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<A NAME="RG27508ST-16">16</A>
Synthesis of Thioamide
1
Pivalaldehyde (7.82 mL, 72.0 mmol) and (S)-2-isocyano-3-methyl-1-butanol (5.46
g, 48.2 mmol) were added to a solution of Na2S2O3 (11.4
g, 72.0 mmol) and PPTS (18.1 g, 72.0 mmol) in H2O (40
mL) at 0 ˚C. The mixture was allowed to stir at
0 ˚C for further 30 min, before the ice bath was
removed and the solution warmed to r.t. Then, H2O was added,
and the product was extracted three times with CH2Cl2.
The combined organic layers were extracted with aq NaHCO3,
KHSO4, and H2O and dried over Na2SO4.
After evaporation of the solvent, the crude product was purified
by flash chromatography (silica gel, hexanes-EtOAc, 7:3) giving
rise to a colorless solid. The diastereomeric thioamides could be
separated by crystallization from benzene, providing (R,S)-1 (1.59 g, 6.9 mmol, 36%) as colorless
crystals; mp 129-130 ˚C. The S,S-isomer
was obtained (1.78 g, 7.7 mmol, 41%) by a second flash chromatography
(silica gel, hexanes-EtOAc, 8:2) as a colorless oil, which
solidified to a wax. R
f
= 0.47 [(S,S)-1] and 0.53 [(R,S)-1] (Et2O).
Compound
(R,S)-1: [α]D
²0 -57
(c 1.3, CHCl3). ¹H
NMR (500 MHz, DMSO-d
6): δ = 0.89,
0.92 (2 d, J = 6.8
Hz, 6 H), 0.94 (s, 9 H), 2.07 (m, 1 H), 3.49 (ddd, J = 9.9,
4.8, 4.6 Hz, 1 H), 3.58 (ddd, J = 9.9,
5.0, 4.8 Hz, 1 H), 4.03 (d, J = 6.0
Hz,
1 H), 4.35 (dddd, J = 9.0,
7.0, 4.8, 4.6 Hz, 1 H), 4.69 (dd, J = 5.0,
4.8 Hz, 1 H), 5.41 (d, J = 6.0
Hz, 1 H), 9.07 (d, J = 9.0
Hz, 1 H). ¹³C NMR (125 MHz, DMSO-d
6): δ = 18.8, 19.1,
26.6, 27.8, 34.8, 59.5, 60.9, 84.1, 202.5. HRMS (CI): m/z calcd for C11H24NO2S [M + H]+:
234.1528; found: 234.1530. Anal. Calcd for C11H23NO2S
(233.37): C, 56.61; H, 9.93; N, 6.00. Found: C, 56.60; H, 9.70;
N, 5.96.
Compound (S,S)-1: [α]D
²0 -92
(c 1.7, CHCl3). ¹H
NMR (500 MHz, DMSO-d
6): δ = 0.89,
0.91 (2 d, J = 6.9
Hz, 6 H), 0.95 (s, 9 H), 2.07 (m, 1 H), 3.50 (ddd, J = 11.0,
4.9, 4.7 Hz, 1 H), 3.60 (ddd, J = 11.0,
5.0, 4.8 Hz, 1 H), 4.05 (d, J = 5.9
Hz,
1 H), 4.34 (dddd, J = 8.8,
6.9, 4.8, 4.7 Hz, 1 H), 4.68 (dd, J = 5.0,
4.9 Hz, 1 H), 5.42 (d, J = 5.9
Hz, 1 H), 9.13 (d, J = 8.8
Hz, 1 H). ¹³C NMR (125 MHz, DMSO-d
6): δ = 19.0, 26.7,
28.0, 34.9, 59.3, 61.0, 84.2, 202.7. HRMS (CI): m/z calcd
for C11H24NO2S [M + H]+:
234.1528; found: 234.1547. Anal. Calcd for C11H23NO2S
(233.37): C, 56.61; H, 9.93; N, 6.00. Found: C, 56.61; H, 9.70;
N, 5.62.
<A NAME="RG27508ST-17">17</A>
Synthesis of the
Thiazoline Ligand (
S
,
S
)-2
Thioamide (S,S)-1 (8.40 g, 36.0 mmol) and Et3N
(11.1 mL, 79.2 mmol) were dissolved in abs. THF (180 mL). This solution
was cooled to 0 ˚C before MsCl (3.09 mL, 39.6 mmol)
in THF (35 mL) was added dropwise. After the addition was complete,
the ice bath was removed and the mixture allowed to warm to r.t.
The mixture was diluted with Et2O and washed with H2O.
