Tandem Enyne Allene-Radical Cyclization: Low-Temperature Approaches to Benz[e]indene and Indene Compounds

In an effort to lower the temperatures required to prepare multicyclic compounds using the tandem enediyne-radical cyclization, we have developed the tandem enyne allene-radical cyclization which proceeds at temperatures as low as 37 degrees C. The reactions were carried out using three different methods for the preparation of the enyne allenes. The first method involved the [3,3] sigmatropic rearrangement of an enediyne followed by a tandem enyne allene-radical cyclization. This reaction could be effected either by thermolysis (150 degrees C) or by AgBF(4) rearrangement followed by heating at 75 degrees C. A second technique utilized a [2,3] sigmatropic shift of an enediyne at -78 degrees C followed by tandem cyclization at 37 or 75 degrees C depending on the substrate. The final method involved the base-catalyzed isomerization of propargyl sulfones which yielded enyne allenes that underwent cyclization at 37 degrees C. These three sequences provide a method for the synthesis of ring systems using conditions that may be compatible with the sensitive functionality needed during the synthesis of complex natural products.