of-fit on F2: 1.016; final R-indices [I >
2r(I)]: R1 = 0.0761, wR2 = 0.171; largest diff. peak and hole: 0.178 and -0.195 e Å-3. CCDC:
895813
Crystallographic data were deposited in CSD under CCDC registration number. These data
can be obtained free of charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
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3. Results and discussion
Commercially available 5,5-dimethyl-1,3-cyclohexanedione (1) were converted to the 6,7-
dihydro-6,6-dimethyl benzofuranone (2) using the method of Matsumoto and Watanabe [8].
As an initial synthetic procedure (Scheme1), treatment of 6,7-dihydro-6,6-dimethyl
benzofuranone (2) with benzylamine, in aqueous ethanol at 150ºC under pressure, [8,9]
produces 1-benzyl-6,7-dihydro-6,6-dimethyl-1H-indol-4(5H)one (3) with a 56% yield. The
single crystal structure of the compound (3) has been obtained.
Then the oxidation of 1-benzyl-6,7-dihydro-6,6-dimethyl-1H-indol-4(5H)-one (3) in the
precence of 3 equivalents of KMnO4 in benzene-acetic acid (10:1) medium is converted to γ-
acetoxy-1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indole derivative (rac-4) in a
64% yield in the presence of a base according to the procedure in the literatüre [10] as shown
in Scheme 1.
It is known that several studies on sources of Mn(OAc)3 oxidation are designated in the
literature. In general, manganese (III) acetate oxidations are identified by α׳-regioselectivity
[11,12,13].
In our previous work, we also showed the selective oxidations of enones by Mn(OAc)3
oxidation [14]. Here, we tried this method for acetoxylation of our indol derivatives, however
we have not any acetoxylation product. So we decided to try another method for acetoxylation
of indol derivatives.
The selective oxidation of enones also leading to α׳-acetoxy enones by potasium
permanganate/carboxylic acid in an organic solvent with high yield according to Demir etal
[10]. Herein we showed, for the first time, that potassium permanganate method in our indol
system and we realize that, when we used KMnO4/acetic acid method in our indole system,
we got only γ-acetoxy product (rac-4) instead of α'-acetoxy product (Scheme 1).
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6
O
O
O
O
O
a
b
Lit.12
N
N
O
O
O
(2)
(1)
(3)
(rac-4)
c
O
*
O
+
N
N
*
OH
O
O
(4)
(5)
Scheme1. a) Benzylamine, EtOH/H2O, heat; b) KMnO4, Benzene/AcOH, reflux; c) Enzyme, solvent, pH=7,
37°C
In the course of biotransformation, enzymatic catalysis has been recently succefully used for
the optical resolution of various functionalized chiral molecules on a large scale such as
alcohols, carboxylic acids and esters in organic solvents [15a]. During the course of our
studies on the biotransformation of rac-4 the screening reactions were examined with varied
lipases.
In order to determine the enzyme that gives the best results, screening with seven different
enzymes (Amano PS, CCL, PFL, CAL, HPL, MJL, and PCL) was carried out as shown in
Table 1. They were tested in four different organic solvents (DMSO, THF, acetonitrile,
toluene) for the kinetic resolution step.
In a distinctive experiments for enzymatic hydrolysis rac-4 was dissolved in an organic
solvent, a phosphate buffer (pH 7.0) was then added the mixture stirred at 37°C in the
presence of an enzyme (Scheme 1).
The compound rac-4 was also hydrolyzed chemically, to understand which peak in the HPLC
spectra corresponds to the hydroxyl enone. This chemical hydrolysis was carried out using
K2CO3 in methanol [15a,b]. The reaction was monitored by TLC and HPLC with a chiral
column using rac-4 and corresponding hydroxyl enone as a reference. When an
approximately 50% conversion was obtained, the crude product was separated by flash
column chromatography to ensure both enantiomers of 4 and 5.
According to the HPLC analysis the reaction provided enantioselectivity for the hydroxy
indol (-)-5 in 13 to 95% e.e. (Table 1)
For the preparative scale, the synthesis of (-)-5 γ-hydroxy-1-benzyl-4,5,6,7-tetrahydro-6,6-
dimethyl-4-oxo-1H-indole (Table 1, entry 5) was used which was obtained the best result
from PFL (Pseudomonas fluorescens lipase) enzyme in DMSO with high enantiomeric
Page 7 of 14

7
excesses (95% ee, 49% yield). The acetoxy enantiomer, however, does not resolve in good
enantiomeric excesses. This may be due to the presence of bulky groups on the ring.
