Protection of Ketones and Aldehydes

In each of the following reaction sequences, explain why a carbonyl group had to be protected  

15(a) If the ketone was not protected, it would react directly with the butyl lithium that is being used to prepare the lithioalkene 

15(b) The ketone is protected in the first step, so that when the alcohol is oxidized, there is only one free ketone to react with the nucleophile it would otherwise react with 

16 Both ketones are unsaturated, but one is sterically considerably more hindered than 

The increasing use of oximes and hydrazones as intermediates in the regio-selective modification of carbonyl compounds has led to an upsurge of interest in seeking mild, specific methods for regeneration of the carbonyl function from these derivatives. AjV-Dimethylhydrazones are hydrolysed rapidly under cupric ion 

Two new methods for dethioacetalization involve oxidative cleavage using sulphuryl chloride and wet silica and transthioacetalization using glyoxylic acid and hydrochloric acid.  

Trimethyl o-formate adsorbed on the commercially available acidic mont-morillonite clay K-10 is a reagent that effects rapid acetalization of carbonyl compounds at room temperature.  

The concept of using enolate ions as protecting groups for ketones has been elaborated in full.  

1-Cycloalkenyl ethyl sulphides result from the reaction of cycloalkanones with ethanethiol in the presence of phosphorus pentoxide.  

The bis(silyloxy)cyclohexadienes (35) and (36) are prepared by treatment of the readily available keto trialkylsilyl enol ethers with lithium di-isopropylamide and chlorotrimethylsilane. The isomeric compound (37) is prepared by treatment of cyclohexane-1,4-dione with triethylamine, zinc(ii) chloride, and chlorotrimethylsilane. 

Deketalization can be accomplished using iodotrimethylsilane in chloroform saturated with propene the alkene is required to trap traces of HI present.  

---

As a consequence, thiols are preferred to alcohols for the protection of aldehyde and ketone groups in synthetic procedures. Thioacetals and thioketals are... 

Dioxanes, like other acetals and ketals, are labile to dilute acid and this property has been exploited in schemes for the protection of aldehydes and ketones as well as for... 

Protection of aldehydes and ketones.1 The reagent reacts with aldehydes or ketones in refluxing ethanol (4-24 hours) to form 3-methylbenzothiazolines (2) in —70-90% yield. Aldehydes usually react so much more readily than ketones that selective protection is possible. Deprotection is effected in 90-98% yield by treatment with AgN03 in aqueous CH3CN at pH 7 or by reaction with HgCl2 in refluxing aqueous CH3CN. 

Protection of aldehydes and ketones.1 Bis-o-nitrobenzyl acetals or ketals are removable in 85-95% yield on irradiation at 350 nm in benzene. The acetals or ketals are easily prepared from 1 by an exchange reaction using 2,2-dimethoxy-propane (1, 268-269) catalyzed by an arenesulfonic acid. In the case of hindered ketones (17-keto steroids), the glycol o-N02C6H4CH(OH)CH2OH (2) can be used. 

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

Ti(ISRi)i (1), Ti(NR,)4, R = CH, or C,H, is an excellent reagent for in situ protection of aldehydes and ketones by formation of adducts that revert to the original carbonyl compounds on aqueous work-up. Both la. R = CH, and lb, R = C,H, react more readily with aldehydes than with ketones, but la is more reactive in general than lb. Thus selective reactions can be conducted on a ketone group in the presence of an aldehyde group (equation I). The method can also be used to carry out selective reaction... 

Protection of aldehydes and ketones. Treatment of a dimethyl or diethyl acetal with 1.5 eq. of trichloroethanoi in refluxing benzene under acid cataly.sis (p-TsOH) gives the mixed acetal use of 4 eq. of the alcohol gives the di-2,2,2-trichloroethyl acetal. 

Oxa-thiolanes and -thianes combine the properties of oxygen and sulfur acetals. Thus they can be cleaved with mercury salts and are also more labile towards acids than the sulfur analogs. In addition, oxazolidines, imidazolidines and thiazolidines can be used for the protection of aldehydes and ketones however, they have not found a broader use in protecting group chemistry. 

The thioacetals (R = H) and thioketals (6) can be used for the protection of aldehydes and ketones since on treatment with dilute acid they are converted back to the original carbonyl substrates.113 They may also be applied in the conversion ofa carbonyl to a methylene group by reaction with Raney nickel, an important general procedure of desulfurisation (Scheme 4).3... 

Br0nsted acidic ionic liquids have been successfully applied to a variety of reactions including esterification of carboxylic acids [55], protection of aldehydes and ketones [56], and cleavage of ethers [57], 3-Methylimidazolium hydrogen sulfate ([hmim][HSO ]) is a Brpnsted acidic ionic liquid that applied for the oxidation of... 

---