Abstract
The thesis discusses several new tungsten and ruthenium mediated cycloaddition reactions and their application in organic synthesis. For convenience and better understanding, the thesis is divided in to three chapters. The first two chapters deals with tungsten mediated cycloaddition reactions and the third chapter deals with ruthenium mediated cycloaddition reactions and application in organic synthesis. The synthesis of organic carbocyclic and heterocyclic systems from acyclic building blocks is typically achieved through the use of either condensation or cycloaddition process. The reactions of Co2(CO)8 with alkynes RCCR¢ form (RCCR¢)-Co2(CO)6 complexes. The versatility of these reactions is well recognized, and they are frequently employed as a key step in synthesis of complex molecules. Development of new synthetic method involving carbenoid intermediates has received considerable attention. Although there are numerous investigations on (RC≡CR¢)Co2(CO)6 complexes, generation of a carbenoid intermediate from this functionality remains entirely unknown. In chapter-I we describe several intramolecular cyclizations via carbenoid intermediates generated from Co2(CO)6 complexation of funtionalized alkynyltungsten compounds. The synthetic utility of these cyclizations is manifested in the synthesis of cyclopentenone derivatives. The cycloaddition of alkynes and alkenes with organic substrate is widely used for the synthesis of carbocyclic and heterocyclic compounds. Epoxides and aziridines are important substances, and the cycloaddition of these molecules with alkenes alkynes is an interesting topic in organic synthesis. In chapter-II we report two useful cyclizations based on alkynyltungsten compounds, and each reaction involves sequential bond-breaking and –forming process. [3+2]-Cycloaddition of epoxides and aziridines with alkynes are hitherto unreported to the best of our knowledge. In this study, we describe a new cycloaddition of common epoxides and trans-aziridines with funtionalized alkynes. These methods are applicable to the enantiocontrolled synthesis of complex bicyclic lactones and lactams. Significant feature of this cycloaddition is that, it provides a one pot synthesis of bicyclic lactones via a tungsten enol ether. In chapter-III we describe the syntheses of various chiral oxacyclic dienes via enyne metathesis using Grubbs Catalyst (PCy3)2Cl2Ru=CHPh. These oxacyclic dienes are designed to bear a (2S)-(alkoxy)ethyl group derived from natural (2S)-ethyl lactate. The role of this substituent is twofold in synthetic application (1) control of diastereoselective Diels-Alder reaction (2) easy degradation into a common functionality after cycloadditon reaction. Our present approach for the synthesis of chiral oxacyclic dienes was extended for the construction of enantiopure tricyclic furan derivatives. Fused bicyclic [n.m.0] structural frameworks represent an important substructure in many natural products, and development of new synthetic approaches to these skeletons continues to be an important goal. Our enyne metathesis approach toward the synthesis of chiral oxacyclic dienes led us to focus on the construction of functionalized chiral oxabicyclic products that contain five-, six- and seven-membered rings by tandem RCM of dienynes using Grubbs ruthenium catalyst. Using this Tandem RCM approach variety of chiral fused bicyclic [n.m.0] rings can be constructed from simple precursors.