One area of natural product synthesis which has been heavily investigated in the last eight decades is the total synthesis of terpenoids. Among terpenoids, the presence of a great deal of stereochemical complexity in combination with a variety of functionalities makes sesquiterpenes challenging targets to the synthetic chemists. As a result synthetic activity in this area continues to flourish. The thesis entitled “Enantiospecific Synthesis of Guaianes and Tricyclic Ring Systems of Elisabethins and Dumsins’’ describes the studies directed towards the synthesis of the guaiane sesquiterpenes and exploratory studies towards elisabethins and dumsin diterpenoids. For convenience, the results are presented in two chapters; viz (1) Enantiospecific Total Synthesis of Guaiane Sesquiterpenes; and (2) Enantiospecific Synthesis of ABC Ring System of the Diterpenoids Elisabethins and Tetranortriterpenoids Dumsins. In each chapter of the thesis, the compounds are sequentially numbered (bold) and references are marked sequentially as superscripts and listed at the end of the chapter. All the spectra included in the thesis were obtained by xeroxing the original NMR spectra. Clavukerin A, the first member of the trisnorguaianes, was simultaneously isolated in 1983 by the research groups of Kitagawa from the Okinawan soft coral Clavularia koellikeri (stolonifer), and Bowden from the Australian soft coral Cespituloria sp. In 1992, Kakisawa and his research group reported the isolation of isoclavukerin A from the Okinawan soft coral Clavularia sp. In the present thesis, formal total syntheses of clavukerin A and isoclavukerin A have been described in the first part of the first chapter. To begin with, (R)-limonene has been transformed into 1-[5-isopropenyl-2-methylcyclopent-1-en-1-yl]pent-4-en-1-one via the 5-isopropenyl-2-methyl-cyclopent-1-en-1-carboxaldehyde. RCM reaction of the enone produced 6,10-dimethylbicyclo[5.3.0]deca-1(10),5-dien-2-one, which on epoxidation generated 1-[(1S,2S,5R) and (1R,2S,5S)-2-isopropenyl-5-methyl-6-oxabicyclo[3.1.0]hex-1yl]pent-4-en-1-ones. These epoxy ketones were then transformed into (6S,7S) and (6R,7S)6,10-dimethylbicyclo[5.3.0]dec-1(10)-en-2-ones, thus completing the formal total synthesis of clavukerin A and isoclavukerin A. In the subsequent parts of the first chapter, enantiospecific total syntheses of the guaiane sesquiterpenes, aciphyllenes A and B, isocalamusenone and 6-epiisocalamusenones, and (6S)- and (6R)-11-hydroxyguaiadienes have been described. Aciphyllene A was isolated in 1983 by Kubota and co-workers from the essential oil of the roots of Lindera glauca. Aciphyllene B was isolated by Konig et al. in 1998 from the liverwort D. hirusta. In 1979, Rohr and co-workers reported the isolation of isocalamusenone from the plant Acorus calamus L, In 2000, Nkunya and colleagues isolated (6R)-11-hydroxyguaiadiene from the root bark of Lettowianthus stellatus, whereas (6S)-11-hydroxyguaiadiene was isolated in 1977 by Bohlmann et al. from the roots of Parthenium hysterophorus. The 5isopropenyl-2-methyl-cyclopent-1-en-1-carboxaldehyde derived from (R)-limonene has been converted into the (3R,4S,6S,7S)- and (3R,4S,6R,7S)-3-acetyl-6,10 dimethylbicyclo[5.3.0]dec1(10)-en-4-ols employing a type II carbonyl ene reaction and Wilkinson’s hydrogenation as the key steps, which have been further converted into aciphyllenes A and B along with their C-6 epimers, (+)-isocalamusenone, 6-epi-isocalamusenone, and (6S)- and (6R)-11-hydroxyguaiadienes. In the second chapter of the thesis, enantiospecific synthesis of the ABC-ring systems of elisabethin group of diterpenoids and tetranortriterpenoids dumsins have been described, starting from the readily available monoterpene (R)-carvone. To begin with, (R)-carvone has been transformed into 1,6,6-triallylcarveol, which on two simultaneous RCM reactions generated the ABC-ring system of the elisabethin group of diterpenoids. An alternative synthetic strategy was also developed for the same compound. Thus, first (R)-carvone has been transformed into 6-allyl-10-isopropenyl-7-methylspiro[4.5]deca-2,7-dien-6-ol, which on ROM-RCM reaction generated the requisite tricyclic alcohol, which on oxidation generated 4,8-dimethyltricyclo[7.4.0.01,5]trideca-3,8,11-trien-7-one, which represents the ABC ring system of elisabethins. Introduction of the second allyl group at the C-7 position of 6-allyl-10isopropenyl-7-methylspiro[4.5]deca-2,6-dien-8-one followed by RCM reaction resulted in the formation of the tricyclic ketones (1S,4S,6R) and (1R,4S,6R)-4-isopropenyl-1-methylbicyclo[4.4.0]decanespiro[5,1']-cyclopenta-3',8-diene-2-ones, which represents the ABC ring system of tetranortriterpenoids dumsins.

