Synthesis of Functionalized Phenanthrenes via Regioselective Oxidative Radical Cyclization

The majority of Sn-mediated cyclizations
are reductive and, thus,
cannot give a fully conjugated product. This is a limitation in the
application of Sn-mediated radical cascades for the preparation of
fully conjugated molecules. In this work, we report an oxidatively
terminated Bu3Sn-mediated cyclization of an alkyne where
AIBN, the commonly used initiator, takes on a new function as an oxidative
agent. Sn-mediated radical transformation of biphenyl aryl acetylenes
into functionalized phenanthrenyl stannanes can be initiated via two
potentially equilibrating vinyl radicals, one of which can be trapped
by the fast 6-endoclosure at the biphenyl moiety in good to excellent
yields. The efficient preparation of Sn-substituted phenanthrenes
opens access to convenient building blocks for the construction of
larger polyaromatics.