3-Substituted prolines: from synthesis to structural applications, from peptides to foldamers.

Among the twenty natural proteinogenic amino acids, proline is unique as its secondary amine forms a tertiary amide when incorporated into biopolymers, thus preventing hydrogen bond formation. Despite the lack of hydrogen bonds and thanks to conformational restriction of flexibility linked to the pyrrolidine ring, proline is able to stabilize peptide secondary structures such as b-turns or polyproline helices. These unique conformational properties have aroused a great interest in the development of proline analogues. Among them, proline chimeras are tools combining the proline restriction of flexibility together with the information brought by natural amino acids side chains. This review will focus on the chemical syntheses of 3-substituted proline chimeras of potential use for peptide syntheses and as potential use as tools for SAR studies of biologically active peptides and the development of secondary structure mimetics. Their influence on peptide structure will be briefly described.

Microfluidic synthesis of palladium nanocrystals assisted by supercritical CO2: tailored surface properties for applications in boron chemistry.

On the surface: A library of organic-inorganic hybrid palladium nanocrystals was synthesized using continuous supercritical microfluidic technology. The nanocatalysts show moderate to excellent activities towards C(Ar)-B and C(Ar)-C(Ar) bond-forming reactions, thus illustrating the relationship between surface properties and modulated catalytic activity.

A NEW APPROACH TO THE SYNTHESIS OF SUBSTITUTED PHENAZINES VIA PALLADIUM-CATALYZED ARYL LIGATIONi.

A new method for the "ligation" of two aromatic rings has been achieved via synthesis of functionalized phenazines by double Buchwald-Hartwig cyclization of a variety of substituted bromoanilines.

Rhodium-catalyzed functionalization of sterically hindered alkenes.

For the first time the rhodium-catalyzed 1,4-addition of organoboranes to hindered Baylis-Hillman adducts, trisubstituted alkenes, affording highly functionalized alkenes, via addition of the organoboranes and hydroxyelimination, is reported. Moreover, preliminary results have shown that, thanks to the use of a monosubstituted chiral diene ligand, enantio-enriched products were easily accessible, while chiral phosphane ligands were completely inappropriate in this reaction.

Room-temperature rhodium-catalyzed asymmetric 1,4-addition of potassium trifluoro(organo)borates.

For the first time the room-temperature rhodium-catalyzed asymmetric 1,4-addition of potassium aryltrifluoroborates to alpha,beta-unsaturated substrates is described. Thanks to the use of a chiral diene as ligand for rhodium and triethylamine as base, to facilitate transmetalation of the boron species, high yields and enantioselectivities were generally achieved. Moreover, the use of such tetravalent boron species offers some improvements compared to the use of boronic acids in term of stability and ease of purification.

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.