The present work deals with the synthesis of pyridine fused heterocyclic compounds, three condensed pyridine systems were selected for this project; Benzo[b]pyridine (Quinoline) (a), Pyrazolo[3,4-b]pyridine (b) and Pyrano[2,3-b]pyridine (c).Various strategies were utilized for introducing an ester functionality on the 4 position of (a) and (b) by utilizing Doebner-vonMiller reaction or additionally the use of Pfitzinger’s synthesis of quinolines. For (c) the ester group was introduced at the 3 position by applying a Knoevenagal condensation of ethyl acetoacetate and 2-hydoxypyridine-3- carbaldehyde. These ester groups in (a), (b) and (c) were converted to their respective carbohydrazides which were condensed with a number of arylcarbaldehydes to furnish their hydrazones. The final cyclization to the 1,3,4-oxadiazoles were effected by using various methods. POCl3 was very useful for this purpose. The products of the systems (a). (b) and (c) were thus synthesized and fully characterized by IR, 1HNMR, 13CNMR and MS and elemental (C,H andN) analyses.The results obtained confirmed the formation of the desired products. The final products were screened for their antioxidant and enzyme inhibition activities. viii Some azabicyclic compounds (I, II, III) were also prepared through ring closure metathesis by using Grubbs catalyst (Second Generation). These compounds were prepared through a multisetp strategy involving alkylation, RCM, decarboxylation, hydrolysis, Curtius reaction and finally cyclization to get the desired bicyclic compounds.These were characterized through 1HNMR, DEPT and HRMS.