Traceless Directing Groups in Radical Cascades: From Oligoalkynes to Fused Helicenes without Tethered Initiators

We
report the first example of a traceless directing group in a
radical cascade. The chemo- and regioselectivity of the initial attack
in skipped oligoalkynes is controlled by propargyl OR moiety. Radical
translocations lead to the boomerang return of the radical center
to the site of initial attack where it assists the elimination of
the directing functionality via β-scission in the last step
of the cascade. The Bu3Sn moiety continues further via
facile reactions with electrophiles as well as Stille and Suzuki cross-coupling
reactions. This selective radical transformation opens a new approach
for the controlled transformation of skipped oligoalkynes into polycyclic
ribbons of tunable dimensions.