Benzylic acetals Lewis acid activated

Metal triflate Lewis acids can also be dispersed in ionic liquids for catalytic applications. Acetylation of alcohols with acetic anhydride and acetic acid has been reported with Cu(OTf)2, Yb(OTf)3, Sc(OTf)3, In(OTf)3, HfClq. (THF)2, and InCl3 in ionic liquids that consist of [BMIM] and the anions BF4, PF, or SbF 166). With lmol% acid, all the catalysts in [BMIMJPF showed >99% acetylation products in acetyl anhydride acetylation of benzyl alcohol. Sc(OTf)3 showed the best yield with recycling, with a 25% drop in yield after two cycles. A relatively long reaction period was needed to obtain a high yield (95-98%) for the acetylation of benzyl alcohol with acetic acid, indicating that the activities of the catalysts were... 

A number of Lewis acidic metal salts have been tested in the acetylation of alcohols with acetic anhydride and acetic acid.[67] Of these, copper(II) triflate showed by far the highest activity, but the recycling potential was low. The recyclability was found to be much better with either Sc(OTf)3 or Yb(OTf)3. The reaction between benzyl alcohol and acetic anhydride proceeded to completion within one hour, whereas with acetic acid two days were required under identical reaction conditions. Of the ionic liquids tested, the best activities were obtained in [C4Ciim][PF6]. 

Oftentimes, even weak Lewis acids can accomplish the deprotection of methyl ethers activated by proximate carbonyl groups. For example, the last step in a synthesis of the protein kinase C inhibitor Calphostin A [Scheme 4,104]177 178 entailed regioselective cleavage of two phenolic methyl ethers using magnesium iodide in THF.179 The method is compatible with benzyl esters, N-Boc groups and N. 0-acetals,180... 

Certain benzylic alcohols, ethers, acetals and thiols, which are normally resistant to reduction by LAH, can be reduced to the hydrocarbon when the substrate is activated with a Lewis acid. AICI3 has been most widely used in this role. Various acetals can also be reduced to ethers using the same reagent (equation 44). A related procedure has been used to convert aldehydes into sulfides (equation 45). ... 

Related to the acetate activating group is the more labile trifluoroacetate group. In a reaction between methylphenol and 2,3,4,6-tetra-(9-benzyl-l-(9-trifluoroacetyl-D-glucopyranose, shown in Scheme 7.23, Allevi et al. [79] observed the formation of a 50% yield of the (3 coupled product. Additional examples illustrating the dependence of the stereochemical outcome on the nature of the Lewis acid are known [107,108]. 

In addition to the formation of anomeric nitriles, similar chemistry allows for the formation of anomeric isocyanides. Requirements for these reactions include utilizing acetates, halides, and imidates as activating groups. Some Lewis acids used to effect these transformations include SnCU, TMSOTf, TMSCN, Et2AlCN, and HgCN. Additionally, both benzylated and acetylated sugars may be used. 

Substitution. The Lewis acidity of Bi(OTf)3 caters to use in activating electron-rich benzyl ethers and acetates to react with enol silyl ethers. By the same token, only the alcohols (instead of halides) are needed in allylation and benzylation of 1,3-dicarbonyl compounds. ... 

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