Retinal pinacolization

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). ... 

Cathodic reduction of retinal leads to a regioselective coupling in the presence of malonic ester to produce the corresponding pinacol (Fig. 26) [129]. Chromium(III) ions facilitate the reduction and favor the regioselective coupling of conjugated dienones to pinacols. A Cr(III)-carbonyl compound complex is evidenced as the reason for the selectivity [130]. 

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. ... 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

---