The photo-dehydro-Diels-Alder reaction (PDDA), a new photochemical route to naphthalenes is presented. The [4+2] cycloaddition takes place between 3-arylynones and arylacetylenes, in which these moieties may be located in two molecules (intermolecular PDDA, 1) or in the same molecule (intramolecular PDDA, 5 and 9). Especially the latter approach is attractive from a preparative point of view and permits a straightforward access to highly functionalized naphthalenes. The irradiation of unsymmetrical reactants 9 provides, in contrast to symmetrical reactants 5, two isomeric naphthalenes. We found that the regioselectivity can be easily influenced by suitably located substituents in the aromatic rings and by blocking undesired positions. Notably, the PDDA may be used for the preparation of binaphthyls, as shown by the formation of 14 and 16. The mechanism of the PDDA was elucidated by spectroscopic analyses and theoretical (DFT) calculations.
A remarkable improvement of both the chemical yield (from 6% to 82%) and the enantiomeric excess (up to > 99%), of (S)-proline catalyzed direct aldol reactions of a wide range of aldehydes with acetone was found when hexasubstituted or pentasubstituted guanidinium salts were added as ionic liquids. Effects of temperature, amount of proline and the type of guandidinium salts on the outcome of the reaction were investigated.
The application of chiral α-acylamino hydroperoxide moieties in asymmetric sulphoxidation and epoxidations was pursued. In order to synthesize new isoquinoline-derived α-acylamino hydroperoxides isoquinolines were treated with (R)-menthyl chloroformate and hydrogen peroxide. Instead of the expected hydroperoxides novel bis(2-menthyloxycarbonyl-1,2-dihydroisoquinolin-1-yl) peroxides were obtained as 1:1 epimeric mixtures by a subsequent reaction. 1:1 mixtures of t-butyl (2-menthyloxycarbonyl-1,2-dihydroisoquinolin-1-yl) peroxides were formed from t-butyl hydroperoxide, isoquinoline and (R)-menthyl chloroformate. For eventual in situ preparation of chiral (R)-menthyloxycarbonyl-1,2-dihydroisoquinolin-1-yl) hydroperoxides and their application in stereoselective O-transfer reactions, isoquinolines were combined with (R)-menthyl chloroformate, hydrogen peroxide and metal alkoxides. These mixtures allowed a stereoselective synthesis of aryl methyl sulphoxides from corresponding sulphides. The stereoselectivity results from a kinetic resolution of racemic sulphoxides formed in the first step wherein the (S)-enantiomer was faster oxidized to the corresponding sulphone. As another type of chiral α-acylamino hydroperoxide moiety piperazinedione hydroperoxide was applied in the transformation of aryl methyl sulphides into sulphoxides. Low enantioselectivities were achieved. The same reagent allowed a high yielding epoxidation of naphthoquinones, but again in modest stereoselectivities.
Abstract. New ionic liquids were developed based on triazolium salts and guanidinium salts. By introduction of functions, such as organocatalytic units, into these units task-specific ionic liquids could be obtained. These products can be used as efficient catalysts in asymmetric syntheses.
Novel 1,2,3-triazolium-tagged proline derivatives were synthesized by copper-catalyzed click-reaction of alkynes with azides and N-alkylation of the resulting 1,2,3-triazoles. They were applied as recyclable organocatalysts in direct asymmetric aldol and Michael reactions with high enantioselectivity and diastereoselectivity. These catalysts performed better than (S)-proline itself; that is to say, a synergistic effect of the triazolium and the proline moiety exists. The reactions could be carried out either in conventional solvents or in ionic liquids. The catalysts were easily recycled and reused several times.
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