Nucleophilic Additions to Fused Bicyclic Five-Membered Ring Oxocarbenium Ions: Evidence for Preferential Attack on the Inside Face

Evidence is provided that nucleophilic attack on five-membered ring oxocarbenium ions occurs
from the inside face of the envelope. An eight-five fused-bicyclic system in which two substituents are
constrained to pseudoequatorial positions underwent nucleophilic addition with selectivity that was
comparable to an unconstrained monocyclic system. On the other hand, a bicyclic six-five analogue
underwent reaction with low selectivity. This observation indicates that minimization of eclipsing interactions
by attacking inside the envelope is not enough to control selectivity, but that the changes in the overall
three-dimensional structure of the ring must be favorable as well. In the bicyclic six-five series, the six-membered ring is accommodated in the cation, but it destabilizes the transition state structure leading to
the first-formed product of inside attack.