The Cropwatch Files
Copyright Tony Burfield 2005
Agarwood oil contains many unusual substances. The elucidation of the structures of many minor components of agarwood oil, especially of Far Eastern origin, have kept many structural organic chemists busy for years, and continue to do so [see Lawrence (1998) for a review of progress from 1973-1998].
The presence of eight sesquiterpenes including agarol and agarospirol together with a- and b- agofuran, were characterised by Battacharrya et al. in 1959 & 1965 [Jain & Battacharrya (1959), Varma, Maheshwari & Battacharrya (1965)]. Maheshwari et al. (1963, 1963a) characterised alpha- and beta-agaofuran, dihydroagarofuran, nor-ketoaragofuran, 4-hydroxydihydroagarofuran and 3,4 dihydroxydihydroagarofuran by solvent extraction of infected agarwood of Indian origin. Subsequent total synthesis of nor-ketoagarofuran by Heathcock & Kelly (1968) showed the formula presented by Maheshwari et al. was incorrect. Pant & Ragosti (1980) used instrumental techniques to show that dihydroagarofuran was dihydro-beta-agarofuran and isodihydroagarofuran was isodihydro-alpha-agarofuran.
Pant & Rastogi (1980) also confirmed the presence of two sesquiterpenes: agarol & gmelofuran in the fragrant wood. Ishihara et al. (1991a) described seven newly identified compounds in agarwood oil from A. agallocha based on the guaiane skeleton, including guaia-1(10),11-dien-15-ol, guaia-1(10),11-dien-15-carboxylic acid, methylguaia-1(10)-11-dien-15-carboxylate, guaia-1(10),11-dien-9-one, 1,1`0-epoxyguaia-11-ene, guaia-1(10),11-dien-15,2-olide and guaia-1(5),11-dien-2-one. In a subsequent paper Ishihara (1991b) described a further 3 sesquiterpenes and ascribed odour descriptions to some characterised components.
Näf et al. (1992, 1995) described components in fresh agarwood oil distilled from some freshly purchased Indian wood (identified as A. agallocha), The authors identified beta-agarofuran, vetispira-2(11),6(14)-dien-7-ol, dihydrokaranone and valerianol as major constituents of Indian agarwood oil. Eight new valencane-, eremophilane- and vetispirane- derivatives were described (see abstract in bibliography) including (3R, 7R, 9R, 10S)-9,10-dimethyl-6-methylene-4-oxatricyclo[7.4.0.03,7]1-tridecene, and compounds synonymous with 2,14-epoxyvetaspira-6-ene and 2,14-epoxyvetaspira-6(14),7-diene were also described (1995), odour descriptions being ascribed to three of the characterised compounds:
Table 1. Odour Descriptions of Compounds from Indian Agarwood
|8,12-epoxy-eremophila-9,11(13)-diene||Woody with vetiver subtonality|
|2,14-epoxy-vetispir-6-ene||Woody with typically agarwood-like, sweet, woody, smoky, phenolic like oak-moss but weak.|
|2,14-epoxy-vetaspira-6,(14),7-diene||Woody but nearly odourless|
Amongst other components, an Indian patent reveals the presence of 1,7-oxoaporphine and liriodenine in the heartwood [Indian Patent (1979) 145857 through Chem Abstr. (1980) 93, 80049f]. Bhandari et al. (1982) confirmed the presence of a coumarinolignan, aquillochin, from A. agallocha.
Indonesian Agarwood (A. malaccensis/A. agallocha).
Jinkohol (2b-hydroxy-(+)-prezizane) was characterised by Nakanishi et al. (1980) following work on a benzene extract of Indonesian agarwood imported via Singapore.
In 1983, two new sesquiterpenes: jinkoh-eremol and jinkohol II were characterised by the same author from “Aquliaria sp; (not A. agallocha) probably A. malaccensis Benth.” from agarwood “collected in Indoneisa and imported via Singapore ”, which they called “type” B agarwood, reserving the description “type A” for agarwood from A. agallocha.
The authors reasoned that “type B” agarwood contained nootkatane (e.g. kusunol, jinkoh-eremol), spirovetivane (e.g. agarospirol) and tricyclic (+)-prezizane (e.g. jinkohol, jinkohol II) type sequiterpenes, and that these structural types also occur in vetiver oil, and may evolve from a common precursor. The authors also indicated that four compounds possessed intense odours (agarospirol, kusunol, jinkohol & jinkohol II) and “appear to be the source of the fragrance of agarwood”.
Subsequently Nakanishi et al. (1984) isolated alpha-agarofuran, (-)-10-epi-gamma-eudesmol and oxo-agarospirol as major constituents from “type B” agarwood.
Yoneda et al. (1984) were able to list the major sesquiterpenes of agarwood from type A and type B agarwoods imported from Indonesia and Vietnam through Singapore. A. agallocha (agawood type A) was found to contain b-agarofuran 0.6%, nor-ketoagarofuran 0.6%, agarspirol 4.7%, jinko-eremol 4.0%, kusunol 2.9%, dihydrokaranone 2.4%, and oxo-agarospirol 5.8%. In type B agarwood the following compounds were identified a-agarofuran, (-)-10-epi-g-eudesmol 6.2%, agarospirol 7.2%, jinkohol 5.2%, jinko-eremol 3.7%, kusunol 3.4%, jinkohol II 5.6%, and oxo-agarospirol 3.1%. From their findings the authors concluded that type A wood exclusively contains nor-ketoagarofuran and dihydrokaranone, but does not contain (-)-10-epi-gamma-eudesmol, jinkohol and jinkohol II, findings which might be used to distinguish the products.
