Scheme 3.8 Synthesis of some 1-(1,5-dimethyl-5-substituted hexyl)-4-methyl benzenes 45 (adapted from Menon et al. 1993).

Synthesis of Zingiberenol and (3-Sesquiphellandrols

The structure of zingiberenol (49) extracted from higher boiling fraction of ginger oil and isolated by chromatography was tentatively established by Varma et al. (1962). NMR studies show a close similarity with ( -eudesmol. In reality, the extracted compound was a mixture of (3-eudesmol with 70% trans ring and 30% cis ring juncture. Some years later, the structure of zingiberenol (49) was determined by its chemical analysis and spectroscopic data (IR, NMR, MS) by Terhune et al. (1975).

Zingiberenol (49) and oxygenated bicyclo (3.3.1) nonanes were synthesized by Paquette and Kinney (1982) and Kinney et al. (1983) using the regiospecific 7-alkylation of the sulfonylcyclohexanone ketal (52). The latter was prepared in 85% yield by Diels-Alder reaction of sulfonylphenylethylene (51) with trimethylsilyl derivative of the 1-methoxy-1,3-butanediene-3-ol (50) followed by ketalization with ethylene glycol. This ketal was alkylated by RBr (R = H2C = CH — CH2-, PhCH2) in the presence of sodium hydride in dimethyl formamide to give the corresponding alkyl cyclohexanone. Ketals (53) were obtained in 77% and 81%, respectively.

Desulfonation and hydrolysis of the 7-alkylated derivatives (53, R = PhCH2) gave mixture of a, (3- and (,^-cyclohexenones (54) in 83% of a 68/32 mixture of 4-benzyl-a,(- and (,^-cyclohexenones, respectively (Scheme 3.10).

a-Zingiberenol (49) and (3-sesquiphellandrene (55) were synthesized from 4-(6-methyl-5-hepten-2-yl)-2-cyclohexen-1-one by Flisak and Hall (1986) according to Scheme 3.11.

The unsaturated ketone (32) was prepared using an one-pot tandem arylation multisteps reaction-hydrolysis sequence. Cis- and trans-(-Sesquiphellandrols have been isolated from ginger oil and their structures determined by IR, UV, NMR, and MS data by Bednarczyk et al. (1975). They are stereoisomers of 5-(1,5-dimethyl-4-hexenyl)-2-methylene-3-cyclo-hexanol.

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