Divergent Mechanisms of the Banert Cascade with Propargyl Azides

Understanding the Fate of the Banert Cascade of Propargylic Azides: Sigmatropic versus Prototropic Pathway

The Banert cascade is an efficient synthetic strategy for obtaining 4,5-disubstituted 1,2,3-triazoles. The reaction can proceed via a sigmatropic or prototropic mechanism depending on the substrate and the conditions. In this work, the mechanisms of both pathways from propargylic azides with different electronic features were investigated using density functional theory, quantum theory of atoms in molecules, and natural bond orbital approaches. The calculated energy barriers were consistent with the experimental data. Three patterns of electron density distribution on the transition structures were observed, which reflected the behaviors of the reactants in the Banert cascade. The stronger conjugative effects were associated with lower/higher free activation energies of sigmatropic/prototropic reactions, respectively. A clear relationship between the accumulation of the charge at the C³ atom of propargylic azides with the energy barriers for prototropic reactions was found. Thus, the obtained results would allow the prediction of the reaction's course by evaluating reactants.

Transition-metal catalyzed oxidative spirocyclization of N-aryl alkynamides with methylarenes under microwave irradiation

A practical synthetic route to construct a variety of 3-benzyl spiro[4,5]trienones was developed via transition-metal Cu/Ag-catalyzed oxidative ipso-annulation of activated alkynes with unactivated toluenes using TBPB as an oxidant under microwave irradiation. This method allows the formation of two carbon-carbon bonds and one carbon-oxygen bond in a single reaction through a sequence of C-H oxidative coupling, ipso-carbocyclization and dearomatization. The advantages of this protocol are its operational simplicity and broad substrate scope, and the ability to afford the desired products in moderate to good yields.

Intercepting the Banert cascade with nucleophilic fluorine: direct access to α-fluorinated NH-1,2,3-triazoles

The treatment of propargylic azides with silver(i) fluoride in acetonitrile was found to yield α-fluorinated NH-1,2,3-triazoles via the Banert cascade. The reaction was regioselective and the products result from an initial [3,3] rearrangement. The reaction is demonstrated on >15 examples with yields ranging from 37% to 86%.

Silver Mediated Banert Cascade with Carbon Nucleophiles

The Banert cascade of propargylic azides can be promoted by simple silver salts, and the triazafulvene intermediate can be intercepted by carbon nucleophiles. Various indoles (>25 examples, up to 92% yield) and electron-rich heterocycles were effective. The Mayr nucleophilicity parameter (N) was found to correlate to the reaction efficiency, which enabled the formation of C_sp³-C_sp² and C_sp³-C_sp³ bonds under otherwise identical conditions from structurally dissimilar nucleophiles.

Structural studies of triazole inhibitors with promising inhibitor effects against antibiotic resistance metallo-β-lactamases

Metallo-β-lactamases (MBLs) are an emerging cause of bacterial antibiotic resistance by hydrolysing all classes of β-lactams except monobactams, and the MBLs are not inhibited by clinically available serine-β-lactamase inhibitors. Two of the most commonly encountered MBLs in clinical isolates worldwide - the New Delhi metallo-β-lactamase (NDM-1) and the Verona integron-encoded metallo-β-lactamase (VIM-2) - are included in this study. A series of several NH-1,2,3-triazoles was prepared by a three-step protocol utilizing Banert cascade reaction as the key step. The inhibitor properties were evaluated in biochemical assays against the MBLs VIM-2, NDM-1 and GIM-1, and VIM-2 showed IC₅₀ values down to nanomolar range. High-resolution crystal structures of four inhibitors in complex with VIM-2 revealed hydrogen bonds from the triazole inhibitors to Arg228 and to the backbone of Ala231 or Asn233, along with hydrophobic interactions to Trp87, Phe61 and Tyr67. The inhibitors show reduced MIC in synergy assays with Pseudomonas aeruginosa and Escherichia coli strains harbouring VIM enzymes. The obtained results will be useful for further structural guided design of MBL inhibitors.