The constitution and stereochemistry of the triterpenoid a-onoceradienediol (a-onocerin), obtained from the roots of Ononis sfiinosa, have been elucidated. a-Onoceradienediol has the molecular formula C,,H,,O,, containing two secondary hydroxyl groups and two exocyclic methylene groups. It must, therefore, be tetracyclic. Removal of either of the secondary hydroxyl groups affords the same deosy-compound and demonstrates the unique symmetry of the molecule.The secondary hydroxyl groups have been proved to be contained in the system *CH,*CH,*CH (OH)*CMe,-in a six-membered ring, and the exocyclic methylene groups to be attached as part of the system CH2*C(:CH2)%H-in a six-membered ring.Dehydrogenation of a-onoceradiene affords 1 : 2 : 5-triniethylnaphthalene in high yield. Ozonolysis of a-onoceradienediol diacetate followed by hydrolysis and oxidation gives a C2* tetraketone which on Wolff-Kishner reduction and then dehydrogenation furnishes 1 : 5-dimethylnaphthalene.Acid-catalysed cyclisation of a-onoceradienediol affords a new series of symmetrical pentacyclic triterpenoids.Based on these and other experiments the constitution and stereochemistry (I ; R = OH) are proposed for a-onoceradienediol. This molecule is thus the first squalenoid to be detected in the vegetable kingdom; it is of importance in formulating theories of triterpenoid biogenesis. I THE triterpenoid, a-onoceradienedio1,t was first isolated a hundred years ago, from the roots of Ononis s$inosa L. (the restharrow) by Hlasiwetz ( J . prakt. Chem., 1855, 65, 419), Since that time its constitution has been studied in a desultory fashion, the earlier work being summarised in Elsevier's " Encyclopaedia " (Vol. XIV and supplement thereto). It has been established that a-onoceradienediol is a disecondary glycol containing two exocyclic methylene groups (or their equivalent) and having the composition CaH,,O, or C, H, O, .The more significant of the earlier work is that of Schulze (2. PhysioZ. Chem., 1936, 238, 35)) who obtained 1 : 2 : 5 : 6-tetramethylnaphthalene by dehydrogenation of a-onoceradienediol, and that of J. Zimmermann (Helv. Cltim. Ada, 1938,21, 853; 1940, 23, 1110 to which more detailed reference is made below. In most respects we have had no difficulty in confirming this earlier work of Schulze and Zimmermann.Extraction of commercial Ononis root gave a-onoceradienediol, conveniently isolated as its diacetate, by the procedure outlined on p. 2645, in about 0.2% yield. We express our best thanks to Messrs. Glaxo Laboratories for carrying out this extraction on a large scale. The first objective was to distinguish between the formul;,e Ca€I,80, and C, H, O, .This was done by studying the hydrogenation of the diacetate in acetic acid solution using a platinum catalyst. Zimmermann (Zoc. cit.) had reported that this afforded two onoceranediol diacetates. By thorough chromatographic fractionation of the hydrogenation product (see p. 2646) we were able to show that three onoceranediol diacetates were in fact produced. Onoceranediol-I diacetate, ch...