110
WERNER BERGMANN
conveniently be divided into several groups on the basis of the degree
and location of unsaturation in their ring structures (18). Such unsaturation significantly influences the direction and magnitude of the optical
rotatory power of the sterols, thereby permitting their preliminary
classification. Very pronounced, negative specific rotations, in excess of
—90°, are characteristic of the presence in ring B of a Δ
5 '
7 -system of
conjugated double bonds (I). A more moderated levorotation of —30
to —45° is typical of sterols with a double bond in the 5,6-position (II),
and rotations of —50 to —70° point to the presence of additional unsaturation in the side chain. Sterols with a double bond in the 7,8position (III) show specific rotations rather close to zero, or somewhat
more negative when accompanied by side-chain unsaturation. Absence
of unsaturation in the ring systems (IV) is revealed by a significant,
positive specific rotation from +10 to +30°, and more pronounced
dextrorotations, of +40 to +50° point to the presence of a double bond
at 8,9 (V). Higher positive rotations generally indicate the presence of
triterpenoids which are not included among the sterols with the exception of such methylsterols as (VI) and (VII).
Additional differences between the sterols are associated with the
side chain, its unsaturation and its size. In many invertebrate sterols
and nearly all plant sterols the eight-carbon side chain of cholesterol is
replaced by a nine-carbon unit with a methyl branch at C-24 (E-H),
or a ten-carbon unit with an ethyl group at this position (J-M). Such
substitution confers asymmetry upon C-24, and hence two sets of isomers,
epimeric at this position, are expected to exist. Although nature is rather
stereospecific and generally synthesizes but one of the two possible
isomers, in this case natural representatives of both epimeric series have
become known. In several instances their absolute configurations have
been rigidly established. Following a proposal made by Fieser in the
forthcoming new edition of his well-known monograph on sterols (14),
a methyl or ethyl group at C-24 will receive the designation a when on
the same side of side chain as the «-oriented methyl group at C-20 (E,
F and J, K), and the designation ß, when in the opposite epimeric position (G, H and L, M).
The most common unsaturation in the side chain is the double bond
at 22,23. As yet it has been found only in sterols with alkyl substituents
at C-24. In all known examples this double bond is frans-oriented, as
may be readily recognized by the characteristic peak at 10.30 μ in the
infrared spectrum (16). The double bond noticeably enhances the levorotatory power of a sterol. A sterol unsaturated at 24,25 (II, R = B) is
thought to represent an intermediate step in the biosynthesis of cholesterol. In animal sterols this side chain (B) has so far been found only
WERNER BERGMANN
conveniently be divided into several groups on the basis of the degree
and location of unsaturation in their ring structures (18). Such unsaturation significantly influences the direction and magnitude of the optical
rotatory power of the sterols, thereby permitting their preliminary
classification. Very pronounced, negative specific rotations, in excess of
—90°, are characteristic of the presence in ring B of a Δ
5 '
7 -system of
conjugated double bonds (I). A more moderated levorotation of —30
to —45° is typical of sterols with a double bond in the 5,6-position (II),
and rotations of —50 to —70° point to the presence of additional unsaturation in the side chain. Sterols with a double bond in the 7,8position (III) show specific rotations rather close to zero, or somewhat
more negative when accompanied by side-chain unsaturation. Absence
of unsaturation in the ring systems (IV) is revealed by a significant,
positive specific rotation from +10 to +30°, and more pronounced
dextrorotations, of +40 to +50° point to the presence of a double bond
at 8,9 (V). Higher positive rotations generally indicate the presence of
triterpenoids which are not included among the sterols with the exception of such methylsterols as (VI) and (VII).
Additional differences between the sterols are associated with the
side chain, its unsaturation and its size. In many invertebrate sterols
and nearly all plant sterols the eight-carbon side chain of cholesterol is
replaced by a nine-carbon unit with a methyl branch at C-24 (E-H),
or a ten-carbon unit with an ethyl group at this position (J-M). Such
substitution confers asymmetry upon C-24, and hence two sets of isomers,
epimeric at this position, are expected to exist. Although nature is rather
stereospecific and generally synthesizes but one of the two possible
isomers, in this case natural representatives of both epimeric series have
become known. In several instances their absolute configurations have
been rigidly established. Following a proposal made by Fieser in the
forthcoming new edition of his well-known monograph on sterols (14),
a methyl or ethyl group at C-24 will receive the designation a when on
the same side of side chain as the «-oriented methyl group at C-20 (E,
F and J, K), and the designation ß, when in the opposite epimeric position (G, H and L, M).
The most common unsaturation in the side chain is the double bond
at 22,23. As yet it has been found only in sterols with alkyl substituents
at C-24. In all known examples this double bond is frans-oriented, as
may be readily recognized by the characteristic peak at 10.30 μ in the
infrared spectrum (16). The double bond noticeably enhances the levorotatory power of a sterol. A sterol unsaturated at 24,25 (II, R = B) is
thought to represent an intermediate step in the biosynthesis of cholesterol. In animal sterols this side chain (B) has so far been found only