Nonasymmetric synthesis

Titanium-based Materials as Catalysts for Nonasymmetric Synthesis... 

Helicenes and helicene-like molecules possessing nonplanar ortho-fused scaffolds exhibit helical chirality. Enantiopure forms of these molecules are potentially applicable to optical or electronic functional materials and chiral reagents [1], Therefore, development of their practical enantioselective synthesis, which can introduce various substituents on their frameworks, is highly attractive. The classical nonasymmetric synthesis of helicenes is based on an oxidative photocyclization of stilbene-type precursors [2]. However, this method suffers from low product yields and low substrate concentrations. Thus, several nonphotochemical synthetic methods have been developed which have been applied to the synthesis of enantioenriched helicenes [3]. Among them, a [2 - - 2 - - 2] cycloaddition strategy is highly attractive because of its convenient operation and its applicability to the catalytic enantioselective synthesis. The synthesis of helicenes and helicene-like molecules by transition-metal-mediated [2 -I- 2 -I- 2] cycloaddition reactions, including enantioselective variants, is summarized in this section. 

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