Optisch aktive 4,5‐Epoxy‐4,5‐dihydro‐α‐ionone und Synthese der stereoisomeren 4,5:4′,5′‐Diepoxy‐4,5,4′,5′‐tetrahydro‐ϵ,ϵ‐carotine und der sterische Verlauf ihrer Hydrolyse

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Abstract
Optically Active 4,5‐Epoxy‐4,5‐dihydro‐α‐ionones; Synthesis of the Stereoisomeric 4,5:4′,5′‐Diepoxy‐4,5,4′,5′‐tetrahydro‐ϵ,ϵ‐carotenes and the Steric Course of their HydrolysisWe prove that epoxidation with peracid of α‐ionone, contrary to a recently published statement, predominantly leads to the cis‐epoxide. Acid hydrolysis affords a single 4,5‐glycol whose structure, established by an X‐ray analysis, shows that oxirane opening occurred with inversion at the least substituted position (C(4)). Stable cis‐and trans‐epoxides are prepared by epoxidation of the C15‐phosphonates derived from α‐ionone. Both the racemic and optically active form are used for the synthesis of the 4,5:4′,5′‐diepoxy‐4,5,4′,5′‐tetrahydro‐ϵ,ϵ‐carotenes having the following configuration in the end groups: meso‐cis/cis, meso‐trans/trans, rac‐cis/trans, rac‐ and (6R, 6′ R)‐cis/cis, rac‐ and (6R, 6′R)‐trans/trans, rac‐ and (6R, 6′R)‐cis/trans, and (6R, 6′ R)‐cis/ϵ. Acid hydrolysis of the cis/cis‐epoxycarotenoids under relatively strong conditions occurs again with inversion at C(4)/C(4′) in case of the cis/cis‐epoxycarotenoids, but at C(5)/C(5′) in case of the trans/trans‐epoxycarotenoids. An independent synthesis of this 4,5,4′,5′‐tetrahydro‐ϵ,ϵ‐carotene‐4,5,4′,5′‐tetrol is presented. The irregular results of the oxirane hydrolysis are explained by assumption of neighbouring effects of the lateral chain. 400‐Mz‐1H‐NMR data are given for each of the stereoisomeric sets. In the visible range of the CD spectra, the (6R, 6R′)‐epoxycarotenoids compared with (6R, 6R′)‐ϵ,ϵ‐carotene exhibit an inversion of the Cotton effects.

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