The Sonogashira Reaction: A Booming Methodology in Synthetic Organic Chemistry

Synthesis of organochalcogen propargyl aryl ethers and their application in the electrophilic cyclization reaction: an efficient preparation of 3-halo-4-chalcogen-2H-benzopyrans

We herein described the synthesis of various organochalcogen propargyl aryl ethers via reaction of lithium acetylide intermediate with electrophilic chalcogen (sulfur, selenium, tellurium) species. Various aryl and alkyl groups directly bonded to the chalcogen atom were used as electrophile. The results revealed that the reaction does not significantly depend on the electronic effects of substituents in the aromatic ring bonded to the chalcogen atom of the electrophilic chalcogen species. Additional versatility in this process was demonstrated with respect to a diverse array of functionality in the aromatic ring at propargyl aryl ethers. These propargyl aryl ethers, bearing the chalcogen group, underwent highly selective intramolecular cyclizations when treated with I(2) or ICl affording 3-iodo-4-chalcogen-2H-benzopyrans. The results demonstrated that the cyclization efficiency was significantly influenced by the steric effects of aromatic ring, since the cyclization reaction gave low yields with aromatic rings having a substituent at orto position than those having no substituent. The reactivity of 3-iodo-4-chalcogen-2H-benzopyrans was also studied. 4-Selenobutyl benzopyrans were treated under Neghishi cross-coupling conditions providing the corresponding 3-aryl benzopyran derivatives in good yields. In addition, using the copper catalyzed cross-coupling reactions with thiols, in the absence of any cocatalyst, we were able to introduce a thiol function in 3-iodo-benzopyran derivatives.

Synthesis, optical, and mesomorphic properties of self-assembled organogels featuring phenylethynyl framework with elaborated long-chain pyridine-2,6-dicarboxamides

A series of new organogelators with pi-conjugated phenylethynyl framework featuring long-chain carboxamides have been synthesized. These organgelators have shown great ability to gel a variety of organic solvents to form stable organogels with the minimum gelation concentration as low as 0.1 wt %. Gelation is completely thermoreversible, and it occurs due to the aggregation of the organogelators resulting in the formation of a fibrous network via a combination of intermolecular hydrogen bonding, pi-pi stacking, and van der Waals interactions that is observed for the xerogels by TEM. The influence of sol-to-gel transition has been explored in detail by variable-temperature 1H NMR, UV-vis absorption, and fluorescence spectroscopy. Aggregation-induced enhanced emission has been observed in these organogelator molecules with an order of higher fluorescence quantum yields from solution to gels. In addition, some molecules also exhibit unique liquid crystalline properties over a large temperature range as revealed by DSC and POM studies.

Iron-catalysed Sonogashira Reactions

A catalytic system has been developed that used an iron/ligand combination for the Sonogashira cross coupling of terminal alkynes with aryl iodides, which affords products in good to excellent yields.

A convenient method for the synthesis of alpha-ethynylphospholes and modulation of their pi-conjugated systems

Mind the (narrow) gap: Alpha-ethynylphospholes generated in situ from the corresponding silyl-capped precursors were converted into a series of alpha-(arylethynyl) phospholes bearing functional substituents as well as an alpha,alpha'-linked terphosphole (see scheme). The terphosphole has a narrow HOMO-LUMO gap owing to efficient pi conjugation over the three phosphole rings.

Efficient highly selective synthesis of methyl 2-(ethynyl)alk-2(E)-enoates and 2-(1'-chlorovinyl)alk-2(Z)-enoates from 2-(methoxycarbonyl)-2,3-allenols

Highly regio- and stereoselective reactions of readily available 2-(methoxycarbonyl)-2,3-allenols 1 with oxalyl chloride in the presence of Et(3)N or DMSO afforded methyl 2-(ethynyl)alk-2(E)-enoates (E)-2 and 2-(1'-chlorovinyl)alk-2(Z)-enoates (Z)-3, respectively, in moderate to good yields.

