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8 For a review see: Andruszkiewicz R.
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Borowski E.
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12
Typical Procedure
for the Preparation of 4a and 4b: The appropriate substrate 2a,b (5 mmol),
and DPPA (1.18 mL, 5.5 mmol) were dissolved in t-BuOH
(20 mL). The reaction flask was flushed with argon, and Et3N
(0.75 mL, 5.5 mmol) was added. The reaction mixture was refluxed
for 4 h and cooled to r.t. The solvent was removed under reduced pressure,
the residue dissolved in EtOAc (50 mL). In the case of 4a, the solution was washed with sat. NaHCO3 solution,
water and finally dried over anhyd MgSO4. After evaporation
of the solvent, the solid residue was crystallized from EtOAc and
hexane to yield 4a (0.77 g, 70%).
Mp: 128-129 °C; [α]D +10.0
(c 1, EtOAc). 1H
NMR (200 MHz, CDCl3): δ = 3.66-3,74
(dd, J
4a,4b = 9.5
Hz, J
4a,5a = 5.6
Hz, 1 H CHCH
2
,
4Ha), 3.85-3.95 (dd, J
4b,4a = 9.5
Hz, 1 H, CHCH
2
, 4
Hb), 5.05-5.10 (dd, J
5a,4b = 9.5
Hz, J
5a,4a = 5.6
Hz, 1 H, CHCH2, 5 Ha),
5.28 (s, 2 H, CH
2
C6H5),
5.9 (br s, 1 H, NH), 7.40(s, 5 H, C
6
H
5
).
Anal. Calcd for C11H11NO4: C, 59.73;
H, 5.01; N, 6.33. Found: C, 59.58; H, 5.12; N, 6.38. In the case of 4b, after evaporation of the solvent, the
product was purified by column chromatography (silica gel, hexane-ethyl
acetate 2:3) furnishing 4b (0.58g, 80%)
which was crystallized from ethyl acetate and hexane. Mp: 95-97 °C; [α]D +20.1
(c 1, EtOAc). 1H
NMR (200 MHz, CDCl3): δ = 3.65-3.75(dd, J
4a,4b = 9.6
Hz, J
4a,5a = 5.5
Hz, 1 H, CHCH
2
,
4 Ha), 3.85 (s, 3 H, CH
3
), 3.86-3.96 (dd, J
4a,4b = 9.6, J
4b,5a = 5.5
Hz, 1 H, CHCH
2
,
4 Hb), 5.01-5.09 (dd, J
5a,4b = 9.6
Hz, J
5a,4b = 5.5
Hz, 1 H, CHCH2, 5 Ha),
6.24
(br s, 1 H, NH). Anal.
Calcd for C5H7NO4: C, 41.38; H,
4.86; N, 9.65. Found: C, 41.22; H, 4.74; N, 9.60.
13
Typical Procedure,
4a,b to 5a-5c: Compound 4a or 4b (9 mmol), DMAP (25 mg) and Et3N
(1.5 mL, 11 mmol) were dissolved in dry THF (25 mL) and cooled in
an ice bath. A solution of (Boc)2O (2.05 g, 9.4 mmol)
in THF (8 mL) was added to a stirred reaction mixture over 20 min.
The temperature of the reaction was kept between 5 °C and
10 °C overnight. Then, equimolar amounts of NaHSO4 (1.32
g, 11 mmol) in water (10 mL) were added. After evaporation of the
solvent under reduced pressure, the white suspension was dissolved
in EtOAc (50 mL), and the solution was washed with dilute NaHSO4 solution,
water and finally dried (MgSO4) and evaporated. The solid
was crystallized from diethyl ethyl ether and hexane to give 5a (2.5 g, 85%). Mp: 130-131 °C; [α]D +24.0
(c 1, EtOAc). 1H
NMR (200 MHz, CDCl3): δ = 1.56 [s,
9 H (CH
3
)C],.
4.00-4.07 (dd, J
4a,4b = 10.7, J
4a,5a = 5.5
Hz, 1 H, CHCH
2
,
4 Ha), 4.14-4.24 (dd, J
5a,4b = 9.5
Hz, J
4a,4b = 10.7
Hz, 1 H, CHCH
2
,
4 Hb), 4.91-4.99 (dd, J
5a,4b = 9.5
Hz, J
5a,4a = 5.5
Hz, 1 H, CHCH2), 5.29 (s,
2 H, CH
2
C6H5),
7.40 (s, 5 H, C
6
H
5
).
