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P. Ferraboschi et al. / Tetrahedron: Asymmetry 21 (2010) 2136–2141
monitoring the reaction progress by HPLC (see Table 3). The sam-
ples for HPLC analyses were prepared by the extraction of an
amount of reaction mixture, at pH 3, evaporation of the solvents
and dilution of the residue with methanol. At 40% conversion
(14 h), the reaction mixture was extracted with TBME
(3 ꢀ 80 mL); the aqueous phase was filtered by suction to remove
the enzyme and after acidification to pH 3, extracted with TBME
(4 ꢀ 60 mL). The organic phase was treated with sodium sulfate
and, after filtration, evaporated under reduced pressure, to afford
(S)-acid 5 (0.32 g, 34%). A sample, after removal of the protecting
group to give the salt of compound 2, was analyzed by HPLC (see
Table 3) showing 98% ee.
washed with water (10 mL) and 15% sodium chloride aqueous
solution (15 mL). After drying over sodium sulfate and filtration,
the solvent was removed under reduced pressure. Crude residue
9 was purified by silica gel column chromatography (1/10). Elution
with hexane/ethyl acetate 9:1 afforded pure 9 (2.38 g, 70%). Ee 98%
(by HPLC, see Table 3). 1H NMR (CDCl3) d 2.21 (m, 1H, NH), 2.82 (m,
1H, CH–N), 2.96 (m, 1H, CH–N), 3.07 (t, 2H, CH2-thienyl, J 7 Hz),
3.72 (s, 3H, CH3O), 4.98 (br s, 1H, CH–COOCH3), 6.85 (d, 1H, H-30,
J 3.4 Hz), 6.94 (m, 1H, H-40), 7.15 (d, 1H, H-50, J 5 Hz), 7.23–7.32
(m, 2H, Ar), 7.35–7.43 (m, 2H, Ar). MS (ESI+) m/z 310 (M+1 with
35Cl), 312 (M+1 with 37Cl).
4.9. Clopidogrel 1
4.5. N-Boc-(S)-2-chlorophenylglycine methylester 6
To a solution of 9 (1 g, 3.23 mmol) in 1,2-dichloroethane
(20 mL) paraformaldehyde (0.11 g, 3.67 mmol) was added; the
mixture was kept under stirring, at reflux, removing water under
azeotropic conditions for 4 h. After cooling to 30 °C, a solution
(10 mL) of hydrochloric acid in dimethylformamide (4 g of HCl in
100 mL of DMF) was added dropwise. The mixture was heated at
reflux (2 h) while monitoring the reaction progress by TLC (tolu-
ene/ethyl acetate 95:5, detection with 5% phosphomolybdic acid
ethanol solution). After cooling at room temperature, water
(200 mL) was added and the pH was adjusted to 7.5 by means of
30% aqueous potassium carbonate solution. Extraction with ethyl
acetate (3 ꢀ 100 mL), followed by washing with water (100 mL),
drying over sodium sulfate, filtration and removal of the solvent
at reduced pressure gave a residue (0.67 g) that was purified by sil-
ica gel column chromatography (1/10, hexane/ethyl acetate 98:2 as
eluant) affording pure 1 (0.52 g, 50%) as an oil. 1H NMR (CDCl3) d
2.87–2.96 (m, 4H, H-6 and H-7), 3.68 (d, 1H, H-4, J 4 Hz), 3.75 (s,
3H, CH3O), 3.80 (d, 1H, H-4, J 4 Hz), 4.97 (br s, 1H, H-8), 6.68 (d,
1H, H-3, J 5 Hz), 7.08 (d, 1H, H-2, J 5 Hz), 7.24–7.36 (m, 2H, H-12
and H-13), 7.44 (d, 1H, H-14, J 7.7 Hz), 7.74 (d, 1H, H-11, J
7.7 Hz). MS (ESI+) m/z 322 (M+1 with 35Cl), 324 (M+1 with 37Cl).
Esterification of (S)-acid 5 (0.6 g, 2.10 mmol) was performed fol-
lowing the procedure previously described for (RS)-5. The crude
(S)-methylester 6 (0.6 g, 95%) was used in the next step without
any further purification. A sample was purified for analytical pur-
poses. [a] [a]D = +117.1.
D = +119.3 (c 1, CHCl3) lit.21
4.6. (S)-2-Chlorophenylglycine methylester 3
To a solution of (S)-6 (0.5 g, 1.67 mmol) in dichloromethane
(10 mL), under stirring at room temperature, a solution of TFA
(0.64 mL, 8.35 mmol) in dichloromethane (10 mL) was added drop-
wise. The reaction progress was monitored by TLC until the disap-
pearance of the starting material (3 h). The solvent was removed at
reduced pressure. To the residue (98% ee by HPLC) dissolved in
water (10 mL), 20% ammonium hydroxide was added until pH 7.
Extraction with dichloromethane (3 ꢀ 20 mL), followed by treat-
ment with sodium sulfate, filtration and evaporation of the solvent
afforded the title compound 3 [0.315 g, 90% from (S)-5, 98% ee].
[a] [a]
D = +123 (c 1, CH3OH) lit.30 D = +134. 1H NMR (CDCl3) d 2.3
(m, 2H, NH2), 3.75 (s, 3H, CH3O), 5.06 (br s, 1H, CH), 7.25–7.33
(m, 2H, Ar), 7.36–7.43 (m, 2H, Ar).
[
a
]
D = +45 (c 1, CH3OH). Ee 98% (by HPLC). For analytical purposes
a sample was transformed into the corresponding hydrogen sulfate
D = +54.8 (c 1, CH3OH) lit.31
D = +55. DSC (5 °C/min) endo-
H 75.769 J/g).
4.7. 2-(2-Thienyl)ethyl 1-p-tolylsulfonate 8
[a
]
[a]
thermic peak of fusion at 178.50 °C (
D
p-Toluenesulfonyl chloride (4.55 g, 23.87 mmol) and triethyl-
amine (3.36 mL, 24.1 mmol) were added to a solution of 2-(2-thie-
nyl)-ethanol 7 (3 g, 23.4 mmol) in diisopropylether (23 mL) and
kept under stirring at room temperature until starting material dis-
appearance (50 h, TLC toluene/ethyl acetate 9:1, detection with a
5% phosphomolybdic acid ethanol solution). The organic phase
was washed with water (15 mL), 30% potassium carbonate aque-
ous solution (10 mL) and water (2 ꢀ 10 mL) and, then, dried over
sodium sulfate, filtered, and evaporated under reduced pressure
to afford tosylate 8 (5.85 g, 86%). 1H NMR (CDCl3) d 2.47 (s, 3H,
CH3Ar), 3.20 (t, 2H, CH2CH2O, J 7 Hz), 4.25, (t, 2H, CH2O, J 7 Hz),
6.82 (d, 1H, H-30, J 3.4 Hz), 6.92 (m, 1H, H-40), 7.16 (d, 1H, H-50, J
5 Hz), 7.34 (d, 2H, Ar, J 8 Hz), 7.78 (d, 2H, Ar, J 8 Hz).
Acknowledgments
This work was financially supported by Università degli Studi di
Milano. We thank Professor Fiamma Ronchetti for helpful
discussions.
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