Chemistry Reference
In-Depth Information
Grignard reaction
24
O
15
Bu
O
O
1
OH
O
OH
HO
24
S
squamocin A
(annonin, rollincicin)
24
R
squamocin D
(asiminacin)
( )
12
BnO
MgBr
1.
2. PCC
128
CHO
O
O
O
O
3. L-selectride
HO
OH
TBSO
124
127
OBn
OBn
( )
13
( )
13
OHC
O
O
O
O
TBSO
OH
OTBS
129
130
OTBS
BrMg
C
6
H
13
OBn
C
6
H
13
131
( )
13
O
O
R
2
R
1
OTBS
OTBS
R
1
= OH, R
2
= H R
2
= OH, R
1
= H
132
133
C
6
H
13
O
squamocin A
squamocin D
( )
11
O
O
R
2
R
1
OTBS
O
OTBS
R
1
= OH, R
2
= H R
2
= OH, R
1
= H
134
135
Scheme 10-24. Total synthesis of squamocin A and D by Emde and Koert.
skeletons (Scheme 10-24). In the second Grignard reaction, two diastereomers 132
and 133 were separated by chromatography. Both of them were used to synthesize
squamocin A and D, respectively. The final procedures in both cases were to intro-
duce the lactone subunit.
For those acetogenins having a C
2
symmetry in the THF region, two directional
synthetic strategies are often adopted, although the efficiency of the desymmetriza-
tion step usually remains a problem. The iodoetherification was used in the synth-
esis of asimicin and trilobacin as the desymmertrization step to differentiate two
chemically equal hydroxyl groups by Ruan and Mootoo
64
(Scheme 10-25). This
step was carried out by using I(coll)
2
ClO
4
as the reagent, affording the THF-
containing intermediate 137. Treatment of mono-THF containing iodide 138 with
acid and base successively and reduction of double bond gave the known compound
139. On the other hand, the iodide 138 was substituted by hydroxide and then con-
verted into mesylate 140. Compounds 139 and 140 were both advanced intermedi-
ates for trilobacin and asimicin in Keinan et al.'s routes, respectively.
65
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