Alkyne‐Based Syntheses of Carbo‐ and Heterohelicenes

[n]Helicenes are polycyclic aromatic molecules in which n ortho-condensed benzene or other aromatic rings give rise to helical chiral and highly conjugated structures. Unique screw shape, helically extended π-system and intriguing electronic and chirooptical properties of these molecules have attracted scientific interest for decades. The applications of helicenes have now expanded into several fields including material science, nanoscience, biochemistry, supramolecular and polymer chemistry. Despite the great theoretical and practical interest in helicenes, their synthesis remains a challenge. Among synthetic approaches to helicenes, alkyne cyclizations have great potential since carbon-rich high-energy alkynes can form aromatic rings via cycloisomerizations. These processes are atom-economic. In addition, alkyne reactions are highly favorable thermodynamically. This review is focused on the alkyne-based syntheses of carbo- and heterohelicenes (2000-2022). These syntheses are obviously multistage. In the review, they are systematized in terms of the key step leading to the formation of a helical shape. The methods are grouped into two categories: 1) metal-free syntheses and 2) transition metal-catalyzed syntheses. This division is very arbitrary as far as catalytic Sonogashira, Suzuki-Miyaura reactions and others are often used to prepare starting materials in the syntheses of the first group. The largest number of publications on the topic is related to the transition metal-catalyzed syntheses of helicenes.