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13 A reviewer proposed a plausible alternative
mechanism for the tandem asymmetric formation of γ-lactams 3 from the intermediate C.
He suggests that a fragmentation of N-O bond (in the bicyclic
intermediate C) could occur to give a nitrogen
cation, stabilized by resonance into the phenyl group, prior to
migration.
17
General Procedure
(Using Catalyst I)
To a CHCl3 (2.7 mL)
solution of the prochiral cyclobutanone 1 (3
mmol) and l-proline (0.18 mmol) was added
a CHCl3 (0.9 mL) solution of nitrosobenzene (0.6 mmol)
over 48 h at 0 ˚C via syringe pump, and the mixture
was stirred for 96 h at that temperature. The crude reaction mixture
was directly loaded on silica gel column without workup, and pure products
were obtained by flash column chromatography (silica gel, hexane-Et2O).
General Procedure (Using Catalyst IV)
In
a glass vial equipped with a magnetic stirring bar, to 0.375 mmol
of the prochiral cyclobutanone 1, catalyst IV (0.075 mmol, 20 mol%) was added,
and the reaction mixture was stirred at ambient temperature for
10-15 min. To the reaction mixture nitrosobenzene (1.13
mmol) was added and stirred at 0 ˚C for the time
indicated in Tables
[¹]
and
[²]
. The crude reaction mixture
was directly loaded on silica gel column without workup, and pure
products were obtained by flash column chromatography (silica gel,
mixture of hexane-Et2O).
4-(4-Chlorophenyl)-5-hydroxy-1-phenylpyrrolidin-2-one
(3a)
Yield 60%; yellow
oil. IR (film): ν = 3400, 1650 cm-¹. ¹H NMR
(300 MHz, CDCl3): δ = 2.68 (dd, 1 H, J = 4.5, 14.1 Hz),
2.93 (t, 1 H, J = 14.4
Hz), 3.26-3.33 (m, 1 H), 5.48 (d, 1 H, J = 5.4
Hz), 7.18-7.72 (m, 9 H). ¹³C
NMR (75 MHz, CDCl3): δ = 36.9, 47.86,
103.2, 120.0, 125.6, 128.6, 128.7, 129.2, 133.6, 137.6, 139.3, 169.8.
MS: m/z (%) = 269
(100) [M+ - 18],
240 (80), 206 (17), 136 (23), 104 (72). The ee was determined to
be 58% ee by chiral-phase HPLC using a Daicel Chiralcel
OJ column (hexane-i-PrOH = 80:20,
flow rate 1.2 mL/min, λ = 254
nm): t
R(major) = 12.1
min; t
R(minor) = 14.4
min.
5-Hydroxy-1,4-diphenylpyrrolidin-2-one
(3b/3b′)
Spectral data refer to
a 95:5 inseparable mixture of two trans-
and cis-diastereomers. Yield 40%;
yellow oil. IR (film): ν = 3400, 1650 cm-¹. ¹H
NMR (300 MHz, CDCl3):
δ = 2.58-2.66
(m, 1 H), 2.74-2.81 (m, 1 H), 2.90 (t, 1 H, J = 14.1 Hz),
3.05 (t, 1 H, J = 13.8
Hz), 3.14-3.20 (m, 1 H), 3.23-3.30 (m, 1 H), 4.58
(br s, 1 H), 4.92 (t, 1 H), 5.48 (d, 1 H, J = 5.4
Hz), 5.55 (dd, 1 H, J = 7.05,
9.6 Hz), 6.74-7.73 (m, 20 H). ¹³C
NMR (75 MHz, CDCl3): δ = 37.1, 38.6,
48.5, 48.8, 92.4, 103.5, 114.6, 118.8, 120.1, 124.9, 125.5, 127.0, 127.2,
127.6, 127.8, 128.5, 128.6, 129.0, 129.1, 129.3, 139.3, 139.5, 170.3,
170.8. MS: m/z (%,
the same for the two diastereomers) = 235 (100) [M+ - 18],
206 (100), 115 (20), 104 (48), 77 (57), 63 (7), 51 (16). The ee
was determined to be 20% ee for the trans-diastereomer
by chiral-phase HPLC using a Daicel Chiralcel OJ column (hexane-i-PrOH = 90:10, flow
rate 1.2 mL/min, λ = 254
nm): t
R(major) =
30.8
min; t
R(minor) = 36.6
min.
