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Heteroatom-substituted alkynes have attracted a significant amount of interest in the synthetic community due to the polarized nature of these alkynes and their utility in a wide range of reactions. One specific class of heteroatom-substituted alkynes combines this utility with the presence of an azole moiety. These N -alkynyl azoles have been known for nearly 50 years, but recently there has been a tremendous increase in the number of reports detailing the synthesis and utility of this class of compound. While much of the chemistry of N -alkynyl azoles mirrors that of the more extensively studied N -alkynyl amides (ynamides), there are notable exceptions. In addition, as azoles are extremely common in natural products and pharmaceuticals, these N -alkynyl azoles have high potential for accessing biologically important compounds. In this review, the literature reports of N -alkynyl azole synthesis, reactions, and uses have been assembled. Collectively, these reports demonstrate the growth in this area and the promise of exploiting N -alkynyl azoles in synthesis.

A new class of potential antitumor agents inspired by the enediyne antitumor antibiotics has been synthesized: the 1,2-dialkynylimidazoles. The aza-Bergman rearrangement of these 1,2-dialkynylimidazoles has been investigated theoretically at the B3LYP/6-31G(d,p) level and experimentally by measuring the kinetics of rearrangement in 1,4-cyclohexadiene. There is a good correlation between the theoretical and experimental results; subtle substituent effects on the initial aza-Bergman cyclization barrier predicted by theory are confirmed by experiment. Yet, despite the ability of these 1,2-dialkynylimidazoles to undergo Bergman rearrangement to diradical/carbene intermediates under relatively mild conditions, there is no correlation between the rate of Bergman cyclization and cytotoxicity to A459 cells. In addition, cytotoxic 1,2-dialkynylimidazoles do not cause nicking of supercoiled plasmid DNA or cleavage of bovine serum albumin. An alternative mechanism for cytotoxicity involving the unexpected selective thiol addition to the N-ethynyl group of certain 1,2-dialkynylimidazoles is proposed.

1,2-Dialkynylimidazoles have been reported to undergo thermal cyclization/rearrangement to diradical and carbene intermediates. Optimization of the synthesis of the 1,2-dialkynylimidazole has provided sufficient material for kinetic and biological studies. The 1,2-dialkynylimidazole is cytotoxic against a wide range of cancer cells and induces apoptosis in A549 cells. Experimentally-determined kinetics of the thermolysis of (E(a) = 30.0 kcal mol(-1)) are in excellent agreement with DFT calculations of the cyclization/rearrangement to diradical and cyclopentapyrazine carbene intermediates (E(a) = 29.7 kcal mol(-1)). Commensurate with the relatively high barrier for cyclization of , no evidence for cleavage of supercoiled DNA under physiological conditions was found; however, under aqueous conditions at 70 degrees C formed a covalent adduct with a model peptide. These studies indicate that if cyclization of is involved in its anticancer activity, the cyclization must be facilitated, perhaps through initial protein binding, which could lead to covalent protein modification.

A cross-coupling reaction of imidazoles with bromoalkynes in the presence of a catalytic amount of CuI is reported. This protocol allows an access to novel N-(1-alkynyl)imidazoles in moderate to good yields.

The interactions between a novel enediyne [1-methyl-2-(phenylethynyl)-3-(3-phenylprop-2-ynyl)-3H-benzimidazolium] (1) and various cytosine-containing oligonucleotides were studied using electrospray ionization mass spectrometry (ESI-MS) in a flow injection analysis mode useful for small volumes. This enediyne ligand, developed as a potential alternative to the highly cytotoxic natural enediynes, some of which have been successfully used as anti-tumor agents, has previously been shown to interact with DNA through frank strand scission as well as via the formation of adducts that lead to 2'-deoxycytidine-specific cleavage. Through ESI-MS, the structures of these adducts were examined and a sequence dependence of the 2'-deoxycytidine-specific cleavage was noted. Collisionally activated dissociation of the observed adducts confirmed the strength of the interactions between the enediyne and DNA and supports a direct linkage between the enediyne and the cytosine nucleobase, likely the result of a nucleophilic attack of the phenylethynyl group by the cytosine amine.

