Ar-Curcumene

The sequence, addition of one equivalent of MeLi to a solution of dithioester (1) and (2) in THF at -78°C, led, after allowing the temperature to rise slowly to 20°C and the usual work up, to dithioacetal (3) in a 79% yield [333]. Dithioacetalization and a Wittig reaction yielded dehydro-ar-curcumene (4). 

Such routes have been used for the synthesis of natural products terpenes such as egomaketone (3-(4-methyl-3-pentenoyl)furan, perillene, ar-turmerone and iso-ar-turmerone [341], ar-curcumene [152], artemisia and isoartemisia ketones [342, 343], damascone [344], macrolides such as maysine [345], and pheromones with the sex attractant of the pine saw-fly [346],... 

Table I shows the quantitative comparison of selective compounds in the hydrocarbon fraction. The total amount of monoterpenes in the steam distilled sample was less than that in the liquid carbon dioxide extract. These compounds could have been lost during distillation. The significant decrease in zingiberene and the concomitant increase in ar-curcumene confirmed that zingiberene was converted to ar-curcumene (16). g-Sesquiphelladrene, another compound which could be converted to ar-curcumene, decreased slightly. The scheme of oxidative conversion of zingiberene and B-sesquiphel-...

Figure 3. Oxidative conversion of zingiberene and B-sesquiphel-landrene into ar-curcumene.

Ginger oil displays considerable compositional diversity but is typically characterized by a high content of sesquiterpene hydrocarbons, including zingiberene, ar-curcumene, (3-bisabolene and (S-sesquiphellandrene. 

The essential oil obtained from the rhizomes of Z. officinale from Cuba contained ar-curcumene (22.1%), zingiberene (11.7%), (3-bisabolene (11.2%) and cadina-1,4-diene (12.5%) (Pino et al., 2004). 

The chemical composition of the essential oils obtained from the hydrodistillation of the rhizomes of the common ginger (Z. officinale) grown in Mauritius was analysed by Fakim et al. (2002). The oil was characterized by the presence of geranial (16.3%), neral (10.3%), zingiberene (9.5%), (3-sesquiphellan-drene (6.3%) and ar-curcumene (5.1%). 

The lemony note is attributed to citrals together with a-terpineol, while P-sesquiphel-landrene and ar-curcumene are regarded as partly responsible for the characteristic ginger flavour. Nerolidol was considered to contribute to the woody note and cis- and frans-P-sesquiphellandrol were suspected as significant contributors to the ginger flavour, fn combination, these compounds accounted for 85% of the taste panel s response. 

GC-MS analysis of rhizome oils from five different Curcuma species shows variations in the major components, e.g. ar-turmerone (2.6-70.3%), a-turmerone (trace 6.2%) and zingiberene (trace-36.8%). Li et al. (1997) analysed a series of oils produced from several Zingiberaceae plants, including the rhizome oil of turmeric, using GC-MS. Thirty-five components were identified and turmerone (49%), ar-curcumene (15%) and ar-turmerone (6.4%) were the major compounds. 

The composition and yield of curcuminoids extracted using the existing techniques, e.g. hydrodistillation, low-pressure solvent extraction, soxhlet and SC-C02 extraction, were compared (Braga et al., 2003). The highest yield was obtained using soxhlet extraction with ethanol and the lowest (2.1%) with hydrodistillation. The best solvent for SC-C02 extraction is a mixture of ethanol and isopropanol. ar-Curcumene, z-(y)-atlantone and ( )-(y)-atlantone constituted about 60% of the light fraction. 

Composition Main constituent is zingiberene (>30%) followed by P-sesquiphelland-rene. (-H)-ar-curcumene, (E,E)-a-famesene and (-)-P-bisabolene are further important sesquiterpene hydrocarbons. a-Terpineol and citral (up to 9% in Australian ginger) result in the citrus note, whereas nerolidol is responsible for the woody-soapy taste [115]. Further constituents are camphene, limonene, phellandrene, bomeol, 1,8-cine-ole, linalool, a-bisabolene, oxygenated sesquiterpene hydrocarbons, etc. For recent analytical data see [116, 117, 118]. 

Analysis of rhizomes of cultivated plants showed significantly higher content of valepotriates and patchouli alcohol than reported in wild plants. The maximum valepotriate content was found at 4.3% (dry matter basis) under nylon net shade. A marked difference in other constituents of ar-curcumene, (3-patchoulene, and y-patchoulene was found in higher percentage than reported from wild plants [62]. 

Remarkably, benzylic stereo-control is also possible in the acyclic series due to the significant configurative stability of the reactive intermediates. An example is the short enantioselective synthesis of (-i-)-ar-curcumene (27) (Scheme 5)... 

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