Introduction of a quaternary stereogenic center to oxindole using cholinesterase-catalyzed asymmetric hydrolysis

Article abstract of DOI:10.1016/S0957-4166(01)00138-0

Prochiral diesters bearing an oxindole skeleton were efficiently prepared from oxindole. Cholinesterase-catalyzed hydrolysis of prochiral dipropionate afforded an optically active monoalcohol of 95% e.e. The obtained monoalcohol might find use as a versat

Full text of DOI:10.1016/S0957-4166(01)00138-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 897–901
Introduction of a quaternary stereogenic center to oxindole using
cholinesterase-catalyzed asymmetric hydrolysis
Koichi Nakazawa, Masaki Hayashi, Masakazu Tanaka,* Mariko Aso and Hiroshi Suemune*
Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
Received 23 February 2001; accepted 19 March 2001
Abstract—Prochiral diesters bearing an oxindole skeleton were efficiently prepared from oxindole. Cholinesterase-catalyzed
hydrolysis of prochiral dipropionate afforded an optically active monoalcohol of 95% e.e. The obtained monoalcohol might find
use as a versatile intermediate in the enantioselective synthesis of indole alkaloids. © 2001 Elsevier Science Ltd. All rights
reserved.
1. Introduction
(Fig. 1), and are also regarded as chiral building blocks
in indole syntheses.5 Herein, we report the asymmetric
hydrolysis of prochiral 3,3-bis(acyloxymethyl)-2-oxin-
dole, which is a potential chiral building block in the
synthesis of the aforementioned natural products.
Synthesis of quaternary stereogenic centers remains a
challenge for synthetic organic chemists.1 Some meth-
ods, such as diastereoselective reaction using chiral
auxiliaries, enantioselective reaction using transition-
metals, and enzymatic procedures, have been reported
for the formation of quaternary stereogenic centers
with stereoselectivity.2 Among them, enzymatic meth-
ods are attractive to many organic and medicinal
chemists because enzymatic reactions require only mild
reaction conditions, react with high selectivity and
accept a wide range of substrates.3 The formation of a
quaternary stereogenic center from either a prochiral or
meso compound by enzymatic methods has been
reported by several groups, including us.4 However,
such induction in an indole skeleton by enzymatic
methods has not yet been studied. Oxindoles bearing a
quaternary stereogenic carbon5 are found in natural
products, such as spirotryprostatins6 and horsfiline7
2. Results and discussion
Prochiral diesters 3 for enzymatic hydrolysis were pre-
pared from oxindole (Scheme 1). Initially, oxindole was
protected as its methoxymethyl (MOM) amide by treat-
ment with MOMCl in 63% yield. After several attempts
the C-(3) position of the oxindole skeleton was success-
fully dihydroxymethylated by treatment with
paraformaldehyde in the presence of K2CO3 to afford
the diol 2 in 96% yield. The obtained diol 2 was
converted into the prochiral diesters 3 by acylation with
acid anhydrides (3a: acetic anhydride, 80%; 3b: propi-
onic anhydride, 85%; 3c: butyric anhydride, 32%; 3d:
O
O
CH3
H3C
N
CH3NHCO2
HN
N
H
N
H3CO
N
CH3
O
N
H
H
O
N
H
CH3
(—)-Horsfiline
Spirotryprostatin B
(—)-Physostigmine
Figure 1.
* Corresponding authors. Tel.: +81-92-642-6604; fax: +81-92-642-6545; e-mail: mtanaka@phar.kyushu-u.ac.jp; suemune@phar.kyushu-u.ac.jp
0957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00138-0

898
K. Nakazawa et al. / Tetrahedron: Asymmetry 12 (2001) 897–901
Scheme 1.
valeric anhydride, 55%; 3e: benzoic anhydride, 76%,
respectively).
Disappointingly, the obtained monoalcohol 4a had low
e.e. (entries 2–4, 30–47% e.e.).
The introduction of the quaternary stereogenic carbon
was first attempted by enzymatic transesterification of
diol 2 using Pseudomonas cepacia lipase (PCL, Amano
PS);4c,8 unfortunately the transesterification reaction did
not proceed.
