SPECIALTOPIC
Enzymatic Hydrolysis of -Chloro Esters
945
was distilled under reduced pressure to give 63 (4.7 g, 95%) as a co-
lourless oil; bp 70°C/3 Torr.
and concentrated in vacuo. The residue was distilled by Kugelrohr
to afford ester 2 (1.1 g, 82%) as a colourless oil.
IR (film): = 1756 (C=O), 1455 (C–O), 1163 cm–1 (C–O).
1H NMR (400 MHz, CDCl3): = 7.5 (2 H, m, C6H5), 7.3 (3 H, m,
C6H5) 5.4 (1 H, s, CHCl), 3.8 (1 H, s, CH3).
13C NMR (CDCl3): = 168.9 (C=O), 135.9 (CH), 129.5 (CH),
129.1 (CH), 128.1 (CH), 59.3 (CHCl), 53.7 CH3).
MS (EI): m/z (%) = 184 (23, M+), 125 (97).
IR (film): = 2963 (C–H), 1652 (C=O), 1464 (C–O), 1171 (C–O),
769 cm–1 (C–Cl).
1H NMR (400 MHz, CDCl3): = 0.9 (t, J = 7 Hz, 3 H, CH3), 1.1–1.6
(m, 2 H, CH2), 1.6–2.2 (m, 2 H, CH2CHCl), 3.7 (s, 3 H, OCH3), 4.1
(t, J = 7 Hz, 1 H, CHCl).
13C NMR (CDCl3): = 159.1 (C=O), 56.6 (CHCl), 44.4 (CH3), 38.4
(CH3), 38.4 (CH2), 35.0 (CH2), 27.0 (CH2), 19.8 (CH3).
MS (CI): m/z = 166.1 (95%, M+).
A similar procedure was employed for the synthesis of esters 1b and
1c, using EtOH or 2-chloroethanol (1.2 equiv) in THF, respectively.
Enzymatic Hydrolysis of -Chloro Esters; General Procedure
Candida cylindracea lipase (Altus 17, CLEC) (3 mg, 10% w/w)
was added to a solution of ester (25 mg, 0.14 mmol) in phosphate
buffer/solvent (5 mL, 80:20) at r.t. At the suitable time (48 h, see
Table 1) the resulting solution was filtered through Celite, acidified
with 0.1 M HCl, and extracted with EtOAc (3 5 mL). The com-
bined extracts were dried (MgSO4) and concentrated in vacuo, giv-
ing ester and acid. Alternatively, reactions were run in H2O–solvent
controlled at pH 7–8 using an autotitrator. The enantiomeric excess
was determined by HPLC or GC analysis (Table 1).
Ester 1b
Yield: 4.9g (95%); colourless oil; bp 98°C/1.5 Torr.
IR (film): = 1752 (C=O), 1455 (C–O), 1158 (C–O), 727 cm–1 (C–
Cl).
1H NMR (400 MHz, CDCl3): = 7.5 (2 H, m, C6H5), 7.3 (3 H, m,
C6H5) 5.4 (1 H, s, CHCl), 4.2 (2 H, q, J = 7.0 Hz, CH2), 1.2 (3 H, t,
J = 7.0 Hz, CH3).
13C NMR (CDCl3): = 168.4 (C=O), 136.1 (CH), 129.4 (CH),
129.0 (CH), 128.1 (CH), 62.8 (CHCl), 59.5 (CH2), 14.4 (CH3).
MS (CI): m/z (%) = 199 (89, MH+), 178 (66), 163 (100), 102 (73).
Resin Bound Phosphonium Salt
Ph3P (1.0 g, 3.8 mmol, 10 equiv), was added to a solution of Merri-
field resin (0.5 g, 1.26 nM/g, 1 equiv) in toluene (15 mL). The mix-
ture was heated at reflux for 16 h, filtered, then washed successively
with toluene (10 mL), H2O (10 mL), EtOH (10 mL) and finally with
CH2Cl2 (10 mL). The resin was used immediately.
HRMS: m/z found for MH+ 199.0526. C10H11ClO2 requires MH
199.0527.
Ester 1c
Yield: 5.3g (91%); pale yellow oil; bp 120°C/1.5 Torr.
IR (film): = 1756 (C=O), 1455 (C–O), 1161 (C–O), 728 cm–1 (C–
Cl).
