Abstract
Abstract Phthalides (isobenzofuranones) are five-membered lactones found in plants. These species possess several important properties such as fungicidal, bacteriocidal, herbicidal and analgesic activities. In chapter 2, we report a novel efficient method for the synthesis of phthalides via a nickel-catalyzed cyclization of o-halobenzoates with aldehydes is described. The reaction of o-bromobenzoate with benzaldehyde in the presence of Ni(dppe)Br2 and zinc metal powder in THF for 24 h at reflux temperature afforded the corresponding phthalide derivatives in good to excellent yields and in high chemoselectivity. On the other hand, treatment of 2-iodobenzoate with benzaldehyde in the presence of a mixture of CoI2(dppe) and zinc metal powder in THF at reflux temperature also led to the cocyclization of phthalide derivative in good yields. This new cyclization reaction provide convenient method for the synthesis of various substituted phthalides in good to excellent yields from easily available starting materials. The transition metal catalyzed [2+2+2] cycloaddition reaction of unsaturated molecules is an efficient method to construct six-membered cyclic compounds with the formation of three new carbon-carbon bonds. However, versions of [2+2+2] cycloaddition generally have encountered difficulties in the control of chemo- and regioselectivity. In chapter 3, we report a cobalt catalyzed [2+2+2] cycloaddition reaction of alkynyl alcohols and amines with propiolates and diynes to give various substituted benzolactones and lactams with high chemo- and regioselectivity in good to excellent yields. The preparation of complex polycyclic pyridine derivatives is an important synthetic goal, because of the utility of these molecules as potential pharmaceuticals. Transition metal-catalyzed [2+2+2] cocyclotrimerization of two alkynes with a nitrile is one of the attractive methods to construct polycyclic pyridine derivatives in highly atom-economical manner. In chapter 4, we report a cobalt catalyzed partially intermolecular cocyclotrimerization of diynes with alkylnitrile to form pyridine derivatives in modest to high yield. We also reported an intramolecular cocyclotrimerization of nitrilediynes to afford polycyclic pyridine derivatives in good to excellent yields. In chapter 5, a cobalt complex/Zn system effectively catalyzes the reductive coupling of alkenes and alkynes in the presence of water to give highly substituted alkenes in very high yields. The reaction of enones and enals requires the use of CoI2(dppe)/Zn/ZnI2 system. The reductive coupling of enones and alkynes also work efficiently in intramolecular way. During the course of our investigation in related reductive coupling reactions we noticed that allenes and enones effectively cyclized to form cyclopentanols with very high diastereoselectivity. Thus, treatment of phenyl allene with methyl vinyl ketone in the presence of CoI2(dppe), Zn, ZnI2 and water in CH3CN gave 3-methylenecyclopentanol derivative in very good yield. The reaction also exhibited excellent chemo- and regioselectivity.