The thesis entitled “Studies on the Ring-opening Reactions of Vinylcyclopropanes, Vinylcyclobutanes, and other Small-ring Systems” is divided into four chapters. Chapter 1: Part A: Bromenium Catalyzed Tandem Ring-opening/Cyclization of Vinylcyclopropanes and Vinylcyclobutanes: A [3+2+1]/[4+2+1] Cascade for the Synthesis of Chiral Amidines. In this part of the Chapter, we discuss our serendipitous results in the reaction of vinylcyclopropanes (VCPs), like Δ2-carene under Sharpless aziridination conditions using chloramine-T and phenyltrimethylammonium tribromide (PTAB) as catalyst in acetonitrile. The reaction follows a [3+2+1] cascade pathway involving acetonitrile (Ritter-type reaction) to give chiral bicyclic amidines in very good yield. The reaction was found to be tolerant to hydroxyl- and keto-functionalities. . existence of a tight-carbocation intermediate. Our attempts to access bridged bicyclic amidines from vinylcyclobutanes like α-pinene resulted in the formation of bicyclo[4.3.1]pyrimidines successfully in moderate yields. Partial racemization of the product was observed and this observation was rationalized through competing cyclization pathways. Vinylcyclopropanes and Vinylcyclobutanes towards the Synthesis of Chiral Amidines. In this part of the chapter, we discuss our computational results obtained from modeling the reaction pathway in gas-phase and solvent dielectrics (acetonitrile). Initially, we modeled the ring-opening process, to visualize the geometrical features and the orbital interactions present in the tight-carbocation intermediate. We also modeled the competing cyclization pathways to justify the racemization observed in the case of α-pinene. Our calculations show that, the free energy of activation for the allylic substitution and the direct substitution pathways are nearly equal. Thus, the formation of both the enantiomers is feasible kinetically. the proposed cascade pathway. Chapter 2: Electrophile-Induced Indirect Activation of C-C Bond of Vinylcyclopropanes: A Masked Donor-Acceptor Strategy for the Synthesis of Z-Alkylidenetetrahydrofurans. In Chapter 2, we discuss the results of introducing VCPs as masked donor-acceptor systems under electrophilic conditions. Our aim was to activate the VCPs with in situ generated bromine electrophile to give a tight-carbocation as discussed in Chapter 1. Further, the tight-carbocation can be used to access novel heterocycles. formation of Z-alkylidienetetrahydrofurans with high stereoselectivity across the exocyclic double bond. An interesting reactivity of benzofuran derived VCPs was observed, where the ring-opening occurred concurrently adjacent to the heteroatom and at the benzylic position to give both cis- and trans-furofuran. methyl group on VCP as a chiral marker. Under our standard reaction conditions, cyclization resulted in the retention of configuration at the phenyl center. The retention of configuration results through a directed attack of hydroxyl group on the tight-carbocation. functionalized tetrahydrofurans Chapter 3: σ-Ferrier Rearrangement of Carbohydrate Derived Vinylcyclopropanes: A Facile Approach to Oxepane Analogs In the present chapter, we have presented the idea of a tight-carbocation through an electrophile-mediated activation of VCPs on carbohydrate derived VCPs through a σ-Ferrier rearrangement. We expected high stereoselectivity at the anomeric center assuming the existence of a tight carbocation intermediate. Reaction of glucose-derived VCPs resulted in the ring-expansion to oxepane analogues, but with poor diastereoselectivity. Similar selectivity was observed even in the case of galacto- derived VCPs. intermediate. The planar oxonium intermediate is a more stable intermediate but reacts with poor facial selectivity. With water as nucleophile, the reaction led to a diene aldehyde through a complete ring-opening of the oxepane formed, followed by the elimination of hydrogen bromide. the unsaturated oxepanes with facial diastereoselectivity. Chapter 4: One-Pot Synthesis of β-Amino/β-Hydroxyselenides and Sulfides from Aziridines and Epoxides. In this chapter, we present details of the reductive cleavage of aromatic disulfide and diselenide bonds mediated by Rongalite. The reagent reacts with disulfides to generate thiolate anion through a two-electron transfer mechanism. The thiolate anion was further utilized for nucleophile-mediated ring-opening of small-ring systems. The reaction of aziridines with aryl disulfides mediated by Rongalite, resulted in regioselective ring-opening to from β-aminosulfides. In the case of trisubstituted aziridines, the reaction led to a regioisomeric mixture of products. The reaction was found to be efficient for the ring-opening of epoxides as well. diselenides and Rongalite, successfully underwent cleavage of diselenide bond followed by ring-opening to give β-aminoselenides. The reaction was successful with epoxides as starting material to yield β-hydroxyselenides