Diels–Alder and Friedel–Crafts-Type Reactions

Abstract

In this chapter, the synthesis of helicenes via Diels–Alder reactions and Friedel–Crafts-type reactions is described. In the first section, the Diels–Alder reactions are discussed, which not only are practical methods for the preparation of symmetric helicenes, but also can be utilized for the large-scale synthesis of helicene derivatives, especially helicene quinones. The major limitation of the methodology is the limited types of dienes and dienophiles. In the second section, Friedel–Crafts-type reaction is introduced, which is one of the first reported and also a useful strategy for the synthesis of helicenes. The synthesis usually needs less than five steps with moderate-to-good yields. Similarly, heterohelicenes including azahelicenes and phosphahelicene can be synthesized by double Friedel–Crafts acylation or phospha-Friedel–Crafts reaction. Two points should be taken into account in Friedel–Crafts-type reaction: (1) polar unsaturated bonds should be well designed and incorporated into the substrates; (2) it is better to introduce the directing or blocking groups to promote the regioselectivity.