Benzylic ethers Lewis acid activated

Posner G. H., Anjeh T. E. N., Carry J. C., French A. N. A New and Efficient Asymmetric Synthesis of an A-Ring Precursor to Physiologically Active 1-a-Hydroxyvitamin D3 Steroids Proc. - NOBCChE 1994 21 383-389 Keywords inverse electron-demand Diels-Alder cycloadditions, (S)-lactate and Lewis acids (-)-Pr(hfc)3 with benzyl vinyl ether... 

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). ... 

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. ... 

In the presence of a Lewis acid, silyl enol ethers can be alkylated with reactive secondary halides, such as substituted benzyl halides, and with chloromethylphenyl sulfide (ClCH2SPh), an activated primary halide. Thus, reaction of the benzyl chloride 10 in the presence of zinc bromide with the trimethylsilyl enol ether derived from mesityl oxide allowed a short and efficient route to the sesquiterpene ( )-ar-turmerone (1.22). Reaction of ClCH2SPh with the trimethylsilyl enol ethers of lactones in the presence of zinc bromide, followed by 5-oxidation and pyrolytic ehmination of the resulting sulfoxide (see Section 2.2), provides a good route to the a-methylene lactone unit common in many cytotoxic sesquiterpenes (1.23). Desulfurization with Raney nickel, instead of oxidation and elimination, affords the a-methyl (or a-alkyl starting with RCH(Cl)SPh) derivatives. ... 

Allyl silanes are rather like silyl enol ethers they react with electrophiles, provided they are activated, for example by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trimethylsilyl triflate. Electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

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