Scheme 3.10 Synthesis of zingiberenol 49 (adapted from Paquette, 1982 and Kinney et al. 1983).


Scheme 3-11 Synthesis of a-zingiberenol 49 and P-sesquiphellandrene 55 from the unsaturated ketone 32.

Precursors of Aroma and Flavoring Compounds

Biosynthesis of terpenoids and sesquiterpenoids in essential oils has been widely studied (Teisseire, 1991, and references cited therein). Biosynthesis of the main sesquiterpene hydrocarbons with a bisabolane skeleton has been reported by Rani (1999) in an Indian ginger oil. Cyclization of farnesyl pyrophosphate isomers (57) to (59) affords ar-cur-cumene, a-zingiberene and P-bisabolyl cation (60) (Scheme 3.12).

The atomic ratios (14C/3H) in ar-curcumene, a-zingiberene and P-bisabolene, which were fed with (2-14C, 2-3H2), (2-14C, 4 R — 3H1) and (2-14C, 5-3H2) mevalonic acid and (1 — 3H2) farnesyl pyrophosphate (FPP) revealed:

1. (2E, 6E)-isomer of FPP (59) is isomerized to (2E, 6E)—isomer (57) without loss of epimeric hydrogen, that is, without a redox process.

2. (2Z, 6E) FPP (57) is cyclized to a bisabolyl cation (60) that is the penultimate precursor of a-zingiberene, ar-curcumene, and P-bisabolene.

3. Two 1,2-hydrogen shifts take place during the formation of a-zingiberene, whereas one 1,2-hydrogen shift was observed during the formation of ar-cur-cumene.

Another interesting work is the first isolation of geranyl disaccharides (61) from ginger and their relation to aroma formation (Sekiwa et al. [2001b,c]) (Figure 3.5).

The precursors and enzyme activities involved in the formation of geraniol and related compounds in ginger were investigated. Repeated chromatography afforded the isolation of a glucoside and three kinds of disaccharides of geraniol from fresh rhizomes of ginger. Their structures were determined by spectroscopic analyses. After incubating, each glycoside with a crude enzyme solution prepared from ginger powder released significant quantities of geraniol. These data suggest that the glycosides exist as precursors or intermediates of geraniol-related compounds in ginger aroma. The nomenclature of these glycosides and amounts of geraniol liberated are given below:

61a: geranyl 6-O-a-L-arabino pyranosyl-P-D-glucopyranoside: 76.6% geraniol 61b: geranyl 6-O-P-D-xylopyranosyl-P-D-glucopyranoside: 19.3% geraniol 61c: geranyl 6-O-P-D-apiopyranosyl-P-D-glucopyranoside: 76.2% geraniol (relative to P-D-pyranoside = 100)

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