Table 1. Enzymatic hydrolysis of 1-benzyl-4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-
7-yl-acetate (rac-4)
Entry
Enzyme
Time(h) Solvent
Alcohol
e.ea(%) Yieldb
Acetate
e.ea (%)
Conversion
%
Ec
1
2
PCL
MJL
48
30.5
DMSO
DMSO
86
81
9.6
0.55
10
0.67
15
0.009
7
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
CCL
HPL
PFL
72
30.5
72
DMSO
DMSO
DMSO
DMSO
THF
THF
THF
THF
THF
74
71
8.7
8.2
1.7
8.4
0.15
4
2.5
3.8
3
10.5
10.4
1.76
12
0.20
4.5
3
7.13
6.3
1.6
4.2
7
13
5.6
1.36
10.8
6.5
3
9
6
1
10.5
10
25
17
6
95
59
74
84
79
13
82
74
59
85
57
51
76
49
Amano PS 192
MJL
CCL
HPL
PFL
21.5
21.5
20
6.5
23
Amano PS 96
3.5
1.5
0.25
0.5
0.2
1
1.2
1.1
0.13
0.27
1.7
1.4
CAL
MJL
CCL
HPL
PFL
CAL
MJL
CCL
HPL
PFL
21.5
29
29
45
27.5
47.5
3.1
3
3.1
3.1
3
THF
1
Toluene
Toluene
Toluene
Toluene
Toluene
Acetonitrile 78
Acetonitrile 81
Acetonitrile 89
Acetonitrile 70
Acetonitrile 76
0.15
0.4
0.13
0.07
1
0.9
0.12
0.24
1.2
1
CAL
6
a Determined by HPLC using Chiralcell OD-H column, UV detection at 254 nm, eluent: hexane\2 propanol
=98:2, flow 1.0 ml min-1 20oC, using racemic compounds as references.
bIsolated yield after flash column chromatography.
c See ref. [18]
We next investigated the transesterification of rac-5a under different conditions as another
alternative route (Scheme 3).
Thus, we examined the transformation of 6,7-dihydro-6,6-dimethyl benzofuranone (2) into the
4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-benzofuran-7-yl acetate (2a) (major product) and
4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-benzofuran-5-yl acetate (2b) (minor product) using
manganese (III) acetate oxidation procedure as shown in Scheme 2.
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8
O
O
O
O
O
a
+
O
O
O
O
O
(2)
(2a)
(2b)
b
b
O
O
HO
N
N
OH
(rac-5b)
(rac-5a)
Scheme 2. a) Mn(OAc)3, AcOH, benzene, reflux; b) Benzylamine, EtOH/H2O, heat
As we reported in a previous study [16], when the reaction time was 20 hours, only 4,5,6,7-
tetrahydro-6,6-dimethyl-4-oxo-benzofuran-5-yl acetate (2b) was produced. In this work, we
realized that when we increased the reaction time, we obtained two products, 2a as the major,
and 2b as the minor product. Because of separation problem of these two products, the
mixture of the two was used for obtaining the indole derivatives [8]. From this reaction, 1-
benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-4(5H)-one (rac-5a) was obtained in
high yield (52% ) as shown in Scheme 2. The single crystal structure of the compound rac-5a
is shown in Figures 2a and b.
Figure 2. (a) ORTEP drawing of the molecule 1-benzyl-7-hydroxy-6,6-dimethyl-6,7-dihydro-1H-indol-4(5H)-
one (rac-5a). Thermal ellipsoids are shown at 50% probability level. C12(R). (b) H-bonding pattern (dashed
lines) along the b-axis in the unit cell. O2-H···O1i = 2.815(3) Å, <(O2-H···O1i )=159º (Symmetry code i=x,
1+y,z).
Page 9 of 14

9
Then we have studied transesterification reaction of rac-5a, with several lipases in different
solvents (DMSO, toluene, THF, acetonitrile) or without solvent were explored. In all cases,
only trace amounts of the products were formed.
It is known that the addition of crown ethers, amino alcohols, and certain bases, is well known
to influence the hydrolysis of esters; bases also increase the rate of transesterification in the
presence of enzymes in apolar organic solvents [17]. Enzyme mediated transesterification was
achieved using several bases (DMAP, 2,4-lutidine, 2,6-lutidine and pyridine). DMAP, in
particularly is the most basic additives according to the others. It is considerably enhanced the
reaction rate. This was the idea behind the approach we adopted. Therefore we decided to use
DMAP as an additive.