The bulk of the acyl function is known to affect the
enantioselectivity of enzymatic hydrolysis reactions.3
This led us to examine the enzymatic hydrolysis of the
dipropionate 3b. Hydrolysis of 3b using cholinesterase
afforded the monoalcohol 4b in 20% yield with a
moderate e.e. of 70%. The hydrolysis of 3b with PLE or
PPL afforded 4b with low e.e. These results encouraged
us to further study the effect of the acyl function on the
selectivity of reactions with cholinesterase.
We next examined the hydrolysis of diesters 3 using
four hydrolytic enzymes. The enzymatic hydrolysis was
performed in 0.1 M phosphate buffer (pH 7.0) at 30°C
and terminated when diol 2 was formed as indicated by
TLC. The results of hydrolyses are summarized in
Table 1. The enantiomeric excess (% e.e.) of the
hydrolyzed products was determined by HPLC using
( )-4 as reference standards. In the case of prochiral
diacetate 3a, hydrolysis using PCL did not proceed at
all, but hydrolyses by PLE, PPL, and cholinesterase4a,9
afforded monoalcohol 4a in 8–53% chemical yields.
Dibutyrate 3c, divalerate 3d, and dibenzoate 3e were
examined in the cholinesterase reactions (entries 8–12),
but hydrolysis of these diacylates 3c–3e either afforded
monoalcohols with very low e.e. (entries 8–10), or did
not proceed (entries 11 and 12). Therefore, we focused
our attention on optimization of the reaction between
dipropionate 3b and cholinesterase. After varying the
Table 1.
Entry
Substrate
Enzymea
Time
Monoalcohol (4)
Recovery (%)
Yield (%)
e.e.b (%)
Sign of [h]
1
2
3
4
5
6
7
8
3a: R=CH3
3a: R=CH3
3a: R=CH3
3a: R=CH3
3b: R=C2H5
3b: R=C2H5
3b: R=C2H5
3c: R=C3H7
3d: R=C4H9
3e: R=Ph
PCL
PLE
PPL
Cholinesterase
PLE
PPL
Cholinesterase
PLE
Cholinesterase
Cholinesterase
PLE
48
1
9
5
0.25
26
8
1.5
55
82
48
0
53
34
8
47
27
20
13
3
1
0
0
38
–
31
47
30
41
12
70
46
2
–
100
40
53
59
20
58
77
49
78
(−)
(+)
(+)
(−)
(+)
(+)
(−)
(+)
–
9
10
11
12
13
26
–
–
81
3e: R=Ph
3e: R=Ph
3b: R=C2H5
–
–
(+)
100
100
40c
PPL
Cholinesterase
48
120
95
a Enzymes: PCL, Pseudomonas cepacia lipase; PPL, porcine pancreatic lipase; PLE, pig liver esterase; cholinesterase, cholinesterase from electric
eel.
b HPLC: column, Daicel Chiralpak AD; eluent, hexane/i-PrOH=80/1; flow rate, 0.5 ml/min; detection, UV (254 nm).
c Prochiral diol 2 was isolated in 16% yield.

K. Nakazawa et al. / Tetrahedron: Asymmetry 12 (2001) 897–901
899
Scheme 2.
reaction temperature and investigating the effect of
added co-solvents in several reactions, we found that
hydrolysis of 3b using cholinesterase afforded mono-
alcohol 4b with high 95% e.e. and 38% yield by an
extended reaction time of 5 days. The monoalcohol 4b
was isolated with the recovered dipropionate 3b (40%)
and the diol 2 (16%). The improvement in e.e. may
result from kinetic resolution of the monoalcohol 4b.
(Taking into consideration the recovered dipropionate
3b and the diol 2, the yield of the homochiral mono-
alcohol 4b was 86%.)
4.1. 1-(Methoxymethyl)-2-oxindole 1
A solution of oxindole (100 mg, 0.75 mmol) in DMF
(1.5 mL) was added dropwise to a stirred suspension of
NaH (60% oil dispersion, 97 mg 2.4 mmol) in DMF
(1.5 mL) at 0°C, and the mixture stirred for 15 min.
Methoxymethyl chloride (0.2 mL, 2.7 mmol) was added
dropwise to the stirred solution, and the solution was
stirred for 3 h at room temperature. The solution was
neutralized with 10% aqueous HCl, extracted with
EtOAc, washed with brine, and dried over Na2SO4.