5-Hydroxy-4-phenethyl-1-phenylpyrrolidin-2-one (3e)
Spectral data worked out
from the 94:6 inseparable mixture of two trans-
and cis-diastereomers 3e/3e′. Yield 65%; orange
oil. IR (film): ν = 3350, 1660 cm-¹. ¹H
NMR (300 MHz, CDCl3): δ = 1.77-2.14
(m, 3 H), 2.36-2.57 (m, 2 H), 2.71 (t, 2 H, J = 7.5 Hz),
3.41 (br s, 1 H), 5.29 (d, 1 H, J = 5.1 Hz),
7.13-7.69 (m, 10 H). ¹³C NMR
(75 MHz, CDCl3): δ = 32.9, 36.0, 42.2,
52.3, 102.7, 119.9, 126.1, 128.2, 128.4, 128.5, 128.6, 139.6, 140.9,
170.4. MS: m/z (%,
the same for the two diastereomers) = 263 (31) [M+ - 18],
172 (100), 106 (14), 91 (60), 77 (27), 65 (10), 51 (8). The ee of
the trans-diastereomer was determined
to be 51% ee by chiral-phase HPLC using a Daicel Chiralcel
OJ column (hexane-i-PrOH = 85:15,
flow rate 1.0 mL/min, λ = 254
nm): t
R(major) = 26.1
min; t
R(minor) = 33.6
min.
4-Hexyl-5-hydroxy-1-phenylpyrrolidin-2-one
(3f/3f′)
Spectral data refer to
a 67:33 inseparable mixture of two trans-
and cis-diastereomers. Yield 60%;
orange oil. IR (film): ν = 3400, 1660 cm-¹. ¹H
NMR (300 MHz, CDCl3):
δ = 0.84-1.59
(m, 26 H), 1.92-2.06 (m, 2 H), 2.27-2.63 (m, 4
H), 4.73 (d, 1 H, J = 9.3
Hz), 5.17 (d, 1 H, J = 4.8
Hz), 5.22 (dd, 1 H, J = 6.9,
9.3 Hz), 6.76-7.72 (m, 10 H). ¹³C
NMR (75 MHz, CDCl3): δ = 14.0, 22.5,
26.6, 26.8, 29.1, 31.6, 33.0, 33.8, 36.1, 37.3, 42.7, 43.3, 91.4,
102.8, 114.5, 118.6, 119.9, 125.2, 128.5, 129.3, 139.7, 144.1, 170.7,
171.0. MS: m/z (%, the
same for the two diastereomers) = 243 (77) [M+ - 18], 172
(100), 158 (26), 130 (14), 104 (24), 77 (33). The ee was determined
to be 38% ee for the trans-diastereomer
and 44% ee for the cis-diastereomer
by chiral-phase HPLC using a Daicel Chiralcel OJ column (hexane-i-PrOH = 95:5,
flow rate 0.8 mL/min, λ = 254
nm): trans-diastereomer: t
R(minor) = 14.3
min(minor); t
R(major) = 17.4
min; cis-diastereomer: t
R(major) = 22.2
min; t
R(minor) = 26.9
min.
Procedure for the Synthesis of
3-Hexyl-1-phenyl-pyrrolidine-2,5-dione (5)
PCC
(85.1 mg, 0.395 mmol) was added to a solution of compounds 3f/3f′ (dr = 67:33;
70 mg, 0.270 mmol) in CH2Cl2 (8 mL), the mixture
was then stirred at r.t. for 2 h. The reaction mixture was filtered
through a Celite pad, concentrated to give the crude mixture, which
was then purified by flash column chromatography (hexane-Et2O = 3:1)
on silica gel to give the pure pyrrolidine-2,5-dione 5. Yield
60%; yellow oil. IR (film): ν = 1774,1701,
1443, 1376 cm-¹. ¹H
NMR (300 MHz, CDCl3): δ = 0.83-1.45
(m, 10 H), 1.58-1.68 (m, 2 H), 1.95-2.03 (m, 1
H), 2.50-2.61 (m, 1 H), 2.91-3.05 (m, 2 H). ¹³C
NMR (75 MHz, CDCl3): δ = 14.0, 22.5,
26.6, 28.9, 29.6, 31.51, 31.54, 34.5, 40.0, 126.4, 128.5, 129.1,
131.9, 175.6, 178.9. MS: m/z (%) = 259
(10) [M+], 188 (35), 175 (100),
147 (10), 119 (30), 93 (16), 77 (7), 55 (14). The ee was determined
to be 40% ee by chiral-phase HPLC using a Daicel Chiralcel
OJ column (hexane-i-PrOH = 95:5,
flow rate 1.2 mL/min, λ = 254
nm): t
R(major) = 41.8
min; t
R(minor) = 44.8
min.