O
O
O
c
+
*
N
*
N
N
O
OH
OH
O
(5)
(rac-5a)
(4)
Scheme 3. c) Enzyme, vinyl acetate, additive (DMAP)
In the transesterification of rac-5a, was performed with various enzymes, in different solvents
(DMSO, THF, acetonitrile, toluene), using vinyl acetate as the acetyl source and the reaction
to take place in a matter of hours, in the presence of DMAP(5 equiv) as is shown in Table 2.
From the lipases examined, it is observed that the MJL in THF, CAL in THF and Amano PS
in acetonitrile gave better conversions with reverse selectivity (˃99% ee). These enzyme
solvent systems preferentially recognized the (R)-5 enantiomers.
Page 10 of 14

10
Table 2. Transesterification of 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-
4(5H)-one (rac-5a) with solvent
Entry Enzyme Solvent
Additive Time(h) Alcohol
Acetate
Conversion Ec
eea Yieldb
eea Yieldb
(%) (%)
(%) (%)
0.2
8
0.6
47
5
5
9
1
3
%
1
2
3
4
5
6
7
8
Amano DMSO
DMAP 11
DMAP 23
DMAP 23
DMAP 11
DMAP 11
DMAP 23
DMAP 23
DMAP 23
DMAP 23
DMAP 23
DMAP 23
82d
0.2
0.1
0.8
39
6
5
9
1
3
5
6
1
15
9
41
15
2
16
2
CCL
CAL
HPL
DMSO
DMSO
DMSO
69d
75d
73d
Amano THF
80d
MJL
HPL
CAL
RAL
HPL
CAL
THF
THF
THF
THF
Toluene
Toluene
>99d 49
87d
>99d
86d
9
10
11
12
13
14
15
16
17
18
69d
0.1
3
3
0.1
8
3
35d
2
Amano Acetonitrile DMAP 21
99d
68
51
1
6
16
3
MJL
PFL
Acetonitrile DMAP 22,5
Acetonitrile DMAP
91d
4
4
7
62
75
84
0.7
0.5
0.05
1
1
CCL
CAL
HPL
RAL
Acetonitrile DMAP 22,5
Acetonitrile DMAP 22,5
Acetonitrile DMAP
0.7
0.06
2
8
53
Acetonitrile DMAP 23
77d
11
13
9
a
Determined by HPLC using Chiralcell OD-H column, UV detection at 254 nm, eluent: hexane\2-
propanol=98:2, flow 1.0 ml min-1 20oC, using racemic compounds as references.
bIsolated yield after flash column chromatography.
c See ref. [18]
d Reverse selectivity
Furthermore, the effect of solvents were also investigated and it was observed that the lipases
from MJL in acetonitrile (91% ee), RAL in THF (86% ee), RAL in acetonitrile (77% ee),
Amano PS in THF (80 % ee), CCL in DMSO (69 % ee), HPL in DMSO (73 % ee), HPL in
THF (87 % ee), HPL in toluene (69 % ee), CAL in DMSO (75 % ee), CAL in THF (99% ee),
and CAL in toluene (35% ee) all exhibited solvent dependent reverse selectivity and (R)-5
was obtained. However, the lipases from PFL in acetonitrile (62 % ee), CCL in acetonitrile
(75% ee), CAL in acetonitrile (84 % ee), and HPL in acetonitrile (53% ee) showed (S)-5
selectivity. These results clearly showed that solvent dependent substrate selectivity for rac-
5a.
For the preperative scale, the synthesis of (R)-5 MJL in THF (Table 2, entry 6) is used and the
product obtained in 49% yield.
In addition, different lipases were also screened for resolving rac-5a in the enantioselective
transesterification of the γ-hydroxy-4-oxo-tetrahydroindole derivative (5), by using vinyl
acetate as an benign acyl donor and DMAP used as an additive, without solvent, as shown in
Table 3.
Page 11 of 14

11
Table 3. Transesterification of 1-benzyl-6,7-dihydro-7-hydroxy-6,6-dimethyl-1H-indol-
4(5H)-one (rac-5) without solvent
Entry
Enzyme
Solvent
Additive
Time
(h)
Alcohol
eea (%)
Acetate
eea (%)
Conversionb
%
Eb
1
2
3
4
5
Amano
MJL
PFL
CAL
HPL
-
-
-
-
-
DMAP
DMAP
DMAP
DMAP
DMAP
7,5
23
7,5
23
8
52
6
10
23
1
7
2
4
12
9
21
3
82d
85
24
0.8
7
90d
42
1
a
Determined by HPLC using Chiralcell OD-H column, UV detection at 254 nm, eluent: hexane\2-
propanol=98:2, flow 1.0 ml min-1 20oC, using racemic compounds as references.
b See ref. [18]
c Reverse selectivity
From this data it can be seen that the lipases, CAL with 90% enantiomeric excess, and MJL
with 82% enantiomeric excess, exhibit reverse selectivity and (R)-5 is obtained. However, the
lipase from Amano PS with 52 % enantiomeric excess, PFL with 85 % enantiomeric excess
and HPL with 42% enantiomeric excess show (S)-5 selectivity.
