On the structure of andrographolide

Abstract

The infrared spectrum of andrographolide shows the presence of a fivemembered unsaturated lactone-ring and a terminal methylene group in the molecule. On catalytic hydrogenation more than four atoms of hydrogen are taken up. A new diacetyl-anhydro-andrographolide is described.For some time we have been engaged in work on the constitution of andrographolide, the amaroid of Andrographis paniculata Nees (Acanthaceae). Recently we received a copy of the Helv. Chim. Acta in which Schwyzer et al. 1 ) publish the results of their work on the same substance. On the whole our results are in agreement with those of the Swiss authors, but there are some points of difference and we are able to offer some more suggestions as to the formula of andrographolide.Unlike the Swiss authors we found the melting point of andrographolide to be quite indefinite. W e observed melting points ranging from 226 to 230' 2 ) in working up different batches of dried leaves. Only in a few cases was the sharper melting point 231-232' observed. In these cases there was no indication of decomposition at the melting point and the melting point remained unaltered after cooling and remelting the same sample.If andrographolide is regenerated from pure andrographolic acid the same indefinite melting point is observed, indicating that impurities are not the cause of the melting point range, but that in all probability the indefinite melting point is caused by an isomerisation.It may be mentioned, that pure andrographolic acid does not give a colour when treated with alcoholic potassium hydroxide. In our opinion therefore this typical colour reaction of andrographolide is not due solely to the presence of a tertiary hydroxyl group -as surmised by the Swiss authors --but the unsaturated lactone-ring plays a role too.Neither by determination of the equivalent weight nor by catalytic 1) R. Schwyzer, H . G. Biswas and P. Karrer, Helv. 34, 652 (1951).2, All melting points in this article are uncorrected.