1,2,4-Triazole acylating reagent

Alkylation, arylation, and acylation of nitrogen in triazole have all been extensively explored and a wide range of reagents have been used. Both the protonated and deprotonated nitrogen substrates have been used. 

Halotriazoles can act as halogenating agents and A-acyltriazoles can act as acyl transfer reagents. Triazole can be used for the synthesis of peptide bonds and is superior to imidazole in that less racemization is observed. It can also be used to transfer the t-butyloxycarbonyl (t-Boc) protecting group to the nitrogen of amino acids. For details see Polya <84CHEC-I(5)733, p. 786). 

The parent 2(3/i/)-oxazolone moiety functions as a bifunctional leaving group when carboxyl groups are activated for acylations and condensations, similar to other five- and six-membered heterocycles such as imidazole, triazole, and 2-pyridinethiol. The excellent leaving ability of a 2(3//)-oxazolone moiety has led to the development of versatile reagents. Thus, 3-acyl- and 3-alkoxycarbonyl-2(3//)-oxazolones serve as ready-to-use -type agents for the regioselective and chemoselective N-protection of amino alcohols, amino phenols and polyamines. 

Deactivation against electrophilic attack accounts for the difficulty or failure of nitration, sulfonation and iV-oxidation of 1,2,4-triazoles proper. However, triazolate anions react readily with electrophilic reagents alkylation and acylation have received much attention but halogenation and addition reactions less. Systematic study of the formation and reactions of salts and metallic complexes is of recent origin. 

Triazoles with alkyl, aryl or acyl substituents on N-1 or N-4 can be quaternized. Because of the mesomeric distribution of the positive charge on triazolium compounds, representations such as (63) are convenient but the equivalent formula (64) may be used to denote the site at which quaternization has taken place (the mesomeric nature of (64) must be kept in mind of course). Trialkyloxonium tetrafluoroborates are powerful quaternizing reagents (70JOC2246). 

The condensation of azides with acyl-Wittig reagents offers a regiospecific synthesis of 1,5-disubstituted 1,2,3-triazoles. ... 

Sulfonyl azides participate in unique CuAAC reactions with terminal alkynes. Depending on the conditions and reagents, products other than the expected triazole 28 [119] can be obtained, as shown in Scheme 10.8. For example, N-sulfonyl azides are converted to N-sulfonyl amidines 29 when the reaction is conducted in the presence of amines [120]. In the aqueous conditions, N-acyl sulfonamides 30 are the major products [121, 122]. 

Some recent highlights include adaptation of Katritzky s benzo-triazole syntheses to ether preparation , generation of acyl radicals by carbonylation , and oxidative ring closures with Fe(C104)3 (as an alternative to the harsher Mn(III) route) . Bis(phosphonium) anhydrides are finding diverse applications, and a number of new vanadium reagents have materialized, e.g. a V(II) complex dimer for cross-coupling of aldehydes as an alternative to the McMurry route . ... 

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