An Unusual Fragmentation of Oxetane-Embedded Tetracyclic Ketal Systems

An unusual route for the synthesis
of functionalized cyclobutane
derivatives starting from functionalized norbornane derivatives is
reported. Base-induced fragmentation of an oxetanol-type moiety embedded
in a tetracyclic norbornyl ketal leads to a cyclobutane-fused derivative
as the major or exclusive product. The fragmentation reaction for
bridgehead-bromine-substituted derivatives was much faster than for
the corresponding chlorine-substituted substrates. The functionalized
cyclobutane product was formed exclusively in high yield in the former
case, while the latter furnished a minor uncyclized side product in
varying yields.