Biomedical Engineering Reference
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H
MeO
MeO
H
O
10
AuCl, PPTS
MeOH, rt
O
O
OPMB
13
O
OPMB
H
[Au]
O
OTBDPS
O
OTBDPS
H
75%
H
64
65
H+
H
H
O
O
10
10
13
O
13
O
O
O
O
O
OTBDPS
OTBDPS
H
H
OPMB
OPMB
HOMe
67
66
H
O
B
H
A
H
O
OH
O
C
D
O
B
A
O
OH
O
O
C
D
H
O
O
OTBDPS
NH
E
H
H
HO
I
OPMB
O
O
68
H
G
O
O
H
F
Azaspiracid
69
SCHEME 4.21
Synthesis of the trioxadispiroketal-containing ring system of azaspiracid by
Forsyth and coworkers.
The mechanism presented in Scheme 4.23 has been proposed to explain the
difference in reactivity, starting from a common intermediate
. While the presence
of the hydroxyl functionality at C21 can favor the formation of allene
74
by
elimination of AuOH [17b], a benzoate group at C21 can favor the presence of a
75
PMBO
OH
OH
S
O
21
RO
S
OH
O
70
O
O
AuCl (5-10 mol%)
PPTS
O
OH
OH
HO
21
OH
OBz
21
+
O
O
O
O
OPMB
OPMB
73
(-)-Ushikulide A
S
S
S
S
71
72
MeOH, 50°C, 12 h
63%
Trace
THF, 50°C, 22 h
24%
63%
SCHEME 4.22
Synthesis of (
)-ushikulide A by Trost and coworkers.
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