Biomedical Engineering Reference
In-Depth Information
OH
Me
Me
Me
Me
Me
Cl
Me
Me
Me
122
116
(3 equiv.), Oxone
®
, Bu
4
NHSO
4
DME, MeCN, H
2
O, pH = 10.5, 0°C, 1.5 h
OH
Me
Me
Me
O
O
Me
Me
Cl
O
O
O
Me
Me
Me
123
21%
(2 steps)
CSA, acetone
rt, 1 h
Me
Me
OH
Cl
O
O
O
O
O
Me
Me
H
Me H
Me H
H
Me
H
Me
H
124
Me
OH
Me
O
O
O
O
Me
H
Me H
H
Me
H
Me
H
Me
HO
Me
Omaezakianol (
121
)
SCHEME 6.25
Total synthesis of omaezakianol via asymmetric multi epoxidation.
Isolated from the red alga
Laurencia omaezakiana
[45], omaezakianol
121
is
believed to be biosynthesized from squalene by cascade cyclizations [46]
(Scheme 6.25).
Corey and coworkers achieved a three-step synthesis of
121
from racemic
chlorohydrin
122
. Hence, exposure of pentaene
122
to Shi's epoxidation condition
afforded pentaepoxide
(Scheme 6.25). The acid-promoted biomimetic epoxide-
opening cascade reaction successfully proceeded to furnish pentacyclic compound
124
123
[47]. The stereochemistry and the enantiomeric excess of the asymmetric
epoxidation and cyclization were confirmed by comparing the NMR spectra and
the optical rotation with those reported for the natural product.
6.3.2.
-Aminoxylation
Optically active
a
-hydroxy-aldehydes and -ketones are important intermediates in
organic synthesis. The MacMillan, Zhong, C
a
ordova, and Hayashi groups indepen-
dently demonstrated that the proline-catalyzed
a
-aminoxylation of aldehydes and
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