Biomedical Engineering Reference
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Me
O
N
Me
Me
H
Me
N
·Cl
2
CHCO
2
H
O
Bn
H
71
(20 mol%)
H
MeO
MeO
HF
B(OH)
2
+
Me
O
EtOAc, 23°C
O
O
Me
84%
69
70
72
(ee = 93%)
Me
Me
73
N
NHTrisyl
86%
t-
BuLi
THF, -78°C
Me
Me
Me
Me
OH
HO
MeO
BBr
3
CH
2
Cl
2
-78°C to 0°C
69%
O
O
Me
Me
Frondosin B
(
68
)
74
SCHEME 6.14
Three-step total synthesis of frondosin B.
6.2.4. Diels-Alder Reaction
The Diels-Alder reaction between a diene and an alkene is one of the most powerful
tools for the construction of a six-membered ring [27]. MacMillan and Northrup
recently reported an asymmetric Diels-Alder reaction between cyclopentadiene
and 4-hexen-3-one catalyzed by 2-(5-methylfuryl)imidazolidinone derivative
75
to
afford the
endo
adduct
in a highly stereoselective fashion (Scheme 6.15) [28].
Hayashi et al., on the other hand, found that
exo
-selective Diels-Alder reaction
proceeded to give
76
78
and
79
with high enantioselectivity by combining diarylpro-
linol silyl ether
and HClO
4
[29].
Isolated from
Strychnos minfiensis
, minfiensine (
77
) has a complex structure
containing a (hydroiminoethano)carbazole skeleton (Scheme 6.16) [30]. MacMillan
and coworkers completed the nine-step total synthesis of
80
starting from commer-
cially available materials and employing a organocatalytic cascade sequence to
construct the tetracyclic pyrroloindoline core. Hence, a Diels-Alder reaction of
2-vinylindole
80
81
and propynal (
82
) catalyzed by secondary amine
83
proceeded to
produce the tricyclic diene
(Scheme 6.16). Subsequent protonation of the enamine
moiety provided the iminium ion
84
, while a 5-
exo
cyclization followed by a NaBH
4
reduction furnished the tetracyclic framework
85
86
in high yield and with excellent
enantioselectivity [31].
The total synthesis of
b
-santalol (
), a sandalwood odorant, was reported
by Fehr et al. via an
exo
selective Diels-Alder reaction between cyclopentadiene (
93
87
)
and crotonaldehyde (
69
) catalyzed by prolinol catalyst
88
(Scheme 6.17). The
exo
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