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General Procedure
for ‘Nonclassical’ Wittig Reaction - Method
B
To a mixture of lactone 9a (2.19
g, 7.78 mmol) in dry toluene (120 mL) were added subsequently the
catalyst 9a˙HCl (0.124 g, 0.39
mmol) and the ylide 10 (4.608 g, 13.23 mmol).
The reaction mixture was refluxed under argon atmosphere for 1 h.
The solvent was then evaporated under reduced pressure, and the
residue was purified by chroma-tography on silica gel column (hexane-EtOAc = 3:1)
to provide the constrained tetramic acid 7a (2.33
g, 7.63 mmol, 93%) as a white solid; mp 111-113 ˚C
(Et2O-heptane), [α]D
²0 205.7
(c 0.32, CHCl3). IR (KBr): νmax = 3105,
2983 (CH), 1668 (C=O), 1644 (C=C), 1185 (COC)
cm-¹. ¹H NMR (300
MHz, CDCl3): δ = 7.26-7.41
(m, 10 H, ArH), 5.55-5.65 (m, 1 H, H-1′, 1 H,
H-2), 5.06 (s, 1 H, H-6), 4.02 (ddd, 1 H, J = 1.0,
6.6, 11.7 Hz, H-3a), 2.66 (ddd, 1 H, J = 4.4,
6.5, 10.9 Hz, H-3A), 1.93 (q, 1 H, J = 22.9
Hz, H-3B), 1.56 (d, 3 H, J = 7.3
Hz, H-2′). ¹³C NMR (75 MHz,
CDCl3): δ = 178.7 (C-6a),
175.6 (C-5), 140.9, 137.7, 129.2, 128.9, 128.8, 127.6, 127.1, 126.2
(ArC), 91.4 (C-6), 90.8 (C-2), 59.6 (C-1′), 48.7 (C-3a),
41.3 (C-3), 18.7 (C-2′). Anal. Calcd for C20H19NO2 (305.14):
C, 78.66; H, 6.27; N, 4.59. Found: C, 80.00; H, 6.51; N, 4.66.
Method C
To a mixture of lactone 9a (0.391 g, 1.39 mmol) in dry toluene
(15 mL) were added subsequently the additive 11 (0.718
g, 1.67 mmol) and Et3N (0.141 g, 1.39 mmol). The reaction
mixture was refluxed under argon atmosphere for 2 h. The white precipitate
deposited was filtered off and washed with toluene. Filtrates were
then evaporated under reduced pressure, and the residue was purified
by chroma-tography on silica gel column (hexane-EtOAc = 3:1)
to provide the constrained tetramic acid 7a (0.401
g, 1.31 mmol, 94%) as a white solid.
<A NAME="RB06411ST-29">29</A>
General Procedure
for the Reduction of Tetramic Acids 7a-h into Corresponding
Compounds 12a-h
To the bicyclic tetramate 7a (2.33 g, 7.63 mmol) dissolved in EtOAc
(190 mL) was added catalyst (0.47 g, 10 mol% Pd/C),
and the resultant suspension was then vigorously stirred under an
hydrogen atmosphere for 2 h. After the reaction was complete, the
catalyst was filtered off and the product purified by flash chromatography
on silica gel column (hexane-EtOAc = 4:1)
to provide the sat. bicyclic lactam 12a (1.82
g, 77%); mp 52-54 ˚C (Et2O-heptane); [α]D
²0
-69.3
(c 0.23, CHCl3). IR (KBr): νmax = 3064,
2929 (CH), 1661 (C=O), 1056 (COC) cm-¹. ¹H
NMR (300 MHz, CDCl3): δ = 7.15-7.32
(m, 10 H, ArH), 5.53 (q, 1 H, J = 7.3 Hz,
H-1′), 4.74 (dd, 1 H, J = 5.2,
10.5 Hz, H-2), 4.50 (td, 1 H, J = 5.5,
7.5 Hz, H-7), 3.86 (dd, 1 H, J = 7.4,
14.2 Hz, H-3a), 2.78 (d, 2 H, J = 5.4
Hz, H-6), 2.51 (ddd, 1 H, J = 5.3, 7.5,
12.6 Hz, H-3A), 1.84 (ddd, 1 H, J = 6.6,
10.6, 12.4 Hz, H-3B), 1.53 (d, 3 H, J = 7.3
Hz, H-2′). ¹³C NMR (75 MHz, CDCl3): δ = 172.6
(C-5), 140.0, 139.4, 128.7, 128.6, 128.0, 127.9, 127.5, 125.8 (ArC),
81.7 (C-2), 74.9 (C-7), 61.2 (C-1′), 50.4 (C-3a), 43.8
(C-6), 38.3 (C-3), 17.7 (C-2′). Anal. Calcd for C20H21NO2 (307.16):
C, 78.15; H, 6.89; N, 4.56. Found: C, 78.23; H, 7.04; N, 4.54.
<A NAME="RB06411ST-30">30</A>
Niwa H.
Okamoto O.
Miyachi Y.
Uosaki Y.
Yamada K.
J.
Org. Chem.
