arin I was a step in the biosynthetic process (Yagi etal, 1978a). Subsequently it was shown that tetrahydroanthracene glycosides were converted to anthraquinone glycosides in the light (Yagi etal., 1983). In contrast, however, aloe emodin appears to be synthesized directly from acetate units (Simpson, 1980; Grün and Franz, 1982).
Aloechrysone was isolated from roots of A. berhana Reynolds (Dagne etal, 1992) and subsequently found in 66 species (Van Wyk etal, 1995a) and then in A. vera (Saleem etal, 1997b). This and prechrysophanol from subterranean stems of A. graminicola Reynolds (Yenesew etal, 1993) are seen as having the chrysophanol pattern subjected to reduction, a double keto-enol transformation and migration of a hydroxyl group and it is postulated that prechrysophanol may be a biosynthetic precursor. Similarly aloesaponol I is related to aloesaponarin I and aloesaponol II to aloesaponarin II. These two compounds were originally isolated from A. saponaria (Yagi etal, 1974) and then found in the roots of 32 species (Dagne etal, 1994). In a further survey, aloesaponol I was found in 70 species and aloesaponol II in 113 species (Van Wyk etal., 1995a). Aloesaponol III and aloesaponol IV (Yagi etal., 1977) relate structurally to helminthosporin and isoxanthorin, respectively. Again, these were first isolated from A. saponaria, while aloesaponol III was then found in 31 other species (Dagne etal., 1994). Recently three tetrahydroanthracenones, gaste-riacenones A, B and C, with related substitution patterns have been isolated from the neighbouring genus Gasteria (Figure 3.6) (Dagne etal, 1996b). A more remote structure, aloebarbendol (Figure 3.6), was recently described from roots of A. vera (Saleem etal, 1997b).
From the investigations described above it appears that tetrahydroanthracenes occur in the roots accompanied by several anthraquinones (cf. Sigler and Rauwald, 1994a) and that the leaves contain some anthraquinones, notably aloe-emodin, while the flowers contain free anthrones. Much more prominent in the leaves are the O- and C-glycosides. The next research step might be a quantitative survey of the compounds in the various organs of a few species to complement the chemotaxonomic surveys.
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