7-oxanorbornanone 181. In the absence of water and at higher temperature, 180 has
the time to be rearranged into the more stable aminoalkyloxybenzyl cation
intermediate 182 that reacts with water to give the cis-exo-5,6-disubstituted
7-oxanorbornanone 183. Saponification of camphanate 183 furnishes ketone 184
and (1S)-camphanic acid (recovery of the chiral auxiliary). Stereoselective exo face
reduction of the ketone with NaBH 4 , followed by benzoylation, provides endobenzoate (À)-185. Its reaction with HBr in AcOH produces bromide (À)-186. Baseinduced elimination of HBr and subsequent hydrolysis of the oxazoline and acetate
moieties liberates (À)-conduramine C [236].
Lewis acids can induce heterolytic cleavage of the ethereal bridge of
7-oxanorbornanes as exemplified with the conversion of 187 into 188 [237], and
of 189 into 190 [238] that imply intermolecular quenching of the nucleophile, and
for the intramolecular displacement of the ethereal bridge of 191 by its
N-benzylamido moiety to give the bicyclic lactam 192 with high stereoselectivity
(S N i displacement) [239] (Scheme 30).
5.2 Base-Induced Ethereal Bridge Opening of 7-Oxabicyclo
[2.2.1]heptanes
When a carbanionic center is engendered β to an ethereal bond, β-elimination
follows readily (E 1cb type elimination). In the case of 7-oxanorbornane2-carboxylic esters, of 7-oxanorborn-2-yl alkyl ketones, of 7-oxanorborn-2-yl sulfones, and of 7-oxanorborna-2-ones, their conjugate bases do not undergo quick
β-elimination for stereoelectronic reasons (small overlap between the HOMO of the
carbanionic center and the LUMO of the adjacent ethereal moiety because of the
OMe
SO 2 Ph
BnO
BnO
OH
HO
OBn
O H
OMe
OMe
O
E
E
O
OMe
OMe
H
OH
Cl
H
O
OMe
E
O
H
OH
BCl 3
BCl 3
O
BnO
SO 2 Ph
PhO 2 S
O
OH
HO
O
NHBn
OH
OH
OMe
BnO
BnO
BnO
O
SO 2 Ph
187
E = COOMe
1) TiCl 4
189
190
+
2) H 2 O
191
192
Me 3 SiOTf/Et 3 N
CH 2 Cl 2 , 23
o
C
78 %
188
52%
38%
NBn
HO
H
O
62%
Scheme 30 Heterolytic cleavage of the ethereal bridge of 7-oxanorbornanes
170
A.J. Moreno-Vargas and P. Vogel