Access to N-cyanosulfoximines by transition metal-free iminations of sulfoxides.

A transition metal-free synthesis of N-cyanosulfoximines from sulfoxides using N-chlorosuccinimide (NCS) as oxidising agent and cyanamide as nucleophilic amine source is reported. The products are obtained in moderate to excellent yields. The protocol enables an easy access to N-cyanosulfoximines from readily available starting materials under inversion of configuration at a preexisting stereogenic center.

Configurational control in stereochemically pure ligands and metal complexes for asymmetric catalysis

Enantioselective synthesis relies on suitable chiral mediators, which, in many cases, owe their stereochemical information to chiral ligands coordinated to metals. Like nature, which uses (diastereomerically pure) enzymes with several stereogenic centers to catalyze biological processes, chemists, for their purposes, tend more and more to turn their attention towards ligands and metal complexes with more than one stereogenic center or element of chirality. Selected issues of the resulting diastereomeric interactions as well as the advantages that result from the use of such complexes in catalysis are presented and discussed here.

Synthesis and application of novel catalytically active polymers containing 1,4,7-triazacyclononanes.

New polymers containing 1,4,7-triazacyclononanes have been synthesised by means of ring opening metathesis polymerisation (ROMP); their complexes with Mn(II) catalyse the oxidation of simple olefins by hydrogen peroxide.

Palladium-catalyzed N-arylation of sulfoximines with aryl bromides and aryl iodides

Various N-arylated sulfoximines have been synthesized in high yield by a direct approach which is based on a palladium-catalyzed cross-coupling strategy. Aryl bromides of variable substitution pattern were found to be the most effective coupling partners, whereas aryl iodides showed a nonpredictable behavior requiring lithium or silver salts as additives to ensure product formation in acceptable yields. Coupling of (S)-2-(2'-bromophenyl)-4-tert-butyloxazoline with enantiomerically pure (-)-(RS)-S-methyl-S-phenylsulfoximine afforded the corresponding product in good yield as a single diastereomer, showing that the palladium-catalyzed arylation proceeds in a stereospecific manner. The reaction with dibromobenzenes yielded the monosulfonimidoyl arenes in all cases, suggesting that the introduction of the sulfonimidoyl moiety deactivates the arene, thus preventing a second coupling step.

Metal-catalyzed asymmetric oxidations.

Recent advances in enantioselective metal-catalyzed oxidations are presented. Two transformations are discussed in detail: (a) asymmetric Baeyer-Villiger reactions of racemic and prochiral cyclic ketones giving optically active lactones, and (b) sulfide oxidations affording enantiomerically enriched sulfoxides. For both reactions, new metal/ligand combinations have been introduced which allow catalysis of these reactions with high enantioselectivities. For some substrates, the ee-values are above 90%. As previously demonstrated, sulfoxides can be iminated to give the corresponding sulfoximines. Under appropriate reaction conditions, this transformation is stereospecific and enantiopure sulfoximines are obtained. These highly oxidized sulfur reagents have now been modified to give beta-amino acid analogues which may be incorporated into peptides. Two variants have been realized: either the sulfoximine-containing pseudo-amino acids are connected with standard alpha-amino acids to give mixed pseudopeptides, or the sulfoxime units are mutually attached to give homooligomers, which can be regarded as beta-peptide analogues.

Zirconium-mediated asymmetric baeyer-villiger oxidation.

Combinations of axially chiral C2-symmetric diols were used as ligands in zirconium-mediated Baeyer-Villiger reactions. The in situ preformed Zr-diol species proved effective in the asymmetric oxidation of bicyclic and monosubstituted cyclobutanones when a hydroperoxide was employed as oxidant. Asymmetric induction could be preserved upon replacement of one out of two enantiopure BINOL ligands by conformationally flexible 2,2'-biphenol.

Catalytic synthesis of aldehydes and ketones under mild conditions using TEMPO/Oxone

[formula: see text] A novel, metal-free oxidation system for the catalytic synthesis of aldehydes and ketones--TEMPO/Oxone--has been developed. An optimized reaction protocol proved especially successful for the synthesis of ketones. Additionally, the influence of quarternary ammonium salts on the catalysis was studied. The mild conditions of this novel procedure were shown to tolerate even sensitive silyl protective groups which can otherwise be cleaved in the presence of Oxone.

Practical and highly enantioselective ring opening of cyclic meso-anhydrides mediated by cinchona alkaloids.

The cinchona alkaloid-mediated opening of prochiral cyclic anhydrides in the presence of methanol leading to optically active hemiesters is described. Very structurally diverse anhydrides are converted into their corresponding methyl monoesters, and either enantiomer can be obtained with up to 99% ee by using quinine or quinidine as directing additive. After the reaction, the alkaloids can be recovered almost quantitatively and reused without loss of enantioselectivity. Additionally, a catalytic protocol which permits the substoichiometric use of quinidine in the presence of easily accessible pentamethylpiperidine (pempidine) is presented.

Synthesis of pseudopeptides with sulfoximines as chiral backbone modifying elements.

The synthesis of pseudopeptides with a chiral alpha-sulfonimidoylcarboxy moiety in the backbone is described. Starting from readily available (Ss)-S-methyl S-phenyl sulfoximine and various cyclic and acyclic alpha-amino acids the desired products are obtained in good yields with peptide coupling methodology. Specific secondary structures caused by intramolecular hydrogen bonds may be adopted. Results of NMR studies to reveal conformational preferences will be discussed.

Silica-supported TEMPO catalysts: synthesis and application in the Anelli oxidation of alcohols.

The application of silica-supported TEMPO as a recyclable catalyst in the Anelli oxidation of alcohols is reported. The catalyst is easily obtained in a one-step reductive amination procedure starting from a commercially available aminopropyl-functionalized silica. Details of the synthesis of the supported catalyst and its analysis by MAS NMR are presented. Various alcohol oxidations according to the Anelli protocol have been carried out and the stability of the applied silica-supported TEMPO has been studied.