Transition metal complexes in the structure, streochemistry and synthesis

Abstract

The thesis entitled "Transition Metal Complexes in Structure, Stereochemistry and Synthesis of Some Naturally Occurring Compounds" consists mainly of two chapters. The first chapter is divided into two parts, and Part I is further divided into three sections. Section I contains a brief introduction to the chemistry of the extractives isolated from Garcinia morella. In Section II, the structure of morellin, a xanthone isolated from the seed coat extract of Garcinia morella, has been discussed. The structure of morellin was established on the basis of X-ray diffraction data of p-bromobenzenesulfonate of morellin by Kartha et al. But morellin readily undergoes isomerisation to a closely related compound, isomorellin, under acidic or basic conditions. Hence, the structure of morellin, as determined by the X-ray analysis, is questionable since this derivative, which has been prepared under basic conditions, might have been that of isomorellin formed through isomerisation. Morellin and isomorellin have the same gross structure but differ in the configuration of the unsaturated aldehyde. Hence, this problem was undertaken with the aim of solving the ambiguity of the configuration around the double bond carrying the aldehyde group in morellin and isomorellin. A brief review on tris(triphenylphosphine)rhodium chloride, (PPh?)?RhCl, which is a versatile homogeneous catalyst for decarbonylation of acid chlorides and aldehydes, is presented. It has been proved that the decarbonylation is stereoselective by Wolborsky et al. Hence, to determine the stereochemistry of ?,?-unsaturated aldehyde in morellin and isomorellin, the decarbonylation technique was chosen. Both morellin and isomorellin have been successfully decarbonylated to the corresponding olefins using (PPh?)?RhCl. The structures of the olefins derived from morellin and isomorellin have been shown to possess the trans and the cis geometry as evidenced from their IR and PMR data. This confirms the configuration of the unsaturated aldehyde in morellin and isomorellin and hence their structures. Further confirmation was obtained from a study of the NMR spectra of morellin and isomorellin, which is presented in this section. Section III is devoted to the studies on the decarbonylation of some natural compounds like nuciferal, auroglaucin and flavoglaucin and a synthetic analogue of nuciferal, 2-methyl-6-(p-methoxyphenyl)hepta-2-en-1-ol. Nuciferal is isolated from Torreya nucifera and its structure was established as 2-methyl-6-(p-tolyl)hept-2-en-1-al. The configuration of the double bond of the unsaturated aldehyde has been assumed to be trans on the basis of its NMR data. This assumption has been confirmed from the decarbonylation studies since the resulting olefin possessed cis-configuration. Similarly, the analogues of nuciferal afforded the cis-olefin on decarbonylation with TTRC. Auroglaucin and flavoglaucin are the main constituents of the mould Aspergillus glaucus. Their structures have been established as the derivatives of 2,5-dihydroxy-3,3-dimethylallyl benzaldehyde. The position of the dimethylallyl substituent was shown to be meta to the aldehyde group by degradation and synthesis of the intermediate. This problem was investigated in order to obtain confirmatory proof for the structures of auroglaucin and flavoglaucin. Decarbonylation of auroglaucin and flavoglaucin afforded the corresponding benzenoid compounds having a 2,3,5,6-tetrasubstituted pattern as shown from their PMR data, thus confirming the position of aldehyde in the parent compounds. In Part II, the isolation of a new compound moreollin from the seed coat of Garcinia morella is described. This compound was isolated from the mother liquors after the removal of morellin. The structure of moreollin is deduced from its physical and chemical data. Like morellin, moreollin also isomerises rapidly to isomoreollin, the structure of which has been conclusively established by partial synthesis from isomorellin. Like morellin and isomorellin, moreollin and isomoreollin differ only in the configuration of the unsaturated aldehyde. The chemistry of moreollin is discussed. Chapter II is divided into two parts. Part I consists of a brief account of the hydrogenations using pentacyanocobaltate(II) complex anion, Co(CN)?²?, as a homogeneous catalyst. The hydrogenation of several conjugated organic substrates with this homogeneous catalyst and the mechanisms involved have been reviewed in detail. Part II contains the results of hydrogenation of certain ?,?-unsaturated carbonyl compounds hitherto not reported using Co(CN)?²? as homogeneous catalyst. Thus, 1-carvone gave dihydrocarvone; ?-ionone gave ?,?-dihydro-?-ionone; mesityl oxide gave 4-methyl-pentan-2-one; cyclohexenone gave cyclohexanone; benzylidene acetone gave 4-phenyl-2-butanone; and citral gave citronellol. To gauge the utility of this catalyst, the hydrogenation of some pharmacologically important steroid hormones, viz., testosterone, its acetate, dienobol, ergosteryl acetate, 5?-androst-1-en-17?-ol-3-one acetate, androst-4-en-3,17-dione and androst-1,4-diene-3,17-dione was investigated. Except for the 5?-androst-1-en-17?-ol-3-one acetate, the hydrogenations of all the other steroid hormones were not successful. The mechanism of hydrogenation was discussed in terms of the product formation and the steric hindrance. Detailed experimental procedures are given at the end of each chapter.