An operationally simple E → Z isomerization of activated dienes, based on the β-ionyl motif intrinsic to retinal, is reported using inexpensive (−)-riboflavin (vitamin B 2 ) under irradiation at 402 nm. Selective energy transfer from photoexcited (−)-riboflavin to the starting E-isomer enables geometrical isomerization. Since the analogous process with the Z-isomer is inefficient, microscopic reversibility is circumvented, thereby enabling a directional isomerization to generate the contrathermodynamic product (up to 99% yield, up to 99:1 Z/E). Prudent choice of photocatalyst enables chemoselective isomerization to be achieved in both inter-and intramolecular systems. The principles established from this study, together with a molecular editing approach, have facilitated the development of a regioselective isomerization of a truncated triene based on the retinal scaffold. Letter pubs.acs.org/OrgLett
Methylenecyclobutanes are found to undergo Wacker oxidation via a semi-pinacol-type rearrangement. Key to a successful process is the use of tert-butyl nitrite as oxidant, which not only enables efficient catalyst turn-over but also ensures high Markovnikovselectivity under mild conditions. Thus, cyclopentanones (26 examples) can be accessed in an overall good yield and excellent selectivity (up to 97 % yield, generally > 99 : 1 ketone:aldehyde ratio). Stereochemical analysis of the reaction sequence reveals migration aptitudes in line with related 1,2-shifts. By introducing a pyox ligand to palladium, prochiral methylenecyclobutanes can be desymmetrized, thus realizing the first enantioselective Wacker oxidation.
Enantioselective methylene insertion into prochiral cyclobutanones is described providing access to chiral β-substituted cyclopentanones as important structural motif in synthesis and natural products. Commercially available trimethylsilyl as well as other silyl diazomethanes act as one-carbon synthon and scandium triflate is found to be a potent Lewis acid catalyst. By using bis(oxazoline) ligands, enantioinduction is achieved for a number of β-substituted cyclopentanones including examples bearing all-carbon quaternary stereocentres.
Methylencyclobutane werden der Wacker Oxidation über eine semi-pinakolartige Umlagerung unterzogen. Der Schlüssel zu einem erfolgreichen Prozess ist die Verwendung von tert-Butylnitrit als Oxidationsmittel, das nicht nur eine effiziente Katalyse ermöglicht, sondern auch eine hohe Markovnikov-Selektivität unter milden Bedingungen gewährleistet. Auf diese Weise können Cyclopentanone (26 Beispiele) in guter Ausbeute und ausgezeichneter Selektivität (bis zu 97 % Ausbeute und generell > 99 : 1 Keton:Aldehyd-Verhältnis) zugänglich gemacht werden. Die stereochemische Analyse der Reaktionssequenz zeigt ein Wanderungsverhalten, das mit verwandten 1,2-Umlagerungen übereinstimmt. Des Weiteren können durch die Einführung eines Pyox-Liganden an Palladium prochirale Methylencyclobutane desymmetrisiert werden, was der ersten enantioselektiven Wacker-Oxidation entspricht.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.