A Late Stage Strategy for the Functionalization of Triazolium-based NHC catalysts

Recent Advances in Enantioselective Organocatalytic Reactions Enabled by NHCs Containing Triazolium Motifs

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

A late-stage diversification via Heck-Matsuda arylation: Straightforward synthesis and cytotoxic/antiproliferative profiling of novel aryl-labdane-type derivatives

In the current study a late-stage diversification of unactivated olefins labd-8(17)-en-15-oic acid (1a) and methyl labd-8(17)-en-15-oate (1b) via Heck-Matsuda arylation is described. The reaction provided straightforward and practical access to a series of novel aryl-labdane-type derivatives (HM adducts 3a-h) in moderate to good yields in a highly regio- and stereoselective manner at room temperature under air atmosphere. The cytotoxic activity of these compounds was investigated in vitro against three different human cell lines (THP-1, K562, MCF-7). Of these, HM adduct 3h showed a selective effect in all cancer cell lines tested and was selected for extended biological investigations in a leukemia cell line (K562), which demonstrated that the cytotoxic/antiproliferative activity observed in this compound might be mediated by induction of cell cycle arrest at the sub-G1 phase and by autophagy-induced cell death. Taken together, these findings indicate that further investigation into the anticancer activity against chronic myeloid leukemia from aryl-labdane-type derivatives may be fruitful.

Enantioselective N-Heterocyclic Carbene-Catalyzed Kinetic Resolution of Anilides

The N-heterocyclic carbene (NHC)-catalyzed enantioselective kinetic resolution of anilides (a kind of hemiaminals) is reported. Upon exposure to the reaction in the presence of an NHC precatalyst and base, catalytic C-O bond formation occurs, providing axially chiral isoindolinones in high yields with excellent enantioselectivities.

Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions

Heterogeneous NHC-catalysis in batch and flow modes is an effective synthetic platform for the production of monoacylglycerols.

Diastereoselective Radical Hydroacylation of Alkylidenemalonates with Aliphatic Aldehydes Initiated by Photolysis of Hypervalent Iodine(III) Reagents

Diastereoselective radical hydroacylation of chiral alkylidenemalonates with aliphatic aldehydes is realized by the combination of a hypervalent iodine(III) reagent and UV-light irradiation. The reaction is initiated by the photolysis of hypervalent iodine(III) reagents under mild, metal-free conditions, and is the first example of diastereoselective addition of acyl radicals to olefins to afford chiral ketones in a highly stereoselective fashion. The obtained optically active ketones are useful chiral synthons, as exemplified by the short formal synthesis of (-)-methyleneolactocin.

Influence of Electronic Effects on the Reactivity of Triazolylidene-Boryl Radicals: Consequences for the use of N-Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

A small library of triazolylidene-boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen-atom donors, as well as that of the corresponding N-heterocyclic carbene (NHC)-boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin-trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene-borane. The new NHC-boranes were also used as co-initiators for the Type I photopolymerization of acrylates. This allowed a structure-reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC-boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.