26
1. Isoprenoids
The structure of 63a, 22,25-oxidoholothurinogenin, C30H44O5, was carefully
and convincingly deduced by Chanley et al. (1966) on the basis of chemical
degradation and spectral determinations.
Presence of a trans heteroannular diene system in the genin, but—as was
pointed out earlier—absent in the glycosides, was based primarily on the
characteristic triple band in the ultraviolet: a maximum at 243 nm (ε 14400)
and two prominent shoulders at 237 nm (ε13430) and 252 nm (ε10390).
Triple substitution of each of the two double bonds was inferred from the
presence in the nmr spectrum of the 3-acetate of only two broad one-proton
signals at 85.25 and 85.51.
Two of the five oxygen functions in 63a are represented by hydroxyl
groups—one secondary that can be oxidized to a ketone, and one tertiary.
22,25-Oxidoholothurinogenin forms monoesters that retain hydroxyl
absorption in the infrared (ca. 3565 cm"
1 ); under forcing conditions it
forms a bistrifluoroacetate, which is devoid of hydroxyl absorption. A fivemembered lactone accounts for two more oxygen atoms. Its major support
rests on the infrared band at 1763 cm"
1 . The lactone appeared to be hindered
since the aglycone was recovered unchanged after prolonged refluxing with
alcoholic potassium hydroxide. The ethereal nature of the fifth oxygen atom
was inferred by exclusion and was positively indicated by a broad one-proton
triplet nmr signal centered at 84.23 (/ = 6 Hz), which was assigned to a
methine hydrogen flanked by a cyclical ether oxygen and a methylene group.
Seven quarternary methyl groups could be distinguished in the nmr
spectrum of 63a. Since a "normal" triterpenoid compound has eight such
groups, it was a reasonable assumption that one methyl group had been
oxidized and had become part of the lactone. Presence of two olefinic
linkages, a lactone and an ether ring pointed to a tetra- rather than a pentacyclic triterpenoid. Tetracyclic triterpenoids of the dammarane or isoeuphane
types were excluded since neither will accommodate a trans heteroannular
diene bearing only two vinyl hydrogen atoms. The methyl signals ranged
from 0.91 to 1.38 ppm in deuteriochloroform and from 0.90 to 1.59 ppm in
pyridine. The signal farthest downfield was assigned to the C-21 methyl
group that is attached to C-20, which in turn is bonded to the lactonic oxygen
atom. The y-lactone ring would then include the oxidized C-18 methyl. This
assumption agrees with the hindered nature of the lactone as well as with
the fact that an unoxidized C-18 methyl would give rise to an nmr signal farther upfield than 0.90 ppm. Two downfield methyl signals at 1.26 and 1.78 ppm
(in CDCI3) that were incompletely resolved were compatible with the gemdimethyl group at C-25 that would be linked to the ether oxygen in line with
their chemical shifts. The other carbon (C-22) that is linked to the ether oxygen
atom bears a single hydrogen atom and has already been assigned. Excellent
support for this unusual side chain came from the mass spectrum of 63a,
1. Isoprenoids
The structure of 63a, 22,25-oxidoholothurinogenin, C30H44O5, was carefully
and convincingly deduced by Chanley et al. (1966) on the basis of chemical
degradation and spectral determinations.
Presence of a trans heteroannular diene system in the genin, but—as was
pointed out earlier—absent in the glycosides, was based primarily on the
characteristic triple band in the ultraviolet: a maximum at 243 nm (ε 14400)
and two prominent shoulders at 237 nm (ε13430) and 252 nm (ε10390).
Triple substitution of each of the two double bonds was inferred from the
presence in the nmr spectrum of the 3-acetate of only two broad one-proton
signals at 85.25 and 85.51.
Two of the five oxygen functions in 63a are represented by hydroxyl
groups—one secondary that can be oxidized to a ketone, and one tertiary.
22,25-Oxidoholothurinogenin forms monoesters that retain hydroxyl
absorption in the infrared (ca. 3565 cm"
1 ); under forcing conditions it
forms a bistrifluoroacetate, which is devoid of hydroxyl absorption. A fivemembered lactone accounts for two more oxygen atoms. Its major support
rests on the infrared band at 1763 cm"
1 . The lactone appeared to be hindered
since the aglycone was recovered unchanged after prolonged refluxing with
alcoholic potassium hydroxide. The ethereal nature of the fifth oxygen atom
was inferred by exclusion and was positively indicated by a broad one-proton
triplet nmr signal centered at 84.23 (/ = 6 Hz), which was assigned to a
methine hydrogen flanked by a cyclical ether oxygen and a methylene group.
Seven quarternary methyl groups could be distinguished in the nmr
spectrum of 63a. Since a "normal" triterpenoid compound has eight such
groups, it was a reasonable assumption that one methyl group had been
oxidized and had become part of the lactone. Presence of two olefinic
linkages, a lactone and an ether ring pointed to a tetra- rather than a pentacyclic triterpenoid. Tetracyclic triterpenoids of the dammarane or isoeuphane
types were excluded since neither will accommodate a trans heteroannular
diene bearing only two vinyl hydrogen atoms. The methyl signals ranged
from 0.91 to 1.38 ppm in deuteriochloroform and from 0.90 to 1.59 ppm in
pyridine. The signal farthest downfield was assigned to the C-21 methyl
group that is attached to C-20, which in turn is bonded to the lactonic oxygen
atom. The y-lactone ring would then include the oxidized C-18 methyl. This
assumption agrees with the hindered nature of the lactone as well as with
the fact that an unoxidized C-18 methyl would give rise to an nmr signal farther upfield than 0.90 ppm. Two downfield methyl signals at 1.26 and 1.78 ppm
(in CDCI3) that were incompletely resolved were compatible with the gemdimethyl group at C-25 that would be linked to the ether oxygen in line with
their chemical shifts. The other carbon (C-22) that is linked to the ether oxygen
atom bears a single hydrogen atom and has already been assigned. Excellent
support for this unusual side chain came from the mass spectrum of 63a,