Synthesis and Application of Tetrahydro-2H-fluorenes by a Pd(0)-Catalyzed Benzylic C(sp³)-H Functionalization

Abstract

A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp³)-H functionalization. Importantly, the success of the cyclization step was dependent on there being substituents at the two positions ortho to the benzylic group to avoid an undesired C(sp²)-H functionalization. This method was subsequently used to prepare the right-hand fragment of the hexacyclic triterpenoid benzohopanes, and therefore represents a powerful tool for the construction of the related compounds. A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp³)-H alkenylation of an enol triflate bearing substituents at its two ortho positions (see scheme). This method was used to prepare the right-hand fragment of benzohopanes.