Ketal functional groups

Hemi-acetal and hemi-ketal functional groups (Fig. 26) are very susceptible to base-catalyzed hydrolysis and are also susceptible to acid catalyzed hydrolysis. In contrast, acetals and ketals are extremely resistant to hydrolysis by base. For acetals and ketals, only acid-catalyzed hydrolysis occurs... 

In another approach to enantioselective 2 + 2 photocycloadditions, the easily available chiral l,3-dioxin-4-ones have been used as synthetic equivalents of acetylacetates in de Mayo reactions. Chiral information is introduced through a ketalic functional group and menthone derivatives efficiently control the facial selectivity, as indicated in Scheme 30 [155-157]. 

The methods used to synthesize the acetal and ketal functional groups have already been considered in a previous volume by the authors [1]. In addition, the polymerization of aldehydes to give polyethers as shown in Eq. (3) has been described in Vol. 1 of this series [2]. 

A carbonyl group cannot be protected as its ethylene ketal during the Birch reduction of an aromatic phenolic ether if one desires to regenerate the ketone and to retain the 1,4-dihydroaromatic system, since an enol ether is hydrolyzed by acid more rapidly than is an ethylene ketal. 1,4-Dihydro-estrone 3-methyl ether is usually prepared by the Birch reduction of estradiol 3-methyl ether followed by Oppenauer oxidation to reform the C-17 carbonyl function. However, the C-17 carbonyl group may be protected as its diethyl ketal and, following a Birch reduction of the A-ring, this ketal function may be hydrolyzed in preference to the 3-enol ether, provided carefully controlled conditions are employed. Conditions for such a selective hydrolysis are illustrated in Procedure 4. 

The dimethyl ketal function (51) is one of the most suitable base stable protecting groups for saturated 5a- and 5/i-3-ketones. It is formed by reaction of the ketone (50) with methanol in the presence of a suitable catalyst. Good selectivity can also be achieved with this group since 2-, 6-, 11-, 12-, 17- and 20-ketones do not form dimethyl ketals under these conditions. The 2-ketone is converted in part to the dimethyl ketal in the presence of homogeneous rhodium catalyst. "" y -Toluenesulfonic acid is the catalyst of... 

Many functional groups are stable to alkaline hydrogen peroxide. Acetate esters are usually hydrolyzed under the reaction conditions although methods have been developed to prevent hydrolysis.For the preparation of the 4,5-oxiranes of desoxycorticosterone, hydrocortisone, and cortisone, the alkali-sensitive ketol side chains must be protected with a base-resistant group, e.g., the tetrahydropyranyl ether or the ethylene ketal derivative. Sodium carbonate has been used successfully as a base with unprotected ketol side chains, but it should be noted that some ketols are sensitive to sodium carbonate in the absence of hydrogen peroxide. The spiroketal side chain of the sapogenins is stable to the basic reaction conditions. 

Deep fluorinalion of alkanes, ethers, acid fmlides, esters, alkyl chlorides, most ketones, ketals, orthoesters, and combinations of these functional groups produces principally the perfluonnated analogues (Table 2) Chlorine substituents (or chloro groups) usually survive fluorination... 

In the mass spectrum (Figure 6) of 3-deoxy-l,2 5,6-di-0-isopropyli-dene-D-xt/Zo-hexofuranose (9) the fragmentations described above are found at m/e 229, 171, 143, 111, and 101. The fragments at m/e 143 and 101 arise by cleavage of C-4-C-5 with charge retention on C-4 and C-5, respectively (see Equations 17 and 18). Scheme 2 summarizes the losses of a methyl group, acetone from the second cyclic ketal function, and... 

This preparation describes a convenient and general method of synthesis of substituted pyrimidines from compounds containing a /3-dicarbonyl group, either intact or as the corresponding ketal. The usefulness of the 2-mercaptopyrimidines is enhanced by the ease of removal of the mercapto group by desulfurization 9 or oxidation 10 and its replacement by other functional groups.1 ... 

Dehydration of cortisone (198) affords the diene 199. This is then converted to ketal 200. The selectivity is due to hindrance about both the 11- and 20-carbonyl groups. The shift of the double bond to the 5,6-position is characteristic of that particular enone. Treatment of protected diene 200 with osmium tetroxide results in selective oxidation of the conjugated double bond at C-16,17 to afford the cis-diol (201). Reduction of the ketone at C-ll (202) followed by hydrolysis of the ketal function gives the intermediate 203. Selenium dioxide has been... 

The selective organosilane reduction of ketone functions can be effected in the presence of a number of other functional groups including epoxides,320,366 ketals,86,367 thioketals,368 other ketones,369,370 /1-lactams,371 alkynes,372 esters,79,80,83,84,87,320,373,374 a-bromides,76,80,83 amides80,83,84,86,276,320,375... 

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. 

In 1999 Uemuraetal. isolatedattenol A (100) and B (101) (Fig. 1.2.5), both marine natural products exhibiting a moderate cytotoxicity against P388 cells [57], from the Chinese Pinna attenuata. Since they are isomeric triols they differ only in the hydroxyl groups involved in the formation of the ketal functionality, so that this results in a l,6-dioxaspiro[4.5]decane and a 6,8-dioxabicyclo[3.2.1]octane unit as the main structural feature of attenol A and B, respectively. 

Vinyltrimethylstannanes react with xenon difluoride in dichloromethane at room temperature in the presence of equimolar (or a 50 Vo excess) of silver trifluoromethanesulfonate and a catalytical (0.1 equiv) quantity of 2,6-di-/e/7-butyl-4-methylpyridine to form the corresponding vinyl fluorides in high to moderate yields.54 57 The substitution reaction is tolerant to various functional groups, such as ketones, esters, carbamates, ketals, ethers, phenol rings and tertiary alcohols. As byproducts corresponding alkenes have been detected due to pro-tiodestannylation. 

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