Protection, aldehydes and

Conversion to acetals is a very general method for protecting aldehydes and ketones against nucleophilic addition or reduction.245 Ethylene glycol, which gives a cyclic dioxolane derivative, is frequently employed for this purpose. The dioxolanes are usually prepared by heating a carbonyl compound with ethylene glycol in the presence of an acid catalyst, with provision for azeotropic removal of water. 

Thioacetals and thioketals are the sulphur equivalents of acetals and ketals and are also prepared under acid conditions (Following fig.). These can also be used to protect aldehydes and ketones, but the hydrolysis of these groups is more difficult. Moreover, the thioacetals and thioketals can be removed by reduction and this provides a method of reducing aldehydes and ketones. 

Hamelin and coworkers protected aldehydes and ketones as acetals and dioxolanes by using orthoformates, 1,2-ethanedithiol, or 2,2-dimethyl-l,3-dioxolane. This acid-catalyzed reaction proceeds in the presence of p-toluenesulfonic acid (PTSA)... 

Acetals and Ketals, There have been several reports of protecting aldehydes and ketones via acetals and ketals prepared from trlmethyl orthoformate (equation 9) 1,2-ethanedlthlol and 1,3-butanedlol, or 1,3-propanedlol. 

A dioxolane can be used in this way to protect aldehydes and ketones from powerful, basic nucleophiles, and makes the first entry in the tour of important protecting groups we shall conduct you through in the next few pages. 

Acetals are used to protect aldehydes and ketones from undesired reactions in basic solutions. 

Aldol Reactions.—The recent use of metalated derivatives of protected aldehydes and ketones in cross-aldol reactions is referred to elsewhere (see refs 68, 72, 73, and 147). Alternative methodology for accomplishing cross-aldol reactions involves the utilization of vinyloxyboranes, and a new method for preparing the latter compounds from ketones has been reported (Scheme 25)." The scope of the related and more established cross-aldolization process involving the titanium(iv) chloride-promoted reaction of silyl enol ethers with aldehydes and ketones has been investigated, and the reaction has been utilized in the synthesis of two naturally occurring compounds. ... 

Glycidyl acetal derivatives, that is, C3 chiral building blocks bearing one stereo-genic center and two different and chemically differentiable fimctions (such as protected aldehyde and epoxide) located on a short carbon skeleton, are of particular... 

H ], 2 ROH, (-H2O) Acetal formation Conversion of an aldehyde or ketone into an acetal. The acetal group can be used to protect aldehydes and ketones. The acetal group is stable to basic conditions, but is removed when subjected to aqueous acidic conditions to regenerate the carbonyl group (a process called hydrolysis). 

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