2. STEROLS: STRUCTURE AND DISTRIBUTION
113
side chain of ergosterol (II, R = F) (23). Like its 24/3-epimer, described
below, it is a probable component of the provitamin D mixtures of invertebrates. Upon irradiation it aifords vitamin D 4 (24).
24ß-Methyl-7-dehydrocholesterol
(7-dehydrocampesterol) (I, R = G),
C 28 H 46 0, m.p. 165°, [a] D —109°; benzoate, m.p. 157°, clear at 164°,
[a] D —50°. It has been prepared by standard reactions from campesterol
(II, R = G) (25). In contrast to its 24a-epimer it acquires only feeble
antirachitic activity upon irradiation (25). The configuration of the 24methyl group is therefore of some significance to vitamin D activity.
Ergosterol (provitamin D 2 ) (I, R = F), C 28 H 44 0, m.p. 168°, [a] D
—132°, infrared spectrum (26); acetate, m.p. 181°, [a] D —90°. It is
appropriately regarded as the most typical sterol of fungi. Its inclusion
among animal sterols is justified since it has been found as a major component of the sterol mixture from a snail and possibly also other mollusks (see Section VIII).
24a-Ethyl-7-dehydrocholesterol
(7-dehydro-/3-sitosterol)
(I, R = J),
C 29 H 48 0, m.p. 144-145°, [a] D -116°; acetate, m.p. 151-152° (27, 28)
[a] D —71°, and 24ß-eihyl-7-dehydrocholesterol
(7-dehydro-y-sitosterol
(?); 7-dehydroclionasterol (29), (I, R = L), C 29 H 48 0, m.p. 138°, [*] D
—98°; acetate, m.p. 140°, [a] D —72°. Both sterols have been prepared
by standard reactions from starting materials of questionable uniformity
and hence cannot be regarded with certainty as pure substances. Their
occurrences in provitamin D mixtures has been suspected (see Section
VIII).
7-Dehydrostigmasterol
[24a-ethyl-7,22-bisdehydrocholesterol; corbisterol (?)], (I, R = K), m.p. 154°, [a] D -113°; acetate, m.p. 172°. It
was first prepared from stigmasterol (II, R = K) (30), and it has been
suggested to be identical with corbisterol from mollusks (see Section
VIII).
2. Group II (Structure II): A
5 -Sterols with Specific Rotations of
[a] D -25° to -70°
This group includes the sterols most commonly encountered in
animals and plants. Whether a sterol is A
5 -unsaturated and of a reasonable degree of purity may be learned from a comparison of its molecular
rotation with those of some of its derivatives. According to the rules of
rotational shift, conversion of all sterols of group II to their acetates or
benzoates should be accompanied by analogous shifts in molecular rotations. Barton (31) found that the acetate shift of A
5 -sterols is —35 ± 10
and the benzoate shift +81 db 10. A newly found, levorotatory sterol
may safely be regarded as belonging to group II when the rotational
shifts of its acetate and benzoate are close to the known values. In-
113
side chain of ergosterol (II, R = F) (23). Like its 24/3-epimer, described
below, it is a probable component of the provitamin D mixtures of invertebrates. Upon irradiation it aifords vitamin D 4 (24).
24ß-Methyl-7-dehydrocholesterol
(7-dehydrocampesterol) (I, R = G),
C 28 H 46 0, m.p. 165°, [a] D —109°; benzoate, m.p. 157°, clear at 164°,
[a] D —50°. It has been prepared by standard reactions from campesterol
(II, R = G) (25). In contrast to its 24a-epimer it acquires only feeble
antirachitic activity upon irradiation (25). The configuration of the 24methyl group is therefore of some significance to vitamin D activity.
Ergosterol (provitamin D 2 ) (I, R = F), C 28 H 44 0, m.p. 168°, [a] D
—132°, infrared spectrum (26); acetate, m.p. 181°, [a] D —90°. It is
appropriately regarded as the most typical sterol of fungi. Its inclusion
among animal sterols is justified since it has been found as a major component of the sterol mixture from a snail and possibly also other mollusks (see Section VIII).
24a-Ethyl-7-dehydrocholesterol
(7-dehydro-/3-sitosterol)
(I, R = J),
C 29 H 48 0, m.p. 144-145°, [a] D -116°; acetate, m.p. 151-152° (27, 28)
[a] D —71°, and 24ß-eihyl-7-dehydrocholesterol
(7-dehydro-y-sitosterol
(?); 7-dehydroclionasterol (29), (I, R = L), C 29 H 48 0, m.p. 138°, [*] D
—98°; acetate, m.p. 140°, [a] D —72°. Both sterols have been prepared
by standard reactions from starting materials of questionable uniformity
and hence cannot be regarded with certainty as pure substances. Their
occurrences in provitamin D mixtures has been suspected (see Section
VIII).
7-Dehydrostigmasterol
[24a-ethyl-7,22-bisdehydrocholesterol; corbisterol (?)], (I, R = K), m.p. 154°, [a] D -113°; acetate, m.p. 172°. It
was first prepared from stigmasterol (II, R = K) (30), and it has been
suggested to be identical with corbisterol from mollusks (see Section
VIII).
2. Group II (Structure II): A
5 -Sterols with Specific Rotations of
[a] D -25° to -70°
This group includes the sterols most commonly encountered in
animals and plants. Whether a sterol is A
5 -unsaturated and of a reasonable degree of purity may be learned from a comparison of its molecular
rotation with those of some of its derivatives. According to the rules of
rotational shift, conversion of all sterols of group II to their acetates or
benzoates should be accompanied by analogous shifts in molecular rotations. Barton (31) found that the acetate shift of A
5 -sterols is —35 ± 10
and the benzoate shift +81 db 10. A newly found, levorotatory sterol
may safely be regarded as belonging to group II when the rotational
shifts of its acetate and benzoate are close to the known values. In-