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Camphoraceous, India ink-like, fatty, putrid

aAccording to Nishimura (2001).

aAccording to Nishimura (2001).

3. Tasting the compounds in each selected peak in combination with ginger oil

4. Tasting combination of compounds in selected peaks with ginger heat chemicals added

The results can be summarized as follows:

1. Neral, geranial, and a-terpineol are responsible for the characteristic lemony flavor and odor of ginger. P-sesquiphellandrene and ar-curcumene partly contribute to the characteristic ginger flavor intrinsic to the spice. Owing to their high threshold values, they have more of a tendency to dilute the flavor intensity of the oil rather than to increase it when they are added to the oil.

2. Nerolidol possesses woody, soapy, or green notes, not very reminiscent of ginger. Cis and ira»s-P-sesquiphellandrol presumably contribute to the significant flavor of ginger as well as other contributors selected by the regression analysis. Bednarczyk and Kramer (1975) concluded that this stepwise regression analysis makes it possible to preselect flavorfully significant compounds in a flavor essence.

More recently, Chau et al. (2001) described a fingerprint analysis by GC/MS of dried and fresh ginger rhizomes from China using chemometrics techniques. Unfortunately, essential oil classification by chemometrics according to its origin and flavoring properties seems to be missing. For such studies, the percentages of the following compounds have to be taken into account: limonene, camphene, 1,8-cineole, neral, geranial, nerol, geran-iol, a-terpineol, geranyl acetate, a-zingiberene, P-sesquiphellandrene, P-bisabolene, (E,E)-a-farnesene, ar-curcumene, nerolidol, zingiberenols, and sesquiphellandrols.

Synthesis of Some Authentic Samples

Synthesis of terpenoids and sesquiterpenoids in flavors and fragrances has been widely studied since the end of the nineteenth century. Synthetic procedures have been reviewed in several books, particularly in Teisseire's (1991) excellent work.

In this section, only zingiberene isomers and its derivatives as well as other important constituents of ginger oils will be briefly described. Spectral data (MS, 'H-NMR, 13C-NMR) and KI on a nonpolar column of sesquiterpene hydrocarbons have been reported by Joulain and König (1998).

(—)-a-Zingiberene was first isolated in a concentrated form by Von Sodem and Rojahn (1900) by a distillation procedure. Its structure was studied by Eschenmoser and Shing (1950), Mills (1952), Arigoni and Jeger (1954), Banerjee (1962), and Joshi and Kulkarni (1965). The latter workers obtained the enamine (31) by condensation of ( + )-citronellal (29) with piperidine (30) in the presence of anhydrous K2CO3. Treatment of enamine (31) with methyl vinyl ketone followed by acetic acid yielded a product that by careful chromatography gave the a,ß-unsaturated ketone (32), a key intermediate that upon treatment with MeMgl gave a tertiary alcohol (33). Then (33) in the presence of oxalic acid gave zingiberene (34) containing 60% of a homoannular diene (Scheme 3.5).

Physical properties of (—)a-zingiberene (35a) were reported by Pliva et al. (1960):

b.p. = 134-135° C/15 mm Hg; d20 = 0.8713; n20 = 1.4937; (a)20 = —119.6° C

It was identified in other essential oils such as Santal Amgris and in Zizyphus spina-christi from Egypt by El-Hamouly and Mohamad (2001). ( + )a-Zingiberene (35b), an enantiomer of the above, has been synthesized for the first time from (—) menthol (36) in nine steps via the crucial intermediate 7(R,S^-isopropyl-10(R^-methyl-1(5V-4-oxobi-cyclo (4.4.0) dec-5-ene (Bhonsle et al. 1994) (Scheme 3.6).

The intermediate ketone (37) is converted to its ketal (38) with migration of the double bond by reacting it with ethanediol in refluxing benzene in the presence of a catalytic amount of p-toluene sulfonic acid. The ketal was then subjected to ozonolysis in heptane, and the subsequent reduction of the ozonide in situ gave the diol ketal (39). Deprotection and dehydration were achieved in a single step by stirring the diol ketal cho cho o

2) AcOH

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