APPLICATION OF SPECTRAL DATA TO INVESTIGATION OF GROSS MOLECULAR STRUCTURE

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    Hypericin, the photodynamically active plant pigment, has a molecular formula C30H16O8. Kuhn–Roth analysis shows two C—Me groups and benzoylation gives a yellow hexabenzoate. Reductive benzoylation of hypericin, on the other hand, affords a blue octabenzoate. These observations led Brockmann and his colleagues to the conclusion that hypericin is a hexahydroxyquinone. The fact that zinc dust distillation of hypericin yields meso-anthrodianthrene (CLXXV) without loss of carbon atoms, allows only two possible extended quinonoid systems (CLXXVI) and (CLXXVII) for hypericin. The shift of the ultraviolet spectrum in alkaline solution and the reactivity of the hydroxyl groups suggested the presence of a number of OH groups in the peri-positions to the quinone carbonyl functions. Since only two of the six hydroxyl functions are acetylated by keten, the remaining four OH groups were assumed to be in peri-positions.

     

    Brockmann61 next devised a spectroscopic method for the identification of parent hydrocarbons of polycyclic hydroxyquinones. This employed the technique of reductive acetylation to the corresponding polyacetoxy-hydrocarbon, whose spectrum closely resembles that of the parent hydrocarbon in shape and band spacings with the maxima shifted slightly to longer wavelengths by the presence of the acetoxyl groups which correspond roughly to methyl groups in their power of bathochromic displacement. Thus 1,4,5-trihydroxyanthraquinone on reductive acetylation gives 1,4,5,9,10-penta-acetoxyanthracene, the latter having λmax 350, 370, 385, and 410 mμ in dioxan. For comparison, anthracene in this solvent has bands at 325, 340, 360 and 378 mμ, the shapes of these curves being very similar. For strict comparison it would be necessary to have 1,4,5, 9,10-pentamethylanthracene available. Since it is often difficult to obtain reference samples of such complexity, the working rule of a red shift of the entire spectrum of 5 mμ for each acetoxyl group added to the hydrocarbon gives generally adequate agreement.

     

    We now return to the reductive acetylation of hypericin. On the basis of partial structures (CLXXVI) and (CLXXVII) we should expect to be able to distinguish between the parent systems generated by zinc treatment, viz. 2,2-dimethylhelianthrene (CLXXVIIIa) and 2,2′-dimethylmesonaphthodianthrene (CLXXVIIIb). The maxima of these hydrocarbons are sufficiently disparate for such a decision to be made, for the former has its longest band at 543 mμ and is red, while the latter absorbs in the visible at 627 mμ and is coloured blue. In the event, reductive acetylation of hypericin gives a blue hexa-acetate whose spectrum is almost identical with that of (CLXXVIIIb) and whose longest wavelength band is at 625 mμ. Now this is at first sight a surprising result, for we should expect a bathochromic shift of 30 mμ of the spectrum of the hydrocarbon by the introduction of six acetoxyl groups. The desmethyl hydrocarbon (CLXXIX) itself, however, has λmax 605 and 660 mμ in the visible region, the 3,3′-dimethyl derivative λmax 609 and 663 and the 10,10′-diacetate 626 and 678 mμ.

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