652
H. Iding et al. / Tetrahedron 60 (2004) 647–653
98.9% area. ISP-MS: 345.3 (MþNaþ), 323.3 (MþHþ). H
NMR (DMSO): 0.87 (t, J¼8 Hz, 3H, CH3), 1.31 (m, 2H,
CH2), 1.52 (m, 2H, CH2), 2.43–2.59 (m, ,2H, CH2), 4.03
(m, 2H, CH2O), 4.40 (m, 1H, CH), 5.03 (m, 2H, CH2O),
6.93 (bs, 1H, CONH2), 7.29–7.39 (m, 6H, Ph and CONH2),
7.60 (d, J,12 Hz, 1H, CONH).
0.7 mL/min, room temperature, 220 nm). [a]D¼216.5
(c¼1.0; DMSO). HPLC: 99% area. ISN-MS: 235.2
1
1
(M2H2). H NMR (DMSO): 2.57–2.72 (m, 2H, –CH2–),
4.72 (m, 1H, –CH–), 6.94 (bs, 1H, CONH2), 7.39 (bs, 1H,
CONH2), 7.46–7.85 (m, 5H, Ph), 8.64 (d, J¼8 Hz, 1H,
–CONH–), 12.6 (s, 1H, COOH).
4.4.5. N-Benzoyl-D-asparagine ethyl ester 2b. A suspen-
sion of 3.5 g (13.2 mmol) ethyl ester rac. 2b in 55 mL 0.1 M
sodium chloride solution, 5 mL 0.1 M sodium phosphate
buffer pH 7.0 and 8 mL THF was vigorously stirred. The pH
was adjusted to 6.5 with 1.0 N hydrochloric acid and the
reaction started by addition of 350 ml Alcalase 2.4 L The pH
was maintained at 6.5 under vigorous stirring by the
controlled addition (pH-stat) of 1.0 N sodium hydroxide
solution. After a consumption of 6.44 mL 1.0 N sodium
hydroxide solution (49% conversion; after 23 h) the reaction
mixture was extracted with 3£50 mL dichloromethane and
3£50 mL ethyl acetate. The combined organic phases were
dried on anhydrous sodium sulfate, evaporated at 35 8C and
the residue dried on HV to give 1.63 g of the ethyl ester 2b
as a white solid (yield: 47%). Enantiomeric excess: 98.2%
(Chiralpak-AD, 25 cm£4.6 mm, 75% n-heptaneþ25%
EtOHþ0.2% TFA, 1 mL/min, room temperature, 220 nm).
[a]D¼þ12.0 (c¼1.1; DMSO). HPLC: .99% area. ISP-MS:
4.4.7. (R)-(1-Benzyl-2,5-dioxo-pyrrolidin-3-yl-carbamic
acid benzyl ester 5. A suspension of 1.12 g of 60% NaH
in 75 mL of THF is treated with 7.50 g of methyl ester 1a
(99.9% (R)-isomer) over 5 min at room temperature. After
20 min, 3.57 mL of benzyl bromide was added followed by
120 mL of DMF. After 3 h, HPLC indicated conversion was
complete. The reaction was quenched with 150 mL H2O and
extracted three times with 120 mL of toluene. The organic
layer was washed with H2O, dried over MgSO4, filtered and
the filtrate evaporated to dryness. The residue was triturated
in 100 mL of TBME, the resultant suspension filtered and
dried (35 8C/10 mbar) to give 8.13 g (90%) of the pure
benzyl imide 5 as white crystals. Analytics: enantiomeric
excess: 93% (Chiracel ODH, 15 cm£300 mm, 85%
n-hexaneþ15% isopropanol, 6 ml/min, 30 8C, 210 nm).
SFC: 100% area. Mp 143.3–144.5 8C (lit. 140.5 8C).11
SFC: 100% area. ISP-MS: 339.2 (MþHþ). IR (Nujol): (NH)
1
3321, (NCO) 1773, 1706, (CO2Bu) 1685 cm21. H NMR
1
287.1 (MþNaþ), 265.3 (MþHþ). H NMR (DMSO): 1.17
(CDCl3, 400 MHz): 2.77 (dd, J¼18, 4 Hz, 1H, COCH2),
3.05 (dd, J¼18, 9 Hz, 1H, COCH2), 4.27 (m, 1H, NCH),
4.70 and 4.65 (2£ d, J¼12 Hz, 2H, NCH2), 5.08 (bs, 2H,
OCH2), 5.23 (bd, 1H, NH), 7.37–7.25 (m, 10H, Ph).
Optically pure material could be obtained from crystal-
lization from CH2Cl2/n-hexane, 72% recovery. Analytics:
enantiomeric excess 99.9%. Mp 145.9–146.7 8C. Anal.
Calcd for C19H18N2O4: C, 67.45; H, 5.36; N, 8.28. Found:
C, 67.25; H, 5.27; N, 8.29.
