Trifluoroacetic anhydride, building block

From the synthetic viewpoint, a particularly interesting trifluoroacetylation reaction of simple vinyl ethers was reported first by Hojo et al in 1976 [73] The scope and limitation of this particular reaction were elaborated intensively, the reaction proved to be of general applicability with practically no restrictions on substituents of the vinyl ether moiety 9] (equation 34) This general validity is particularly beneficial because a trifluoroacetylated vinyl ether is the synthetic equivalent of a specifically protected tnfluoromethyl-substituted 1,3-dicarbonyl compound 19], thus the reaction provides access to a broad spectrum of variously substituted synthetic building blocks with selective reactivities on each carbon acceptor (a) and donor (d) center (equation 35) Obviously, such building blocks can react as heterodiene systems m cycloaddition reactions [< 74] or can be treated with a wide variety ot 1,2 or 1 3-dinucleophihc species to give any desired tnfluoromethyl-substituted carbocychc or heterocyclic system [8 75] Treatment of simple vinyl ethers with an excess of trifluoroacetic anhydride at elevated temperature leads to doubly acylated products [76] Comparable acylation reactions occur with vinyl thioethers [73], and the mesoiomc l,3-oxathiol-4-ones show, at least in a formal sense, similar behavior [77] (equation 36)... 

The [l,2,4]triazolo[l,5- ]pyridine system is usually constructed by the closure of the triazole ring either by oxidative formation of the NN bond or by condensation of the N-aminopyridine derivatives . Various compounds 78 were obtained by the oxidation of the amidine 77 with Mn02 (Scheme 47) <2003IJB2901, 2005ARK(xiii)21>. A dehydrative cyclization by treatment of formamidoximes 79 with trifluoroacetic anhydride (TFAA) has been used in the synthesis of 80 (Scheme 48), which are key building blocks in the preparation of DPP (IV) inhibitors <2005EJ03761, 2006JME3614>. 

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