Palladium-catalyzed and electrophilic cyclization reactions are among the most studied in synthetic organic chemistry. However, these methodologies cannot always be generally applied to obtain good yields of a desired class of compounds. Many factors, including the reaction conditions, the nature of the ligands, and the reactivity of the involved species determine the product and the yield. This thesis describes several useful and novel palladium-catalyzed and electrophilic cyclization methodologies that have been optimized to obtain biologically and industrially useful carbocycles and heterocycles in good to excellent yields. These methods are tolerant of a number of functional groups including, a halogen moiety, which is further subjected to palladium-catalyzed Sonogashira, Suzuki-Miyaura and CO insertion reactions.;Chapter 1 describes the synthesis of 3,4-disubstituted 2H-benzopyrans by the electrophilic cyclization of propargylic aryl ethers. Emphasis has been placed on the iodocyclization reactions of these ethers using I 2 and ICl as the electrophiles. The regioselectivity of unsymmetrical propargylic aryl ethers has also been studied.;An innovative one step palladium-catalyzed aminocarbonylative cyclization is explored in Chapter 2. Our method uses only one atmosphere of CO and requires no specialized equipment to obtain an important class of heterocycles called phthalimides, starting from commercially available primary amines and the corresponding ortho-halobenzoates.;Arynes and organopalladium species both are very reactive and, therefore, their coupling reactions are limited in the literature, particularly due to the harsh conditions generally required to obtain arynes. A novel palladium-catalyzed aryne annulation to obtain the important class of carbocycles known as 9-fluorenylidenes from ortho-halostyrenes and o-silylaryl triflates in the presence of CsF is described in Chapter 3. The methodology has also been extended to the synthesis of 9,10-phenanthrenes from the corresponding ortho-halo allylic benzenes.;Obtaining a good sized library of important potential pharmacophores (e.g., indoles) is desirable in medicinal or pharmaceutical fields. Chapter 4 describes the synthesis of highly substituted indoles in compliance with Lipinski rules. The palladium-catalyzed reactions employed have achieved substitution at the 3-position of the indole moiety, which is difficult due to the electron-rich nature of the C-I bond. The chemistry has been successfully transferred to a solid support and diversity has been achieved at the 5-position by various cleavage reactions
