Total Synthesis Of Sesquiterpenes Acorenols, Chamigrenes And Laurokamurene B; And Enantiospecific Synthesis Of ABC-Ring System Of A-Nor And Abeo Pentacyclic Triterpenes

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 make them ideal targets for developing and testing new synthetic strategies for efficient articulation of carbocyclic frameworks. The thesis entitled “Total synthesis of sesquiterpenes acorenols, chamigrenes, and laurokamurene B; and Enantiospecific synthesis of ABC-ring system of A-nor and abeo pentacyclic triterpenes” describes the studies directed towards the total synthesis of the sesquiterpenes mentioned in the title and exploratory studies towards triterpenoids. 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 sesquiterpenes acorenols, containing an interesting spiro[4.5]decane carbon framework, was first isolated in 1970 by the research group of Tomita from the wood of Juniperus rigida. Recently, in 2003, Braun and coworkers reported the isolation of epi α- and epi β-acorenols along with α- and β-acorenols from the sandal wood oil Santalum spicatum. Total synthesis of all the four acorenols has been described in the first part of the first chapter of the thesis. Initially, a model study has been carried out for the spirocyclopentannulation of cyclohexanone employing a combination of Ireland ester Claisen rearrangement and ring closing metathesis reaction to furnish methyl 4-methylspiro[4.5]dec-3-en-1-carboxylate. The same methodology has been extended for the total synthesis of all the four acorenols starting from cyclohexane-1,4-dione via cis and trans isomers of methyl 4-methyl-8-methylene-spiro[4.5]dec-3-ene-1-carboxylate. Total synthesis of β-chamigrene, γ-chamigrene and laurencenone C, containing spiro[5.5]undecane carbon framework, has been described in the second part of the first chapter. As a model study, cyclohexanone has been transformed into 1,5,5-trimethylspiro-[5.5]undec-4-en-3-one employing a combination of Ireland ester Claisen rearrangement and intramolecular type-II carbonyl ene reactions. The methodology has been extended to chamigrenes starting from cyclohexane-1,4-dione via methyl 2-(1-isopropenyl-4-oxocyclo-hexyl)-2-methylpropanoate and 5,5-dimethyl-1,9-ismethylenespiro[5.5]undecan-3-ol. The marine sesquiterpenes laurokamurenes were first isolated in 2006 by Mao and Guo from Laurencia okamurai Yamada. First total synthesis of (±)-laurokamurene B has been described in the first part of the second chapter. To begin with Ireland ester Claisen rearrangement of but-2-enyl 2-methylpropionate furnished methyl 2,2,3-trimethylpent-4-enoate, which was then transformed into 4,5,5-trimethyl-3-(4-methylphenyl)hepta-1,6-dien-3-ol. RCM reaction followed by reductive deoxygeneation transformed 4,5,5-trimethyl-3-(4-methylphenyl)hepta-1,6-dien-3-ol into (±)-laurokamurene B. Subsequently, an enantioselective total synthesis of (+)-laurokamurene B has been accomplished. Stereoselective hydrogenation of methyl campholenoate furnished methyl 2-[(1S,3S)-2,2,3-trimethyl-cyclopent-1-yl]acetate, which was then transformed into (+)-laurokamurene B via degradation of the two carbon side chain and introduction of the aryl moiety, which established the absolute configuration of laurokamurenes. The third chapter deals with the enantiospecific generation of ABC-ring system of A-nor and abeo 4(3 → 2) tetra and pentacyclic triterpenes. To begin with (R)-carvone was identified as B-ring of ABC-ring system of A-nor and abeo tetra and pentacyclic triterpenes, as the absolute configuration at the C-5 position of the targets correlate to the stereo centre of carvone, and isopropenyl group can serve as the C-4 carbon of the targets along with the two gem-dimethyl groups. A lithium liquid ammonia mediated cyclisation of δ,ε-unsaturated esters was employed for the construction of the A ring and an RCM reaction was opted for the construction of the C ring. (R)-Carvone has been converted into 2-(1-ethoxyethoxy)-1,3,7,7-tetramethylbicyclo[4.3.0]non-3-en-8-ol via lithium liquid ammonia mediated cyclisation of methyl 2-(1-ethoxyethoxy)-6-isopropenyl-1,3-dimethylcyclohex-3-enyl]acetate, which was then transformed into 4-methoxymethoxy-2,5,5,9-tetramethyltricyclo[7.4.0.02,6]tridec-11-en-8-one via the RCM reaction of 3,4-bisallyl-8-methoxymethoxy-4,6,9,9-tetramethylbicyclo-[4.3.0]nonan-3-one. The strategy has been further extended to the synthesis of 4-methylene-2,5,5,9-tetramethyltricyclo[7.4.0.02,6]tridec-11-en-8-one, which contains the ABC ring system of abeo 4(3→2) tetra and pentacyclic triterpenes.