Retinal electroreduction

The intermediate 4-(t-butylthio)-but-3-en-2-one has been used in a new synthesis of isorenieratene [0,0-carotene(69)]. Electroreduction of retinal (70) in the presence of diethyl malonate or diethyl ethylmalonate gave the pinacol 15,15 -dihydro-/S,/8- carotene-15,15 -diol (71)... 

When an alkenic bond is conjugated with the carbonyl group, the carbonyl carbon and the p-carbon become the two reactive centers and a variety of products can be obtained depending on the medium. Presence of water in the reduction of 4-methyl-2-cyclohexenones results in a mixture of products. However, when the p-position is substituted such as in retinal (11), pinacolization takes place to form the pinacol (12) in 89% yield, provided that the electroreduction is carried out in an aprotic medium in the presence of a mild proton donor, such as diethyl malonate (equation 6). ... 

The effective electroreductive pinacolization has been exemplified in the terpene series, i.e. aldehydes such as retinal and ketones such as a- or j8-ionone give the corresponding pinacols in good yields when electrolyzed in a DMF-sodium per-chlorate-Hg system or with a Pt cathode in the presence of tin [8, 9]. 

In the presence of Cr" /3-ionone, /3-ionylideneacetaldehyde (160), and retinal undergo electroreduction to pinacols (161)—(163). A mechanism involving reduction of a complex between Cr " and the carbonyl compound is proposed. ... 

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