Hydroxy ketals

Ketones and aldehydes (51) are oxidized to a-hydroxy ketals and acetals (52) by electrolysis in MeOH containing halide ion under basic conditions (Eq. 9) [88]. 

Scheme 20 Anodic conversion of ketones to a-hydroxy ketals.

Iodine may be employed as a mediator to achieve a-hydroxylation of carbonyl compounds. In basic methanolic solution containing iodide, oxidation was reported to lead to a-iodo ketones, which further reacted to give a-hydroxy ketals [171]. The electrolysis of alkylidenemalonates in the presence of iodide as a mediator has been shown to yield cyclopropane derivatives [172]. 

Direct activation methods are not normally successful for the oxidation of alcohols using hydrogen peroxide. An advantage of this is that alcohols can often be used as solvents for other oxidations, which do not use direct activation methods. However, hydroxy ketals can be cleaved with aqueous hydrogen peroxide, and the ene-diol of L-ascorbic acid can be oxidized with sodium perborate.185 Under the conditions employed, it is likely that the active oxidant is free hydrogen peroxide rather than any boron species. 

Isopropylideneglycerol, a five-membered cyclic hydroxy ketal from acetone and glycerol, is prepared in 90% yield by removing the liberated water by an azeotropic distillation. In another procedure, calcium carbide is added directly to the reaction mixture as a desiccant. Acetaldehyde and benzaldehyde, unlike acetone, react with glycerol to form a mixture of the five- and six-membered cyclic hydroxy acetals. Alkoxy acetals are made by the acetalization of a,/3-olefinic aldehydes in weakly acidic solutions however, the addition of alcohol to the double bond may not go to completion. ... 

The 1,2-addition reaction of organoaluminums may be extended to a-hydroxy ketals. An interesting example of this ketal variant appears in equation (14). 

Table 12 Addition of Organoaluminiums to a-Hydroxy Ketals (equation 14)...

Thus treatment of the cyclohexanone ketal 36 with 2-pyridyllithium gave the hydroxy ketal 37 which by hydrolysis and a two-carbon chain extension sequence yielded the diol 38. Acid treatment of 38 produced a mixture of the hydroxy lactone 39 and the unsaturated... 

Oxidation of chiral ketals.2 The chiral ketal 1 is oxidized by Rc.->07 and 2,6-lutidinc to 2-hydroxy ketal 2 with high (>99 1) enantiosclectivity. This reaction is believed to... 

---