This dissertation has been divided into four chapters. Each chapter has its own numbering of compounds and references. The general introduction describes the importance of natural products and the drugs based on them. The chapter 1 deals with the phytochemical studies on the chemical constituents of Xanthium strumarium Linn. The introduction provides a review of the earlier contributions made in the chemistry and pharmacology of the genus Xanthium and a brief account of the present work. Studies undertaken on different fractions of methanolic extract (XS-HX, XS-DC, XS-EA, XS-BU and XS-ME) of the air dried aerial parts of X. Strumarium Linn. showed weak to moderate antibacterial and weak antioxidant activity except ethyl acetate fraction (XS-EA) which exhibited moderate to high antibacterial and antifungal activity (Table-2, 3) while significant antioxidant activity (Table-4) was observed among all fractions. Studies undertaken on the bioactive ethylacetate fractions led to the isolation and structure elucidation of eight known compounds. The known compound (1) is reported for the first time from X. strumarium Linn. The constituents obtained are listed below. I. Lupenyl acetate (1) II. Stigmasterol (2) III. β-Sitosterol (3) IV. Palmitic acid (4) V. β-Amyrin (5) VI. Oleanolic acid (6) VII. β-Sitosterol-3-O-β-D- Glucopyranoside (7) VIII. Ferulic acid (8) The structure of all the isolated compounds have been determined through various spectroscopic techniques such as, IR, EIMS, HR-EIMS, 1H-NMR, 13C-NMR, 2DNMR and also by comparison of their spectral data with those reported in literature. Chapter 2 is about characterization and bioassay screening of thirty five (35) synthesized derivatives of 2, 3-diaminonaphthalenimidazole (65-99). Out of these thirty five naphthalenimidazoles, twenty six (26) (65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 79, 80, 82, 84, 85, 86, 88, 90, 91, 93, 94, 95, 96, 97, 98) are newly synthesized compounds. All synthesized derivatives showed interesting in vitro enzyme inhibitory (urease, tyrosinase, acetylcholinesterase and butrylcholinesterase inhibitory), antimicrobial and antioxidant activities. Two compounds 81 and 85 revealed potent in vitro tyrosinase inhibitory activity. On the other hand compounds 65, 66, 68, 69, 71, 79, 88 and 94 were found moderately active for this activity. When tested for their in vitro butrylcholinesterase inhibitory activity, three compounds 65, 66 and 79 exhibited good activity while compounds 67, 81, 82 and 89 showed moderate butrylcholinesterase inhibitory activity but all synthesized compounds were found inactive for acetylcholinesterase inhibitory activity. In urease inhibitory activity two compounds 71 and 90 revealed good activity while moderate activity was observed in compounds 65, 66, 68, 81 and 82. All synthesized derivatives when screened for their anti-microbial activity, only two compounds 90 and 92 were found exhibiting remarkable activity against bacterial strains B. cereus, B. subtilis, S. epidermidis, S. paratyphi A, Enterobacter and S.dysenteriae. Significant activity against Enterobacter and S.dysenteriae was displayed by 99 and moderate activity was found in compounds 65, 74, 81, 82, 85 and 94 against various tested bacterial strains. All synthesized compounds showed weak antifungal activity. For in vitro antioxidant activity, the compounds 65, 68, 77, 90, and 99 revealed promising whereas compounds 79, 82, 85 and 95 showed good antioxidant activity. Chapter 3 describes the synthesis, structure elucidation and biological activity of fifteen (15) amides of piperic acid (42-56). Five compounds (47, 51, 52, 54, 56) are new amides. All the synthesized derivatives were evaluated for their in vitro anti-microbial, nematicidal and anti-oxidant activity. In the case of antimicrobial activity compound 54 was found the most active against all applied bacterial strains except S. pneumoniae, compound 49 showed excellent activity for P. vulgaris and 53 was good against P. stutzeri where as compound 44, 46, 47 and 48 showed good activity against P. aeruginosa. It was determined that compound 50 was active against S. aureus, P. stutzeri and P. aeruginosa. Compound 52 showed good activity against P. aeruginosa and E. coli whereas compound 56 exhibited activity against E. coli only. It was observed that among all the synthesized amides only compound 54 showed antifungal activity against all applied fungal strains. When screened for nematicidal activity compounds 42, 43, 45, 47, 52 and 56 were found possessing excellent nematicidal activity against root-knot nematode, Meloidogyne incognita, where as compounds 44, 50 and 54 have significant mortality rate. During antioxidant testing three compounds 44, 49 and 51 showed significant and two compounds 46 and 54 showed moderate DPPH radical scavenging property. Chapter 4 deals about introduction of biological activities and all the protocols used to determine the inhibitory potential of all fractions and synthesized compounds.