General asymmetric hydrogenation of 2-alkyl- and 2-aryl-substituted quinoxaline derivatives catalyzed by iridium-difluorphos: unusual halide effect and synthetic application.

A general asymmetric hydrogenation of a wide range of 2-alkyl- and 2-aryl-substituted quinoxaline derivatives catalyzed by an iridium-difluorphos complex has been developed. Under mild reaction conditions, the corresponding biologically relevant 2-substituted-1,2,3,4-tetrahydroquinoxaline units were obtained in high yields and good to excellent enantioselectivities up to 95%. With a catalyst ratio of S/C = 1000 and on a gram scale, the catalytic activity of the Ir-difluorphos complex was maintained showing its potential value. Finally, we demonstrated the application of our process in the synthesis of compound (S)-9, which is an inhibitor of cholesteryl ester transfer protein (CETP).

Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to α,ß-unsaturated ketones with DIFLUORPHOS and SYNPHOS analogues.

Applications of electron-deficient DIFLUORPHOS and SYNPHOS analogues in the rhodium-catalyzed asymmetric conjugate addition of boronic acids to α,ß-unsaturated ketones afford the 1,4-addition adducts in yields up to 92% and with 99% ee. Particularly, a Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to nonsubstituted maleimide substrates using the (R)-3,5-diCF(3)-SYNPHOS ligand is also reported. This protocol provides access to various enantioenriched 3-substituted succinimide units of biological interest, in high yields and good to excellent ee up to 93%, which could be upgraded up to 99% ee, after a single crystallization.

C-C bond formation via C-H bond activation using an in situ-generated ruthenium catalyst.

We report here our full results concerning the possibility of generating in situ from a stable and readily available ruthenium(II) source a highly active ruthenium catalyst for C-H bond activation. The versatility of this catalytic system has been demonstrated, as it offers the possibility of modifying the electronic and steric properties of the catalyst by fine-tuning of the ligands, allowing functionalization of various substrates. Aromatic ketones and imines could be easily functionalized by the reaction with either vinylsilanes or styrenes, depending on the electronic and steric nature of the ligand. Moreover, variable-temperature NMR experiments and the isolation of a ruthenium intermediate complex provided some insights into the generation of the active catalytic ruthenium species in this reaction.

Rhodium-catalyzed formation of stereocontrolled trisubstituted alkenes from Baylis-Hillman adducts.

Efficient and general conditions for the formation of stereodefined trisubstituted alkenes by using the rhodium-catalyzed reaction of unactivated Baylis-Hillman adducts with either organoboronic acids or potassium trifluoro(organo)borates are reported (see scheme).We report here efficient and general conditions for the formation of stereodefined trisubstituted alkenes using the rhodium-catalyzed reaction of unactivated Baylis-Hillman adducts with either organoboronic acids and potassium trifluoro(organo)borates. The use of the [{Rh(cod)OH}(2)] precursor gave very fast coupling reactions under low catalyst loading, very mild reaction conditions (from room temperature up to 50 degrees C) and without the need of additional phosphane ligands. Based on the new reaction conditions, the reaction, originally limited to Baylis-Hillman adducts derived from esters, could be extended to a large variety of Baylis-Hillman adducts, bearing either keto, cyano or amido functionalities. Moreover, the reaction of Baylis-Hillman adducts bearing esters functionality was improved and could be conducted at lower temperature using lower catalyst loading.

Access to enantioenriched alpha-amino esters via rhodium-catalyzed 1,4-addition/enantioselective protonation.

