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Half penny Postage Stamps received for amou. ts under £1. THE CHEMICAL NEWS, 16, Newcastle Street, Farringdon Street, LONDON, E.C The best and brightest Photographic Weekly for the Beginner or Advanced Worker. PROFUSELY ILLUSTRATED. Office: 51, LONG ACRE, LONDON, W.C. CHEMICAL NEWS, May 11, 1917 THE Fischer's Synthesis of Depsides and Tanning Substances FISCHER'S SYNTHESIS OF DEPSIDES AND By Prof. J. A. WILKINSON, M.A, F.C.S. UNDER the generic name of tannin, a large number of vegetable substances from different sources have been included, which have been classified at various times in various ways, according to the point of view chosen. The experiments undertaken in this case were limited to the tannin of gall apples and some substances which belong to the same type. The first experiments carried out by Fischer and Freudenberg had for their object the ques in whether the glucose, fi st observed by Strecker in 1852.4. was in reality a component of the tannin molecule or merely an accidental adulteration. For this purpose they used as raw materials: (a) Acidum tannicum leviss. puriss. (Merck's parest preparation); (b) ditto (Kahlbaum), both of which they subjected to purification by special methods devised for this purpose. Briefly these consisted in (1) Extraction with ether, which gave a yield of 60 per cent; (2) extrac tion with ethyl acetate gave a yield of 65 per cent; (3) preparation of the potassium salt and subsequent acidification with N.H2SO4Aq. The object was to get a preparation as pure as it was possible to get. The purified tannin so prepared was so hygroscopic that great care had to be taken to exclude the moisture of the atmosphere when weighing. On analysis the following numbers were obtained: 217 fully estimated by several methods, from the acid portion of the molecule. Examination and analysis of the osazone proved conclusively that dextrose was present and the acid gallic acid. If the hydrolysis were not completed, in addition to the gallic acid and glucose, a dark coloured substance remained, which was called the tannin residue. As a final check, artificial mixtures of gallic acid and dextrose were also made and similarly treated. The acid used was 5 per cent H2SO4Aq, but in one case, Strecker's method, using 11 per cent H2SO4Aq and heating for twenty-four hours at 400° was employed. The results obtained are shown in Table A. These experiments lead to the conclusion that in the purest tannin investigated 1 mol. glucose is combined with about 10 mols. gallic acid, and no other phenolcarboxylic acia is present. In order to elucidate its structure still further, an attempt was made to isolate intermediate produces, but this was found to be impossible owing to their properties, in consequence of which the synthetic method was adopted. No free carboxyl being present, as the above experiments showed, the method of combination of the glucose with the gallic acid was ind.cated as being in the form of an ester, and these conditions are satisfied if tannin be consilered as a compound for ned by the union of I mal. glucose with 5 mols. digallic acid, similar to pentacetylglucose. Digallic acii, C6H2(OH);CO OC6H2(OH)2COÓH, is a didep side, which was synthetically prepared in a pure condition, first by Fischer in the manner indicated above, but, as it is extremely difficult to work with, an attempt was first made with gallic acid in order to obtain pentagalloyl glucose, C6H706 OC C6H2(OH)35, and test its properties. In this The methods hitherto used for the complete acylation of glucose were found to be unavailable for the preparation of pentagalloyl glucose, and a new method had to be worked out. This was done, and consisted in bringing together the glucose and the chloride of the acid dissolved in dry chloroform, together with quinoline. manner pentatrimethylcarbonatogalloyl glucose (mol. wt. 18105), CH62056 or C6H-06[OC.C6H2(OCOOCH3)3`5, was obtained as a granular colourless amorphous powder sintering about 95°, [a) v 20 34 3+ (04). By careful saponification with excess alkali in aqueous acetone solution at ordinary temperature this yielded pentagalloyl glucose, C41H32026 (mol. wt. 940 26) as a yellow amorphous powder, softening about 150° and begining to decompose at 160, [ 20 31-35°. D This substance shows a startling similarity to tannin, the chief differences being the optical rotation and the amount of gallic acid obtained on hydrolysis with sulphuric acid. It possesses a strong astringent and bitter, but not acid, taste. It is soluble in most of the organic solvents, gives coloration with ferric chloride (inks), and is precipi tated by the same reagents. Analysis gave The benzoylation of glucose was also carried out, and in this case two stereoisomers were obtained which could be separated by crystallisation, according as a- or 3-glucose was used, but in the case of the corresponding galloyl. compounds, owing to their amorphous nature, this was impossible. From analogy and other considerations Prof. Fischer does not consider that the pentagalloyl glucose is a single uniform substance, but probably a mixture of isomers. In spite of this, he states that with its preparation the chapter of the principal synthesis seems to be concluded, though not yet fully completed, owing to the fact that tannin itself has not yet been synthesised. A large number of other experiments have been carried out, some dealing with other commercial tannins, which were purified and hydrolysed as above, others being attempts at synthesis along the above lines, but these have not yielded |