The work presented in this thesis describes the synthesis, characterization and bioevaluation of different classes of compounds, i.e., 3,4-dihydropyrimidine-2thiones, thioureas, benzamides and 2-aroyliminothiazolines. In some cases, docking and computational studies have also been carried out. In addition, a new multistep synthetic methodology of tropane auxiliary which is used as a stereo-directing element to achieve α-alkylation of aldehydes has been developed. A series of fourteen 3,4-dihydropyrimidine-2-thiones (248a-n) were synthesized by a green protocol, and their structures were characterized by spectroanalytical data. The compounds were obtained in high yields by efficient annulation of mesityl oxide (4methyl-pent-3-en-2- one) with anilines in the presence of potassium thiocyanate. The reaction is essentially metal-catalyst- and solvent-free, as mesityl oxide itself is the solvent as well as the reactant. The compounds were tested for their ability to inhibit the lymphoid tyrosine phosphatase PTPN22, and 5 of the 14 compounds exhibited IC50 values in the mid micro-molar range, with the most potent hit being the compound 248d, having the methoxy substituent at the 2- position of the phenyl ring with an IC50 value 18± 1 μM, and the second most potent compound 248c with an IC50 value of 45± 3 μM, having methyl substituents at both 2- and 4- position of the phenyl ring. A series of twelve new aryl thiourea derivatives of 4-aminophenazone (252a-l) has been synthesized. The 4-aminophenazone which is also called as ‘4-aminoantipyrine’ or ‘ampyrone’ belongs to a class of non-steroidal anti-inflammatory drugs (NSAID’s) responsible for a broad spectrum of medicinal and therapeutic applications. So, based on the biomedical importance of this drug, the newly-synthesized aryl thiourea derivatives of 4- aminophenazone were screened in vitro against alkaline phosphatase enzyme found in the intestine of calf as well as also evaluated for their antioxidant and cytotoxic potential. Among the tested compounds, the 2-methyl derivative 252b of the series was found to be the most potent compound showing greater inhibition potential against alkaline phosphatase, besides, displaying greater antioxidant potential. The 3-nitro member 252i of the series came out to be the most active member while screening the synthesized series for their cytotoxic potential using brine shrimp assay. Apart from these bioassays, kinetic analysis of the most potent member of the series on basis of IC50 value 252c was performed in order to find the mechanism of enzyme inhibition. The results suggested that the compound 1-(1,5dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)3-m-tolylthiourea 252c is a non-competitive inhibitor of calf intestinal alkaline phosphatase, i.e., it lowers the enzyme concentration by showing non-competitive binding mode with enzyme. The synthesis of a series of different substituted N-(2,3-dimethyl-5-oxo-1-phenyl-2,5dihydro- 1H-pyrazol-4-yl)benzamides was carried out by making use of 4aminophenazone, a compound of great interest in medicinal chemistry. These compounds possess potential biological applications and were screened against human recombinant alkaline phosphatase including human tissue-nonspecific alkaline phosphatase (h-TNAP), tissue specific human intestinal alkaline phosphatase (h-IAP), human placental alkaline phosphatase (h-PLAP) and human germ cell alkaline phosphatase (h-GCAP). These compounds were also tested for their inhibitory potential against recombinant human and rat ecto-5′-nucleotidases (h-e5-NT & r-e5NT, respectively). All benzamide derivatives inhibited APs to a lesser degree than 6 e5NT. The reported compounds are of considerable interest for further applications in the field of medicinal chemistry as these compounds have potential to bind nucleotide protein targets.153 An efficient synthesis of new aroyl thiourea derivatives of ferrocene (261a-q) was also accomplished. Ferrocene substituted benzoyl chloride was reacted with potassium thiocyanate to give the corresponding isothiocyanate intermediate which on reaction with different substituted anilines, afforded thiourea derivatives in good yields. The synthesized series of compounds were evaluated for their in vivo locomotor activity that was carried out inside the mice using diazepam as standard drug. Pharmacokinetic parameters such as absorption, distribution metabolism, LD50 and other characteristics were determined for active members of the synthesized series, i.e., 261d (4-methoxy substituted) and 261g (3- chloro substituted). These compounds were also screened in silico for their pharmacokinetic profiling using Admet-SAR software. Synthesis of 2-imino-1,3-thiazolines was accomplished in two steps. In the first step, isomeric chloro benzoyl thiourea derivatives (265a-c) were synthesized via reaction between in situ formed isomeric chloro benzoyl isothiocyanates with aqueous ammonia. Structure of one of the crystalline isomeric, i.e., 2-chlorobenzoyl thiourea derivative 265a was determined through single crystal X-ray crystallography and its vibrational properties were determined as well.175The conformational analysis of the crystal structure 265a revealed that a local planar structure is preferred with opposite orientation between the C=O and C=S (preference of S-conformation over Uconformation), thus, forming a pseudo six-membered ring structure that promotes a C=O…N-H intramolecular hydrogen bond. The second step involved the heterocyclization reaction between different cyclizing agents such as alpha halo carbonyl compounds and diethyl oxalate with synthesized isomeric chloro benzoyl thiourea derivatives to form a series of novel 2-imino-1,3thiazoline derivatives (266, 267, 268, 269a-c). The synthesized derivatives were screened for their antileishmanial activity using Amphotericin B as reference standard. The screening results showed that the derivative 266c having chloro group at para position to be most potent and active member of the series. An efficient multistep strategy was devised for the synthesis of racemic 1-methyl tropane auxiliary (±)-270. The alkylation potential of the aldenamine (±)-292 derived from racemic tropane auxiliary has been tested in order to exploit the 5-membered ring’s effect on facial selectivity. The dr of α-alkylated diastereomeric iminium ions was determined for the racemic system from 1H-NMR analysis. The experimental findings indicated a dr of 64:36 which are in close agreement with the computationally-determined results (dr of 68:32). It was further concluded from the collaborative computational study and laboratory experimental findings that the presence of 5-membered ring has caused a surprising change in the precise ground state structural orientation in the enamine species. The enamine exocyclic double bond was indicated as leaning more over the face of 6-membered ring (fig. 39) than the 5-membered pyrrolidine ring, thus, directing alkylation towards the face of 5membered ring, i.e., preference for Si compared to Re addition by ~2 kJ mol-1. All the synthesized compounds were characterized on the basis of their physiochemical parameters and spectral data.