Depside hydrolysis products

Chemically, the procedure of hydrolytic degradation of depsides and identification of the hydrolysis products remains a general and definitive procedure. For example the new depside 2-O-methylconfluentic acid (237) has been isolated from the lichen Lecidea fuscoatra and the structure deduced from spectroscopic data 190). Hydrolysis of the corresponding methyl ester, methyl 2-0-methylconfluentate (238) with methanolic potassium hydroxide gave 2,4-dimethoxy-6-(2-oxoheptyl)benzoic acid (239) and methyl 4-hydroxy-2-methoxy-6-pentylbenzoate (240) thus establishing the structure of this depside (Scheme 27). 

In a number of cases where only fragments of lichen material were available (and hence only traces of the new depsides present often as mixtures), microhydrolysis of the depside with concentrated sulfuric acid followed by comparative t.l.c. analysis of the hydrolysis products has proved a particularly effective method of primary structural elucidation. This technique was used to identify the new depside 2-O-methylperlatolic acid (241) present in Pertusaria tuberculifera Nyl. 80). Microhydrolysis of (241) with sulfuric acid led to the formation of 2,4-dimethoxy-6-pentylbenzoic acid (242) and 2,4-dihydroxy-6-pentylbenzoic acid (243), and these acids were identified by comparison of the Rp values on t.l.c. with those of the authentic acids in three independent solvent systems 80) (Scheme 28). 

Although further transformations of the hydrolysis products (e.g. decarboxylation, lactonisation) are sometimes observed under these conditions, in general they cause no confusion. Although such experiments do not provide absolute proof of structure without isolation and characterisation of the new depsides and the hydrolysis products they certainly do provide compelling evidence for them. In a number of cases the structures assigned by this method have subsequently been confirmed by comparisons of natural and synthetic material 111, 120). 

High performance liquid chromatography (h.p.l.c.) has also been effectively employed as an analytical tool in examining depsides and their hydrolysis products in microextracts (67, 73, 87). The major advantage of this technique is that it yields quantitative information about the components present in crude extracts. 

The structure of 3,5-dichloro-2 -0-methylanziaic acid (208) was assigned from the spectroscopic properties and the nature of the hydrolysis products 183). Further evidence for this structure was forthcoming from the partial synthesis of methyl 3,5-dichloro-2,4,2 -tri-0-methylanzeate (252) both from anziaic acid (253) and the new depside (208) (Scheme 29) 183). 

The structural elucidation of the new depside 2-O-methylconfluentic acid (237) 190) via the nature of the hydrolysis products has been mentioned previously (Scheme 27). The total synthesis of this compound has also been reported 119) and was the first instance where a depside containing an oxidized side chain had been synthesized. Here additional difficulties arise because of the close proximity of the reactive benzyl ketone... 

Culberson and Kristinsson (1969) 7 Depsides, 11 hydrolysis products, precoated plates... 

Hydrolysis is performed ideally by dissolving 0.1-5 mg of the depside in 0.05-1 ml of concentrated sulfuric acid at — 10°-0 C, and after 10-30 minutes adding crushed ice. The hydrolysis products are extracted in ether, and the etheral extract can be used directly for chromatography. 

The ester linkage in depsides can be hydrolyzed with concentrated H2SO4 at 0°C or with aqueous or methanolic KOH. The corresponding phenolic carboxylic acids, their methylesters, or their decarboxylation products are obtained. Hydrolysis is especially easy with depsides having a/5-positioned carbonyl group in the 6 and 6 side chains. One therefore obtains from... 

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