Conjugate addition of potassium trifluoro(organo)borates 2 to dehydroalanine derivatives 1, mediated by a chiral rhodium catalyst and in situ enantioselective protonation, afforded straightforward access to a variety of protected alpha-amino esters 3 with high yields and enantiomeric excesses up to 95%. Among the tested chiral ligands and proton sources, Binap, in combination with guaiacol (2-methoxyphenol), an inexpensive and nontoxic phenol, afforded the highest asymmetric inductions. Organostannanes have also shown to participate in this reaction. By a fine-tuning of the ester moiety, and using Difluorophos as chiral ligand, increased levels of enantioselectivity, generally close to 95%, were achieved. Deuterium labeling experiments revealed, and DFT calculation supported, an unusual mechanism involving a hydride transfer from the amido substituent to the alpha carbon explaining the high levels of enantioselectivity attained in controlling this alpha chiral center.

Anti-Markovnikov hydroarylation of styrenes catalyzed by an in situ generated ruthenium complex.

An efficient and practical catalytic system for the anti-Markovnikov ruthenium-catalyzed hydroarylation of styrenes with acetophenone, allowing a straightforward access to bibenzyl backbones, is described for the first time: this process, involving regioselective C-H bond activation, is complementary to a Friedel-Crafts type reaction giving the branched adduct.

Sterically hindered benzophenones via rhodium-catalyzed oxidative arylation of aldehydes.

Efficient cross-coupling, allowing a straightforward access to congested benzophenones, between aromatic aldehydes and potassium aryltrifluoroborates, is described in the presence of a rhodium/tri-tert-butylphosphane catalyst system and acetone as cosolvent. The use of the stable phosphonium salts of tri-tert-butylphosphane prevented the use of highly oxidizable tri-tert-butylphosphane and allowed a careful control of the stoichiometry with the rhodium.

Asymmetric Pauson-Khand-type reaction mediated by Rh(I) catalyst at ambient temperature.

An efficient asymmetric PKR mediated by Rh(I) catalyst at ambient temperature was developed. The reaction utilizing a Rh(I) catalyst bearing a (R)-3,5-diMeC4H4-BINAP ligand at 18-20 degrees C under a reduced partial pressure of CO (0.1 atm) provided PKR products in high chemical yield as well as high enantioselectivity.

Captive breeding of the barndoor skate (Dipturus laevis) at the Montreal Biodome, with comparison notes on two other captive-bred skate species.

In 1997, the Montreal Biodome obtained five barndoor skates (Dipturus laevis) from the waters off Boston, Massachusetts. Six years later, those specimens began reproducing, and the first egg case was collected in November 2003. Since then, 73 hatchlings have been born and raised. Egg cases were observed year round, and annual fecundity was measured for the first time: one female laid 69 eggs in 2005, 85 in 2006 and 115 in 2007. Egg incubation was longer than believed previously, ranging from 342 to 494 days. Hatching occurred throughout the year. Hatchlings averaged 193 mm total length and 128 mm disk width and weighed 32 g. They were fed krill and diced fish. All but one survived the first month. A photo identification system was useful in recognizing two groups of 10 specimens during their first year, and transponders could be inserted in the wing muscles of 1-year-old skates. Total lengths at birth and at age 2 were similar to the data reported from the wild, suggesting a similar growth pattern in captivity. The reproduction characteristics of the barndoor skate were compared with those of two other skate species currently bred at the Montreal Biodome, the winter skate (Leucoraja ocellata) and the thorny skate (Amblyraja radiata). Zoo Biol 27:145-153, 2008. (c) 2008 Wiley-Liss, Inc.

Cycloisomerization of 1,n-enynes: challenging metal-catalyzed rearrangements and mechanistic insights.

Metal-catalyzed cycloisomerization reactions of 1,n-enynes have appeared as conceptually and chemically highly attractive processes as they contribute to the highly demanded search for atom economy and allow the discovery of new reactions. Since the pioneering studies with palladium by the research group of Barry Trost in the mid-1980s, several other metals have been identified as excellent catalysts for the rearrangement of enyne skeletons. Moreover, the behavior of 1,n-enynes may be influenced by other functional groups such as alcohols, aldehydes, ethers, alkenes, or alkynes, thus enhancing the molecular complexity of the synthesized products. Apart from the intrinsic rearrangements of 1,n-enynes, several tandem reactions incorporating intramolecular trapping agents or intermolecular partners have been discovered. This Review aims to highlight the main contributions in this field of catalysis and to propose and comment on the mechanistic insights of the recent discoveries.

