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10 Preparative HPLC conditions: Nucleosil
50-5; 32 × 237 mm; heptane-EtOAc,
25:1; 26 mL/min; t
R
(Z,Z) = 31.5
min, t
R
(E,Z) = 38.0
min; baseline separation with 200 mg/injection.
11
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13 The relative and absolute configuration
was assigned based on the established stereochemical course of the
CAGC. The enantiomeric excess was determined by Mosher ester ¹H NMR
analysis. To synthesize Mosher esters, aldehydes 14 or 18 were reduced to the corresponding primary
alcohols with NaBH4 in methanol, followed by esterification
with Mosher’s acid.¹7
14
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18 α-Keto Ester 6:
To a solution of [Cu{(S,S)-tert-Bu-Box}(H2O)2](SbF6)2 (12; 81 mg, 0.094 mmol, 8 mol%)
in CH2Cl2 (3 mL, 3 mL/mmol 6) was added (Z,Z)-7 (0.5 g,
1.18 mmol, 1 equiv) in CH2Cl2 (3 mL, 3 mL/mmol 6) at r.t. The solution was stirred for
24 h, then the solvent was evaporated and the residue filtered through
a short plug of silica gel (cyclohexane-EtOAc, 1:1). Evaporation
of the solvent and purification by flash chromatography (cyclohexane-EtOAc, 50:1→20:1)
furnished 6 (478 mg, 96%) as a
clear oil. R
f
= 0.34
(cyclohexane-EtOAc, 10:1); ¹H NMR (CDCl3, 400
MHz): δ = 7.62-7.64 (m, 4 H),
7.36-7.45 (m, 6 H), 5.81 (ddd, J = 8.7,
10.3, 17.3 Hz, 1 H), 5.00-5.08 (m, 2 H),
3.80 (s, 3 H), 3.65-3.67 (m, 2 H), 3.56-3.63
(m, 1 H), 2.67-2.73 (m, 1 H), 1.11 (d, J = 6.8 Hz,
3 H), 1.04 (s, 9 H); ¹³C
NMR (CDCl3, 100 MHz): δ = 196.7 (C),
161.8 (C), 136.8 (CH), 135.6 (4 × CH),
133.3 (C), 133.1 (C), 129.7 (2 × CH),
127.6 (4 × CH), 117.5 (CH2),
64.1 (CH2), 52.7 (CH3), 48.3 (CH), 42.6 (CH),
26.7 (3 × CH3), 19.2 (C),
12.5 (CH3); IR (neat): 2930, 2860, 1730, 1430 cm-¹;
Anal. Calcd for C25H32O4Si: C,
70.72; H, 7.60. Found: C, 70.6; H, 7.6; [α]²5
D +38.1
(c 1.0, CHCl3).
α-Keto
Ester 19: To a solution of [Cu{(S,S)-tert-Bu-Box}(H2O)2](SbF6)2 (12, 384 mg, 0.44 mmol, 8 mol%)
in CF3CH2OH (11 mL, 2 mL/mmol 7) was added (Z,Z)-7 (2.36 g,
5.55 mmol, 1 equiv) in CH2Cl2 (17 mL, 3 mL/mmol 7) at r.t. The solution was stirred for
24 h, then the solvent was evaporated and the residue filtered through
a short plug of silica gel (cyclohexane-EtOAc, 1:1). Evaporation
of the solvent and purification by flash chromatography (cyclohexane-EtOAc,
50:1→20:1) furnished 19 (2.24
g, 95%) as a clear oil. R
f
= 0.34
(cyclohexane-EtOAc, 10:1); ¹H NMR (CDCl3,
400 MHz): δ = 7.62-7.64 (m, 4 H),
7.36-7.45 (m, 6 H), 5.54 (ddd, J = 17.1,
10.4, 9.3 Hz, 1 H), 4.98-5.08 (m, 2 H),
3.80 (s, 3 H), 3.57-3.66 (m, 3 H), 2.75-2.83 (m,
1 H), 1.03-1.07 (m, 12 H); ¹³C
NMR (CDCl3, 100 MHz): δ = 196.6 (C),
161.7 (C), 135.5 (4 × CH), 135.0 (CH), 133.2
(C), 133.1 (C), 129.7 (2 × CH), 127.6
(4 × CH), 118.5 (CH2), 65.3
(CH2), 52.7 (CH3), 48.0 (CH), 42.4 (CH), 26.7 (3 × CH3),
19.2 (C), 12.4 (CH3); IR (neat): 2930, 2860, 1730, 1430
cm-¹; Anal. Calcd for C25H32O4Si:
C, 70.72; H, 7.60. Found: C, 70.8; H, 7.5; [α]²5
D +4.3
(c 0.99, CHCl3).
