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Allene Cyclization Chemistry: Application to the Syntheses of Helical Polycyclic Aromatic Hydrocarbons and the ABCD Ring Core of the Camptothecin Family of Alkaloids Weixiang Dai Thermolysis of the benzannulated enyne-allenes provides an efficient pathway to a variety of novel helical polycyclic aromatic hydrocarbons, which are represented by a remarkably beautiful diindenophenanthrene with four congested phenyl substituents. For example, its X-ray structure shows that the four phenyl substituents are essentially parallel to one another but are virtually perpendicular to the helical axis of the diindenophenanthrene ring system. Viewing from the direction perpendicular to the helical axis, the structure resembles that of a segment of a spiral staircase having four parallel steps. The Wagner-Meerwein rearrangement was successfully applied to 11H-benzo[b]fluorene11-methanols and related fluorene derivatives leading to highly conjugated aromatic systems, which are prone to protonation for subsequent intramolecular electrophilic aromatic substitution reactions, leading to polycyclic aromatic hydrocarbons with novel architectures not easily attainable by other synthetic methods. Resolution of fluorenylmethanols derived from 15, 16-diaryl-diindeno[2,1-b:1’,2’-h]phenanthrenes was successfully achieved by using Mosher’s reagent, providing a good opportunity to obtain the pure enantiomers of the corresponding helical polycyclic aromatic hydrocarbons. Thermolysis of the benzannulated enyne-allenes bearing a heteroaromatic substituent at the allenic terminus produced a variety of indeno-fused heteroaromatic ring systems, which are of interest as potential candidates for pharmacological applications. The simplicity of this synthetic method encouraged us to explore the corresponding enallene-isonitrile cyclizations. Thermolysis of the pyridodiynes provides an excellent synthetic pathway to the analogs of the ABCD ring core of the camptothecin family of alkloids. The facile thermal cyclization of the pyridodiynes compared with the corresponding benzodiynes encouraged us to explore the corresponding pyridoannulated nitrile-allene cyclizations. Thermolysis of the pyridoannulated nitrile-allenes provides a very efficient synthetic pathway to the parent and substituted ABCD ring cores of the camptothecin family of alkaloids. This metal-free, base-catalyzed cyclization method may provide insight for new cyclization pathways and could be suitable for large scale synthesis and it represents the first successful allenic nitrile cyclization.

Allene cyclization chemistry: Application to the syntheses of helical polycyclic aromatic hydrocarbons and the ABCD ring core of the camptothecin family of alkaloids