Regioselective 1,4-trifluoromethylation of α,β-enones using ‘protect-in-situ’ methodology

In a convenient and efficient procedure, the nucleophilic 1,4-trifluoromethylation of several α,β-enones using (trifluoromethyl)trimethylsilane was achieved. The high regioselectivity of the reaction has been reached by blocking the carbonyl moiety of the electrophile with a bulky aluminum-centered Lewis acid.     

Preparation and catalytic applications of partially fluorinated binaphthol ligands

New partially fluorinated binaphthols were obtained using a copper-catalyzed oxidative coupling. The corresponding enantiomerically pure compounds were prepared by fractional crystallization of the corresponding bis(menthyl)carbamates. Nucleophilic aromatic substitution using oxygen- and carbon-based nucleophiles resulted in functionalized derivatives without concomitant racemization. The titanium(IV) complexes of these ligands are catalytically active in the asymmetric oxidation of sulfides.     

Synthesis,characterization and properties of methylaminocellulose

Homogeneously prepared tosylcelluloses (TC) with degrees of substitution (DS) of DS_Tos 0.1–1.8 were used as intermediates for the synthesis of methylaminocelluloses (MAC) by nucleophilic substitution with methylamine. TC with DS_Tos up to 1.1 were shown to be valuable intermediates for selective synthesis of MAC with DS_MA varying from 0.1 to approximately 1. No nucleophilic substitution was observed at higher DS_Tos. At the chosen reaction conditions (60 °C, 48 h) residual tosyl moieties remained unchanged and little hydrolysis took place. The samples obtained were characterized by means of elemental analysis, FTIR and ¹³C CP/MAS NMR spectroscopy. ¹³C CP/MAS NMR spectroscopy was found to be an efficient tool for quantification of DS_MA. Furthermore, the swelling behaviour in water was investigated and preliminary tests concerning the bilirubin adsorption capacity of MAC were carried out.     

Toward phenanthridin-2-ylidene: electrophilicity versus acidity in planar-constrained C-aryl iminium salts

A phenanthridinium salt was prepared in four steps, including an intramolecular CH-arylation and a hydride abstraction reaction. Treatment with sterically demanding bases does not lead to the corresponding carbene, but rather to addition products: the planar-constrained geometry significantly enhances the electrophilicity over the acidity of C-aryl iminium salts.     

The synthesis of N-substituted ureas II: Nucleophilic substitution of ureas at the carbonyl group

Summary N-Arylsubstituted ureas undergo exchange of the N-residue upon reaction with amines. Using kinetic measurements, investigation of product distribution, regioselectivity, catalysis, and substrate influences, it was shown that this reaction proceeds via a second order nucleophilic substitution at the urea carbonyl center. By means of semiempirical calculations using the MNDO method the alternative mechanism of fragmentation was investigated.

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Zur Synthese N-substituierter Harnstoffe II: Nucleophile Substitution von Harnstoffen an der Carbonylgruppe

Zusammenfassung N-Arylsubstituierte Harnstoffe erleiden bei der Reaktion mit Aminen einen Austausch des N-Restes. Durch kinetische Messungen, Untersuchung der Produktverteilung, Regioselektivität, Katalyse und des Einflusses des Edukts konnte abgeleitet werden, daß es sich hiebei um eine nucleophile Substitution zweiter Ordnung am Carbonylzentrum des Harnstoffs handelt. Durch semiempirische Rechnungen mit Hilfe der MNDO Methode wurde der alternative Reaktionsmechanismus einer Fragmentierung untersucht.

     

Efficient synthesis of 2- and 3-substituted-2,3-dihydro [1,4]dioxino[2,3-b]pyridine derivatives

A versatile new approach for the synthesis in three steps of 2-substituted-2,3-dihydro[1,4]dioxino[2,3-b]pyridines B via a Smiles rearrangement using easily available reagents is described. A study illustrating the influence of experimental conditions on the progress of the reaction is reported.     

A new and versatile Ugi/SNAr synthesis of fused 4,5-dihydrotetrazolo[1,5-a]quinoxalines

A combinatorial synthetic route yielding fused tetrazolo[1,5-a]quinoxalines is described. The use of 2-fluorophenylisocyanide in the Ugi-tetrazole reaction (tetrazole-U-4CR) followed by a nucleophilic aromatic substitution (S_NAr) affords the tricylic tetrazolo[1,5-a]quinoxaline moiety in good yields and with high diversity.     

Chlorodimethylaluminum-promoted nucleophilic addition of lithium pentamethylcyclopentadienide to aliphatic aldehydes and DDQ-mediated carbon-carbon bond cleavage of the adducts providing the parent aldehydes

Treatment of aliphatic aldehyde with lithium pentamethylcyclopentadienide in the presence of chlorodimethylaluminum provided the corresponding carbinol in excellent yield. The carbinol returns to the parent aldehyde and pentamethylcyclopentadiene by the action of a catalytic amount of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ).     

Pentamethylcyclopentadienide in organic synthesis: nucleophilic addition of lithium pentamethylcyclopentadienide to aromatic aldehydes and carbon-carbon bond cleavage of the adducts affording the parent aldehydes

Treatment of aromatic aldehyde with lithium pentamethylcyclopentadienide provided the corresponding carbinol in excellent yield. The carbinol returns to the parent aldehyde and pentamethylcyclopentadiene upon exposure to an acid or due to heating. The combination of the two reactions can represent a protection of aromatic aldehyde.     

Elektrophile Substitution und nucleophile Addition an 3,4-Dihydro-5(1H)-pyrromethenonen

3,4-Dihydro-5(1H)-pyrromethenones are easier attacked at themeso-position by electrophiles than 5(1H)-pyrromethenones. This is demonstrated both by aMannich-type-substitution or deuterium-exchange-experiments and by the addition of O-, S-, and N-Nucleophiles to the exocyclic double bond of the model-dihydropyrromethenone (Z)-1 under very mild reaction conditions. Applying these results to the chemistry of 2,3-dihydro-bilatrienes-abc, their chemical characteristics—especially their tautomeric behavior and their dominant C-5-selectivity towards electrophiles—become better understandable.