LABIATE 225
Pickard and Littleburyl have isolated from the oil an isomer of
menthol, which occurs in small quantity, and which they have named neo-
menthol. This body is of particular interest in regard to the stereo-
chemistry of this important body.
These chemists pointed out that a ketone of the configuration of
menthone should exist in both the cis and trans forms, one of Which
may be termed menthone, the other isomenthone. When the keto
grouping is reduced to the carbinol complex, two optically inactive
menthols should exist for each of the ketones, and each of these should!
yield its pair of optically active menthols. It is proposed to term the
optically inactive alcohols menthol and neomenthol in the case of men-
thone, and isomenthol and isoneomenthol in the case of isomenthone.
They artificially prepared the isomers, menthol and neomenthol, and re-
solved each into its optically active varieties, so that the characters of
four out of the eight possible optically active menthols are, to some ex-
tent, known. They have also shown that, in addition to Z-menthol,
tf-neomanthol is present in Japanese peppermint oil.
The starting-point of the experimental work was the reduction pro-
duct of thymol, by finely divided nickel. The unchanged thymol is
removed by washing with a solution of soda; alcohols are transformed
into hydrogen phthalic esters, which are then removed by sodium car-
bonate. The ketones remaining are treated with semi-carbazide in the
usual manner, and a mixture of two semi-carbazones obtained, from/
which menthone and isomenthone can be obtained. Fractional crystal-
lisation of the hydrogen phthalic esters from acetic acid yielded two pure
products—the less soluble melting at 177° and the more soluble one at
130°. The former on hydrolysis yields pure neomenthol (inactive) v
melting at 51°.
The ester melting at 130
°
yields inactive menthol melting at 34°.
Pure menthol (inactive) is characterised by its melting-point (34°),
and by those of its hydrogen phthalate (130°), and hydrogen succinate
(85° to 86°). It also forms a phenyl-carbamate melting at 102° to 104°.
By repeated recrystallisations of the brucine or cinchonine salt of the
hydrogen phthalate ester, from acetone or alcohol, the nearly pure
laevo-rotatory form can be obtained, whilst by the use of cinchonidine,.
the dextro-rotatory variety can be separated, i-menthol was found to
be identical in melting-point and specific rotation with the natural
menthol, which forms the principal ingredient of the peppermint oils,,
etc., whilst ^-menthol has the same melting-point and an equal rotation^
but in the opposite direction. Strangely enough, the dextro and laevo
varieties differ considerably in odour, and each forms a crystalline
benzoate, whilst the benzoate of the inactive variety is an uncrystallis-
able oil.
Neomenthol in the inactive condition melts at 51°, and forms a
hydrogen phthalate melting at 177°, and a phenyl-carbamate melting at
114°. On oxidation it yields menthone, identical with that formed by
the oxidation of menthol, thus proving that the two bodies are stereo-
isomers. By fractional crystallisation of brucine salt of the hydrogen
phthalate, the dextro variety was obtained, whilst by similar treatment
of the quinine salt of the hydrogen succinate Z-neomenthol was obtained.
The specific rotation of these optically active neomenthols is + 19'6°..
(^1) Jour. Chem. Soc., 101 (1912), 109.
VOL. I. 15