Synthesis of novel polycyclic aromatic compounds via cascade cyclizations of benzannulated enyne -allenes

Abstract

Synthesis of Novel Polycyclic Aromatic Compounds via Cascade Cyclizations of Benzannulated Enyne-Allenes Yanzhong Zhang Under mild conditions, a diindeno-fused 4H-cyclopenta[def]phenanthrene was successfully synthesized through the thionyl chloride-induced cascade cyclizations of the corresponding enediynyl propargylic alcohol with the formation of four new rings in one operation. The diindeno-fused 4H-cyclopenta[def]phenanthrene has a 41-carbon framework represented on the surface of C 60. A variety of helicenes with one or two phenyl substituents at the most sterically hindered position were also successfully synthesized through a cascade sequence of reactions involving a Schmittel cyclization reaction of the corresponding benzannulated enyne-allenes followed by an intramolecular radical-radical coupling reaction to afford the formal Diels-Alder adducts. The helical structures were produced either by increasing the ortho annulation number of the central aromatic ring systems or by replacing the phenyl substituents with two longer 4-(tert-octyl)phenyl groups. The structures of these helicenes were established by NMR studies and in some cases were confirmed by X-ray structure analyses. The existence of a helical twist is manifested with a set of AB 1 H NMR signals with a large geminal coupling constant of the diastereotopic methylene hydrogens on the five-membered rings. The racemization barriers of these helicenes were also studied by temperature dependent studies. Due to steric hindrance, the rotation of the phenyl substituent in the helical compounds is restricted. The rotational barriers of the phenyl substituent were also studied by temperature dependent studies. iii Dedicated to My wife and my parents iv ACKNOWLEDGEMENT I would like to express my sincere appreciation to my research advisor, Dr. Kung K. Wang, for his instructive guidance and continuous encouragement during the course of my dissertation research. His patience and calmness in solving problems and his willingness to share his knowledge have been an inspiration. I was quite fortunate to have been associated with such an outstanding chemist and a patient teacher. I am greatly benefited by his great interest and extensive knowledge of chemistry. I would also like to express my gratitude to my research committee members, Dr.