Ketone 5: To a solution of 15 (1.1
g, 2.87 mmol, 1 equiv) in CH2Cl2 (5.7 mL,
2 mL/mmol) was added pyridine (0.91 g, 11.5 mmol, 4.0 equiv)
and Dess-Martin periodinane (1.6 g, 3.8 mmol, 1.3 equiv).
The white suspension was stirred at r.t. for 5 h before being quenched
with sat. Na2S2O3. The phases were
separated, and the aqueous phase was washed with CH2Cl2.
The combined organic layers were dried (MgSO4), concentrated
under reduced pressure and the crude product was purified by flash
chromatography (cyclohexane-EtOAc, 20:1) to yield 5 (0.98 g, 89%) as a colorless
oil. R
f
= 0.27
(cyclohexane-EtOAc, 20:1); ¹H NMR (CDCl3, 400
MHz): δ = 7.64-7.66 (m, 4 H),
7.37-7.45 (m, 6 H), 5.77-5.86 (m, 1 H),
5.00-5.08 (m, 2 H), 3.67 (d, J = 5.2
Hz, 2 H), 2.82 (m, 1 H), 2.43-2.49 (m,
1 H), 2.12 (s, 3 H), 1.04-1.05 (m, 12 H); ¹³C
NMR (CDCl3, 100 MHz): δ = 211.8 (C), 137.7
(4 × CH), 135.6 (CH), 133.4 (2 × C),
129.7 (2 × CH), 127.6 (4 × CH),
116.9 (CH2), 64.3 (CH2), 48.6 (CH), 47.6 (CH),
29.4 (CH3), 26.8 (3 × CH3),
19.3 (C), 13.7 (CH3); IR (neat): 2960, 2858, 1713, 1428
cm-¹; Anal. Calcd for C24H32O2Si:
C, 75.74; H, 8.47. Found: C, 75.6; H, 8.3; [α]²5
D +31.8
(c 0.97, CHCl3).
Acetal 17: To a solution of 22 (106
mg, 0.23 mmol, 1 equiv) in THF (2 mL, 8.6 mL/mmol) and
HMPA (0.3 mL), was added t-BuLi (0.16
mL, 0.26 mmol, 1.1 equiv, 1.6 M in pentane) at -78 ˚C.
The solution was stirred at -78 ˚C for
15 min before a solution of iodoacetaldehyde dimethyl acetal (150
mg, 0.69 mmol, 3.0 equiv) and LiI (109 mg, 0.81 mmol, 3.5 equiv)
in THF (1 mL) was added. After stirring for 30 min, the reaction
mixture was diluted with sat. NaHCO3 and extracted with
CH2Cl2. The combined organic layers were dried
(MgSO4) and concentrated under reduced pressure and the
crude product was purified by flash chromatography (cyclohexane-EtOAc,
50:1→20:1) to afford unreacted 22 (20
mg, 19%) and 17 (77 mg, 61%). R
f
= 0.20
(cyclohexane-EtOAc, 20:1); ¹H NMR (CDCl3,
400 MHz): δ = 7.68-7.70 (m, 4 H),
7.38-7.45 (m, 6 H), 5.69-5.78 (m, 1 H),
5.06-5.17 (m, 2 H), 4.81 (t, J = 4.0
Hz, 1 H), 3.56-3.57 (m, 2 H), 3.30-3.40
(m, 7 H), 2.72-3.03 (m, 3 H), 2.62-2.69
(m, 1 H), 2.51-2.57 (m, 1 H), 2.18-2.22
(m, 1 H), 1.78-1.98 (m, 3 H), 1.05-1.10
(m, 12 H). ¹³C NMR (CDCl3,
100 MHz): δ = 136.7 (CH), 135.6 (4 × CH),
133.6 (C), 133.5 (C), 129.6 (2 × CH), 127.6
(4 × CH), 118.1 (CH2), 102.7
(CH), 66.8 (CH2), 57.2 (C), 53.5 (CH3), 52.5
(CH3), 46.0 (CH), 39.5 (CH2), 35.8 (CH), 26.9
(3 × CH3), 25.8 (CH2),
25.5 (CH2), 24.5 (CH2), 19.3 (C), 9.9 (CH3);
IR (neat): 2930, 2900, 2860, 1430 cm-¹; Anal.
Calcd for C30H44O3S2Si:
C, 66.13; H, 8.14. Found: C, 66.2; H, 8.1; [α]²5
D +21.0
(c 1.01, CHCl3).
