Titanium complexes trifluoromethanesulfonic acid

Protection of phenols by the foregoing methods is complicated by the rapid Friedel-Crafts rearrangement of the nascent rm-butyl ether. By using trifluoro-methanesulfonic add at -78 PC, the rate of /erf-butyl ether formation is fast and the Friedel-Crafts alkylation does not compete [Scheme 4.126].226 Similarly, attempts to deprotect phenol ferf-butyl ethers with trifluoroacetic acid or titanium tetrachloride may give complex mixtures, again as a result of Friedel-Crafts alkylation of the phenol but this side reaction can be suppressed by using a catalytic amount of trifluoromethanesulfonic acid in 2.2,2-trifluoroethanol as solvent at -5 DC. 

In the presence of an amino acid-derived boronate (e.g. 2) or a diamine-tin(II), complex,as Lewis acids, optically active aldol products are obtained in good yields (eq 8). In the addition of KSAs to iminies, a diphosphonium ditriflate (eq 9) or an acidic montmorillonite clay has been claimed to give better results than the originally reported Lewis acids (titanium(IV) chloride and trimethylsilyl trifluoromethanesulfonate ). The products of this reaction are valuable intermediates for the synthesis of p-lactams. Two excellent reviews covering this area have recently appeared. ... 

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