Studies in sesquiterpenes

Abstract

The present thesis, entitled “Studies in Sesquiterpenes,” consists of four parts, which are suitably subdivided into chapters.

PART I — Introduction This part is introductory in treatment and gives a broad survey, with relevant references, of the different types of naturally occurring sesquiterpenes. After discussing several methods of their isolation, a detailed account is given of the different methods of structure determination, making use of modern as well as classical physical and chemical investigative techniques. The stereochemistry and absolute configurations of some of the well?known sesquiterpenoids are also included. In conclusion, developments in the biogenesis of terpenoids are reviewed, with particular emphasis on cadinene, neinene, and related sesquiterpenes of the eudalene–cadalene investigation, which form the subject matter of the present thesis.

PART II — Syntheses of Naphthalenic Hydrocarbons Related to Eudalene This part consists of three chapters, each devoted to synthetic studies on naphthalenic hydrocarbons related to eudalene. These hydrocarbons are expected to be important as reference substances for the labelling of double bonds in naturally occurring eudalene?type sesquiterpenes. CHAPTER 1 The synthesis of 4,5?diisopropyl?eudalene starting from p?xylene and its characterization are described. A simpler route for the preparation of 4?ethyl eudalene, compared to that known in the literature, is also provided. CHAPTER 2 This chapter contains the synthesis and characterization of 4,5?diethyl eudalene. CHAPTER 3 This chapter presents the results of investigations on two projected routes for the synthesis of 3,8?diethyl eudalene, using:

The half?ester resulting from the condensation of diethyl succinate and 2,4?dimethyl acetophenone, and Certain reactions of 7,8?dihydro?1,3,6?trimethylnaphthalene.

PART III — Syntheses in the ??Selinene Series This part deals with synthetic studies in the ??selinene series. ??Cyerone, a sesquiterpene ketone of previously established structure and isolated from the essential oil of Cyerus rotundus, serves as the starting material. The following transformations are described:

Stereospecific reduction of the naturally occurring ketone to trans?dihydro???cyerone, Sodium–alcohol reduction to dihydro???cyerol, Purification via the 3,5?dinitrobenzoate, yielding the thermodynamically more stable isomer, Additional relevant transformation reactions.

PART IV — Synthetic Investigations on (+)?Cadinene This part describes synthetic studies on (+)-cadinene, an important naturally occurring sesquiterpene hydrocarbon. It is divided into two chapters. CHAPTER 1 This chapter describes an attempted synthesis of cadinene via condensation of isoprene with cryptone. An improved procedure for preparing (+)-cryptone is presented. The results of tert?butyl chromate oxidations of several alkyl cyclohexenes—conducted to explore an alternative route to cryptone—are also included. CHAPTER 2 This chapter describes a different synthetic approach to cadinene starting from anisole, involving a sequence of reactions leading to the successful synthesis of 1?isopropyl?7?methoxy?tetralin. Its oxidation to 4?isopropyl?6?methoxy?tetralone?1, a key starting material, is also described. From this intermediate, the further elaboration of the (+)-cadinene molecule is considered feasible.