7-Ketodehydroabietic acid

Ketodehydroabietic acid R1 = R2 = O 7-Hydroxydehydroabietic acid R1 = OH R2 = H Figure 17 Structures of some constitutive antimicrobial terpenoids. 

Parallel analyses by IR, thin-layer chromatography (TLC), and gas chromatography-mass spectrometry (GC-MS) of organic remains adhering to shards of ancient amphoras excavated in the harbor of Carthage (Tunisia) identified these remains as pine pitches. Capillary GC of methylated acid fractions showed abietic acid, dehydroabietic acid, and 7-ketodehydroabietic acid as the principal components. Two-dimensional TLC of untreated ether extracts revealed abietic acid in 12 of 31 samples and dehydroabietic acid in 26 of 31 samples. IR spectra of solid, raw samples indicated the presence of isopropyl groups, characteristic of the abietane skeleton, in 80% of the samples. Rapid and convenient analysis by TLC and IR was, in most cases, sufficient to identify pine resin products even after extensive pyrolytic and oxidative degradation. 

Reference Standards. Reference standards for TLC, GC, and MS were obtained as follows. Pure abietic acid (mp 172-173 °C) was used as received. Dehydroabietic acid was prepared by oxidation of abietic acid with selenium dioxide to hydroxy abietic acid and subsequent dehydration with glacial acetic acid (11). 7-Ketodehydroabietic acid was prepared by oxidation of dehydroabietic acid with potassium permanganate and isolation of the product by way of the Girard reagent T (12). Pyroabietic acid (a mixture of dehydro- and dihydroabietic acids) was prepared by heating abietic acid with 10% palladium on charcoal to 250 °C for 1 h (13). 

On aging, and more rapidly on heating, abietic acid is oxidized first by disproportionation (i.e., without need of oxygen) to dehydroabietic acid (structure 1) and then (but only if exposed to oxygen) to 7-ketodehydroabietic acid (structure 2). The skeletal frequencies of dehydroabietic acid lie at 1110, 1130, and 1175 cm"1 (14),... 

Sample 7 contained more than 20% unchanged abietic acid, more than 50% dehydroabietic acid, but very little 7-ketodehydroabietic acid. This composition is not expected of a pine pitch but of a pine resin that has been through slow disproportionation in an anaerobic environment. This residue is therefore that of a pine resin added to wine to make the retsina for which Greece was noted in antiquity as it is today. Sample 1 contained only 1% unchanged abietic acid, but more than half of the dehydroabietic acid was oxidized to 7-ketodehydroabietic acid, a finding that indicates the presence of air during the preparation of the pitch. 

Samples 10 and 12 are virtually identical neither contained any unchanged abietic acid, and both contained about twice as much 7-ketodehydroabietic acid as dehydroabietic acid. Both are extensively decarboxylated, as indicated by their low total acid content. Therefore, these samples are pitches that were prepared by heating a pine resin for a long time or at a high temperature with ready access to oxygen, as would be the case with stirring in a shallow vessel. 

Beckman rearrangement of the oxime of 7-ketodehydroabietate with phosphoric acid gives the expected lactam (50) and a decarbonylated amine (51). The dihalocarbene adduct of methyl abietate is formed " across the 7(8) double bond. The photo-oxidation of methyl isopimarate, which has a A -double bond, gives 7a-hydroxysandaracopimaric acid methyl ester. However, in the pimaric acid series both the 7a- and 14a-allylic alcohols are formed. 

A soln. of methyl 7-ketodehydroabietate, 90%-ferf-butyl hydroperoxide, and some coned. H2SO4 in acetic acid heated 55 hrs. at 50-55° 5 (6)-dehydro-... 

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