Nucleophilic Carbenes as Organocatalysts
169
Table 2 Reaction of cinnamaldehyde and derivatives with activated ketones a
Entry
Ar
R
5
Yield (%)
l/u b
1 c
Ph
CF 3
a
84
66:34
2 c,d
Ph
CF 3
a
84
64:36
3 e
Ph
CF 3
a
92
68:32
4 c 4-(MeO)C 6 H 4
CF 3
b
92
66:34
5 c 4-(Me 2 N)C 6 H 4
CF 3
c
74
70:30
6 f
Ph
C(O)Me
d
55
58:42
7
P h
C O 2 Me
e
78
50:50
8 c 4-(MeO)C 6 H 4
CO 2 Me
f
94
47:53
9 4-(Me 2 N)C 6 H 4
CO 2 Me
g
98
44:56
a General reaction conditions: IMes·HCl (0.05 mmol), DBU (0.05 mmol),
THF (2.5 ml), cinnamaldehyde derivative (0.5 mmol), ketone (1.0 mmol),
16 h at rt. Yield given for the isolated mixture of diastereomers
b Determined by GC-MS
c Reaction conditions: IMes·HCl (0.05 mmol), KOtBu (0.1 mmol), THF
(3 ml); cinnamaldehyde derivative (1 mmol), ketone (2.0 mmol), 16 h at rt
d 30-mmol scale
e 10-mmol scale
f Run at 60 ◦ C
5
Conjugate Umpolung of Crotonaldehyde Derivatives
Crotonaldehyde derivates, aliphatically substituted α,β-unsaturated aldehydes were also successfully used in the NHC-catalyzed lactone formation (Scheme 11). Good yields up to 90% and good stereoselectivities
up to 93:7 were obtained in these transformations. In these cases, DBU
was found to give better results than KOtBu.