THE STEREOISOMERIC FORMS OF MENTHONE. 173- 
yielded a sparingly soluble semicarbazone melting, after 
careful purification, at 219 — 220°, and was apparently 
identical with the inactive menthone obtained by Wallach 
(a) from /\*-p-menthen-3-one, synthesised in turn from 
1,3,4-trioxyterpane, and .(b) from 1,4-methylhexanone 
(Annalen, 1908, 362, 272). The semicarbazone described by 
Wallach melted in each instance at 210 — 212°, but the lower 
value is readily accounted for by the presence of a more 
fusible isomeric semicarbazone (compare Wallach, *‘*Terpene 
und OCampher,’’ 1914, 377), which we have now isolated 
in a state of purity. The same optically inactive isomen- 
thone was evidently also obtained by Pickard and Little- 
bury (Trans. Chem. Soc., 1912, 101, 109), by the catalytic 
hydrogenation of thymol, the melting point of the semicar- 
bazone in this instance being 217°. 
Hitherto, no definite correlation appears to have been 
established between inactive isomenthone and the various. 
preparations of d-isomenthone which have been described - 
from time to time. It was therefore of considerable 
interest to find that levo-rotatory piperitone, extracted 
from the essential oil of Hucalyptus dives, yielded a highly 
dextro-rotatory isomenthone when hydrogenated in the 
presence of colloidal palladium. Asan example, a specimen 
of I-piperitone having «} —48°05° yielded a menthone which 
after purification had o? +58°33°. This result indicates. 
that optically active piperitone, when hydrogenated under 
appropriate conditions, yields an optically active isomen- 
thone having a reversed and enhanced rotatory power. 
Through the courtesy of Dr. Simonsen, of the Forest 
Research Institute, Dehra Dun, India, we were able to 
submit a specimen of d-piperitone, obtained from the 
essential oil of the Himalayan grass, Andropogon Jwar- 
