pathway involved generation of a diradical, the support for
3
which comes from trapping9b with O2 and also from the
Scheme 1. Various Cyclization Pathways
fact that the solvent polarity does not affect the kinetics of
the reaction.9a In this paper, we provide chemical evidence
to support the accepted mechanism and, in the process,
disprove the involvement of any ionic intermediate by
studying the reactivity of unsymmetrical bispropargyl sul-
fones with donor and acceptor moieties. The method
also demonstrates a strategy for achieving selectivity in
GB rearrangement involving unsymmetrical sulfones. The
involvement of a diradical intermediate and the overall
results are in good agreement with those obtained by
computations.
We would like to address the problem by making
sulfones of the type X equipped with aromatic rings of
a different electronic nature (Scheme 2). Under basic
conditions, X should isomerize to the bisallenic sulfone
A which can then undergo GB cyclization to produce
products represented by F and G. Our notion is that
if ionic intermediates are involved, the product ratio
should be dependent upon the stability of the ions (car-
bocation and carbanion). Thus the ions should be lo-
cated as shown in structure B. The rearrangement then
should involve the electron-deficient aromatic ring to
produce F as the major product. The reactivity of benzyl
cations and anions supports such an argument.12 On the
other hand, for a diradical pathway, the radical R to the
electron-rich aromatic ring will be nucleophilic in char-
acter and hence the major product formed is expected to
involve the electron-rich aromatic ring, as is represented
by structure G.
interaction with external sources. Nicolaou et al.10 first
attempted to utilize the GB cyclization chemistry in bisalle-
nic sulfones. However, the DNA cleavage exhibited by these
molecules mostly involved a Michael addition of DNA-base
followed by Maxam-Gilbert type cleavage11 and not
through H-abstraction.10 It is widely believed that the GB
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