Removal of the solvent afforded an oily residue, which
was purified by column chromatography on silica gel
(30% EtOAc in hexane) to give 1 (84 mg, 63%) as a
In the formation of the aldehyde 5b (Scheme 2), oxida-
tion of the free hydroxyl group of (+)-4b was attempted
by several methods including PCC, PDC, and Swern
oxidations. However, using these reagents the oxidation
afforded a complex mixture. Finally, it was found that
oxidation with Dess–Martin periodinane afforded alde-
hyde 5b, which was unstable at room temperature;
therefore, the aldehyde was isolated as its dimethyl-
hydrazone derivative (+)-6b in 54% yield.
colorless solid: mp 75–77°C; IR (KBr) 1710 cm−1; H
NMR (CDCl3): l 7.24–7.31 (m, 2H), 7.01–7.11 (m, 2H),
5.13 (s, 2H), 3.61 (s, 2H), 3.35 (s, 3H); EIMS m/z: 177
(M+, 40), 146 (89), 43 (100).
1
4.2. 3,3-Bis(hydroxymethyl)-1-(methoxymethyl)-2-oxin-
dole 2
A mixture of 1 (568 mg, 3.2 mmol), K2CO3 (1.4 g, 10
mmol), and (HCHO)4 (1.9 g, 18 mmol) in THF (30 mL)
was stirred at room temperature for 10 min. The mix-
ture was neutralized with 10% aqueous HCl, extracted
with EtOAc, washed with brine, and dried over
Na2SO4. After removal of the solvent, the residue was
purified by column chromatography on silica gel (80%
EtOAc in hexane) to afford 2 (729 mg, 96%) as color-
less solid: mp 98–101°C; IR (KBr) 3400 (br), 1680 cm−1;
1H NMR (CDCl3): l 7.32–7.38 (m, 2H), 7.08–7.18 (m,
2H), 5.16 (s, 2H), 3.96–4.10 (m, 4H), 3.32 (s, 3H),
2.23–2.27 (m, 2H); EIMS m/z: 237 (M+, 34), 206 (100);
FAB(+)HRMS calcd for C12H16NO4 (M++H) 238.1079,
found 238.1079.
3. Conclusion
Prochiral diesters 3 bearing an indole skeleton were
prepared from oxindole. Enzymatic hydrolysis of dipro-
pionate 3b using cholinesterase from electric eel pro-
ceeded in enantioselective manner to afford the
optically active monoalcohol (+)-4b of 95% e.e. Synthe-
sis of natural products using (+)-4b is currently
underway.
4. Experimental
4.3. 3,3-Bis(acetoxymethyl)-1-(methoxymethyl)-2-oxin-
dole 3a
1H NMR spectra were determined at 270 MHz unless
otherwise noted. CH2Cl2 was distilled from P2O5. THF
was purchased from Kanto Chemical Corp., and used
without distillation. PLE, PPL, and cholinesterase from
electric eel were purchased from Sigma Corp., and PCL
(Amano PS) was given by Amano Pharmaceutical
Corp. (Japan), and all were used as received. Infrared
spectra were recorded on a Jasco A-100 spectrometer
(KBr or neat). EIMS, FABMS, and HRMS spectra
were taken on a Jeol JMS 610H or Jeol SX102
spectrometer.
A solution of 2 (40 mg, 0.168 mmol), pyridine (0.05
mL, 0.672 mmol), and Ac2O (0.05 mL, 0.504 mmol) in
CH2Cl2 (1 mL) was stirred at room temperature for 4 h.
The solution was diluted with 5% aqueous NaHCO3,
extracted with EtOAc, washed with brine, and dried
over Na2SO4. After removal of the solvent, the residue
was purified by column chromatography on silica gel
(20% EtOAc in hexane) to give 3a (43 mg, 80%) as a
colorless oil: IR (neat) 1740, 1710 cm−1; 1H NMR

900
K. Nakazawa et al. / Tetrahedron: Asymmetry 12 (2001) 897–901
(CDCl3): l 7.29–7.37 (m, 2H), 7.06–7.14 (m, 2H), 5.17
(s, 2H), 4.65 (d, J=10.8 Hz, 2H), 4.26 (d, J=10.8 Hz,
2H), 3.33 (s, 3H), 1.93 (s, 6H); FAB(+)HRMS calcd for
C16H19NO6 (M+) 321.1213, found 321.1187.
