3-Ionone pinacols

Similarly, the pinacols of p-ionone could be obtained in 75% yield by carrying out the electrolysis in the presence of chromium(III) chloride. Touboul and Dana observed a unique case of enantiomeric recognition when they reduced a series of racemic l,9,10,10a-tetrahydro-3(2/ -phenanthrones (13). Pina-colization occurred with formation of a new carbon-carbon bond preferentially between identical enantiomers, as shown in equation (7).. ... 

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 electrochemically initiated reactions coupling at the p-carbon to give 1,6-diones is preferred even 2,2,6,6-tetramethyl-4-hepten-3-one couples selectively via this mode. Pinacol formation is favored when -coupling is sterically hindered. Thus polyenones, such as retinal, are hydrodimerized to the corresponding pinacol in the presence of efficient hydrogen donors (equation 50). In the absence of added hydrogen donors the yield of retinal pinacol is much reduced, even though a- and p-ionone are efficiently coupled under these conditions (equation 51). ... 

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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