Abstract
Agarwood is the fragrant resinous heartwood obtained from certain trees in the genus Aquilaria belonging to the family Thymelaeaceae. 2-(2-Phenylethyl)chromones and characteristic sesquiterpenes are the main classes of aromatic compounds isolated from agarwood. Although there are many sesquiterpenes, relatively few 2-(2-phenylethyl)chromones have been determined in agarwood by GC-MS. After analysis of the MS spectra of eighteen 2-(2-phenylethyl)chromone derivatives isolated from agarwood and identified by NMR spectroscopy, together with the reported MS data and characteristic of structures of 2-(2-phenylethyl)chromones, the MS characterization, fragmentation patterns and characteristic fragment peaks for the compounds were deduced and a table summarizing MS characterization of 2-(2-phenylethyl)chromones in agarwood is presented. All the 2-(2-phenylethyl)chromones previously reported in agarwood are substituted by methoxy or/and hydroxy groups, except for one compound. Due to the fact they all possess the same basic skeleton (molecular weight: 250) and similar substituent groups (methoxy or hydroxy groups), a formula (30m + 16n = MW − 250) is provided to calculate the number of methoxy (m) or hydroxy (n) groups according to molecular ion peak or molecular weight (MW). We deduced that the characteristic fragmentation behaviors of the 2-(2-phenylethyl)chromones are the cleavages of the CH2-CH2 bond between chromone moiety and phenyl moiety. Thus, characteristic fragment ions, such as m/z 91 [C7H7], 107 [C7H6+OH], 121 [C7H6+OCH3], 137 [C7H5+OH+OCH3] are formed by different substituted benzyl moieties, while characteristic fragment ions such as m/z 160 [C10H8O2], 176 [C10H7O2+OH], 190 [C10H7O2+OCH3], 220 [C10H6O2+OCH3×2] are formed by different substituted chromone moieties. Furthermore, rules regarding to the relationship between the positions of hydroxy or methoxy groups and the relative abundances of benzyl and chromone fragment ions have been deduced. Elucidation of how the positions of hydroxy or methoxy groups affect the relative abundances of benzyl and chromone fragment peaks is also provided. Fifteen unidentified compounds of an artificial agarwood sample analyzed by GC-MS, were preliminary determined as 2-(2-phenylethyl)chromones by analysis of their MS characterization and by comparison of their MS spectra with those of 18 standard compounds or 2-(2-phenylethyl)chromones reported in literature according to the above-mentioned methods and rules. This report will be helpful for the analysis and structural elucidation of 2-(2-phenylethyl)chromones in agarwood by GC-MS, and provides fast and reliable characterization of the quality of agarwood.
Highlights
Agarwood is a fragrant resinous wood obtained from certain trees in the genus Aquilaria belonging to the family Thymelaeaceae [1,2]
2-(2-Phenylethyl)chromone derivatives 1–18 were isolated from Chinese agarwood in our laboratory, and their structures identified by NMR analysis [10,11,12,13]
We found that usually the substitution of a benzyl moiety has a greater impact than the substitution of chromone moiety, and the effect of methoxy substitution is similar to that of no substitution, while the opposite is true of hydroxy substitution; the p-substituted (4′) groups have a greater impact than other positions on the relative abundance of fragments
Summary
Agarwood is a fragrant resinous wood obtained from certain trees in the genus Aquilaria belonging to the family Thymelaeaceae [1,2]. The precious, high-priced, fragrant agarwood, called chen-xiang in Chinese, gaharu and kalambak in Malaysia, kanankoh and jinkoh in Japan, oudh, eaglewood, agar, ghara, and aloeswood, has been used for centuries as incense in Buddhist, Hindu and Islamic ceremonies [1,2] It plays an important role in Chinese Traditional Medicine for use as a sedative and carminative, and to relieve gastric problems, coughs, rheumatism and high fever [3]. This report should be helpful for the analysis and structural elucidation of 2-(2-phenylethyl)chromones in agarwood by GC-MS, and provides a scientific basis for the fast, simple, and sound evaluation of the quality of agarwood
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