in hexane gave diol 2 (6 mg, 16%). 4b: a colorless oil; 95%
e.e.; [h]2D5 +49.0 (c 0.27, CHCl3); IR (neat) 3450 (br), 1710
cm−1; 1H NMR (CDCl3): l 7.30–7.37 (m, 2H), 7.07–7.26
(m, 2H), 5.19 (d, J=10.9 Hz, 1H), 5.14 (d, J=10.9 Hz,
1H), 4.73 (d, J=10.9 Hz, 1H), 4.37 (d, J=10.9 Hz, 1H),
3.85–4.02 (m, 2H), 3.33 (s, 3H), 2.33 (br s, 1H), 2.18 (q,
J=7.6 Hz, 1H), 2.17 (q, J=7.6 Hz, 1H), 0.97 (t, J=7.6
Hz, 3H); FAB(+)HRMS calcd for C15H20NO5 (M++H)
294.1341, found 294.1336.
4.4. 1-(Methoxymethyl)-3,3-bis(propionyloxymethyl)-2-
oxindole 3b
Compound 3b was prepared from 2 and propionic
anhydride, analogously to the preparation of 3a: a
1
colorless oil; 85%; IR (neat) 1730 (br) cm−1; H NMR
4.9. (+)-[3-(Hydroxymethyl)-1-(methoxymethyl)-2-oxoin-
dolin-3-yl]methyl acetate 4a
(CDCl3): l 7.05–7.36 (m, 4H), 5.17 (s, 2H), 4.69 (d,
J=11.2 Hz, 2H), 4.25 (d, J=11.2 Hz, 2H), 3.34 (s, 3H),
2.18 (q, J=7.6 Hz, 2H), 2.17 (q, J=7.6 Hz, 2H), 0.97
(t, J=7.6 Hz, 6H); FAB(+)HRMS calcd for C18H23NO6
(M+) 349.1525, found 349.1528.
Hydrolysis of 3a by PPL afforded 4a (34%, 47% e.e.) as
a colorless oil: [h]2D5 +19.5 (c 0.89, CHCl3); IR (neat) 3480
(br), 1720 (br) cm−1; 1H NMR (CDCl3): l 7.20–7.37 (m,
2H), 7.07–7.27 (m, 2H), 5.19 (d, J=10.9 Hz, 1H), 5.14
(d, J=10.9 Hz, 1H), 4.68 (d, J=10.9 Hz, 1H), 4.40 (d,
J=10.9 Hz, 1H), 3.84–4.10 (m, 2H), 3.32 (s, 3H), 2.50
(br, 1H), 1.91 (s, 3H); EIMS m/z: 279 (M+, 68), 248 (100).
4.5. 3,3-Bis(butyryloxymethyl)-1-(methoxymethyl)-2-
oxindole 3c
Compound 3c was prepared from 2 and butyric anhy-
dride analogously to the preparation of 3a: a colorless
4.10. (−)-[3-(Hydroxymethyl)-1-(methoxymethyl)-2-
oxoindolin-3-yl]methyl butyrate 4c
1
oil; 32%; IR (neat) 1730 (br) cm−1; H NMR (CDCl3):
l 7.27–7.36 (m, 2H), 7.05–7.12 (m, 2H), 5.17 (s, 2H),
4.70 (d, J=10.9 Hz, 2H), 4.24 (d, J=10.9 Hz, 2H), 3.34
(s, 3H), 2.14 (t, J=7.6 Hz, 4H), 1.46 (sextet, J=7.5 Hz,
4H), 0.78 (t, J=7.6 Hz, 6H); FABMS m/z: 377 (M+).
