In the first part of this thesis is related to the regioselectivity in ene reaction of singlet oxygen with cyclic alkenes. The photooxygenation of 1-methyl-, 2,3-dimethyl-, 1,4-dimethylcyclohexa-1,4-dienes, 1,2,3,4,5,8-hexahydronaphthalene (16) and 2,3,4,7-tetrahydro-1H-indene (17) which are readily available through Birch reduction, yielded the ene products. The formed endocyclic dienes were trapped by the addition of singlet oxygen to give corresponding bicyclic endoperoxy-hydroperoxides. In the case of 1-methylcyclohexa-1,4-diene (13) and 1,4-dimethyl-cyclohexa-1,4-diene (15), cis-effect determined the product distribution. Photooxygenation of 2,3-dimethylcyclohexa-1,4-dienes (14) gave mainly exocyclic olefin, which was attributed to the lowered rotational barrier of the methyl group and increased reactivity of the methyl groups. Photooxygeneation of 1,2,3,4,5,8-hexahydronaphthalene (16) and 2,3,4,7-tetrahydro-1H-indene (17) shows importance of the geometry of the allylic hydrogen in the ground state. In the second part of this thesis is related to the stereoselective synthesis of carbaheptopyranose derivatives. Two new carbaheptopyranoses, 5a-carba-6-deoxy-&alpha / -DL-galacto-heptopyranose (184) and 5a-carba-6-deoxy-&alpha / -DL-gulo-heptopyranose (185) have been prepared starting from cyclohexa-1,4-diene. The addition of dichloroketene to cyclohexa-1,4-diene followed by subsequent reductive elimination and Baeyer-Villiger oxidation formed the bicyclic lactone 188. Reduction of the lactone moiety followed by acetylation gave the diacetate 182b with cis-configuration. Introduction of additional acetate functionality into the molecule was achieved by singlet oxygen ene-reaction. The formed hydroperoxide 189 was reduced and then acetylated. The double bond in triacetate was further functionalized either by direct cis-hydroxylation using OsO4 or epoxidation followed by ring-opening reaction to give the hepto-pyranose derivatives 184 and 185.

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.