Acetal ketone protecting group

Hydrates, hemiacetals, acetals, and protecting groups 17-9 Addition of Amines to Aldehydes and Ketones... 

The ability to convert a protective group to another functional group directly without first performing a deprotection is a potentially valuable transformation. Silyl-protected alcohols have been converted directly to aldehydes, ketones, bro-mides, acetates, and ethers without first liberating the alcohol in a prior deprotection step. 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Enol acetates are not very stable compounds and this limits their use as protecting groups. They are readily hydrolyzed to the parent ketones by acids and bases. 

A -Dien-3-ol esters e.g., acetates) have greater utility as reaction intermediates than as protecting groups. They are prepared from A" -3-ketones by reaction with the acetic anhydride"" or by exchange with isopropenyl acetate. 

A review discusses the condensation of aldehydes and ketones with glycerol to give 1,3-dioxanes and 1,3-dioxolanes. The chemistry of 0 0 and 0 S acetals has been reviewed, and a recent monograph discusses this area of protective groups in a didactic sense. ... 

Acetals are useful because they can act as protecting groups for aldehydes and ketones in the same way that trimethylsilyl ethers act as protecting groups for alcohols (Section 17.8). As we saw previously, it sometimes happens that one functional group interferes with intended chemistry elsewhere... 

Acetal (Section 19.10) A functional group consisting of two -OR groups bonded to the same carbon, R2C(OR )2-Acetals are often used as protecting groups for ketones and aldehydes. 

Protective Groups for Diols. Diols represent a special case in terms of applicable protecting groups. 1,2- and 1,3-diols easily form cyclic acetals with aldehydes and ketones, unless cyclization is precluded by molecular geometry. The isopropylidene derivatives (also called acetonides) formed by reaction with acetone are a common example. 

Nitrophenyl)ethylene glycol was used to protect simple aldehydes and ketones, as well as some steroids. Acetals were prepared under acid catalysis, leading, in the case of chiral carbonyl compounds to diaste-reoisomers. The photochemical removal of the protecting group was in several instances complicated by the instability of some carbonyl derivatives to irradiation at 350 nm otherwise, yields were in the range of 83-90% (see Scheme 19). 

Dave and co-workers have reported a successful synthesis of 2,2,4,4-tetranitroadamantane (117) which uses the mono-protected diketone (113) as a key intermediate. In this synthesis (113) is converted to the oxime (114) and then treated with ammonium nitrate and nitric acid in methylene chloride to yield the em-dinitro derivative (115). This nitration-oxidation step also removes the acetal-protecting group to leave the second ketone group free. Formation of the oxime (116) from ketone (115), followed by a similar nitration-oxidation with nitric acid and ammonium nitrate, yields 2,2,4,4-tetranitroadamantane (117). In this synthesis the protection strategy enables each carbonyl group to be treated separately and thus prevents the problem of internal nitroso dimer formation. 

Axenrod and co-workers reported a synthesis of TNAZ (18) starting from 3-amino-l,2-propanediol (28). Treatment of (28) with two equivalents of p-toluenesulfonyl chloride in the presence of pyridine yields the ditosylate (29), which on further protection as a TBS derivative, followed by treatment with lithium hydride in THF, induces ring closure to the azetidine (31) in excellent yield. Removal of the TBS protecting group from (31) with acetic acid at elevated temperature is followed by oxidation of the alcohol (32) to the ketone (33). Treatment of the ketone (33) with hydroxylamine hydrochloride in aqueous sodium acetate yields the oxime (34). The synthesis of TNAZ (18) is completed on treatment of the oxime (34) with pure nitric acid in methylene chloride, a reaction leading to oxidation-nitration of the oxime group to em-dinitro functionality and nitrolysis of the A-tosyl bond. This synthesis provides TNAZ in yields of 17-21 % over the seven steps. 

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