Base-free Mizoroki-Heck reaction catalyzed by rhodium complexes.

A base-free rhodium-catalyzed Mizoroki-Heck (M-H) reaction using potassium aryltrifluoroborates as the arylating agent of alkenes and acetone as a green "oxidant" is described. Thanks to the ready availability of organoboranes, this reaction should constitute an interesting alternative to conventional M-H reactions using aryl halides.

RuCl3/PPh3: an efficient combination for the preparation of chiral 1,3-anti-diols through catalytic hydrogenation.

[reaction: see text] An efficient economical alternative to the commonly used Evans' reagent for the diastereoselective reduction of beta-hydroxy ketones is reported. Thus, ruthenium-mediated hydrogenation of enantioenriched beta-hydroxy ketones using RuCl3 associated to achiral monophosphines allowed the preparation of a series of 1,3-anti-diols in good yields and with a high level of diastereoselectivity. A short screening of ligands pointed out PPh3 as the most effective phosphine, and PCy3 afforded the 1,3-diols with an unexpected moderate syn selectivity.

Gold-catalyzed hydroamination/cycloisomerization reaction of 1,6-enynes.

An efficient Au(I) catalytic system is described for the hydroamination/cycloisomerization reaction of functionalized 1,6-enynes. The reaction leads to carbo- and heterocyclic amino derivatives in good to excellent yields. The cyclizations were conducted in the presence of PPh(3)AuCl/AgSbF(6) catalyst in THF or dioxane at room temperature. The use of allyloxycarbonyl carbamate has allowed the formation of free amino derivatives via sequential Au- and Pd-catalyzed reactions.

Anti-bacterial activity of some Brazilian medicinal plants.

Extracts from various organs of 25 plants of Brazilian traditional medicine were assayed with respect to their anti-bacterial activities against Escherichia coli, a susceptible strain of Staphylococcus aureus and two resistant strains of Staphylococcus aureus harbouring the efflux pumps NorA and MsrA. Amongst the 49 extracts studied, 14 presented anti-bacterial activity against Staphylococcus aureus, including the ethanolic extracts from the rhizome of Jatropha elliptica, from the stem barks of Schinus terebinthifolius and Erythrina mulungu, from the stems and leaves of Caesalpinia pyramidalis and Serjania lethalis, and from the stem bark and leaves of Lafoensia pacari. The classes of compounds present in the active extracts were determined as a preliminary step towards their bioactivity-guided separation. No extracts were active against Escherichia coli.

Carbinol derivatives via rhodium-catalyzed addition of potassium trifluoro(organo)borates to aldehydes.

Reaction of potassium aryltrifluoroborates with aldehydes, in the presence of a rhodium catalyst, afforded carbinol derivatives in high yields under mild aqueous conditions; this efficient reaction proved to be general, allowing the production of highly hindered diarylmethanols and aliphatic aldehydes were also reactive under these conditions.

Multidrug resistance reversal agent from Jatropha elliptica.

As part of an ongoing project to identify plant natural products as resistance-modifying agents, bioassay-guided fractionation of an extract of Jatropha elliptica (Pohl) Muell Arg. led to the isolation of a penta-substituted pyridine, namely 2,6-dimethyl-4-phenyl-pyridine-3,5-dicarboxylic acid diethyl ester (8). The structure was established by spectroscopic methods. This known compound was assayed for in vitro antibacterial and resistance-modifying activities against strains of Staphylococcus aureus possessing the MsrA and NorA resistance efflux mechanisms. Antibiotic efflux studies indicated that (8) acts as an inhibitor of the NorA efflux pump and restores the level of intracellular drug concentration.