820. The chemistry of gum labdanum. Part II. The structure of labdanolic acid

Abstract

The structure of labdanolic acid, a constituent of gum labdanum, has IN Part I the isolation was described of labdanolic acid, a new bicyclic diterpene hydroxyacid, C,H3,03. It has now been shown to have structure (I).* Labdanohc acid was obtained by relatively mild hydrolysis of its methyl ester, showing that its carboxyl group was unhindered and not situated in the sterically hindered 4-position as in the majority of diterpene acids. The hydroxyl group, on the other hand, was unreactive, being acetylated only under vigorous conditions. On attempted mild oxidation of methyl labdanolate with chromic acid the ester was recovered unchanged. Reduction of the ester with lithium aluminium hydride gave the diol (111) which on mild acetylation afforded a monoacetate.Dehydration of methyl labdanolate with phosphoryl chloride in pyridine gave a homogeneous product (IV), the infrared spectrum of which indicated the presence of a vinylidene group (bands at 890 and 1645 cm.-l). This was confirmed by ozonolysis, formaldehyde and a keto-ester (V) being obtained. Reduction of the dehydration product, which was also formed when methyl labdanolate was acetylated under vigorous conditions, gave the alcohol (VI). This was characterised by conversion with osmium tetroxide in pyridine into a trio1 which is provisionally formulated as (VII), addition being assumed on the less hindered (a) side of the molecule (see below, however).The keto-ester (V) formed an oxime but not a 2 : 4-dinitrophenylhydrazone. These observations together with the carbonyl bands in the infrared spectrum at 1735 cm.-l (ester band) and 1712 cm.-l (aliphatic or 6-membered ring ketone band) suggested that the keto-ester contained a somewhat hindered keto-group in a six-membered ring. The formation of a hindered keto-group in a side chain appeared unlikely as it could only be accounted for by a branched side chain unlike any found in bicyclic diterpcnes. In turn, methyl labdanolate must have contained the grouping (VIII), with the hydroxyl group in the equatorial conformation to account for the formation of an exocyclic rather than an endocyclic double bond on dehydration.Dehydrogenation of the unsaturated alcohol (VI) with a platinum-charcoal catalyst gave a trimethylnaphthalene. It was identified as the 1 : 2 : 5-trimethyl isomer by comparison of its ultraviolet absorption spectrum with the spectra of the trimethylnaphthalenes described by Heilbronner, Frohlicher, and Plattner and by formation of the 1 : 3 : 5trinitrobenzene adduct. In addition 1 : 2 : 5 : 6-tetramethylnaphthalene was obtained and characterised as its 1 : 3 : 5-trinitrobenzene adduct.* The nomenclature used in this paper is based on the parent hydrocarbon (11) which is provisionally This It is to avoid confusion in been shown to be (I).The hydroxyl group was accordingly tertiary. called labdane and, on the advice of the Editor, is numbered on the principles used for steroids. hydrocarbon was previously called agathane and numbered differently. numbering that a new name is suggested.

Constituents of the Trunk Resin ofEperua purpurea

“…The already known labd-8 (20)-en-15oic acid (III a) was identified on the basis of its spectral properties, and the absolute stereochemistry was established by comparison with the data reported in the literature for the acid and its keto-derivative (III c) obtained by ozonolysis [8].…”

Qualitative evaluation of composition of the volatile fraction in commercial samples of Cistus incanus L.

“…Independent of the analytical method assumed, with each analyzed volatile fraction, the presence of the two compounds, namely, manoyl oxide and 13-epi-manoyl oxide, was established. These are the derivatives of the natural bicyclic diterpene labdane, which is the main component of the labdanum resin, characteristic substance excreted by the plants belonging to the Cistus genus [26,27].…”

Constituents of the Trunk Resin ofEperua leucantha

“…Mass spectral data were acquired and processed by a Finnigan Incos Data System. Identification of the individual components of the essential oil was made on the basis of comparison of retention times with those obtained in previous analyses (4) and literature mass spectral data.…”