Chemistry Reference
In-Depth Information
BnCO
2
O
O
3 step
s
81
BnCO
2
H
H
ent
-LBA
3
(2 eq)
PrCl
:
CH
2
Cl
2
(1:1)
-80°C, 48 h
74% yield
O
>99% ee
to
(-)-Caparrapi oxide
H
BnCO
2
O
19
H
21% ee
BnCO
2
O
14
BnCO
2
H
O
27% ee
LBA
3
(2 eq)
PrCl
:
CH
2
Cl
2
(1:1)
-80°C, 48 h
H
to
BnCO
2
O
O
3 step
s
73% yield
86
H
H
98% ee
(+)-8-Epicaparrapi oxide
Scheme 3.25.
they demonstrated that chiral LBA catalyzes the asymmetric allylation and crotylation
to various aldehydes to afford products in excellent yields and moderate to good enan-
tioselectivity [46,47] . The LBA
4
was found to exert a strong infl uence on the diastereo-
facial selectivity: High
anti, syn
diastereoselectivity was obtained in crotylation with
matched LBA
4
. Mechanical studies indicated that Lewis acid precludes a well-orga-
nized cyclic transition state due to steric hindrance, whereas a smaller activator such as
proton is allowed to have chair-like transition structure, which might increase enanti-
oselectivity using LBA (Scheme 3.26). Later, a new
C
2
- symmetric BINOL - derived diol
was prepared by Rauniyar and Hall and applied toward LBA for the asymmetric allyl-
boronation of aldehydes [48] .
3.5. CHIRAL PHOSPHORIC ACID CATALYSIS
[49]
Chiral phosphoric acid PA
1a
(Fig. 3.5), derived from simple (
R
) - BINOL, has already
been employed as a chiral resolving reagent [50], and its lanthanide salt was used as a
catalyst for the hetero D-A reaction [51]. However, phosphoric acid itself had not been
used as a chiral catalyst until the research groups of Akiyama et al. [52] and Uraguchi
and Terada [53] independently reported the design of phosphoric acids derived from
