Ketal protecting groups

Removal of the ketal protecting groups, followed by oxidation with sodium dichromate ia acetic acid gave... 

The cleavage proceeds by initial reduction of the nitro groups followed by acid-catalyzed cleavage. The DNB group can be cleaved in the presence of allyl, benzyl, tetrahydropyranyl, methoxy ethoxy methyl, methoxymethyl, silyl, trityl, and ketal protective groups. 

It is possible to change the stereochemical result of the alkylation by replacing the 3-ketal protecting group with a A -enol ether. This structural change eliminates a severe 1,3-diaxial interaction to a-face methylation and results in the formation of the 5a-methyl steroid (15) in about 35% yield, ... 

Removal of the ketal protecting group then permits... 

An expeditious route to the cyclic sulfamide HIV-1 protease inhibitors of type 145 and 146 (tetrahydro-l,2,7-thiadiazepine 1,1-dioxide derivatives) from 141 and 142 hinges on palladium-catalysed amidation reactions. These reactions of 144 and 143 were microwave promoted and provided, after removal of the cyclic ketal protecting group, moderate to good yields of (145, 57%) and (146, 66%) for example with R = NHCOCH2-2-naphthyl <06T4671>. 

This ketal protective group is resistant to basic and nucleophilic reagents but is readily removed by aqueous acid. Formaldehyde, acetaldehyde, and benzaldehyde can also used as the carbonyl component in the formation of cyclic acetals. They function in the same manner as acetone. A disadvantage in the case of acetaldehyde and benzaldehyde is the possibility of forming a mixture of diastereomers, because of the new stereogenic center at the acetal carbon. 

Difluorocyclopentadiene was prepared via a pyrolysis reaction the difluoro-methylene centre was installed (Eq. 119) using a DAST fluorination [318]. A fluorobutenolide building block was prepared by a Wadsworth-Emmons reaction of isopropylidene glyceraldehyde removal of the ketal protecting group led to the formation (Eq. 120) of the unsaturated lactone in acceptable overall yield... 

Note that removal of ketal protective groups usually necessitates acid catalysis. 

Renunal of ethylene hemithioacetal and -ketal protecting group, 1,3-Oxathiolanes when treated with chloramine-T in water or ethanol under mild conditions give the corresponding aldehydes or ketones in good yields. For example, 1,4-oxathiaspiro-[4.4]nonane (1) affords cyclopentanone (2) in 91 % yield when treated with chloramine-T in 85 % CHjOH-HjO at 25° for 2 min. 

The phosphorylation of ascorbic acid under basic conditions has been studied extensively. The synthesis of ascorbic acid 2-phosphate, 22, has been reported by Seib et al. (9) via treatment of 5,6-O-isopropylidene ascorbic acid, with phosphorus oxychloride under highly basic conditions (pH 12). Hydrolysis of the ketal protecting group of the 2-phosphate intermediate 20 affords the 2-O-phosphate 22, whose structure has been... 

W.J. Kerr and co-workers carried out the total synthesis of (+)-taylorione starting from readily available (+)-2-carene and using a modified Pauson-Khand annulation with ethylene gas as the key step. The key terminal alkyne intermediate was prepared by the Corey-Fuchs reaction. Interestingly, the ketal protecting group was sensitive to the excess of CBr4, so the addition of this reagent had to be monitored carefully to cleanly transform the aldehyde to the desired dibromoolefin. 

Completion of the monomeric unit was achieved via Swem oxidation and Arbuzov reaction with trimethylphosphite to give the phosphonate 75 in 31% overall yield from the resolved amino alcohol. Dimeric olefination was effected upon deprotonation to yield the macrocycle 76, which could be converted in 81% yield to optically active vermiculine (56) via sequential cleavage of the thioketal and ketal protecting groups. 

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