Synthesis of natural polyacetylenes bearing furan rings

The first total syntheses of four new polyacetylene compounds have been achieved using convergent routes, which involved Cadiot--Chodkiewicz copper-catalyzed cross-coupling reactions to sp-sp centers as the key steps. 19-Furan-2-ylnonadeca-5,7-diynoic acid (1), 19-furan-2-ylnonadeca-5,7-diynoic acid methyl ester (2), 2-pentacosa-7,9-diynylfuran (3), and 21-furan-2-ylhenicosa-14,16-diyn-1-ol (4) were stable and could be readily identified, isolated, and purified in high overall yields.

The Liebeskind-Srogl C-C cross-coupling reaction

Although a plethora of highly selective and reliable methods for the construction of C-C bonds are known to organic chemists, there is growing interest in the development of new protocols that offer different or orthogonal reactivity to that of existing methods. In 2000, Liebeskind and Srogl described a mechanistically unprecedented transition-metal-catalyzed cross-coupling of thioesters with boronic acids to produce ketones under neutral conditions. This desulfitative cross-coupling process is catalytic in palladium(0), stoichiometric in copper(I), and applicable to a range of organosulfur derivatives and nucleophilic organometallic reagents. In this Minireview, we highlight recent applications of this intriguing cross-coupling reaction in modern organic synthesis, with an emphasis on cases in which traditional methods for C-C bond formation have failed.

Convenient methods for preparing pi-conjugated linkers as building blocks for modular chemistry

Simple, straightforward and optimized procedures for preparing extended pi-conjugated linkers are described. Either unsubstituted or 4-donor substituted pi-linkers bearing a styryl, biphenyl, phenylethenylphenyl, and phenylethynylphenyl pi-conjugated backbone are functionalized with boronic pinacol esters as well as with terminal acetylene moieties allowing their further use as building blocks in Suzuki-Miyaura or Sonogashira coupling reactions.

Steroidal bivalent ligands for the estrogen receptor: design, synthesis, characterization and binding affinities

Steroidal bivalent ligands for the estrogen receptor (ER) were designed using crystal structures of ERalpha dimers as a template. The syntheses of several 17alpha-ethynylestradiol-based bivalent ligands with varying linker compositions and lengths are described. The binding affinities of these bivalent ligands for ERalpha and ERbeta were determined. In the two series of bivalent ligands that we synthesized, there is a clear correlation between linker length and binding affinity, both of which reach a maximum at the same tether length. Further studies are underway to explore aspects of bivalent ligand and control compound binding to the ERs and their effects on ER dimer formation; these results will be reported in a subsequent publication.

Development and biological evaluation of a novel aurora A kinase inhibitor

STOP DIVIDING: In the quest for antitumorigenic compounds, aurora A kinase has recently emerged as a potential drug target. In this paper three novel aurora inhibitors (shown in the illustration) have been tested for their biological activity in cultured cells. One of them (TC-28) appears to be a promising specific aurora A inhibitor in vivo. The aurora kinase family groups several serine/threonine kinases with key regulatory functions during cell division. The three mammalian members, aurora A, B and C, are frequently over-expressed in human tumors and the aurora A gene is located in a genomic region frequently amplified in breast and colon cancer. All these data have fuelled the idea that aurora kinases are promising targets for anticancer therapy. Indeed some inhibitory compounds are currently being evaluated in clinical trials. However, it was recently shown that mutations in the targeted kinase can confer resistance to a broad range of inhibitors and render patients resistant to treatments. Moreover, aurora A over-expression results in increased resistance to antimitotic agents. The development of new compounds targeting aurora A is therefore highly relevant. We describe here the synthesis of three novel aurora kinase inhibitors, TC-28, TC-34 and TC-107. We report their properties as aurora inhibitors in vitro and their effect on human tissue culture cell lines. Interestingly, our results show that TC-28 has properties compatible with the specific inhibition of aurora A, in vivo.

