2. STEROLS: STRUCTURE AND DISTRIBUTION
117
methyl-A
7 >
22 -cholestadienol (stellasterol) (III, R = H) are believed to
be some of the major sterols of starfish. They have not yet been isolated
in a uniform state (see under Echinoderms, Section IX).
24-Methylene-A
7 -cholestenol (episterol) (III, R = D), C 2 8H 46 0, m.p.
151°, [ a ] D —50°; acetate, m.p. 162°, [ a ] D —4°. The structure of this
compound, which is one of the minor yeast sterols, has been derived
from degradation studies (47) and optical rotations (31); it needs confirmation by synthesis. This sterol may be viewed as an analog of 24methylenecholesterol. As this is found among the A
5 -sterol mixtures of
marine invertebrates, so will episterol probably be encountered in corresponding A
7 -sterol mixtures.
24a-Ethyl-A
7 -cholestenol
(A
7 -stigmastenol) (III, R = J), C29H50O,
m.p. 145°; [a] D +8°; acetate, m.p. 157°, [a] D +7°; and 24a-ethyl-A
7 >
22 -
cholestadienol (α-spinasterol) (III, R = K), C 29 H 48 0, m.p. 174°, [a] D
—3°, infrared spectrum (26); acetate, m.p. 187°, [a] D —5°. They are
the principal A
7 -sterols of plants and may well occur in the complex
A
7 -sterol mixtures from certain marine invertebrates, such as starfish.
24a-Ethyl-A
7 >
22 -cholestadienol (chondrillasterol) (III, R = M), C 29
H 48 0, m.p. 169°, [a] D —2°, infrared spectrum (26); acetate, m.p. 175°,
[a] D —1°. This sterol has been isolated from a sponge and an alga (48).
4. Group IV (Structure IV): Sterols Saturated in the Ring System with
Specific Rotations of [a] D -f 15° to -\-30°
All known sterols belonging to this group are completely saturated
with the exception of neospongosterol (IV, R = H) which carries a
double bond in the side chain. They may therefore readily be distinguished from sterols of other groups by their inertness toward bromine
and other double-bond reagents. When quite pure such sterols do not
give any of the typical color reactions. Saturated sterols may be freed
from unsaturated impurities by treatment with acetic anhydride and
concentrated sulfuric acid in carbon tetrachloride. In this reaction only
the unsaturated impurities form colored, acidic products, separable
from the unreacted saturated sterols by alkali (49). The differences
between the molecular rotations of ring-saturated sterols and of their
acetates and benzoates are —34 ± 11 and +2 ± 3, respectively (31).
The last value is particularly characteristic and useful in gauging the
uniformity of a saturated sterol.
Only in certain sponges have saturated sterols so far been found to
be the principal components of sterol mixtures. In other cases they
often represent such a small fraction of the mixture that they are easily
overlooked. It is probable that eventually the saturated derivatives of
all known sterols will be isolated from natural sources.
117
methyl-A
7 >
22 -cholestadienol (stellasterol) (III, R = H) are believed to
be some of the major sterols of starfish. They have not yet been isolated
in a uniform state (see under Echinoderms, Section IX).
24-Methylene-A
7 -cholestenol (episterol) (III, R = D), C 2 8H 46 0, m.p.
151°, [ a ] D —50°; acetate, m.p. 162°, [ a ] D —4°. The structure of this
compound, which is one of the minor yeast sterols, has been derived
from degradation studies (47) and optical rotations (31); it needs confirmation by synthesis. This sterol may be viewed as an analog of 24methylenecholesterol. As this is found among the A
5 -sterol mixtures of
marine invertebrates, so will episterol probably be encountered in corresponding A
7 -sterol mixtures.
24a-Ethyl-A
7 -cholestenol
(A
7 -stigmastenol) (III, R = J), C29H50O,
m.p. 145°; [a] D +8°; acetate, m.p. 157°, [a] D +7°; and 24a-ethyl-A
7 >
22 -
cholestadienol (α-spinasterol) (III, R = K), C 29 H 48 0, m.p. 174°, [a] D
—3°, infrared spectrum (26); acetate, m.p. 187°, [a] D —5°. They are
the principal A
7 -sterols of plants and may well occur in the complex
A
7 -sterol mixtures from certain marine invertebrates, such as starfish.
24a-Ethyl-A
7 >
22 -cholestadienol (chondrillasterol) (III, R = M), C 29
H 48 0, m.p. 169°, [a] D —2°, infrared spectrum (26); acetate, m.p. 175°,
[a] D —1°. This sterol has been isolated from a sponge and an alga (48).
4. Group IV (Structure IV): Sterols Saturated in the Ring System with
Specific Rotations of [a] D -f 15° to -\-30°
All known sterols belonging to this group are completely saturated
with the exception of neospongosterol (IV, R = H) which carries a
double bond in the side chain. They may therefore readily be distinguished from sterols of other groups by their inertness toward bromine
and other double-bond reagents. When quite pure such sterols do not
give any of the typical color reactions. Saturated sterols may be freed
from unsaturated impurities by treatment with acetic anhydride and
concentrated sulfuric acid in carbon tetrachloride. In this reaction only
the unsaturated impurities form colored, acidic products, separable
from the unreacted saturated sterols by alkali (49). The differences
between the molecular rotations of ring-saturated sterols and of their
acetates and benzoates are —34 ± 11 and +2 ± 3, respectively (31).
The last value is particularly characteristic and useful in gauging the
uniformity of a saturated sterol.
Only in certain sponges have saturated sterols so far been found to
be the principal components of sterol mixtures. In other cases they
often represent such a small fraction of the mixture that they are easily
overlooked. It is probable that eventually the saturated derivatives of
all known sterols will be isolated from natural sources.