Acetals, Ketals

Mixtures of glycerol with other substances are often named as if they were derivatives of glycerol eg, boroglycetides (also called glyceryl borates) are mixtures of boric acid and glycerol. Derivatives, such as acetals, ketals, chlorohydrins, and ethers, can be prepared but are not made commercially, with the exception of polyglycerols. 

Several reports iu the Hterature describe the preparation and characterization of low, medium, and high vinyl polybutadienes (55—69). Each of these references used polar modifiers including chelating diamines, oxygenated ether compounds, acetals, ketals, and compounds of similar stmctures (56—64). 

Kinetic studies of acetal/ketal formation from cyclohexanone, and hydrolysis (3 X 0 N HCl/dioxane-H20, 20°), indicate the following orders of reactivity ... 

Hydroxy steroids afford 18,20-lactones in 48-74 % yield in the presence of other substituents such as acetates, ketals and saturated and unsaturated ketones. However only a 24% yield is reported in the presence of an 11-ketone . Tertiary 20-hydroxy-20-methyl pregnanes still give 18,20-lactones in 45% yield although a competitive cleavage of the 17,20-bond is also observed. 

Nearly every substitution of the aromatic ring has been tolerated for the cyclization step using thermal conditions, while acid-promoted conditions limited the functionality utilized. Substituents included halogens, esters, nitriles, nitro, thio-ethers, tertiary amines, alkyl, ethers, acetates, ketals, and amides. Primary and secondary amines are not well tolerated and poor yield resulted in the cyclization containing a free phenol. The Gould-Jacobs reaction has been applied to heterocycles attached and fused to the aniline. 

Reductive alkylations have been carried out successfully with compounds that are not carbonyls or amines, but which are transformed during the hydrogenation to suitable functions. Azides, azo, hydrazo, nitro and nitroso compounds, oximes, pyridines, and hydroxylamines serve as amines phenols, acetals, ketals, or hydrazones serve as carbonyls 6,7,8,9,12,17,24,41,42,58). Alkylations using masked functions have been successful at times when use of unmasked functions have failed (2). In a synthesis leading to methoxatin, a key... 

Carb-29. Hemiacetals and hemithioacetals 2-Carb-30. Acetals, ketals and their thio analogues 2-Carb-31. Names for monosaccharide residues... 

Carb-30. Acetals, ketals and their thio analogues... 

Glycosides were originally defined as mixed acetals (ketals) derived from cyclic forms of monosaccharides. 

The alkoxyl group OR is not a leaving group, so these compounds must be converted to the conjugate acids before they can be hydrolyzed. Although 100% sulfuric acid and other concentrated strong acids readily cleave simple ethers, the only acids used preparatively for this purpose are HBr and HI (10-71). However, acetals, ketals, and ortho esters are easily cleaved by dilute acids. These compounds are... 

Acetals, " ketals, and ortho esters react with Grignard reagents to give, respectively, ethers and acetals (or ketals). The latter can be hydrolyzed to aldehydes... 

Reduction of Acetals, Ketals, Hemiacetals, Hemiketals, and Orthoesters... 

Table 20. organosilane reduction of acetals, ketals, and hemiketals... 

TABLE 20. ORGANOSILANE REDUCTION OF ACETALS, KETALS, AND HEMIKETALS (Continued)... 

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