Hydrolysis of 3c by PLE afforded 4c (13%, 46% e.e.) as
a colorless oil: [h]2D5 −18.3 (c 0.27, CHCl3); IR (neat) 3450
(br), 1720 (br) cm−1; 1H NMR (CDCl3): l 7.26–7.37 (m,
2H), 7.06–7.15 (m, 2H), 5.18 (d, J=10.9 Hz, 1H), 5.15
(d, J=10.9 Hz, 1H), 4.75 (d, J=11.0 Hz, 1H), 4.37 (d,
J=11.0 Hz, 1H), 3.99 (dd, J=9.2, 11.2 Hz, 1H), 3.87 (dd,
J=3.6, 11.2 Hz, 1H), 3.34 (s, 3H), 2.32 (m, 1H), 2.13 (t,
J=7.4 Hz, 1H), 2.12 (t, J=7.4 Hz, 1H), 1.44 (sextet,
J=7.4 Hz, 2H), 0.77 (t, J=7.4 Hz, 3H); FABMS m/z:
308 (M++H).
4.6. 1-(Methoxymethyl)-3,3-bis(pentanoyloxymethyl)-2-
oxindole 3d
Compound 3d was prepared from 2 and valeric anhy-
dride analogously to the preparation of 3a: a colorless
1
oil; 55%; IR (neat) 1730 (br) cm−1; H NMR (CDCl3):
l 7.27–7.35 (m, 2H), 7.04–7.12 (m, 2H), 5.17 (s, 2H), 4.70
(d, J=11.0 Hz, 2H), 4.23 (d, J=11.0 Hz, 2H), 3.34 (s,
3H), 2.15 (t, J=7.6 Hz, 4H), 1.34–1.45 (m, 4H), 1.08–1.19
(m, 4H), 0.80 (t, J=7.3 Hz, 6H); FABMS m/z: 406
(M++H).
4.11. (+)-[3-Dimethylhydrazonomethyl)-1-(methoxy-
methyl)-2-oxoindolin-3-yl]methyl propionate 6b
A solution of 4b (18 mg, 61 mmol) in CH2Cl2 (0.5 mL)
was added dropwise to the stirred solution of Dess–Mar-
tin periodinane (130 mg, 0.31 mmol) in CH2Cl2 (1 mL)
at 0°C, and the solution was stirred for 2 h. Then a
solution of dimethylhydrazine (4.0 mg, 66 mmol) in
CH2Cl2 (0.5 ml) was added dropwise at 0°C and the
mixture was stirred for 1 h. The solution was diluted with
5% aqueous NaHCO3, extracted with EtOAc, washed
with brine, and dried over Na2SO4. After removal of the
solvent, the residue was purified by column chromatog-
raphy on silica gel (20% EtOAc in hexane) to leave 6b
(11 mg, 54%, 95% e.e.) as a colorless oil: [h]2D1 +44.2 (c
0.70, CHCl3); IR (neat) 1710 (br), 1610 cm−1; 1H NMR:
l 7.26–7.37 (m, 2H), 7.05–7.13 (m, 2H), 6.36 (s, 1H), 5.22
(d, J=10.9 Hz, 1H), 5.12 (d, J=10.9 Hz, 1H), 4.93 (d,
J=10.9 Hz, 1H), 4.48 (d, J=10.9 Hz, 1H), 3.35 (s, 3H),
2.78 (s, 6H), 2.08 (m, 2H), 0.85 (t, J=7.6 Hz, 3H);
EI-HRMS calcd for C17H23N3O4 (M+) 333.1688, found
333.1676.
4.7. 3,3-Bis(benzoyloxymethyl)-1-(methoxymethyl)-2-
oxindole 3e
Compound 3e was prepared from 2 and benzoic anhy-
dride analogously to the preparation of 3a: colorless
1
solids; 76%; mp 108–111°C; IR (KBr) 1720 cm−1; H
NMR (CDCl3): l 7.84–7.88 (m, 4H), 7.50–7.57 (m, 2H),
7.30–7.47 (m, 6H), 7.06–7.14 (m, 2H), 5.21 (s, 2H), 4.92
(d, J=10.9 Hz, 2H), 4.79 (d, J=10.9 Hz, 2H), 3.32 (s,
3H); FABMS m/z: 446 (M++H).
