Synthesis of carbohelicenes and derivatives by "carbenoid couplings"

Short synthetic sequences to carbohelicenes have been achieved under therma l conditions, without using photochemistry and high dilution. Couplings of aromatic bis(bromomethyl) moieties, in the presence of an excess of LiHMDS, are key reactions in the final ring closures to carbohelicenes. These opti mized, quick and efficient reactions occur at 0 degrees C within 10 min, an d often provide [5]-helicene, [5] helicene derivatives and [7]-helicene in similar to 75% yield. Preliminary data questioned the formation of carbenoi d anions and carbenes in the so called "carbenoid couplings". (C) 1999 Else vier Science Ltd. All rights reserved.