1987,
52:
2941
<A NAME="RB06411ST-31">31</A>
General Procedure
for the Reduction of Bicycles 7a-h into Corresponding Tetramic
Acids 13a-h
To the bicyclic tetramate 7a (1.013 g, 3.3 mmol) dissolved in MeOH
(50 mL) was added catalyst (0.203 g, 10 mol% Pd/C),
and the resultant suspension was then vigorously stirred under an
hydrogen atmosphere for 4 h. After the reaction was complete, the
catalyst was filtered off and the product purified by flash chromatography
on silica gel column (hexane-EtOAc = 4:1)
to provide the 5-arylalkyltetramic acid 13a (0.648
g, 64%) as colorless oil; [α]D
²0 11
(c 0.42, CHCl3). ¹H
NMR (300 MHz, CDCl3): δ = 7.01-7.35
(m, 10 H, ArH), 5.70 (q, 1 H, J = 7.3
Hz, H-1′′), 3.58 (dd, 1 H, J = 2.9,
6.7 Hz, H-5), 3.05 (s, 1 H, H-3), 2.68 (ddd, 2 H, J = 4.1,
11.4, 13.7 Hz, H-2′A), 2.38 (ddd, 1 H, J = 6.6,
11.3, 13.4 Hz, H-2′B), 2.13 (dddd, 2 H, J = 3.1,
6.6, 11.2, 14.3 Hz, H-1′A), 1.80-1.92 (m, 1 H,
H-1′B), 1.73 (d, 3 H, J = 7.3
Hz, H-2′′). ¹³C NMR
(75 MHz, CDCl3): δ = 207.0
(C-4), 169.2 (C-2), 140.0, 138.0, 128.9, 128.6, 128.3, 128.2, 127.6,
126.4 (ArC); 65.6 (C-5), 50.9 (C-1′′), 41.8 (C-3),
33.3 (C-2′), 29.5 (C-1′), 18.4 (C-2′′).
<A NAME="RB06411ST-32">32</A> For a recent paper stating on this
equilibrium, see:
Storgaard M.
Dörwald FZ.
Peschke B.
Tanner D.
J. Org. Chem.
2009,
74:
5032 ; and the references cited therein
<A NAME="RB06411ST-33">33</A>
General Procedure
for the Debenzylation Process
A solution of 12a (1.591 g, 5.16 mmol) in dry THF (30
mL) was added to liquid NH3 (40 mL) at -78 ˚C.
Small pieces of Na (15-20 equiv) were added until the reaction
mixture remained blue, and the mixture was stirred at -78 ˚C.
After 30 min of the reaction, the solution was then quenched with aq
NH4Cl. The residual ammonia was evaporated, and the mixture
was extracted three times with Et2O. The combined organic
layers were dried over Na2SO4, filtered, and evaporated
in vacuo. The residue was purified by column chromatography on silica
gel column (MeOH-EtOAc = 1:10) to provide
ultimately (4S,5S)-4-hydroxy-5-phenyl-ethylpyrrolidin-2-one
(8a) in yield 80% (0.85 g); mp
104-106 ˚C (Et2O); [α]D
²0 -18.4
(c 0.114, MeOH). ¹H
NMR (300 MHz, CDCl3): δ = 7.14-7.30
(m, 5 H, H-Ar), 4.25 (t,
1 H, J = 4.3
Hz, H-4), 3.52 (dd, 1 H, J = 7.0,
11.7 Hz, H-5), 2.67 (t, 2 H, J = 7.2
Hz, H-2′), 2.53 (dd, 1 H, J = 5.8,
17.2 Hz, H-3A), 2.28 (dd, 1 H, J = 1.4,
17.2 Hz, H-3B), 1.91-2.05 (m, 1 H, H-1′A), 1.80-1.90
(m, 1 H, H-1′B). ¹³C NMR (75 MHz,
CDCl3): δ = 176.8
(C-2), 141.1, 128.5, 128.3, 126.1 (ArC), 68.6 (C-4), 59.2 (C-5),
41.0 (C-3), 32.3 (C-2′), 30.4 (C-1′).
<A NAME="RB06411ST-34">34</A>
Synthesis of Products
14
By using same procedure such as from the substrate 12g as starting material, the expected
(2R,3aS,7S)-2-tert-butyl-hexahydrofuro[3,2b]-pyrrol-5-one (14g)
was isolated in 73% yield; mp 145-147 ˚C
(Et2O); [α]D
²0 13.5
(c 0.1, MeOH). ¹H
NMR (300 MHz, CDCl3): δ = 4.74
(t, 1 H, J = 5.6
Hz, H-7), 4.31 (t, 1 H, J = 5.5
Hz, H-3a), 3.78 (dd, 1 H, J = 4.8,
10.9 Hz, H-2), 2.40-2.68 (m, 2 H, H-6), 1.86 (dd, 1 H, J = 4.8, 13.2
Hz, H-3A), 1.40-1.49 (m, 1 H, H-3B), 0.88 (s, 9 H, H-2′′). ¹³C
NMR (75 MHz, CDCl3): δ = 177.7
(C-5), 85.5 (C-2), 59.6 (C-7), 51.1 (C-3a), 38.9 (C-6), 34.1 (C-3), 32.9
(C-1′′), 25.9 (C-2′′).