After drying of the organic layer (Na2SO4)
and evaporation of the solvent a colorless solid (7.44 g, 34.4 mmol,
96% yield) was obtained. The crude product was crystallized
twice (hexane) giving colorless crystals (3.02 g, 14.0 mmol, 39% yield);
mp 88-90 ˚C. R
f
= 0.19
(hexane-Et2O, 8:2). [α]D
²0 -57
(c 1.4, CHCl3). ¹H NMR
(500 MHz, CDCl3): δ = 0.96
(d, J = 6.8
Hz, 3 H), 0.99 (s, 9 H), 1.03 (d, J = 6.8
Hz, 3 H), 1.97 (m, 1 H), 3.04 (dd, J = 10.5,
10.1 Hz, 1 H), 3.29 (dd, J = 10.5,
8.7 Hz, 1 H), 3.61 (br s, 1 H), 4.05 (d, J = 4.0
Hz, 1 H), 4.17 (dddd, J = 10.1, 8.7,
6.5, 1.1 Hz, 1 H). ¹³C NMR (125 MHz,
CDCl3): δ = 18.9,
19.6, 25.9, 32.8, 35.2, 35.8, 79.5, 81.6, 172.8. HPLC: column: LiChrosorb
Si 60, hexane-Et2O (90:10), flow: 2.0 mL/min; t
R = 11.80
min. HRMS (CI): m/z calcd for C11H22NOS [M + H]+:
216.1422. Found: 216.1447. Anal. Calcd for C11H21NOS
(215.35): C, 61.35; H, 9.83; N, 6.50. Found: C, 61.17; H, 9.50;
N, 6.36.
Synthesis of the Thiazoline
Ligand (
R
,
S
)-2
Ligand
(R,S)-2 was prepared according to the same procedure
from (R,S)-1 (466
mg, 2.00 mmol). The crude product was purified by flash chromatography
giving rise to colorless crystals (363 mg, 1.69 mmol, 85% yield);
mp 69 ˚C. R
f
= 0.38
(hexane-Et2O, 8:2). [α]D
²0 -71
(c 1.5, CHCl3). ¹H
NMR (500 MHz, CDCl3): δ = 0.97
(d, J = 6.8 Hz,
3 H), 1.005 (s, 9 H), 1.006 (d, J = 6.8
Hz, 3 H), 1.98 (m, 1 H, 5-H), 3.07 (dd, J = 10.9,
9.1 Hz, 1 H), 3.33 (dd, J = 10.9,
9.0 Hz, 1 H), 3.59 (d, J = 4.5
Hz, 1 H), 3.99 (m,
1 H), 4.23 (dddd, J = 9.1,
9.0, 6.4, 1.7 Hz, 1 H). ¹³C NMR (125
MHz, CDCl3): δ = 19.1,
19.4, 26.0, 32.6, 35.4, 35.7, 79.5, 81.5, 171.8. HPLC: column: LiChrosorb
Si 60, hexanes-Et2O (90:10), flow: 2.0 mL/min; t
R = 5.65
min. HRMS (CI): m/z calcd for
C11H22NOS [M + H]+:
216.1422; found: 216.1445. Anal. Calcd for C11H21NOS
(215.35): C, 61.35; H, 9.83; N, 6.50. Found: C, 60.93; H, 9.65;
N, 6.45.
<A NAME="RG27508ST-18">18</A>
General Procedure
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2 Additions towards Aldehydes
A
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mmol) was added to the ligand (S,S)-2 (4.3 mg,
0.02 mmol, 2 mol%) in toluene (2 mL) in a Schlenk tube
under argon. The mixture was stirred at r.t. for 30 min before the
aldehyde (1 mmol) in toluene (1 mL) was added. After 22 h, 1 N HCl
was added. After stirring for 10 min the product was extracted with
Et2O (twice) and the enantiomeric ratio of the crude
product was determined by GC using a chiral cyclodextrin column. Afterwards
the crude product was purified by flash chromatography
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General Procedure
for Arylations of Aldehydes
The pinacol ester of phenylboronic
acid (306 mg, 1.5 mmol) was dissolved in toluene (4 mL) in a Schlenk
tube. A 1 M solution of ZnEt2 in hexane (1.5 mL, 1.5
mmol) was added, and the mixture was heated to 60 ˚C
for 12 h. After cooling to r.t., this solution was added to the
ligand (S,S)-2 (5.4 mg, 0.025 mmol, 5 mol%)
in another Schlenk tube. After stirring for 10 min at r.t., the
aldehyde was added in hexane (1 mL). The reaction was monitored
by TLC. After complete conversion, sat. NH4Cl solution
was added to quench the reaction and the aqueous layer was extracted
with CH2Cl2 (2 ×). The combined organic
layers were dried and evaporated. The product was purified by flash
chromatography, and the enantiomeric ratio was determined by HPLC
(Chiracel OD-H).
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The ee was determined by HPLC using
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