Chemoenzymatic routes
1345
organic layer was dried on MgSO4 and concentrated under
reduced pressure. The N-protected amino acid (L) was
obtained after recrystallization in a mixture of diethyl
ether/hexane. (8a): Yield: 41% (891 mg) as white. Rf: 0.6
(20%MeOH/CH2Cl2). HPLC Rt: 8.9 min (gradient of ACN
J = 7.1 Hz).13C NMR (300 MHz, Acetone-d6) d (ppm)
14.5(2 CH3), 22.5(CH3), 41(CH2), 62.6(2 CH2), 67(C),
122.6(CH), 125.1(CH), 137(CH), 149.5(CH), 157.6(C),
168.5(2C=O), 169.7(C=O). MS (ESI) m/z 309 (M ? H)?.
1
in water-0.1% TFA (v/v), 15 min). H NMR (300 MHz,
Diethyl 2-acetamido-2-(methylpyridin-3-yl)malonate (12b)
DMSO-d6) d (ppm) 3.25(m, 2H), 4.05(m, 1H), 4.15
(m, 2H), 4.45 (m, 1H), 7.15(m, 5H), 7.3(m, 2H), 7.55(d, 1H,
J = 7.45 Hz), 7.75(d, 2H, J = 7.35 Hz), 8.35(m, 2H). MS
(ESI) m/z 389 (M ? H)?. [a]D = ?21, c = 1 in MeOH;
ee[ 98%. (9a): Yield: 33% (743 mg) as pale yellow solid.
Mp = 41–43°C. Rf: 0.85 (20%MeOH/CH2Cl2). HPLC Rt:
9.33 min (gradient of ACN in water-0.1% TFA (v/v),
15 min). [a]D = -36, c = 1 in DMF;7, 6 ee[ 98%. See 7a
for NMR analysis.
The same procedure as for 12a was used to synthesize 12b
with 3-bromomethyl pyridine (2.58 g) in place of 2-bro-
momethyl pyridine. Yield: 70% as transparent crystals.
Mp = 94–95°C. Rf: 0.75 (AcOEt). HPLC Rt: 7 min (gra-
dient of ACN in water-0.1% TFA (v/v), 15 min). 1H NMR
(300 MHz, Acetone-d6) d (ppm) 1.25(t, 6H, J = 7,1 Hz),
2.05(s, 3H), 3.6(s, 2H), 4.25(q, 4H, J = 7.1 Hz), 7.3
(m, 1H), 7.4(s, 1H), 7.5(d, 1H, J = 7.1 Hz), 8.3(s, 1H),
8.45(d, 1H, J = 7,1 Hz).13C NMR (300 MHz, Acetone-d6)
d (ppm) 14(2CH3), 22.5(CH3), 36(CH2), 63(2CH2), 68(C),
124(CH), 132.5(C), 138.5(CH), 149(CH), 152(CH),
168(2C=O), 170.05(C=O). MS (ESI) m/z 309 (M ? H)?.
Enzymatic resolution of 7b by a-CT (8b, 9b)
The same procedure as for 8a and 9a was also used to
synthesize 8b and 9b with 7b (2.25 g, 5.6 mmol) in place
of 7a. (8b): Yield: 40% (869 mg) as white powder. Rf: 0.58
(20%MeOH/CH2Cl2). HPLC Rt: 8.8 min (gradient of ACN
Diethyl 2-acetamido-2-(methylpyridin-4-yl) malonate (12c)
1
in water-0.1% TFA (v/v), 15 min). H NMR (200 MHz,
The same procedure as for 12a was used to synthesize 12c
with 4-bromomethyl pyridine (2.58 g) in place of 2-bro-
momethyl pyridine. Yield: 60% as transparent crystals.
Mp = 118–121°C. Rf: 0.5 (AcOEt/cyclohexane 1/1).