The thesis entitled “Chemistry of Tetrathiomolybdate and Tetraselenotungstate: Studies on Aziridine and Epoxide Ring Opening Reactions” is divided into five chapters. (For Formulas and Equations Refer PDF File) Chapter 1: Part 1: Synthesis of β-Sulfonamidodisulfides and β-Sulfonamidosulfides using Benzyltriethylammonium Tetrathiomolybdate In this chapter, a comprehensive study of general and effective one step procedure for the synthesis of β-sulfonamidodisulfides directly from optically pure N-tosyl aziridines using benzyltriethylammonium tetrathiomolybdate [BnEt3N]2MoS4 as sulfur transfer reagent in a regio manner under neutral conditions without the use of any Lewis acid or base has been reported. Additionally, we have demonstrated regio- and stereospecific ring opening of di- and trisubstituted aziridines using [BnEt3N]2MoS4 to synthesize substituted β-sulfonamidodisulfides in good yields. This methodology is extended to the synthesis of an optically pure unnatural amino acid with the disulfide bridge and a cyclic seven membered disulfide. Synthesis of a variety of β-sulfonamidosulfides involving cleavage of disulfide bonds assisted by tetrathiomolybdate and the use of masked thiolate for the synthesis of β-sulfonamidosulfides involving multi-step reactions in a one pot is also demonstrated. Chapter 1: Part 2: Synthesis of β-Sulfonamidodiselenides using Tetraethylammonium Tetraselenotungstate In this chapter, we report the results of regio- and stereospecific, nucleophilic ring opening of chirally pure N-tosyl aziridines with tetraethylammonium tetraselenotungstate [Et4N]2WSe4 as selenium transfer reagent to afford a number of β- sulfonamidodiselenides in good yields. Using this methodology, carbohydrate derived β- sulfonamidodiselenides from the corresponding carbohydrate derived aziridines have been synthesized. These enantiopure diselenide derivatives have the potential to be used as chiral ligands in diethyl zinc addition to aldehydes. Chapter 2: Ring Opening of Aziridine/Epoxide, Disulfide Formation, Reduction of Disulfide Bond and Michael Reaction In this chapter, we report a systematic study of tetrathiomolybdate mediated tandem regio- and stereospecific ring opening of aziridines, disulfide formation, in situ reduction of disulfide bond followed by Michael reaction in an one pot operation to give a variety of β-sulfonamidosulfides in good yields. The main advantage of this methodology is that four reactions involving three components take place in a one-pot operation. Chapter 3: Part 1: New Thia-aza Payne type Rearrangement Mediated by Benzyltriethylammonium Tetrathiomolybdate In this chapter, reaction of aziridinemethanol sulfonate esters with tetrathiomolybdate to give thiirane derivatives as the major product and cyclic disulfides as minor product under mild reaction conditions via an unprecedented thia-aza-Payne type rearrangement have been presented. Interestingly, when the reaction of tetrathiomolybdate was carried out with 2-aziridino-cyclohexanol derivatives it resulted in the formation of thia-bicyclo[3.1.1]heptane or dithia-bicyclo[3.2.1]octane derivatives. Chapter 3: Part 2: New selena-aza Payne Type Rearrangement Mediated by Tetraethylammonium Tetraselenotungstate In this chapter, reaction of tetraselenotungstate with simple N-tosyl aziridinemethanol tosylates to give allyl amine derivatives as the only product via an unprecedented selena-aza-Payne type rearrangement is discussed. When the methodology is extended to disubstituted N-tosyl aziridinemethanol tosylates, regio- and stereospecific ring opening of aziridines occurs to afford allyl amine derivatives as the major products and cyclic five membered diselenides as the minor products in good yields. Chapter 3: Part 3: Synthesis of Sulfur and Selenium Heterocycles by Azirdine Ring Opening followed by Cyclization In this chapter, studies on the synthesis of sulfur and selenium-heterocycles by aziridine ring opening followed by cyclization of N-tosyl aziridino-ethanol tosylates using tetrathiomolybdate as a sulfur transfer reagent and tetraselenotungstate as a selenium transfer reagent respectively are presented. Chapter 4: Tetrathiomolybdate Mediated Ring Opening of bis-Aziridines, bis-Epoxides and Aziridino-epoxides In this chapter, studies on the synthesis and ring opening of bis-aziridines, bis-epoxides and aziridino-epoxides with tetrathiomolybdate as the sulfur transfer reagent are presented. This has resulted in the synthesis of optically active sulfur heterocycles ranging from three membered to eight membered ring systems with excellent stereo and regio- control in good yields. Chapter 5: Part 1: Synthesis of Conformationally Locked, Bridged, Bicyclic Mono and Disulfides In this chapter, work related to the synthesis of conformationally locked bridged bicyclic disulfides and sulfides from cis-aziridino-epoxides by ring opening of both aziridines and epoxides in a tandem fashion using tetrathiomolybdate as a sulfur transfer reagent has been discussed. Comparative studies on the behavior of conformationally locked disulfides which has the dihedral angle close to zero (φ = 0) with disulfides having larger dihedral angles (φ>90) have been presented in this chapter. Some correlations have been made on the physicochemical characteristics of the disulfides with change in the dihedral angles. Chapter 5: Part 2: Synthesis of Conformationally Locked, Bridged, Bicyclic Diselenides In this chapter, work related to the development of a general synthetic methodology for the synthesis of conformationally locked, bridged diselena-bicyclo[3.2.1]octane skeleton by regio- and stereospecific, tandem nucleophilic ring opening of cis-1,4-aziridino-epoxides with tetraselenotungstate in one-pot are presented. To compare the behavior of conformationally locked diselenides which has the dihedral angle close to zero (φ = 0) with diselenides having larger dihedral angles (φ > 90), we have synthesized the acyclic diselenide (see chapter 1.2) and cyclic diselenide by regio- and stereospecific ring opening of simple aziridine and bis-aziridine respectively with tetraselenotungstate. Some correlations have been made on the physicochemical characteristics of the diselenides with change in the dihedral angles.