Abstract
The HBF4 (0.1 equiv)-catalyzed Mannich-type reactions of
ketene silyl acetals with aldimines proceeded smoothly in water in
the coexistence of as low as 1 mol% of SDS. Furthermore,
the Mannich-type reaction also took place in water in the absence
of SDS by means of 0.3 equiv of HBF4 to afford the corresponding β-amino
esters in high yields.
Key words
Brønsted acid - surfactant - Mannich-type
reactions - Schiff bases - water
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General Experimental
Procedure for Protocol A (entry 3 of Table 1). Aq solution
of HBF4 (8 µL, 0.0080 mmol, 1.3598 mol/L)
was added to a mixture of N -benzylidene-p -anisidine(1 )
(17 mg, 0.0805 mmol), ketene silyl acetal(2a ) (48 µL,
0.241 mmol), SDS (8 µL, 0.0080 mmol, 0.10027 mol/L
solution) in H2 O (0.5 mL) at r.t. After being stirred
at the temperature for 0.5 h, the reaction was quenched by addition
of sat. NaHCO3 and CH2 Cl2 . The
aq layer was extracted with CH2 Cl2 and the
combined organic layers were washed with brine, dried over anhyd
Na2 SO4 , and concentrated to dryness. Purification
of the crude mixture by preparative TLC (SiO2 , hexane:ethyl
acetate = 10:1, v/v) gave 3a in 94% yield.
13 Because H3 PO4 also
worked equally as a Brønsted acid for the present Mannich-type
reaction, presence of the fluoride ion, which might be generated
from HBF4 , is not essential for the Mannich-type reaction.
Authors are grateful to a referee for pointing out the issue.
14
General Experimental
Procedure for Protocol B (entry 5 of Table 1). Aq solution
of HBF4 (48 µL, 0.0480 mmol, 1.0 mol/L)
was added to a mixture of N -benzylidene-p -anisidine(1 )
(33.6 mg, 0.159 mmol), ketene silyl acetal (2a ) (99 µL,
0.477 mmol) in H2 O (1.0 mL) at r.t. After being stirred
at the temperature for 0.5 h, the reaction was quenched by addition
of sat. NaHCO3 and CH2 Cl2 . The
aq layer was extracted with CH2 Cl2 and the
combined organic layers were washed with brine, dried over anhyd
Na2 SO4 , and concentrated to dryness. Purification
of the crude mixture by preparative TLC (SiO2 , hexane:ethyl acetate = 10:1,
v/v) gave 3a in 81% yield.
15
Manabe K.
Mori Y.
Wakabayashi T.
Nagayama S.
Kobayashi S.
J.
Am. Chem. Soc.
2000,
122:
7202