stem extract of A. saponaria, A. sabaea, A. barbadensis, and A. vera exhibited the presence of aloesaponol I, aloesaponol II, aloin, aloesaponarin I, aloesaponarin II, cycloartanol, 24-methylene-cycloartanol, ziganein, ziganein-5-methyl ether, lophenol, 24-methyl-lophenol, 24-ethyl-lophenol, and cosmosiin; the compounds were identified by their spectral data (Rubina et al. 2009; Yagi et al. 1974; Blitzke et al. 2000; Abd-Alla et al. 2008, 2012; Kumar et al. 2017b). The leaf extract of A. berhana, Aloe rivae, A. megalacantha, A. barbadensis, and South African Aloe species A. pulcherrima resulted in the characterization of chrysophanol, acemannan, aloin, aloe emodin, aloechrysone, protocatechuic acid, barbaloin, aloin A, aloinoside, natoloin, and 7-hydroxy barbaloin (Choche et al. 2014; Boudreau and Beland 2006; Dagne and Alemu 1991; Amoo et al. 2014) and aloeresin E and aloeresin F from A. peglerae (van Heerden et al. 1996).
The γ-coniceine, coniine, N-methyltyramine, and O,N-dimethyltyramine alkaloids were isolated and identified from leaf extract of A. gillilandii, Aloe ballyi, Aloe ruspoliana, Aloe ibitiensis, Aloe deltoideodonta, and A. viguieri (Dring et al. 1984). Root extract of Aloe species was used for isolation of chrysophanol and asphodelin. The other compounds are derived through 1-methyl-8-hydroxyanthraquinone pathway, viz aloesaponarin I and aloesaponarin II. The reported results show that isoeleutherol is a useful chemotaxonomic compound that helps in characterization of Saponarieae series (van Wyk et al. 1995a). A. saponaria was investigated for isolation of phenolic compounds and revealed the presence of aloesaponol III, aloesaponol IV, chrysophanol, helminthosporin, and isoxanthorin. The identity of aloesaponol III and aloesaponol IV were confirmed to be 1-oxo-4,8,9-trihydroxy-6-methyl-1,2,3,4-tetrahydroanthracene and 1-oxo-4,8,9-trihydroxy-2-methoxy-6-methyl-1,2,3,4-tetrahydroanthracene by spectral analysis (Yagi et al. 1977a,b). The exudate of A. nyeriensis var. kedongensis leaves afforded six compounds, viz 4-methoxy-6-(2′,4′-dihydroxy-6′-methylphenyl)-pyran-2-one, its 2′-O-β-D-glucopyranoside, and the 2″-O-p-coumaroyl ester of aloenin, l,2,8-trihydroxy-6-methylanthraquinone, its 2-O-β-D-glucopyranosyl ester, and the corresponding 10-C-β-D-glucopyranoside nataloin (Conner et al. 1987). The 6-hydroxy-3,5-dimethoxy-2-methyl-1,4-naphthoquinone, ancistroquinone C, 5,8-dihydroxy-3-methoxy-2-methyl-1,4-naphthoquinone, malvone A, droserone, droserone-5-methyl ether, and hydroxydroserone have been isolated from Aloe dawei (Abdissa et al. 2014).
The A. elgonica leaf exudate was examined for anthraquinones, and several other phytochemicals were found such as aloe emodin, aloenin, aloeresin, aloeresin B, emodin-10-C-β-glucopyranoside linked through C-10 to C-7 of the anthraquinone aloe emodin (Conner et al. 1990a,b). Similarly, four bianthraquinoid pigments (A, B, C, and D) were characterized from rhizome of A. saponaria. Besides bianthraquinoid pigments, the (+)-asphodelin; 1,1′,8,8′,10-pentahydroxy-3,3′-dimethyl-10,7′-bianthracene-9,9′,10′-trione; 1,1′,8,8′-tetrahydroxy-3,3′-dimethyl-4,7′-bianthracene (10′H, 10′H)-9,9′,10-trione; and 1,1′,8,8′,10-pentahydroxy-3,3′-dimethyl-10,7′-bianthracene (10′H, 10′H)-9,9′-dione structures were also established by analyzing the spectral data (Crosswhite and Crosswhite 1984; Reynolds and Dweck 1999; Rajasekaran et al. 2005; Loots et al. 2007; Yagi et al. 1978, 1983; Speranza et al. 1993). The geographical conditions affect the production of aloeresin A, aloesin, and aloin and distributed as major compounds in A. ferox leaf exudate. Along with major compounds, aloinoside A and aloinoside B found in western parts of the Cape and aloeresin C and 5-hydroxyaloin A aloin found in all the areas of Cape (van Wyk et al. 1995b). The levels of Barbaloin were determined in Aloe species in the Kew. The maximum concentration of barbaloin was found in exudates of young leaves but the concentration decreased in older leaves (Groom and Reynolds 1987). The littoraloside was isolated from A. littoralis leaf exudate along with littoraloin, deacetyllittoraloin, and 10-hydroxyaloin B, and their structures were confirmed by spectral analysis (Dagne et al. 1998b). The prechrysophanol, chrysophanol, helminthosporin, (R)-aloesaponol I, (R)-aloesaponol II, aloesaponarin I, aloesaponarin II, and laccaic acid D methyl ester were isolated from A. graminicola (Yenesew et al. 1993; Lakshmi and Rajlakshmi 2011).