4.8. (+)-[3-(Hydroxymethyl)-1-(methoxymethyl)-2-oxoin-
dolin-3-yl]methyl propionate 4b
A
suspension of 3b (55 mg, 0.16 mmol) and
cholinesterase (0.2 mg) in phosphate buffer (0.1 M,
pH=7.0, 6.0 mL) was stirred at 30°C for 120 h. The
solution was extracted with EtOAc, and dried over
Na2SO4. Removal of the solvent afforded an oily residue,
which was purified by column chromatography on silica
gel. The fraction eluted with 30% EtOAc in hexane
afforded the recovered 3b (23 mg, 40%), with 50% EtOAc
in hexane afforded 4b (18 mg, 38%), and with 80% EtOAc
References
1. Corey, E. J.; Guzman-Perez, A. Angew. Chem., Int. Ed.
1998, 37, 338–401.

K. Nakazawa et al. / Tetrahedron: Asymmetry 12 (2001) 897–901
901
2. For example, methods developed in our group. See: (a)
Kato, K.; Sakai, K.; Suemune, H. Tetrahedron 1994, 50,
3315–3326; (b) Tanaka, M; Imai, M.; Fujio, M.;
Sakamoto, E.; Takahashi, M.; Eto-Kato, Y.; Wu, X. M.;
Funakoshi, K.; Sakai, K.; Suemune, H. J. Org. Chem.
2000, 65, 5806–5816.
3. (a) Suemune, H. Yakugaku Zasshi 1992, 112, 432–446; (b)
Wong, C.-H.; Whitesides, G. M. Enzymes in Synthetic
Organic Chemistry; Pergamon: Oxford, 1994; (c) Kato, K.;
Suzuki, H.; Tanaka, H.; Miyasaka, T.; Baba, M.;
Yamaguchi, K.; Akita, H. Chem. Pharm. Bull. 1999, 47,
1256–1264.
4. (a) Suemune, H.; Harabe, T.; Xie, Z. F.; Sakai, K. Chem.
Pharm. Bull. 1988, 36, 4337–4344; (b) Prasad, K.; Ester-
mann, H.; Chen, C. P.; Repic, O.; Hardtmann, G. E.
Tetrahedron: Asymmetry 1990, 1, 421–424; (c) Watanabe,
N.; Sugai, T.; Ohta, H. Chem. Lett. 1992, 657–660; (d)
Itoh, T.; Ohara, H.; Takagi, Y.; Kanda, N.; Uneyama, K.
Tetrahedron Lett. 1993, 34, 4215–4218; (e) Fujita, T.;
Tanaka, M; Norimine, Y.; Suemune, H.; Sakai, K. J. Org.
Chem. 1997, 62, 3824–3830; (f) Narisano, E.; Riva, R.
Tetrahedron: Asymmetry 1999, 10, 1223–1242; (g) Akai, S.;
Naka, T.; Fujita, T.; Takebe, Y.; Kita, Y. Chem. Commun.
2000, 1461–1462.
5. (a) Lee, T. B. K.; Wong, G. S. K. J. Org. Chem. 1991, 56,
872–875; (b) Ashimori, A.; Bachand, B.; Overman, L. E.;
Poon, D. J. J. Am. Chem. Soc. 1998, 120, 6477–6487; (c)
Matsuura, T.; Overman, L. E.; Poon, D. J. J. Am. Chem.
Soc. 1998, 120, 6500–6503.
6. (a) Edmondson, S.; Danishefsky, S. J.; Lorenzine, L. S.;
Rosen, N. J. Am. Chem. Soc. 1999, 121, 2147–2155; (b)
Wang, H.; Ganesan, A. J. Org. Chem. 2000, 65, 4685–
4693.
7. (a) Jossang, A.; Jossang, P.; Hadi, H. A.; Sevenet, T.;
Bodo, B. J. Org. Chem. 1991, 56, 6527–6530; (b) Fischer,
C.; Meyers, C.; Carreira, E. M. Helv. Chim. Acta 2000, 83,
1175–1181.
8. P. cepacia lipase (PCL, Amano PS) was previously used as
P. fluorescens lipase (PFL).
9. (a) Deardorff, D. R.; Matthews, A. J.; McMeekin, D. S.;
Craney, C. L. Tetrahedron Lett. 1986, 27, 1255–1256; (b)
Johnson, C. R.; Penning, T. D. J. Am. Chem. Soc. 1988,
110, 4726–4735; (c) Johnson, C. R.; Senanayake, C. H. J.
Org. Chem. 1989, 54, 735–736.
.