Ketals acyclic

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

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. 

As with isolated rings, individual heterorings in fused systems which are synthetic equivalents of acyclic subunits, e.g. lactone, ketal, lactam, and hemiketal, can be disconnected. 

H2Sil2, CDCI3, —42°, 1-10 min, 100% yield. Aromatic ketals are cleaved faster than the corresponding aliphatic derivatives, and cyclic ketals are cleaved more slowly than the acyclic analogues, such as dimethyl ketals. Substituted ketals such as those derived from butane-2,3-diol, which react only slowly with Mc3SiI, can also be cleaved with H2Sil2. If the reaction is run at 22°, ketals and acetals are reduced to iodides in excellent yield. 

The C2-symmetric epoxide 23 (Scheme 7) reacts smoothly with carbon nucleophiles. For example, treatment of 23 with lithium dimethylcuprate proceeds with inversion of configuration, resulting in the formation of alcohol 28. An important consequence of the C2 symmetry of 23 is that the attack of the organometallic reagent upon either one of the two epoxide carbons produces the same product. After simultaneous hydrogenolysis of the two benzyl ethers in 28, protection of the 1,2-diol as an acetonide ring can be easily achieved by the use of 2,2-dimethoxypropane and camphor-sulfonic acid (CSA). It is necessary to briefly expose the crude product from the latter reaction to methanol and CSA so that the mixed acyclic ketal can be cleaved (see 29—>30). Oxidation of alcohol 30 with pyridinium chlorochromate (PCC) provides alde-... 

Acyclic and cyclic sp -hybridized ketal-containing y-functionalized organolithium compounds can be generated using an arene-catalyzed Uthiation at low temperature. In the case of acyclic precursors 171 (R = H) it was necessary to lower the temperature to -90 °C in the Uthiation step under DTBB catalysis (4%) in order to avoid decomposition of intermediates 172. Final electrophilic substitution reaction of these intermediates with electrophiles occurred with retention of configuration at temperatures ranging between... 

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. 

Diperoxyketals. Some commercially available d (tert-alkylperoxy)ketals and their corresponding 10-h half-life temperatures (determined in dodecane) are listed in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually followed by P-scission of the resulting alkoxy radicals (40). For acyclic diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

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