Total Synthesis Of Sesquiterpenes, Seychellene, Trachyopsanes And Bisepoxysecocalamenenes

Abstract

Among Nature's creation, terpenoids are more versatile and exciting natural products. In a remarkable display of synthetic ingenuity and creativity, nature has endowed terpenes with a bewildering array of carbocyclic frameworks with unusual assemblage of rings and functionalities. This phenomenal structural diversity of terpenes makes them ideal targets for developing and testing new synthetic strategies for efficient articulation of carbocyclic frameworks. The thesis entitled “Total Synthesis of Sesquiterpenes Seychellene, Trachyopsanes and Bisepoxysecocalamenenes” describes the studies directed towards the total synthesis of the sesquiterpenes mentioned in the title. For convenience, the results are presented in three chapters; viz (1) First Enantiospecific Total Synthesis of Seychellene; (2) Enantiospecific First Total Synthesis of Trachyopsanes; and (3) Total Synthesis of Bisepoxysecocalamenenes. 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. The tricyclic sesquiterpene (−)-seychellene, containing an interesting tricyclo[5.3.1.03,8]undecane carbon framework, was isolated in 1967 by the research group of Hirose from the leaves of Pogostemon cablin Benth. An enantiospecific total synthesis of seychellene has been described in the first chapter of the thesis. To begin with, (R)-carvone has been transformed into 10-(1-methylethylidene)-3,8-dimethyl-tricyclo[5.3.1.03,8]undecan-2-one employing a tandem intermolecular Michael addition followed by intramolecular Michael addition reaction and intramolecular alkylation reactions. Degradation of the isopropylidene group followed by methylenation transformed 10-(1-methylethylidene)-3,8-dimethyltricyclo[5.3.1.03,8]-undecan-2-one into norseychellene. This methodology has been extended to the first enantioselective total synthesis of (+)-seychellene and (−)-seychellene via (S)-3-methylcarvone and (R)-3-methylcarvone, respectively. The marine sesquiterpene 2-isocyanotrachyopsane was isolated in 1996 by Fusetani and co-workers from the nudibranch Phyllidia varicosa. 2Isocyanotrachyopsane shows potent antifouling activity. During a search for DNA damaging agents, in 1997, Patil and co-workers reported the bioassay guided isolation of two new sesquiterpenes 2-formylaminotrachyopsane and N-phenethyl-N'-2-trachyopsanylurea from a sponge collected in Palau, Axinyssa aplysinoides Dendy 1922. In the second chapter of the thesis enantioselective first total synthesis of 2formylaminotrachyopsane and 2-isocyanotrachyopsane, establishing the absolute configuration of the natural products, has been described. To begin with, (R)-carvone has been transformed into a neopupukeandione employing a tandem double Michael reaction and intramolecular rhodium carbenoid CH insertion reactions. Neopupukea-nan-4-ol was transformed into 2-formylaminotrachyopsane by an acid catalyzed biomimetic rearrangement followed by Ritter reaction. Dehydration of formamide group in 2-formylaminotrachyopsane led to 2-isocyanotrachyopsane. In 1998, the research group of Weyerstahl reported the isolation of two new sesquiterpenes 1,10;7,10-bisepoxy-1,10-seco-calamenene and 6,7;7,10-bisepoxy-6,7-seco-calamenene from the essential oil Hedychium gardnerianum Roscoe. Presence of an interesting benzofused dioxabicyclo[3.2.1]octane framework coupled with the fact that the structures of the natural products were assigned on the basis of the spectral data of a mixture of two compounds prompted us to investigate the total synthesis to confirm their structures. The third chapter of the thesis describes the first total synthesis of these two compounds using an intramolecular ketalisation reaction. p-Cresol was converted into 2-(methoxymethoxy)-5-methylisobutyrophenone, which was further transformed in three steps into 1,10;7,10-bisepoxy-1,10-seco-calame-nene. Catalytic hydrogenation of 1,10;7,10-bisepoxy-1,10-seco-calamenene led to litseachromolaevane A, a new sesquiterpene isolated from an anti-HIV fraction of the leaves and twigs of Litsea verticillata Hance by Fong and co-workers in 2003. For the synthesis of 6,7;7,10-bisepoxy-6,7-seco-calamenene, m-cresol was converted into the 6-(methoxymethoxy)-6-[2-(methoxymethoxy)-4-methylphenyl]-2-methylheptan-3-one, which was transformed into 6,7;7,10-bisepoxy-6,7-seco-calamenene by an intramolecular ketalisation reaction.