Chemistry Reference
In-Depth Information
X
NO
2
ent
-HBM
3
(5 mol %)
THF
OMe
R
1
OH
55-98% yield
>20:1-14:1
anti, syn
-selecitve
OMe
NO
2
X
+
OMe
R
1
CHO
OMe
X
NO
2
ent
-HBM
4
(10 mol %
)
OMe
R
1
THF
OH
70-87% yield
10:1-5:1
syn, syn
-selecitve
OMe
Nu
Boc
BocHN
NO
2
NH
R
H
H
ent
-HBM
3
OMe
O
R
1
H
R
OH
OMe
O
NHBoc
Felkin-Anh model
Nu
Boc
Boc
N
BocHN
NO
2
H
N
R
H
H
ent
-HBM
4
OMe
O
R
1
O
R
OH
OMe
H
H
H-bond chelation
model
Scheme 3.21.
Brønsted acid not only increases its acidity but also restricts the orientation of proton,
which may allow enantiofacial recognition of one prochiral face of olefi n.
In 2003, Yamamoto et al. described modifi ed LBAs
1a
and
1b
, from optically active
monoalkylated - 1,2 - diarylethane - 1,2 - diol · SnCl
4
, which was shown to be superior to the
original BINOL-based LBA for the enantioselective protonation of silyl enol ethers and
ketene disilyl acetals [38]. Based on the X-ray crystal structure of a related LBA complex
of
1a
, the authors were able to specify the conformational direction of the H-O bond of
LBA, which has some asymmetric inductivity. The stereochemical course in the enanti-
oselective protonation would be controlled by a linear O- H · · · π bonding interaction with
an initial step (Scheme 3.22). Another chiral LBA using TiCl
4
was developed by Nakashima
and Yamamoto and applied to the enantioselective protonation of silyl enol ethers [39].
In 1999, the Yamamoto group reported the enantioselective biomimetic cyclization
of polyprenoids catalyzed by chiral LBA, which is the fi rst example of a proton-induced
enantioselective polyene cyclization in synthetic chemistry [40]. He and his colleagues
developed a new chiral LBA
2
containing a pyrogallol skeleton for enantioselective
polyene cyclizations as an “artifi cial cyclase ” [41] . Various
trans
- fused tricyclic skeletons
have been synthesized with good enantioselectivities in the presence of chiral LBA
2
.
Interestingly, the LBAs derived from pyrogallol derivatives are more effective Brønsted
acids than BINOL derivatives for preparing LBA with SnCl
4
(Scheme 3.23 ).
