Structure and Interrelationship of Orientin and Homo-orientin.

Since the first reported occurrence of orientin and homo-orientin in plant material 2 it has been recognized that these compounds possess a luteolin (3'.4'.5.7-tetrahydroxyflavone) nucleus. However, the identity of the aliphatic portions of the molecules and the interrela­ tionship of the two compounds have been a matter of speculation and uncertainty2_4. The structures and interconvertibility of the compounds can, however, now be satisfactorily explained in the light of the following findings. Oxidation of orientin and homo-orientin tetramethyl ethers5 with aqueous ferric chloride under conditions previously described 6 and examination of the reaction mixtures for the presence of sugars has yielded the same result for both compounds and in each case ana­ lysis has revealed the presence of two sugars, arabinose and glucose, identical with authentic arabinose and glucose respectively by chromatography in benzene — butan-l-ol —pyridine —water 1 : 5 : 3 : 3, by vol., in butan-l-ol —acetic acid —water 20 : 5 : 11, by vol. and in 75% aqueous phenol. The presence of a pentose and a hexose in each case indicates that oxidative fission of the aliphatic residue occurs not only between C (l) and C ( 2 ) of the glycosyl residue as in the case of barbaloin6 but also between C (l) of the glycosyl residue and the point of attachment of this residue to the flavone nucleus. The previously reported results for the C-glycoside barbaloin 6 were confirmed in this laboratory with authentic barbaloin, m. p. 146 — 148°, isolated from commercial aloin, when only traces of glucose could be detected. Treatment of the periodate-oxidized, borohydridereduced tetramethyl ethers of orientin and homo-orientin by refluxing in N-aqueous hydrochloric acid for 15 min. resulted, in each case, in the liberation of gly­ cerol. A similar result was obtained by H ansel and R im pler 7 for homo-orientin isolated from Vitex agnus castus. Contrary to the findings of H ay and H aynes 6,