(t, J¼8 Hz, 3H, CH3), 2.57–2.72 (m, 2H, CH2), 4.19 (q,
J¼8 Hz, 2H, CH2O), 4.75 (m, 1H, CH), 6.95 (bs, 1H,
CONH2), 7.40 (bs, 1H, CONH2), 7.46–7.85 (m, 5H, Ph),
8.75 (d, J¼8 Hz, 1H, CONH).
4.4.6. N-benzoyl-D-asparagine benzyl ester 2d and
N-benzoyl-L-asparagine 4.
A suspension of 2.50 g
(7.66 mmol) benzyl ester rac. 2d in 115 mL 0.1 M sodium
chloride solution, 10 mL 0.1 M sodium phosphate buffer pH
7.0 and 20 mL acetone was vigorously stirred. The pH was
adjusted to 6.5 with 1.0 N hydrochloric acid and the reaction
started by addition of 250 ml Alcalase 2.4 L. The pH was
maintained at 6.5 under vigorous stirring by the controlled
addition (pH-static) of 1.0 N sodium hydroxide solution.
After a consumption of 3.479 mL 1.0 N sodium hydroxide
solution (46% conversion; after 17.9 h) the reaction mixture
was extracted with 2£125 mL dichloromethane. The
combined organic phases were dried on anhydrous sodium
sulfate, evaporated (at 35 8C bath temperature) and the
residue triturated in 20 mL TBME overnight. The solid was
filtered off and dried on HV to give 1.10 g of the benzyl ester
2d as a white solid (yield: 44%). Enantiomeric excess .99%
(Chiracel-ODH, 15 cm£2.1 mm, 87% n-heptaneþ13%
iPrOHþ0.1% TFA, 0.1 mL/min, room temperature,
220 nm). [a]D¼þ13.2 (c¼1.2; DMSO). HPLC: .99%
4.4.8. (R)-3-Amino-1-benzyl-pyrrolidine-2,5-dione dia-
cetic acid salt 6. A solution of 7.80 g of the benzyl imide
5 (93% (R)-isomer) in 160 mL of acetic acid was treated
with 0.78 g of 10% Pd/C (Degussa 1835) and hydrogenated
at 30 8C for 20 min whereupon TLC and HPLC indicated
the reaction was complete. The reaction mixture was
filtered, evaporated and the residue crystallized from
EtOAc and n-hexane to give 5.80 g (78%) of the pure
amine acetate salt 6 as white crystals. Analytics: enantio-
meric excess: 91% as trifluoroacetamide (GC (BGB-177):
15 m£0.25 mm, carrier gas: He; program: 150 8C-200 8C at
1 8C /min; injector temp. 210 8C; FID: 220 8C). HPLC 100%
area. GC 99.8% area (as free amine). ISP-M.S: 205.2
(MþHþ). 1H NMR (CDCl3, 1.6 equiv. AcOH) 2.08 (s,
2£CH3CO2, 6H), 2.50 (dd, J¼18, 5.4 Hz, COCH2, 1H),
3.05 (dd, J¼7.8, 18 Hz, COCH2, 1H), 3.92 (dd, J¼5.4,
7.8 Hz, 1H, NCH), 5.64 (bs, 4H, NH), 4.65 (s, 2H, PhCH2,
7.32 (m, 5H, Ph). MA (1.6 equiv. AcOH): Calcd for
C11H12N2O2. 1.6 equiv. C2H4O2: C, 56.79; H, 6.18; N,
9.33. Found: C, 56.66; H, 6.33; N, 9.23.
1
area. ISP-MS: 349.5 (MþNaþ), 327.3 (MþHþ). H NMR
(DMSO): 2.61–2.78 (m, 2H, CH2), 4.84 (m, 1H, CH), 5.14
(s, 2H, OCH2), 6.97 (bs, 1H, CONH2), 7.32–7.84 (m, 11H,
2£Ph and CONH2), 8.83 (d, J¼8 Hz, 1H, CONH).
The aqueous phase was acidified to pH 2 with 25%
hydrochloric acid. The formed precipitate was stirred at
1 8C overnight and filtered off. The filter cake was washed
with 10 mL 10 mM hydrochloric acid and dried on HV to
give 0.77 g of 4 as a white powder (yield: 43%).
4.4.9. (R)-1-Benzyl-pyrrolidin-3-ylamine 7. A solution of
10.87 g of the amine salt 6 in 100 mL of H2O was treated
with 100 mL of CH2Cl2 followed by 67.60 mL of 1 N
NaOH at room temperature to pH 8.0. After saturation with
NaCl, the mixture was extracted seven times with 100 mL
of CH2Cl2, dried over MgSO4 and evaporated at 35 8C/
10 mbar to give 6.32 g (97%) of the free base as a pale
Enantiomeric
excess:
.99%
(Chiralpak-AD,
25 cm£4.6 mm, 85% n-heptaneþ15% EtOHþ0.12% TFA,