Scheme 3.6 Synthesis of (+) a-zingiberene 35b (adapted from Bhonsle et al. ).
(39) at room temperature with 10% sulfuric acid in ethanol to yield the keto alcohol
(40), which reacts with methylmagnesium iodide to afford the diol (41). This latter was subjected to dehydration with phosphorus oxychloride in pyridine yielding ( + ) a-zingiberene (35b) in a poor yield.
An alternative approach was proposed by dehydration of the ketoalcohol with phosphorus oxychloride in pyridine to yield ketoalkene which on treatment with MeMgl gave the alkene alcohol and by dehydration the expected compound in 59% yield. Several syntheses of ( + ) a-zingiberene have been reported by Chen and Ho (1989) Joshi and Kulkarni (1965).
7-epi-Zingiberene (35c), a diastereoisomer of the natural product, was isolated from wild tomato leaves (Breeden and Coates, 1994, 1995).
Structure elucidation was based upon its 1H-NMR, 13C-NMR, IR, UV, and MS. All data for the two diastereoisomers are identical except for C-9 and C-15 13C-NMR resonance. The (4S, 7S) stereochemistry of 7-epi-zingiberene was proven by dehydroge-nation of (7^)-ar-curcumene.
Substituted a-zingiberenes (38) have been synthesized from a-zingiberene via the intermediate unsaturated ketone (37) by Wang et al. (1990). Reacting the ketone with BuMgBr followed by treatment with 20% H2SO4 gave 85% of the butyl derivative (38; R = Bu) (Scheme 3.7).
Scheme 3.7 Synthesis of substituted a-zingiberene 38 from the unsaturated ketone 37.
Another process for the preparation of 1-(1,5-dimethyl-5-substituted hexyl)-4-methyl-benzenes (45) has been patented by Menon and Rao (1993). By reacting a-zingiberene with 2,3-dihydro-5,6-dicyano-1,4-benzoquinone (42) in dry benzene at reflux with a-zingiberene affords (a,b)-ar-curcumene (43), subsequently converted into 1-(1,5-dim-ethyl-5-chlorohexyl)-4-methylbenzene (tertiary chloride) (44) by bubbling anhydrous HCl gas through the mixture. Its reaction with alcohols in the presence of zinc oxide affords the corresponding ar-curcuminyl alcohol derivatives (45; R = H, CH3, CHO, COCH3) in moderate yields (Scheme 3.8). Odors of the respective compounds were described.
An easy synthesis of ar- (or a-)-curcumene (43) was carried out by Birch (1965). By reacting the 6-methyl-5-hepten-2-one (46) with the 4-methylphenylmagnesium bromide (47), the intermediate alcohol (48) was obtained. Upon treatment with sodium-ammonia in ethanol, it gave the .expected compound (43).
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