Bicyclic ketone 29 has been converted into rac-2,5-epoxy-6(E),8(E)megastigmadiene 37 in three steps (Scheme 4) [86]. Compound 37 and its (6Z),
(8E)-stereoisomer 38 have been found (0.02 %) in the extract of Osmanthus
[91]. They can be prepared also by acidic treatment of diol 39, which can be
obtained from alcohol 40 [92].
The 7-oxanorbornanol derivative 41 has been isolated from sun-cured Greek
tobacco [93, 94]. It is formed together with 43 upon acidic treatment of allylic
epoxide 42, a product of degradation of violaxanthin (Scheme 5) [95].
A synthesis of rac-farnesiferol-C, rac-33, is presented in Scheme 6.
Baeyer–Villiger oxidation of ketone 44 resulted in the formation of the
7-oxabicyclo[2.2.1]heptane 46, not in the expected hydroxyl lactone 45. Standard
Barbier–Wieland degradation of 46 (CH 2 N 2 , then PhMgBr followed by acidic
O
OH
O
O
OH
OH
C 4 H 7 MgBr
37
38
HO
O
29
1) PhNCO/Py
2) 225
o
C
40
39
+
36
H 3 O
+ (cat.)
25% (from 40)
ratio 37/38 = 4.7:1
1) Isoprenyl acetate,
PTSA
2) LiAlH 4 , ether
O
O
Scheme 4 Synthesis of rac-2,5-epoxy-6(E,Z ),8(E)-megastigmadiene
Scheme 5 Acid-induced isomerization of a 3,4-epoxycyclohexanol into a 7-oxanorbornanol
O
O
O
O
X
X
O
OH
33, X = H
34, X = OMe
35
Fig. 5 Sesquiterpenoid 7-oxanorbornanes from plants
148
A.J. Moreno-Vargas and P. Vogel