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 beta-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.