This clearly suggests that both esterification and transesterification processes leads to very
effective kinetic resolution as demonstrated by the high enantiomeric excess values, but
surprisingly in the transesterification reaction reverse selectivity is also observed. In the
transesterification process, only HPL in DMSO showed limited selectivity for acetoxy enone
(-)-4 (47% ee, Table 2) while in the esterification reaction any selectivity for acetoxy enone
values was extremely low.
In this paper we report two strategies for enzymatic resolution of 4-oxo-tetrahydroindol
derivatives. If it is considered to the first strategies according to the Scheme 1, 1-benzyl-6,7-
dihydro-6,6-dimethyl-1H-indol-4(5H)one (3) has been synthesized by using indol formation
procedure (benzylamine in aqueous EtOH at 150ºC under pressure) with a good yield (56 %).
In the acetoxylation step which has been easily performed the desired product (rac-4) in the
presence of KMnO4/ acetic acid method in our indol system, surprisingly, we got only γ-
acetoxy-4-oxo-tetrahydroindole derivative. Nevertheless the yield of rac-4 was very low
(25%). Furthermore, we tested a series of enzymes and solvents for the enantioselective
hydrolysis of acetoxy enone rac-4, provided only R enantiomer for hydroxyl enone but the
acetoxy enantiomer was not resoved in good enantiomeric excesses shown in Table1.
In the second route (Scheme 2), firstly acetoxylation of benzofuranone derivative was
synthesized in the presence of Mn(OAc)3, AcOH method. We previously synthesized of (2b)
with very high yield and high regioselectivity, when the reaction time was 20 hours. Then, we
suggest that, Mn(OAc)3, AcOH method which has been generated i) high regioselectivity for
α-acetoxy benzofuranone derivatives when the reaction time is 20 hours, ii) high
regioselectivity for γ-acetoxy benzofuranone, when the reaction time is 40 hours. The second
route showed that the 2a was obtained with very high yield (86%). After treatment of indol
procedure with benzylamine rac-5a was obtained in high yield (52%). This routes could be
usefull for transesterification reactions results. In the transesterification reaction of rac-5a,
both of R and S enantiomer could be obtained for hydroxyl enantiomer with high
enantioselectivity, high conversion value and high E values with respect to the esterification
Page 12 of 14

12
reactions. Additionally, in the transesterification reactions, the acetoxy enantiomer also
resolved in 47%ee and high rate of conversion (39), high E value (15) according to overall
enzymatic route as is shown in Table 2.
4. Conclusion
This work presents the first effective synthesis of γ-acetoxy-4-oxo-tetrahydroindole (4) and γ-
hydroxy-4-oxo-tetrahydroindole (5) derivatives, which are important targets in the drug
industry, through enzymatic kinetic resolution. The enantioselective synthesis of 1-benzyl-
4,5,6,7-tetrahydro-6,6-dimethyl-4-oxo-1H-indol-7-yl acetate (4) was carried out with a 47%
enantiomeric excess via a transesterification process, the esterification reaction showed very
low selectivity. The enantioselective synthesis of 1-benzyl-6,7-dihydro-7-hydroxy-6,6-
dimethyl-1H-indol-4(5H)-one (5) was realized in both the lipase-catalyzed esterification and
transesterification reactions with high enantiomeric excesses. Generally much better results
were achieved when transesterification reaction was conducted with specific lipases in certain
solvent systems. In the transesterification reaction both (S)-5 and (R)-5 enantiomers were
obtained as alcohols whereas, in the enzyme-mediated hydrolysis of rac-4 only the (S)-5
enantiomer was obtained as an alcohol in high optical purity. Therefore, esterification and
transesterification can be considered as complementary with respect to the final product
composition. This method enables a simple new entry to the synthesis of γ-acetoxy-4-oxo-
tetrahydro indol derivatives, which are important precursors for farmacologically interesting
compound.
Acknowledgments
Financial support from Yildiz Technical University (BAP-2010-01-07-YL01) is gratefully
acknowledged. We are also thankful to Assoc. Prof. Dr. Ertan Sahin from the Department of
Chemistry, Ataturk University for providing x-ray crystal structures.
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