Other Synthetic Methods

Abstract

In this chapter, other methods for the preparation of racemic helicenes are introduced. First, helical skeletons can be constructed by radical cyclizations or cascade radical cyclizations, but the regioselectivity may be a problem for some cases. By oxidative coupling method, BINOL analogues are widely utilized for the preparation of oxahelicenes, while iterative modular synthesis is employed to prepare extended helicene structures and longer helicenes. Some azahelicenes can also be expediently prepared by oxidative homocouplings. Similarly, thiahelicene can be synthesized by DDQ/Lewis acid-mediated Scholl reaction to construct three C–C bonds via a one-pot strategy. Compared with photochemical cyclization, oxidative coupling method has the advantages of short reaction time and simple purification process. Particularly, heterohelicenes including helicenium cations, azahelicenes, thiohelicenes, azabora[6]helicene, and boraoxa[6]helicene can be prepared by microwave reaction of suitable amine or salts, carbenoid coupling, Cu-mediated oxidative coupling, one-pot [4+2]benzannulation strategy, Pictet–Spengler reaction, double electrophilic borylation, and other heteroatom arylation reactions.