Synthesis of carbohelicenes and derivatives by "carbenoid couplings"

Short synthetic sequences to carbohelicenes have been achieved under thermal 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 optimized, quick and efficient reactions occur at 0 degrees C within 10 min, and often provide [5]-helicene, [5] helicene derivatives and [7]-helicene in similar to 75% yield. Preliminary data questioned the formation of carbenoid anions and carbenes in the so called "carbenoid couplings". (C) 1999 Elsevier Science Ltd. All rights reserved.