Cocatalysis by pivalic acid or copper bromide allows a very fast, clean, and high-yielding palladium-catalyzed coupling of a large array of aryl, thienyl, and pyridyl halides with cyclic nitrones, including DMPO. The study of the reaction conditions, scope, and mechanism is presented. Applied to the chiral nitrone MiPNO, this transformation provides a straightforward access to enantiopure α-methyl α-arylglycine esters.

Various functionalized aryl boronic esters derived from hexylene glycol and pinacol were prepared in excellent yields according to a simple, safe procedure. The metal-halogen exchange reaction between i PrMgCl 3 LiCl and aryl iodides is performed at 0 °C in the presence of a cyclic borate ester (MPBO i Pr or PinBO i Pr); the organomagnesium intermediate is immediately trapped in situ so that no accumulation of hazardous reactive species can occur. The reaction is very selective, and particularly clean crude products are obtained. The scope of the procedure and the tuning of reaction parameters are investigated.

The preparation of cyclic hexapeptides from N-hydroxy tripeptides building blocks is described. Introduction of an unsaturated chain on the hydroxamate oxygen followed by fragment coupling leads to N,N'-dialkenoxy hexapeptides that are efficiently cyclized through a ring-closing metathesis reaction. The length of the alkene chains allows the modulation of the ring size: the synthesis of 17- and 18-membered cycles is reported.

The resolution of chiral nitrones via derivatization of hydroxylamines was applied to MiPNO, a new, stable, easily prepared chiral cyclic nitrone. The application of MiPNO in totally regio- and diastereo-selective 1,3-dipolar cycloaddition reactions provides an expeditious enantioselective access to unusual gamma-hydroxy alpha-amino acids.

The asymmetric hydrogenation of methyl 3,5‐dioxohexanoate (1) into mixtures of 3,5‐dihydroxyesters (2) and 3‐hydroxylactones (3) has been reinvestigated with a variety of ruthenium catalysts. Catalysts bearing diphosphanes which possess axial chirality such as (S)‐MeO–Biphep give predominantly (3R,5S)‐anti‐2 in up to 78% de and 95% ee, affording an efficient synthesis of (S)‐6‐methyl‐5,6‐dihydro‐2‐pyrone [(S)‐5] in up to 95% enantiomeric excess. On the contrary, some Ru‐{amidophosphane‐phosphinite} complexes catalyze sluggishly the formation of syn‐2 in up to 92% de but with poor enantiomeric excesses. In all cases, methyl (R)‐3‐hydroxy‐5‐oxohexanoate [(R)‐11] is the exclusive primary hydrogenation product, which can be isolated in high yields and enantiomeric excesses up to 78%. Further hydrogenation of enantiomerically enriched (R)‐11 in a separate experiment affords (3R,5R)‐syn‐2 in high diastereomeric and enantiomeric excesses (up to 80% and 98%, respectively), provided a Ru‐(R)‐Binap‐type catalyst is used.

International audienc

2,4‐Dioxoesters 1a–c are selectively converted into optically active 2‐hydroxy‐4‐oxoesters 2a–c by hydrogenation with chiral rhodium‐aminophosphane‐phosphinite catalysts (82–88% ee) or ruthenium‐bisphosphane catalysts (52–67% ee). Direct one‐pot hydrogenation of 2,4‐dioxoesters 1a–c to 2‐hydroxy‐4‐butyrolactones 4a–c proceeds in high yields; catalytic activities, chemo‐, dia‐, and enantioselectivities are strongly dependant upon the nature of the substrate and the catalyst.

International audienceNew chiral nitroxides based on the imidazolidin-4-one skeleton, and the corresponding hydroxylamines, have been prepared from cyclic nitrones by a straightforward reaction sequence. They were evaluated as catalysts in the aerobic oxidation of benzyl alcohol using different co-catalysts. Both the imidazolidinone nitroxides and hydroxylamines were proven to catalyze the reaction, with the ring substituent having an effect depending on the co-catalytic system. In some cases, rapid oxidation to benzaldehyde was accomplished at room temperature under an atmospheric O2 pressure. Moreover, atroposelective desymmetrization was achieved during the aerobic oxidation of a diol catalyzed by an enantiopure imidazolidinone nitroxide. Finally, the electrochemical behavior of the new hydroxylamines and nitroxides was investigated by cyclic voltammetry, which gave insights into the observed catalytic properties