Brønsted Acid Catalysis in Visible‐Light‐Induced [2+2] Photocycloaddition Reactions of Enone Dithianes

1,3‐Dithiane‐protected enones (enone dithianes) were found to undergo an intramolecular [2+2] photocycloaddition under visible‐light irradiation (λ =405 nm) in the presence of a Brønsted acid (7.5–10 mol %). Key to the success of the reaction is presumably the formation of colored thionium ions, which are intermediates of the catalytic cycle. Cyclobutanes were thus obtained in very good yields (78–90 %). It is also shown that the dithiane moiety can be reductively or oxidatively removed without affecting the photochemically constructed ring skeleton.     

Catalytic 1,3-difunctionalisation of organic backbones through a highly stereoselective, one-pot, boron conjugate-addition/reduction/oxidation process

A simple one-pot, three-step synthetic route to chiral 1,3-amino alcohols and 1,3-diols has been established. Considering the overall stereocontrol of the synthetic protocol, the first and key step is an enantioselective β-boration of α,β-unsaturated imines and ketones, respectively. The enantioselectivity provided by the Cu(I) catalyst modified with Josiphos- and Mandyphos-type ligands has been examined. The oxidative substitution of the boryl unit with a hydroxyl group proceeds with complete retention of configuration at the C(β)-atom. In parallel, the stoichiometric reduction of the imino or carbonyl group provides a second stereogenic centre. Depending on the nature of the reducing reagent, exceptionally high diastereoselectivity is achieved, especially for syn-1,3-amino alcohols and 1,3-diols.     

Substrate-controlled diastereoselective aziridination of alkenes using 3-acetoxyaminoquinazolinone in the presence of hexamethyldisilazane

8-Phenylmenthol derived α,β-unsaturated esters were aziridinated highly diastereoselectively using 3-acetoxyamino-2-ethylquinazolinone. The yields of these aziridines were greatly improved in the presence of hexamethyldisilazane.     

Chromium-catalyzed pinacol-type cross-coupling: studies on stereoselectivity

A chromium-catalyzed pinacol-type cross-coupling reaction between alpha,beta-unsaturated carbonyl compounds and aldehydes is reported. Even sterically demanding substrates could be coupled to afford the corresponding pinacols in good yields. Systematic studies concerning the origin of the diastereoselectivities led to the proposal of a mechanism for this synthetically useful reaction. Acroleins with alpha-branched alkyl side chains were coupled to give the corresponding syn pinacols, while on the other hand, acroleins with less bulky substituents furnished the anti derivatives. The effects of both the substrates and the reagents on the diastereo- and enantioselectivities were investigated. An unexpected catalytic formation of cyclopropanols was found.