Synthesis and Absolute Stereochemistry of Chiral γ-Ionone and Dihydro-γ-ionone

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The optical resolution of (±)-α-cyclogeranic acid (4) with (-)-α-methylbenzylamine gave (-)-(S)-α-cyclogeranic acid (4). Epoxidation of (-)-(S)-methyl α-cyclogeranate (5), followed by reduction with lithium aluminum hydride, gave mainly (-)-(1R, 2R)-2-hydroxy-2, 6, 6-trimethyl-1-cyclohexanemethanol (7b), which was oxidized with pyridinium chlorochromate to give (-)-hydroxyaldehyde 8. The reaction of aldehyde 8 with 2-oxopropylidenetriphenylphosphorane gave (-)-(1'S, 2'R)-2', 3'-dihydro-2'-hydroxy-α-ionone (9). The thermal decomposition of (1'S)-2'-acetoxy-2', 3'-dihydro-α-ionone (10) gave (-)-(S)-α-ionone (2) and (+)-(S)-γ-ionone (1). The partial hydrogenation of (+)-(S)-γ-ionone (1) gave (+)-(S)-dihydro-γ-ionone (3).