Anal. Calcd for C16H19NO6: C, 59.18;
H, 5.96; N, 4.36. Found: C, 59.34; H, 5.92; N, 4.22. In the same
manner compound 5c was obtained (1.54 g,
70%). Mp: 76-78 °C; [α]D +36.8
(c 2.5, MeOH). 1H
NMR (200 MHz, CDCl3): δ = 1.55 [s,
9 H (CH
3
)C],
3.87 (s, 3 H, CH3), 4.00-4.08 (dd, J
4a,4b = 10.7, J
4a,5a = 5.5
Hz, 1 H, CHCH
2
,
4Ha), 4.14-4.25 (dd, J
5a,4b = 9.5 Hz, J
4a,4b = 10.7
Hz, 1 H, CHCH
2
,
4 Hb), 4.89-4.97 (dd, J
5a,4b = 9.5
Hz, J
5a,4a = 5.5
Hz, 1 H, CHCH2). Anal. Calcd
for C10H15NO6: C, 48.98; H, 6.17;
N, 5.71. Found: C, 49.17; H, 6.22; N, 5.58.
14
Synthesis of
5b: (5S)-5-Benzyloxycarbonyl-2-oxazol-idinone 4a (1.99 g, 9 mmol) was dissolved in THF
(30 mL) and Et3N (10 mL). The reaction mixture was cooled
to
-20 °C. A solution of benzyl
chloroformate (2.6 mL, 18 mmol) in THF (3.3 mL) was slowly added
over 20 min. The stirred reaction mixture was then kept at -10 °C
overnight. The reaction mixture was quickly neutralized with a stoichiometric
amount of cold 10% aq solution of NaHSO4 and
extracted with EtOAc (2 × 50 mL). The
extracts were washed with water, dried (MgSO4) and evaporated.
The solid residue was dissolved in a small volume of hot EtOAc and
diluted with a large volume of diethyl ether. Crystals were collected
to give 5b (2.4g, 80%). Mp: 112-114 °C; [α]D +24.0
(c 2.5, EtOAc). 1H
NMR (200 MHz, CDCl3):
δ = 4.04-4.12
(dd, J
4a,4b = 10.6, J
4a,5a = 5.5
Hz, 1 H, CHCH
2
, 4Ha),
4.18-4.28 (dd, J
5a,4b = 9.5
Hz, J
4a,4b = 10.6
Hz, 1 H, CHCH
2
,
4 Hb), 4.93-5.01 (dd, J
5a,4b = 9.5
Hz, J
5a,4a = 5.5
Hz, 1 H, CHCH2), 5.27 (s,
2 H, CH
2
C6H5-benzyl
ester), 5.31 (s, 2 H, CH
2
C6H5-benzyloxycarbonyl
group), 7.35-7.45 (br s, 10 H, 2 × C
6
H
5
).
Anal. Calcd for C19H17NO6: C, 64.22;
H, 4.82; N, 3.94. Found: C, 64.34; H, 4.68; N, 3.88.
15a
Representative Experimental Procedure: Compound 5a-5c (5
mmol) was quickly dissolved in dry THF (50 mL) and cooled under
argon to -40 °C. Benzyltrimethyl-ammonium
hydroxide (5.5 mL of 40% solution in methanol, 10.5 mmol)
was added over 20 min to the solution of 5a-5c and stirred for an additional 1 h. HOAc
(1.1 mL) and water (2.5 mL) were then added and the reaction mixture
was warmed to r.t. and the solvent was evaporated under reduced pressure.
The residue was extracted with EtOAc (2 × 50 mL).
The extracts were washed, dried over MgSO4 and evaporated.
The solid residue was crystallized from a mixture of EtOAc and hexane
to yield 6a (0.77 g, 75%). Mp: 93-94 °C; [α]D +6.4
(c 2.5, MeOH). Similarily, 6b was also obtained (0.89 g, 75%).
Mp: 117-119 °C; [α]D +3.2
(c 2.5, MeOH). Physical and spectral
data for 6a and 6b are
in agreement with literature values, see ref.
[15b]
15b
Burger K.
Windeisen E.
Pires R.
J.
Org. Chem.
1995,
60:
7641
16
Preparation
of 7a: Compound 5a (0.64 g, 2 mmol)
was dissolved in a mixture of THF (10 mL) and MeOH (15 mL) and palladium
on activated carbon (5%, 50 mg) was added. The reaction
flask was flushed with hydrogen and the hydrogenation was performed
at atmospheric pressure for 1 h at r.t. The catalyst was filtered
off and the solvents were evaporated. The residue was crystallized
from a mixture of EtOAc and hexane to give 7a (0.43g,
94%). Mp: 99-100 °C; [α]D +28.2
(c 1, MeOH). 1H NMR
(500 MHz, CDCl3): δ = 1.56 [s,
9 H (CH
3
)C],.
4.08-4.11 (dd, J
4a,4b = 10.7, J
4a,5a = 5.4
Hz, 1 H, CHCH
2
,
4 Ha), 4.21-4.25 (dd, J
5a,4b = 9.8 Hz, J
4a,4b = 10.7
Hz, 1 H, CHCH
2
,
4 Hb), 4.95-4.98 (dd, J
5a,4b = 9.8
Hz, J
5a,4a = 5.4
Hz, 1 H, CHCH2). Anal. Calcd
for C9H13NO6: C, 46.75; H, 5.67;
N, 6.06. Found: C, 46.78; H, 5.52; N, 5.98.