Nagashima et al. (1983) found the following compounds in an oil distilled from agarwood collected in Cambodia: alpha-agarofuran, ar-curcumene, nerolidol, agarospirol, benzyl acetone, nor-ketoagarofuran, kusunol, & jinkoh-eremol, as well as characterising new components: dihydrokaranone, karanone, oxo-agoarospirol & iso-agarospirol.
Chinese Agarwood (Aquilaria sinensis)
Yoshi et al. (1978) elicidated the structure of the highly oxygenated chromone, agarotetrol from Jinkoh (A. sinesis). In a series of published studies, Yang and Cheng (1983, 1986) Yang et al., 1989a, 1989b, 1990) identified the presence of benzylacetone, p-methoxybenzylacetone, anisic acid and beta-agarofuran as well as the sesquiterpenes baimuxinic acid, baimuxinal, baimuxinol, dehydrobaimuxinol &isobaimuxinol.
Xu et al. (1988) characterised six compounds in Chinese agarwood (A. sinensis):
Yang et al. (1989) from an ether fraction of an alcohol soluble extract of A. sinensis distinguished a new chromone, 6-hydroxy-2-[2-(4'-methoxylphenyl)ethyl] chromone and the other five known chromones: 2-(2-phenylethyl)chromone, 6-methoxy-2-(2-phenylethyl)chromone , 6.7-dimethoxy-2-(2-phenylethyl)chromone , 6-methoxy-2[2-(3'-methoxyphenyl)ethyl]chromone and 6-hydroxy-2-(2-phenylethyl) chromone. Later Yang & Wang (1990) elucidated the structure of three 2-(2-phenylethyl) chromone derivatives isolated from the ethyl acetate soluble fraction of the alcoholic extract of Aquilaria sinensis (Lous.) Gilg. These were: 5,8-dihydroxy-2-(2-p-methoxyphenylethyl) chromone and 6,7-dimethoxy-2-(2-p-methoxyphenylethyl) chromone. This was followed by Yagura (2003) who identified 5-hydroxy-6-methoxy-2-(2-phenylethyl)chromone, 6-hydroxy-2-(2-hydroxy-2-phenylethyl)chromone, 8-chloro-2-(2-phenylethyl)-5,6,7-trihydroxy-5,6,7,8-tetrahydrochromone, 6,7-dihydroxy-2-(2-phenylethyl)-5,6,7,8-tetrahydrochromone were isolated from a methanol extract of withered wood of Aquilaria sinensis.
Hsu (1996) notes the presence of agarospirol, agarol, agarofuran, dihydroagarofuran, 4-hydroxyagarofuran, 3,4-hydroxydihydroagarofuran, and norketoagarofuran in infected extracted wood, and the presence of benzylacetone, p-methoxybenzylacetone, hydrocinnamic acid, p-methoxyhydrocinnamic acid, agarospirol, agarol, agarofurans and agarotetrol in the essential oil, the inference being this is from non-infected wood.
The presence of the cytotoxic compounds 1,3-dibehenyl-2-ferulyl glyceride, which is novel, and 12-O-n-deca- 2, 4, 6-trienoylphorbol-13-acetate isolated from the stem bark of an A. malaccensis tree growing in Thailand was demonstrated by Gunasekera et al (1981)
Vietnamese Agarwood: Aquilaria agallocha.
Ishara et al. (1993) found the following compounds, amongst others, in a Vietnamese agarwood extract, describing their odours as follows:
|selina 3,11-dien-14-al||floral woody with smoky sandalwood nuance|
|dehydrojinkoh-eremol||woody, slightly balsamic, bitter|
Table 2. Odour Descriptions of Compounds from Vietnamese Agarwood extract (Ishara et al. 1993)
and in a further paper Ishara et al. (1993b) describe the volatiles from extracts of 4 different agarwoods, concluding that major sesquiterpene volatile in Kanankoh (Murrasaki from Vietnam) was oxo-agarospirol, and from two samples of Kanankoh (Ryoku-yu & Cha-yu – both from Vietnam) the major volatiles were the compounds below:
In a third study Ishara et al. (1993c) looked at the volatiles from ground agarwood from a heated flask, capturing the volatiles in a Tenax trap. Clear differences in trapped volatiles composition were seen between Kanankoh (Ryoku-ku) and Jinkoh (Bateikel) agarwoods, with the latter showing a higher propotion of lower molecular weight materials such as 3,4-dimethoxyphenol, benzylacetone and benzaldehyde, but with a more complex picture with higher molecular weight materials. Lower levels of jinkoh-eremol, kusunol and guaia-1(10),11-dienal were shown by the Jinkoh (Bateikel) agarwood, but it had higher levels of beta-agarofuran, neopetasane and dihydrokaranone.