HPLC Rt: 6.8 min (gradient of ACN in water-0.1% TFA
MeOH-d4) d (ppm) 3.25(m, 2H), 4.15(m, 1H), 4.3(m, 2H),
4.6(m, 1H), 7.3(t, 2H, J = 7.45 Hz), 7.4(t, 2H, J = 7.25 Hz),
7.55(t, 2H, J = 7.4 Hz), 7.75(d, 2H, J = 7.35 Hz), 8 (m, 2H),
8.75(m, 2H). MS (ESI) m/z 389 (M ? H)?. [a]D = -31 in
DMF; ee[ 98%. (9b): Yield: 46% (1,035 mg) as white
powder. Mp = 53–55°C. HPLC Rt: 9.2 min (gradient of
ACN in water-0.1% TFA (v/v), 15 min).[a]D = ?16 in
MeOH; ee[ 98%. See 7b for NMR analysis.
1
(v/v), 15 min). H NMR (300 MHz, Acetone-d6) d (ppm)
1.25(t, 6H, J = 7.1 Hz), 2.05(s, 3H), 3.6(s, 2H), 4.25
(q, 4H, J = 7.1 Hz), 7.05(d, 2H, J = 7.1 Hz), 7.4(s broad,
1H, NH), 8.45(d, 2H, J = 7,1 Hz).13C NMR (300 MHz,
Acetone-d6)
d (ppm) 14(2CH3); 23(CH3), 38(CH2),
63(2CH2), 68(C), 126(2CH), 145(C), 150(2CH),
168(2C=O), 170.05(C=O). MS (ESI) m/z 309 (M ? H)?.
Diethyl 2-acetamido-2-(methylpyridin-2-yl)malonate (12a)
To the stirred solution in ACN (100 mL) of diethyl ace-
tamidomalonate (2.17 g, 10 mmol, 1 equiv) and KHMDS
(5.98 g, 30 mmol, 3 equiv) in cooled ethyl alcohol bath
(-40°C), 2-bromomethyl pyridine (2.58 g, 1.5 equiv)
diluted in ACN (5 mL) was slowly added (30 min) with a
syringe pump. The temperature of the stirred mixture raised
until room temperature overnight. The solvent was evap-
orated under reduced pressure. The residue was diluted in
ethyl acetate (100 mL), washed with saturated solution
of ammonium chloride (80 mL), dried on MgSO4 and
concentrated under reduced pressure. 12a was purified
by chromatography on silicagel column with AcOEt/
cyclohexane 1/1 mixture as eluent.
Ethyl (D, L)-2-acetamido-3-(pyridin-2-yl)propanoate (13a)
13a was prepared according to the procedure of Cooper
et al. 1996. Yield: 50% as white powder. Mp = 72–74°C.
HPLC Rt: 3.2 min (isocratic conditions ACN-water (9:1)
1
containing 0.1% TFA (v/v)). H NMR (200 MHz, CDCl3)
d (ppm) 1.15(t, 3H, J = 7.1 Hz), 2.0 (s, 3H), 3.35(m, 2H),
4.15(q, 2H, J = 7.1 Hz), 5(m, 1H), 7.15(m, 3H), 7.65
(m, 1H), 8.55(d, 1H, J = 7.1 Hz). MS (ESI) m/z 237
(M ? H)?.
Ethyl (D, L)-2-acetamido-3-(pyridin-3-yl)propanoate (13b)
Yield: 70% as transparent crystals. Mp = 82.4–86°C.
Rf: 0.45 (AcOEt/cyclohex 1/1). HPLC Rt: 6.5 min (gradient
of ACN in water-0.1% TFA (v/v), 15 min). 1H NMR
(200 MHz, Acetone-d6) d (ppm) 1.3(t, 6H, J = 7.1 Hz),
1.95(s, 3H), 3.75(s, 2H), 4.20(q, 4H, J = 7.1 Hz), 7.05
(d, 1H, J = 7.1 Hz), 7.15(m, 2H), 7.7(m, 1H), 8.45(d, 1H,
The same procedure as for 13a was used to make 13b with
12b in place of 12a. Yield: 60% as white powder.
Mp = 112–115°C. HPLC Rt: 2.9 min (isocratic conditions
1
ACN-water (9:1) containing 0.1% TFA (v/v)). H NMR
(200 MHz, CDCl3) d (ppm) 1,25(t, 3H, J = 7.1 Hz),
123