Acetals carbonyl group protection

Acetalization (see Addition reactions to carbonyl groups, Protection of aldehydes and ketones)... 

Carbanions derived from aldehydes or a-dicarbonyl compounds, with one of the carbonyl groups protected as a dimethyl acetal, also react with o-iodoanilines to give indoles319. 

Logically, an extension of this sequence would lead to d" -synthons, for which a few corresponding reagents exist (Scheme 2.9). In practice, compound 4 is likely to display competing d - and d" -reactivity [12], In the case of compound 5, there is no relation of the reactivity at C-4 to the functionality at C-1 rather the acetal moiety is masking the carbonyl group, protecting it from the donor reactivity at C-4. 

The reagent (138), for photolabile carbonyl group protection, oxidized carbonyl compounds R COR (alcohols, ethers, and esters) under mild acidic or neutral conditions to the photosensitive acetals (139). The oxidation was presumably via hydride abstraction by the tritylium ion generated from (138) under acidic conditions. The primary and secondary alcohols showed unexpected similarities in reactivity, which had been rationalized by proposing slightly different mechanisms. High protection/de-protection efficiencies and remarkable dark stability of the acetals has been reported. ... 

It is well known tirat acetalization reaction is an important synthetic protocol useful not only for the carbonyl group protection when working wittiin multiftmctional organic molecules, but also for the synthesis of fragrances and related compoxmds [43]. 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

In a synthesis of sativene a carbonyl group was protected as a 2,4-DNP while a double bond was hydrated with BH3/H202/0H . Attempted protection of the carbonyl group as a ketal caused migration of the double bond protection as an oxime or oxime acetate was unsatisfactory since they would be reduced with BH3. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

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