[reaction: see text] On the basis of density functional calculations, the isomerization of skipped azaenediynes (C-alkynyl-N-propargylimines) to azaenyne allenes and subsequent rapid aza-Myers-Saito cyclization to alpha,5-didehydro-3-picoline were predicted. We prepared the N-propargylimine of 1-phenyl-3-tri(isopropyl)silylprop-2-yn-1-one, which undergoes proto-desilylation and isomerization to an azaenyne allene when treated with tetrabutylammonium fluoride. In the presence of 1,4-cyclohexadiene, this azaenyne allene affords 6-phenyl-3-picoline and other products corresponding to the trapping of an alpha,5-didehydro-3-picoline diradical.

[reaction: see text] Simple, acyclic 3-aza-3-ene-1,5-diynes undergo an aza-Bergman rearrangement to a fleeting 2,5-didehydropyridine (2,5-ddp) intermediate that rapidly ring-opens to beta-alkynylacrylonitrile products. In an effort to access longer-lived 2,5-ddp intermediates, we have prepared heterocyclic 3-aza-3-ene-1,5-diynes. The thermolysis of one such heterocyclic aza-enediyne does not afford products derived from trapping a 2,5-ddp intermediate but rather cyclopropanes that appear to arise from a carbene intermediate and a product that appears to be a trapping product from a 2,3-ddp intermediate.

The (Z)-hex-1,5-diyne-3-ene reactive core common to the enediyne antitumor antibiotics undergoes a Bergman cyclization after proper activation to afford reactive diradical intermediates that are responsible for initiating DNA cleavage. Direct modification of the enediyne core has been proposed as a method to permit cancer cell-specific triggering of the diradical-generating cyclization. For example, 3-aza-3-ene-1,5-diynes undergo an aza-Bergman cyclization to afford the fleeting 2,5-didehydropyridine diradicals. While protonation of these aza-enediynes can afford products of diradical trapping, the hydrolytic instability of the 3-aza-3-ene-1,5-diyne moiety prevents its use in pH-triggered DNA cleaving anticancer agents. Recently, more hydrolytically stable systems incorporating the 4-aza-3-ene-1,6-diyne moiety were developed. We report here studies of the 4-aza-3-ene-1,6-diyne-containing benzimidazolium salt AZB002 [1-methyl-2-(phenylethynyl)-3-(3-phenylprop-2-ynyl)-3H-benzimidazolium tetrafluoroborate] and two structurally related heterocycles that lack the aza-enediyne functionality, AZB016 [1,3-dimethyl-2-(phenylethynyl)-3H-benzimidazolium triflate] and AZB004 [3-methyl-2-(phenylethynyl)benzothiazolium triflate]. The interaction of these compounds with supercoiled DNA, a double-stranded DNA fragment, and a short DNA duplex oligonucleotide was investigated. There are three distinct DNA interactions exhibited by AZB002: a frank strand scission leading to the relaxation of supercoiled DNA and formation of at least two different DNA adducts, one of which leads to cytosine-specific cleavage after piperidine/heat treatment. In contrast, analogues lacking the aza-enediyne functionality either fail to interact with DNA (AZB016) or cleave DNA at guanine residues, presumably through alkylation of the N-7 position (AZB004). We also investigated the cytotoxicity of AZB002 and the related heterocyclic compounds AZB004 and AZB016 and find that only the DNA interactive compounds AZB002 and AZB004 display significant cytotoxicity. In particular, AZB002 is cytotoxic against a wide range of cancer cell lines.

A copper-catalyzed N-alkynylation of pyrrole and a gold-catalyzed spiroketalization were key steps in the total synthesis of the pyrrole spiroketal alkaloid shensongine A. The preparation of this alkaloid is concise and amenable to the rapid synthesis of a diverse library of compounds. [ABSTRACT FROM AUTHOR]

Screening of a variety of ligands and reaction conditions for the copper-catalyzed cross-coupling of alkynyl bromides with pyrroles, reveals that the use of the phenanthroline ligand 4,7-dimethoxy-1,10-phenanthroline affords a range of ynpyrroles in good to moderate yields. Furthermore, the utility of these ynpyrroles is demonstrated in the preparation of a series of pyrrolo[2,1-c][1,4]oxazin-1-ones and a formal total synthesis of the pyrrole natural product peramine. [ABSTRACT FROM AUTHOR]