Acetals and ketals acyclic

Acyclic monothio acetals and ketals can be prepared directly from a carbonyl compound or by transketalization, a reaction that does not involve a free carbonyl group, from a 1,3-dithiane or 1,3-dithiolane. They are cleaved by acidic hydrolysis or Hg(II) salts. 

A tertiary radical can be formed by elimination during AF of ter/-butyl methyl and ethyl ethers thus, isolation of the respective perfluoro-/erf-butyl ethers, e.g. 1, occurs in only 36 and 42% yield.28 Significant quantities of perfluoro(2-methylpropane) (2) are also isolated. The longer alkyl chains (ethyl and larger) appear to be slightly less prone to scission than the methyl group. Apparently, carbonyl fluoride is more readily eliminated than trifluoroacetyl fluoride, a phenomenon observed during AF of esters.29 Elimination becomes most serious in the special class of polyethers called ortho esters, e.g. 3-5.30 Cyclic ortho esters, acetals and ketals are much less affected than acyclics. 

Cleavage of ethers1 and acetals.2 This reagent cleaves cyclic and acyclic acetals and ketals, including p-methoxyethoxymethyl (MEM), methoxymethyl (MOM), and... 

Ionic hydrogenation occurs when alkyl- or aryl-silicon hydrides reduce carbonium ions. " Thus, acetals and ketals can be reduced by trialkylsilanes in the presence of 5-10% of ZnCh. Yields of 50-85% of ether are obtained from acyclic acetals (equation 15), but cyclic acetals give appreciable amounts of a by-product (8 equation 16), which is believed not to be the result of further reduction. 

An elegant procedure for generating ct-cyano ethers from acyclic or cyclic acetals and ketals is their reaction with r-alkyl isocyanides in the presence of an equimolar amount of TiCU (Scheme 18). 72 7]je exchange of alkoxy groups against cyanide functions proved to be very important for the preparation of cyanoquinones. ... 

Carbonyl functions often are protected as thioacetals, because these derivatives are stable to acids and bases. Dithio-acetals and -ketals generally are prepared by reaction of the carbonyl compound with a thiol in the presence of an acid catalyst. This transformation has been used in carbohydrate chemistry for a long time to lock aldoses in their acyclic forms (see equation 26). In recent years, this blocking principle for carbohydrates has been exploited in several chiral pool syntheses. Several other methods for... 

Acyclic acetals and ketals can also be prepared. A convenient route involves acid-catalyzed exchange with an orthoester... 

---