Related to the water elimination from cyclohexa-1,4-diols, one can cite the
intramolecular nucleophilic displacement of the 4-hydroxycylohex1-yl tosylate
derived from diol 109 into 7-oxanorbornane 110 (Scheme 14) and the synthesis
of rac-farnesiferol C that uses an intramolecular displacement of a cyclohexa-1,4diol-derived lactone (conversion of 45 into oxanorbornane 46, Scheme 6).
With the total syntheses of (À)-3
0 ,6
0 -epoxyauraptene, (À)-23, Lewis acidpromoted isomerization of alk-3-enyloxiranes into 7-oxanorbornane systems has
been illustrated (Scheme 2). The protic acid-promoted isomerization of
7-oxabicyclo[4.1.0]heptan-3-ol into 7-oxanorbornanol derivatives has been illustrated with the conversion of 42 into 41. The iodoetherification of
4-alkylidenecyclohexanols into 1-(1-iodoalkyl)-7-oxanorbornanes has been exemplified in the total synthesis of glycinoeclepin A by Murai et al. (intermediate 72,
73; Scheme 10), by Mori et al. (intermediate 80; Scheme 11), and by Tanino
et al. (96, 97; Scheme 13). Alternatively, the mercurioetherification of
4-alkylidenecyclohexanols can be used to generate 7-oxanorbornane derivatives
as shown by Corey (conversion of 90 into 91, Scheme 12) in his total synthesis of
glycinoeclepin A.
We can add to these methods the acid-catalyzed cyclization of the natural
germacranolide gallicin that generates, among several products, the
1,4-epoxyeudesmanolide 116 that contains a trans-fused decalin moiety
(Scheme 13). Under the same conditions, the closely related 8α-hydroxygallicin,
117, is cyclized into shonachalin B with a cis-fused decalin system. These reactions
proceed through cationic intermediates 114, 115, and 118 [154] (Scheme 16).
Padwa et al. have applied the 1,3-dipolar cycloaddition of carbonyl ylide 120 to
cylopent-2-en-1-one to generate oxanorbornanone 121 as a 4:1 mixture of exo- and
endo-isomers. The process starts with the reaction of diazoketone 119 with
Rh 2 (OAc) 4 [155, 156] (Scheme 17).
An intramolecular version of this cycloaddition has been developed by the same
authors for the preparation of polycyclic systems 122 [157] and 123 (single
diastereomers) [158] (Scheme 18). The latter compound has been converted into
O
OH
O
O
OH
O
O
OH
O
O
O
O
Me
H
O
OH
O
OH
O
OH
O
OH
H
O
O
O
OH
H
+ H
+
+ H
+
- H
+
- H
+
114
116
117
118
115
Shonachalin B
Gallicin
45%
35%
Scheme 16 Protic acid-promoted cyclizations of germacranolides
160
A.J. Moreno-Vargas and P. Vogel
intramolecular nucleophilic displacement of the 4-hydroxycylohex1-yl tosylate
derived from diol 109 into 7-oxanorbornane 110 (Scheme 14) and the synthesis
of rac-farnesiferol C that uses an intramolecular displacement of a cyclohexa-1,4diol-derived lactone (conversion of 45 into oxanorbornane 46, Scheme 6).
With the total syntheses of (À)-3
0 ,6
0 -epoxyauraptene, (À)-23, Lewis acidpromoted isomerization of alk-3-enyloxiranes into 7-oxanorbornane systems has
been illustrated (Scheme 2). The protic acid-promoted isomerization of
7-oxabicyclo[4.1.0]heptan-3-ol into 7-oxanorbornanol derivatives has been illustrated with the conversion of 42 into 41. The iodoetherification of
4-alkylidenecyclohexanols into 1-(1-iodoalkyl)-7-oxanorbornanes has been exemplified in the total synthesis of glycinoeclepin A by Murai et al. (intermediate 72,
73; Scheme 10), by Mori et al. (intermediate 80; Scheme 11), and by Tanino
et al. (96, 97; Scheme 13). Alternatively, the mercurioetherification of
4-alkylidenecyclohexanols can be used to generate 7-oxanorbornane derivatives
as shown by Corey (conversion of 90 into 91, Scheme 12) in his total synthesis of
glycinoeclepin A.
We can add to these methods the acid-catalyzed cyclization of the natural
germacranolide gallicin that generates, among several products, the
1,4-epoxyeudesmanolide 116 that contains a trans-fused decalin moiety
(Scheme 13). Under the same conditions, the closely related 8α-hydroxygallicin,
117, is cyclized into shonachalin B with a cis-fused decalin system. These reactions
proceed through cationic intermediates 114, 115, and 118 [154] (Scheme 16).
Padwa et al. have applied the 1,3-dipolar cycloaddition of carbonyl ylide 120 to
cylopent-2-en-1-one to generate oxanorbornanone 121 as a 4:1 mixture of exo- and
endo-isomers. The process starts with the reaction of diazoketone 119 with
Rh 2 (OAc) 4 [155, 156] (Scheme 17).
An intramolecular version of this cycloaddition has been developed by the same
authors for the preparation of polycyclic systems 122 [157] and 123 (single
diastereomers) [158] (Scheme 18). The latter compound has been converted into
O
OH
O
O
OH
O
O
OH
O
O
O
O
Me
H
O
OH
O
OH
O
OH
O
OH
H
O
O
O
OH
H
+ H
+
+ H
+
- H
+
- H
+
114
116
117
118
115
Shonachalin B
Gallicin
45%
35%
Scheme 16 Protic acid-promoted cyclizations of germacranolides
160
A.J. Moreno-Vargas and P. Vogel