Cyclization agents/cyclizations trifluoroacetic anhydride

The syntheses of dithiopyr (1), thiazopyr (2) and related compounds [4, 5] begin with a Hantzsch-type base-catalyzed intermolecular cyclization [22], which provides the dihydropyridines 4 (R = Me or Et). Two equivalents of methyl or ethyl trifiuoroacetoacetate (5, R = Me or Et) are allowed to react with one equivalent of isovaleraldehyde (6) in the presence of a base, like piperidine, in a suitable solvent at temperatures varying from room temperature to reflux. The intermediate dihydroxytetrahydropyran (structure not shown) is converted into the dihy-droxypiperidine 7 by reaction with a nitrogen source, such as ammonium hydroxide or ammonia gas. Reaction of 7 with a dehydrating agent, such as concentrated sulfuric acid, toluenesulfonic acid, or trifluoroacetic anhydride, gives a mixture... 

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