The 2-norbornyl cation is an old topic in physical-organic chemistry. Whether in classical or non-classical form (partial bridged form) it has been one of the focus of discussion. Currently the experimental data and theoretical calculations favorably support the idea that 2-norbornyl cation is not in the classical form in the stable-ion condition. In this paper, first, we will show that a 3-center-2-electron π-complex is formed by the collapse of 2-norbornyl cation. Further, using different theoretical methods (B3LYP, MP2) with different basis sets (6-31+G, 6-31G(d, p), 6-311G(d, p), 6-311G(2d, p)), we find that there is a trend for the 3-center-2-electron π-complex to approach the Cs symmetry, and this π-complex oscillates within the numerical limits of the perfect Cs symmetrical configuration. The stabilization energies of the π-complex are 13.87 Kcal/mol and 19.47 Kcal/mol by B3LYP/6-31+G and MP2/6-31+G, respectively. Second, our calculations also show that the transition state between 2-norbornyl cation and 3-norbornyl cation is formed by a 3, 2-proton shift, not the generally accepted 3, 2-hydride shift. The activation energy of this 3, 2-proton shift is 10.9 Kcal/mol. Detailed structural changes in the optimization process and the formation of transition state (also a 3-center-2-electron π-complex) between 2-norbornyl cation and 3-norbornyl cation will also be included.

Recent years have witnessed a great deal of interest in the design and synthesis of small molecules, which can mimic complex carbohydrates of vital importance in various life processes. Carbasugars constitute one such class of molecules among several others, in which ring oxygen of sugar is replaced by a methylene unit. Several approaches have been reported in the literature for the synthesis of carbasugars emanating both from carbohydrate and non-carbohydrate sources. While the carbohydrate-based approaches require extensive prote'ction-deprotection maneuver, the non-carbohydrate-based approaches generally have problems of diastereoselection and introduction of chirality. In the context of synthesis of carbasugars, we envisaged that a suitable derivative of i bicyclo[2.2.1]heptane (norbornyl system), could serve as a carbasugar equivalent provided the inherent cyclohexane ring could be disengaged through a tactical cleavage of C1-C7 or C4-C7 bonds. In the present thesis entitled "Synthesis of carbasugars and other structurally related motifs", we have established the carbasugar-norbornyl system equivalence by using 7-* oxobicyclo[2.2.1]hept~5-en-2~yl acetate as precursor . While the tactical cleavage of C1-C7 bond was employed in the synthesis of carbasugars, the C4-G7 bond cleavage provided access to a new class of carbasugars i.e. "confused" carbasugars* The quest for stronger and specific inhibitors of glycosidases led us to synthesize a diverse array of polyoxygenated compounds in this new family of carbasugars. The thesis has been organized under five main sections: I. Introduction, II. Results & Discussion, III. Experimental, IV. Spectra, and V. References where we have narrated our I synthetic efforts with suitable literature citations.

Carbohydrates are ubiquitous and important biomolecules. Initially thought to be dull, energy storing moieties, the importance of carbohydrates and their conjugates, glycoproteins and glycopilids, in cellular communication and various related processes has been well established. Carbohydrate recognition events are involved in the progression of various diseases, as the binding of pathogens to the host cells is carbohydrate mediated. Also the malfunctioning of carbohydrate processing enzymes has been implicated in life-threatening diseases. Thus there is tremendous interest in the design of molecules which can mimic the carbohydrates and provide insights into the mechanisms of action of carbohydrate processing enzymes. Such designer glycomimics possess several advantages over the parent molecules. In this regard the synthesis of small molecules based on the polyhydroxylated cyclohexane framework has gained vital importance. Some of the efforts of several research groups actively working on the design and synthesis of glycomimics have culminated in therapeutics and resulted in the development of many synthetic routes to polyoxygenated cyclohexanoids emanating from either the chiral pool or from aromatics and other non-carbohydrate sources. Nevertheless, the design of a general and variable strategy to access these cyclohexitols is essential. Our quest for a general and more versatile strategy for accessing several of the polyoxygenated cyclohexanoids led to the development of a new norbornyl based approach. The important feature of our approach involves extraction of the inherent cyclohexanoid from the norbornyl scaffold. The present thesis entitled “Norbornyl system revisited: Exploring a versatile building block for the syntheses of natural products and analogues” delineates our synthetic endeavors. The thesis is represented in two parts “Part 1: Synthesis of polyoxygenated cyclohexanoids and azepanes” is subdivided into Introduction, Results and Discussion, Summary, Experimental, Spectra and References sections and describes our synthetic efforts towards various polyoxygenated cyclohexanoids and azepanes. Introduction deals briefly about the importance of glycomimics and synthetic approaches from the literature towards these polyhydroxylated cyclohexanoids. Our findings constitute the Results and Discussion section wherein we delineate the synthesis of a versatile cyclohexanoid building block through a Grob like Wharton fragmentation on an suitably crafted norbornyl scaffold. The synthetic utililty and versatility of this building block are explored in subsections titled Carbasugars, Cyclitols, Gabosines, Aminocyclitols and Azepanes. The synthesis of several polyhydroxylated cyclohexanoid natural products and analogues is discussed. “Part 2: Synthetic studies towards the novel diterpenoid rameswaralide” deals with the elaboration of the versatile norbornyl building block towards the synthesis of a novel 5-7-6 fused diterpenoid rameswaralide. This part is again divided into Introduction, Results and Discussion, Summary, Experimental, Spectra and References sections. The Introduction briefs the relevance and importance of total synthesis of natural products, with a mention of terpenoids. The structure and biological significance of rameswaralide and related molecules is discussed. In the Results and Discussion our synthetic studies towards rameswaralide are delineated. Restructuring the norbornyl framework to a 5,5 fused all cis Corey lactone and its further amplification through ring closing metathesis and Diels-Alder protocols are described.