1H NMR (400 MHz, CDCl3): = 7.5 (2 H, m, C6H5), 7.4 (3 H, m,
C6H5) 5.4 (1 H, s, CHCl), 4.4 (2 H, t, J = 6.5 Hz, CH2O), 3.7 (2 H, t,
J = 6.5 Hz, CHCl).
13C NMR (CDCl3): = 168.1 (C=O), 135.5 (CH), 129.6 (CH),
129.1 (CH), 128.1 (CH), 65.9 (CHCl), 59.1 (CH2) 41.4 (CH2Cl).
Competitive Racemisation Studies; General Procedure
Racemising agent (1 equiv) was added to a solution of ester (S)-1a
(25 mg, 0.14 mmol) and acid (R)-3 (25 mg, 0.15 mmol) in phos-
phate buffer–solvent (5 mL, 80:20) or alternatively pH controlled
H2O–solvent (80:20). At the suitable time (16 h, see Table 2) the re-
sulting solution was filtered through silica gel, the phases separated
and the aqueous layer acidified with 0.1 M HCl and extracted with
EtOAc (3 5 mL). The combined extracts were dried (MgSO4) and
concentrated in vacuo, giving ester (R/S)-1a and (R)-3 that was
analysed using chiral HPLC (Table 2).
MS (EI): m/z = 233 (10%, M+).
Anal. Calcd for C10H10Cl2O2: C, 51.5; H, 4.3. Found: C, 51.2; H,
4.3.
The Dynamic Kinetic Resolution of Ester 1a
2-Chloropentanoic Acid (4)5
Candida cylindracea lipase (Altus 17, CLEC) (3 mg, 10% w/w)
was added to a solution of 1a (25 mg, 0.14 mmol) and racemising
agent (0.1–0.6 equiv) in phosphate buffer–solvent (5 mL, 80:20) or
pH controlled H2O–solvent (5 mL, 80:20). At a suitable time (24 h)
the resulting solution was filtered through Celite, the phases sepa-
rated and the aqueous phase acidified with 0.1 M HCl and extracted
with EtOAc (3 5 mL). The combined extracts were dried (MgSO4)
and concentrated in vacuo, to give ester 1a and acid 3 which were
analysed by chiral HPLC or GC.
NaNO2 (2.97 g, 0.043 mol) was added in portions (slowly) to a so-
lution of norvaline (4.46 g, 0.038 mol) in ethereal HCl at 0°C. After
16 h at r.t., the reaction was poured into H2O and extracted with
CH2Cl2 (3 10 mL). The extract was washed with NaHCO3 and
brine, dried (MgSO4) and concentrated in vacuo. The residue was
distilled by Kugelrohr (132 °C/2 Torr) giving acid 4 (3.67g, 71%)
as a colourless oil.
IR (film): = 3170 (COO–H), 1713 (C=O), 772 (C–Cl), 751 cm–1
(C–Cl).
1H NMR (400 MHz, CDCl3): = 9.1 (br s, 1 H, CO2H), 4.3 (dd, 1
H, J = 8.2, 5.9 Hz, CHCl), 2.0 (m, 1 H, CH2), 1.5 (m, 2 H, CH2), 1.0
(t, 3 H, J = 7.4 Hz, CH3).
Selected HPLC and GC Data
Ester 1a
Chiralcel OD, 25 cm, 99:1 hexane–isopropanol, 1 mL/min, R: 6.5
min, S: 7.1 min.
13C NMR (CDCl3): = 175.2 (C=O), 57.0 (CHCl), 36.7 (CH2), 19.3
(CH2), 13.3 (CH3).
Acid 3
MS (CI): m/z = 136.9 (72%, M+).
Chiralcel OD, 25 cm, 99:1 hexane–isopropanol–formic acid,
240:10:1, 1 mL/min, R: 13.97 min, S: 15.08 min.
Methyl 2-Chloropentanoate (2)
Ester 2 and acid 4 were more easily analysed by chiral GC.
Concd HCl (3 drops) were added to a stirred solution of 4 (1.0g,
8.33 mmol) in MeOH (10 mL) at r.t. After 16 h, the reaction was
poured into aq sat. NaHCO3 solution (10 mL) and extracted with
EtOAc (3 10 mL). The combined extracts were dried (MgSO4)
Ester 2
Gammadex-120, 30 m, 110°C, 15.90 min and 16.21 min.
Synthesis 2001, No. 6, 943–946 ISSN 0039-7881 © Thieme Stuttgart · New York