Preparation of Disubstituted Phenyl Propargyl Alcohols, their Use in Oxathiolene Oxide Synthesis, and Evaluation of the Oxathiolene Oxide Products as Anticarcinogenic Enzyme Inducers

A number of alkynols have been prepared by Sonogoshira coupling of propargyl alcohol to disubstituted aromatic halides. Chelation controlled addition of organometallic nucleophiles to these alkynols was then affected followed by the addition of sulfur dioxide. This methodology was used to prepare a number of oxathiolene oxides which have been screened as NQO1 (quinone oxidoreductase) inducers.

On the mechanism of nitrosoarene-alkyne cycloaddition

The thermal reaction between nitrosoarenes and alkynes produces N-hydroxyindoles as the major products. The mechanism of these novel reactions has been probed using a combination of experimental and computational methods. The reaction of nitrosobenzene (NB) with an excess of phenyl acetylene (PA) is determined to be first order in each reactant in benzene at 75 degrees C. The reaction rates have been determined for reactions between phenyl acetylene with a set of p-substituted nitrosoarenes, 4-X-C(6)H(4)NO, and of 4-O(2)N-C(6)H(4)NO with a set of p-substituted arylalkynes, 4-Y-C(6)H(4)C[triple bond]CH. The former reactions are accelerated by electron-withdrawing X groups (rho = +0.4), while the latter are faster with electron-donating Y groups (rho = -0.9). The kinetic isotope effect for the reaction of C(6)H(5)NO/C(6)D(5)NO with PhC[triple bond]CH is found to be 1.1 (+/-0.1) while that between PhC[triple bond]CH/PhC[triple bond]CD with PhNO is also 1.1 (+/-0.1). The reaction between nitrosobenzene and the radical clock probe cyclopropylacetylene affords 3-cyclopropyl indole in low yield. In addition to 3-carbomethoxy-N-hydroxyindole, the reaction between PA and o-carbomethoxy-nitrosobenzene also affords a tricyclic indole derivative, 3, likely derived from trapping of an intermediate indoline nitrone with PA and subsequent rearrangement. Computational studies of the reaction mechanism were carried out with density functional theory at the (U)B3LYP/6-31+G(d) level. The lowest energy pathway of the reaction of PhNO with alkynes was found to be stepwise; the N-C bond between nitrosoarene and acetylene is formed first, the resulting vinyl diradical undergoes cis-trans isomerization, and then the C-C bond forms. Conjugating substituents Z on the alkyne, Z-C[triple bond]CH, lower the calculated (and observed) activation barrier, Z = -H (19 kcal/mol), -Ph (15.8 kcal/mol), and -C(O)H (13 kcal/mol). The regioselectivity of the reaction, with formation of the 3-substituted indole, was reproduced by the calculations of PhNO + PhC[triple bond]CH; the rate-limiting step for formation of the 2-substituted indole is higher in energy by 11.6 kcal/mol. The effects of -NO(2), -CN, -Cl, -Br, -Me, and -OMe substituents were computed for the reactions of p-X-C(6)H(4)NO with PhC[triple bond]CH and of PhNO and/or p-NO(2)-C(6)H(4)NO with p-Y-C(6)H(4)C[triple bond]CH. The activation energies for the set of p-X-C(6)H(4)NO vary by 4.3 kcal/mol and follow the trend found experimentally, with electron-withdrawing X groups accelerating the reactions. The range of barriers for the p-Y-C(6)H(4)C[triple bond]CH reactions is smaller, about 1.5 and 1.8 kcal/mol in the cases of PhNO and p-NO(2)-PhNO, respectively. In agreement with the experiments, electron-donating Y groups on the alkyne accelerate the reactions with p-NO(2)-C(6)H(4)NO, while both ED and EW groups are predicted to facilitate the reaction. The calculated kinetic isotope effect for the reaction of C(6)H(5)NO/C(6)D(5)NO with PhC[triple bond]CH is negligible (as found experimentally) while that for PhC[triple bond]CH/PhC[triple bond]CD with PhNO (0.7) differs somewhat from the experiment (1.1). Taken together the experimental and computational results point to the operation of a stepwise diradical cycloaddition, with rate-limiting N-C bond formation and rapid C-C connection to form a bicyclic cyclohexadienyl-N-oxyl diradical, followed by fast tautomerization to the N-hydroxyindole product.