Chemical studies of some constituents of Zingiber officinale

Abstract

Essential Oil of Ginger An exhaustive examination of the sesquiterpenes of ginger oil has been carried out. By using a combination of fractional distillation at reduced pressure and chromatography on silver nitrate treated alumina all of the major sesquiterpenes of ginger oil, except zingiberene, have been isolated in a substantially pure form. The presence of (+)-ar-curcumene was confirmed and the absolute configuration at the one asymmetric centre determined for the first time as (I). The bisabolene, shown to be present by previous workers, was conclusively demonstrated to be (-)-b -bisabolene. Two previously unknown sesquiterpenes were isolated. One of these, (1' R, 6S)-2-methyl-6-(4'-methylene cyclohex-2'-enyl) hept-2-ene (II), for which we propose the trivial name (-)- b -sesquiphellandrene, was characterised by the preparation of a solid nitrosite, physical constants, and infrared spectra. The structure was deduced mainly by spectral means while the stereochemistry was revealed by isomerisation to (-)-zingiberene. Isolation of the other new sesquiterpene proved laborious and difficult not allowing a final conclusion as to its structure. However, the chemical and spectral information obtained did reveal it to be a dicyclic sesquiterpene probably closely related to the cadalenic sesquiterpenes. A method was developed for the prediction of the retention times of sesquiterpenes based on the bisabolane and farnesane carbon skeletons. The Epoxide Dehydrogenation Technique The Hildebrand and Sutherland epoxide dehydrogenation technique for positioning double bonds in hydroaromatic systems has been extended and validated. In doing so the structure of dysoxylonene ((p)- d -cadinene, {p)-(III)) was confirmed and the structure of (+)- d -cadinene (III) conclusively demonstrated for the first time. Oleoresin of Ginger By a combination of solvent extraction and dry column chromatography on silica gel, the major pungent principle occurring in fresh ginger oleoresin was isolated. This was characterised as qgingerolq by the preparation of a crystalline methyl ether which had almost identical properties to those described for gingeryl methyl ether by Lapworth and co-workers. Spectral analysis and chemical degradation revealed gingerol and gingeryl methyl ether to have structures (IV) and (V) respectively. These structures are analogous to one of the two possible structures suggested for the compounds by Lapworth and co-workers but they differ in the length of the attached side chain. Another less important pungent constituent was isolated which was shown to have structure (VI) by chemical and spectral means and by analogy with gingerol. The presence of an additional compound having structure (VII) was inferred from degradative experiments. To distinguish the various gingerols it is suggested that the name gingerol be prefixed by a numeral indicating the number of carbon atoms in the aldehyde produced on alkaline degradation e.g. C6-gingerol for the major constituent. Oxidation of C6-gingerol and hydrolysis of the product yielded a glycol of known configuration thus demonstrating that the absolute configuration of the sole asymmetric centre in C6-gingerol is S according to the Cahn, Ingold, Prelog convention. Extraction methods and column chromatography permitted the isolation of another major pungent principle of commercially prepared oleoresins. Spectral and chemical studies showed that this compound had structure (VIII) originally assigned to shogaol by Nomura. C6-Gingerol dehydrated to shogaol under conditions which could be expected to occur during the preparation of ginger oleoresin. It is therefore suggested that shogaol is possibly not a natural compound. The occurence of palmitic acid in substantial quantities in ginger oleoresin was shown for the first time. Parts of this thesis have been published in Aust. J. Chem, 1966, 19, 283 doi:10.1071/CH9660283 and Tetrahedron Let, 1968, 519 doi:10.1016/S0040-4039(01)98796-9.