ALKENES trifluoromethanesulfonic anhydride

Trifluoromethanesulfonates of alkyl and allylic alcohols can be prepared by reaction with trifluoromethanesulfonic anhydride in halogenated solvents in the presence of pyridine.3 Since the preparation of sulfonate esters does not disturb the C—O bond, problems of rearrangement or racemization do not arise in the ester formation step. However, sensitive sulfonate esters, such as allylic systems, may be subject to reversible ionization reactions, so appropriate precautions must be taken to ensure structural and stereochemical integrity. Tertiary alkyl sulfonates are neither as easily prepared nor as stable as those from primary and secondary alcohols. Under the standard preparative conditions, tertiary alcohols are likely to be converted to the corresponding alkene. 

The reactions of iminium complex 668, generated in the reaction of 4-(dimethylamino)-l,l,l-trifluorobut-3-en-2-one 667 with phosphoryl chloride or trifluoromethanesulfonic anhydride, with pyrroles proceeded stereospecifically to give finally (after hydrolysis of intermediates 669) the ( )-isomers of 2-vinylpyrroles 670 (Scheme 133) <1998T119, 1999RCR437>. The reactions with trifluoroacetyl alkenes depicted in Equation (160) and Schemes 132 and 133 are also applied to indole systems <1999RCR437>. 

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