PARASURAMAN ET AL.
3
40 mol %) at room temperature and the mixture was
stirred at that temperature for 48 h. After the completion
of reaction, the mixture was purified by flash column
chromatography (SiO2, hexane: ethyl acetate = 7:3) to
afford the corresponding major hDA adducts 6a–i. The
diastereoselectivity (dr) of the obtained hDA adducts
782, 700. 1H NMR (500 MHz, CDCl3) δ 7.27–7.30 (m,
2H), 7.19–7.22 (m, 3H), 3.67 (q, J = 4.6 Hz, 1H), 3.56 (dd,
J = 10.0, 6.0 Hz, 1H), 3.31 (s, 2H), 3.12–3.18 (m, 1H), 2.83
(q, J = 6.7 Hz, 1H), 2.66 (dd, J = 13.5, 7.7 Hz, 1H), 1.04–
1.06 (m, 21H). 13C NMR (125 MHz, CDCl3) δ 138.26,
129.38, 128.68, 126.66, 77.39, 77.15, 76.89, 66.24, 54.89,
38.94, 18.06, 11.96. MS (FAB): m/z: 307 [M]+, HRMS
(FAB): calcd. for C18H33N1O1Si1 m/z 307.55; found:
307.64.
1
were determined by the crude reaction mixture by H
NMR. The enantiomeric excess (ee) of 6a–i were
determined by HPLC (CHIRALPAK IB, hexane:
iPrOH = 90:10 or 95:5, 0.6 ml,1.0 ml/min, λ = 245 nm).
2.6 | (S)-1-([1,1,1,1,3,3,3-heptamethyl-
2-{trimethylsilyl}-1-trisilan-2-yl]oxy)
3-phenylpropane-2-amine- propane-
2-amine (2o)
2.3 | General procedure for the synthesis
of catalysts 2a–o
To a solution of the corresponding amino alcohol 1a–o
(6.79 mmol, 1 eq) in dry CH2Cl2, cooled at 0 C or ꢀ30 C
were added corresponding trialkyl silyl chloride or
trifluoromethane sulfonate (1.2 eq) and triethylamine
(1.2 eq) for 10 min under argon atmosphere. The solution
was stirred for 24 h at room temperature. After the
completion of reaction, the reaction mixture was diluted
with water and extracted with dichloromethane. Organic
layer was separated and dried with sodium sulfate.
Solvent was evaporated under reduced pressure, the
obtained crude mixture was purified by flash column
chromatography (SiO2, hexane: ethyl acetate = 7:3 and
MeOH:CH2Cl2 = 9:1) to give the corresponding amino
silyl ether catalysts 2a–o.
Colorless liquid. 88% yield. [α]D24 = ꢀ17.24 (c = 0.58,
CH2Cl2). IR (neat) cmꢀ1: 2947, 2892, 1558, 1540, 1070,
769, 699. 1H NMR (500 MHz, CDCl3) δ 7.27–7.29 (m,
2H), 7.17–7.20 (m, 3H), 3.37 (q, J = 4.6 Hz, 1H), 3.31 (dd,
J = 9.5, 6.6 Hz, 1H), 2.99–3.04 (m, 1H), 2.72 (q,
J = 6.3 Hz, 1H), 2.54 (q, J = 6.9 Hz, 1H), 1.45 (d,
J = 22.3 Hz, 2H), 0.17 (s, 27H). 13C NMR (125 MHz,
CDCl3) δ 139.16, 129.38, 128.53, 126.31, 77.39, 77.15,
76.89, 71.98, 54.82, 40.47, 0.39. MS (FAB): m/z: 397 [M]+,
HRMS (FAB): calcd. for C18H39N1O1Si4 m/z 397.86;
found: 397.91.
3 | RESULTS AND DISCUSSION
2.4 | (S)-1-phenyl-3-([triethylsilyl]oxy)
propane-2-amine (2l)
3.1 | Preparation of catalysts and co-
catalysts
Light yellow oil. 89% yield. [α]D24 = +6.24 (c = 0.48,
CH2Cl2). IR (neat) cmꢀ1: 3027, 2875, 1602, 1495, 1097,
794, 699.1H NMR (500 MHz, CDCl3) δ 7.25–7.30 (m, 2H),
7.20 (td, J = 5.0, 2.1 Hz, 3H), 4.11 (s, 2H), 3.62 (q,
J = 4.6 Hz, 1H), 3.49 (dd, J = 9.7, 6.3 Hz, 1H), 3.21 (qd,
J = 6.8, 4.3 Hz, 1H), 2.81 (dq, J = 46.5, 6.9 Hz, 2H),
0.92–0.99 (m, 9H), 0.56–0.63 (m, 6H). 13C NMR
(125 MHz, CDCl3) δ 138.31, 129.37, 128.61, 126.56, 77.41,
77.17, 76.91, 65.35, 54.62, 38.90, 6.90, 4.42. MS (FAB):
m/z: 265 [M]+, HRMS (FAB): calcd. for C15H27N1O1Si1
m/z 265.47; found: 265.69.
The desired amino silyl ethers 2a–n30,31 having several
trialkylsilyl groups on oxygen atom at α-position
were easily prepared by the reactions of the
corresponding β-amino alcohols 1a–n with XOTf (X:
TMS [trimethylsilyl]),30 XCl (X: TES [triethylsilyl]),31
XOTf (X: TIPS [triisopropylsilyl]),31 and XCl (X:
TBDMS [tert-butyldimethylsilyl]),31 respectively, in
moderate to good yields (46%–90%)(Scheme 2).
Furthermore, the bulkiest β-amino silyl ether catalyst
2o having a super silyl TTMSS (tris[trimethylsilyl]
silyl)31 group on oxygen atom was also easily prepared
from the reaction of the corresponding amino alcohol
with TTMSSCl in good yield (88%). Furthermore,
several N-Cbz32 and N-Boc-amino acids 3a–s as co-
catalysts were also easily derived from the
corresponding commercially available nonprotected
acyclic and sterically bulky cyclic amino acids, respec-
tively (Scheme 2).
2.5 | (S)-1-phenyl-3-([triisopropylsilyl]
oxy)propane-2-amine (2m)
Light yellow oil. 90% yield. [α]D24 = +1.81 (c = 0.55,
CH2Cl2). IR (neat) cmꢀ1: 2941, 2864, 1558, 1105,