reaction of 2-methylfurane (sylvan) with 2-chloroacrylonitrile catalyzed by ZnI 2
gives a mixture of 7-oxanorbornene cycloadducts that are hydrogenated into
7-oxanorbornanes 27. Their alkaline hydrolysis (KOH/t-BuOH/H 2 O) generates
ketone 28. Double α-dimethylation with MeI and t-BuOK produces 29, the Wittig
methylenation of which gives alkene 30. Hydroboration of 30 followed by oxidative
work-up and subsequent transformation to a mesylate furnishes 31 that is then reacted
with 7-hydroxycoumarin to yield racemic endo-3
0 6
0 -epoxyauraptene, rac-32.
3.3 Sesquiterpenoid 7-Oxabicyclo[2.2.1]heptanes
Creticacumarin 34, an oxidized form of farnesiferol-C 33 [87, 88], has been isolated
from Turkish species of the genus Anthemis [89]. Sesquiterpenol 35 has been found
in Artemisia barrelieri [90] (Fig. 5).
O
OAr
SnCl 4
H
OAr
HO
OAr
HO
O
OAr
OAr
O
O
HO
O
O
O
O
SnCl 4 (cat.)
CH 2 Cl 2
0
o
C
10 min
(-)-23
BF 3 ×Et 2 O
BF 3 ×Et 2 O
22
24
25
26
(cat.)
(cat.)
Ts: 4-MeC 6 H 4 SO 2
KatsukiSharpless
epoxidation
18
19
20 (ee = 95%)
21
72%
60%
1) TsCl/Py (60%)
2) NaI/acetone (88%)
3) NaBH 3 CN (37%)
O
Scheme 2 Total asymmetric synthesis of 3
0 6
0 -epoxyauraptene
O
O
O
O
O
OMs
O
O
O
O
O
O
CN
Cl
O
Cl
CN
+
29
27
28
Ph 3 PCH 2
1) BH 3 ×THF
2) H 2 O 2 /OH
-
3) MsCl/Py
30
31
32
Ms: MeSO 2
1) ZnI 2 , 0
o
C,
14 days
2) H 2 , Pd/C
KOH
t-BuOH
MeI
t-BuOK
61%
83%
53%
22%
45%
45%
Scheme 3 Synthesis of the 2-epimer of racemic 3
0 ,6
0 -epoxyauraptene
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
147