A. broomi Compound
Figure 3.10 Esters of barbaloin derivatives
chrysophanol dimers, A, B, C and D from A. saponaria (Figure 3.12) (Yagi et al, 1978b) and a pair of diastereomers based on aloe-emodin from A. elgonica, present as gluco-sides (Conner etal., 1990b) and then found in A. vera (Choi etal., 1996).
Dimer A was found to be identical to (+) asphodelin (4,7'-bichrysophanol) (Gonzalez etal, 1973) and was identified in the roots of 32 Aloe species (Dagne etal, 1994). Another type of dimeric molecule has been found in two distantly related genera, Kniphofia and Bulbine. Here 2',4'-dihydroxy-6'-methoxyacetophenone is attached to either chrysophanol (knipholone) (Dagne and Steglich, 1984) or chrysophanol anthrone (knipholone anthrone) (Dagne and Yenesew, 1993).
Other phenolic compounds have been isolated from Aloe species and it is likely that many more will follow. Flavonoids are very widely distributed in plants but have not featured in aloe exudates until recently, when the flavones isovitexin and apigenin, the dihydroflavonol dihydroisorhamnetin and the flavanone narigenin (Figure 3.13) were reported from 31 Aloe species accompanied by a number of unidentified compounds (Viljoen etal, 1998). Both aglycones and glycosides were present. Isovitexin was the major phenolic in Sections Graminaloe and Leptoaloe while naringenin and dihydroisorhamnetin occured in Series Superpositae, Rhodacanthae and Echinatae.
Aloenin, first isolated from A. arborescens and then found in 12% of over 200 Aloe species surveyed (Reynolds, 1985b), is the O-glucoside of a phenol-pyran-2-one dimer (Figure 3.14) (Suga etal, 1974; Hirata etal, 1976; Hirata and Suga, 1978).
Figure 3.12 Dimeric compounds.
A. elgonica dimer A
The coumaroyl ester of an O-O-diglucoside was subsequently found in a Kenya aloe drug sample and named aloenin B (Speranza etal, 1986a). Later the coumaroyl ester of aloenin itself was described from A. nyeriensis, together with the aglycone (Conner etal, 1987). A breakdown product ('process product') isolated from Cape aloes was shown to be orcinol linked by a methylated methylene bridge to a phenyl residue (Figure 3.14) reflecting part of the aloenin structure (Speranza etal, 1994). An even simpler compound, methyl-^-coumarate, was isolated from Cape aloes (Graf and Alexa, 1982). Another dimeric compound isolated from the roots of A. vera consisted of two 4-hydroxy-6-methoxybenzopyran moieties (Figure 3.15) joined by a C-C bond (Saleem etal, 1997a) which relates distantly to the chromones. Feralolide (Figure 3.15), isolated as a minor component of Cape aloes, was shown to be a dimer with a methylene bridge of 2, 4-
dihydroxyacetophenone and 6, 8-dihydroxyisocoumarin (Speranza etal, 1993a). It was subsequently found in A. vera (Choi etal, 1996) while an O-glucoside was isolated from A. hildebrandtii Baker (Veitch etal, 1994).
A number of structures based on the naphthalene and tetralin nuclei have been assigned to aloe components from time to time. Tetrahydroanthracenes could be regarded as naphthalene derivatives with a fused cyclohexane ring but because their hydroxylation patterns relate to those of the anthraquinones they are more conveniently described separately. A substituted naphthalene diglucoside has been isolated from A. plicatilis (L.)Miller and named plicatiloside (Wessels etal, 1996) (Figure 3.16). It was then found in 19 other Aloe species (Viljoen etal, 1999). Underground stems of A. saponaria contain isoeleutherol-5-O-glucoside (Figure 3.16), a derivative of a substituted naphthoic acid (Yagi etal., 1977b), while the aglycone was found more recently in eight Aloe species (Dagne etal, 1994) and then in 18 species (Van Wyk etal., 1995a). Three 1-methyltetralins (derivatives of 5,6,7,8-tetrahydronaphthalene) were isolated from commercial Cape aloes (Figure 3.16). First the aglycone, feroxidin, was characterized as 3,6,8-trihydroxy-1-methyltetralin (1,3,6-trihydroxy-8-methyl-5,6,7,8-tetrahydronaphthalene) (Speranza etal, 1990, 1991) with 6S, 8S configuration. Then the 3-O-glucoside (Feroxin A) and its p-coumaric acid ester (Feroxin B) were described (Speranza etal, 1992). Three naphtho [2,3-C] furans bearing some structural resemblance to a reduced isoeleutherol were characterized from Cape aloes (Figure 3.16) (Koyama etal., 1994).
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