Organocatalysis--after the gold rush

The use of secondary amines as asymmetric catalysts in transformations of carbonyl compounds has seen tremendous development in recent years. Going from sporadic reports of selected reactions, aminocatalysis can now be considered as one of the methods of choice for many asymmetric functionalizations of carbonyl compounds--primarily of aldehydes and ketones. These functionalizations have been published at a breathtaking pace over the last few years--during the "golden age" and "gold rush" of organocatalysis. This tutorial review will firstly sketch the basic developments in organocatalysis, focussing especially on the use of secondary amines as catalysts for the functionalization of aldehydes and alpha,beta-unsaturated aldehydes, with emphasis on the mechanisms of the transformations and, secondly, outline recent trends within central areas of this research topic. Lastly, we will present our guesses as to where new developments might take organocatalysis in the years to come.

Hemostatic factors and renin in Greenland Eskimos on a high eicosapentaenoic acid intake. Results of the Fifth UmanaK Expedition

The Fifth UmanaK expedition compared the fatty acid composition of platelets, bleeding times before and after ingestion of acetylsalicylic acid, 24-hour urinary tetranorprostanedioate, creatinine and Na output, as well as plasma renin, serum electrolytes and antithrombin III in 20 Greenland Eskimos and 20 Danes. The results indicate that the prostaglandin production was not inhibited in the Eskimos, and that the antiaggregatory prostanoids predominate in Eskimos compared to Danes. Although blood pressure and 24-hour urinary Na output were similar, the plasma renin level was significantly higher in the Eskimos on a high eicosapentaenoic acid intake.

Manipulating the torsion of molecules by strong laser pulses

We demonstrate that strong laser pulses can induce torsional motion in a molecule consisting of a pair of phenyl rings. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis, connecting the two phenyl rings, allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by an overall rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for and generalizes the experimental findings.

Temporal profile of calcineurin phosphatase activity during acute allograft rejection in the heterotopic rat heart transplantation model

BACKGROUND

Regardless of the extensive worldwide use of calcineurin inhibitors, little is known about the behavior of calcineurin phosphatase (CaN) during acute allograft rejection. The aim of this study was to investigate the temporal profile of CaN during acute allograft rejection and reveal if it can be utilized as a pharmacodynamic marker to identify and monitor the rejection process.

METHODS

The heterotopic cervical rat heart transplantation model was used (dark Agouti to Lewis). We performed 25 control isogeneic and 46 allogeneic transplantations. Rats were sacrificed at various postoperative time points. CaN activity was measured in isolated peripheral blood and spleen mononuclear cells and in graft heart homogenates. CaN activity was measured as the release of radiolabeled phosphate from a previously phosphorylated 19 amino acid peptide.

RESULTS

We have shown that CaN's activity levels are not significantly altered during acute allograft rejection in peripheral blood mononuclear cells and in spleen-isolated mononuclear cells. CaN's intragraft activity decreased with time in both rejectors and controls, and was significantly lower in the allogeneic group.

CONCLUSIONS

CaN failed as a pharmacodynamic biomarker of acute allograft rejection in the heterotopic rat heart transplantation model. Further research is required in order to reveal the precise role of CaN during acute allograft rejection.

Asymmetric organocatalysed [1,3]-sigmatropic rearrangements

The first organocatalysed enantioselective [1,3]-sigmatropic O- to N-rearrangement reactions are presented. The reactions take place under regio- and enantioselective control, and are catalysed by cinchona alkaloids. Two reactions have been developed the first one is the rearrangement of imidates to amides, while the other rearrangement occurs from carbamates to amines via a decarboxylation. Both transformations give nitrogen protected beta-amino acid derivatives as the product. These novel asymmetric organocatalysed [1,3]-sigmatropic O- to N-rearrangement reactions provide a reliable and efficient synthetic method for obtaining enantioenriched beta-amino acid derivates in good yield from racemic starting materials.

On the origin of the stereoselectivity in organocatalysed reactions with trimethylsilyl-protected diarylprolinol

The origin of the enantioselectivity in the TMS-protected (TMS=trimethylsilyl) prolinol-catalysed alpha-heteroatom functionalisation of aldehydes has been investigated by using density functional theory calculations. Eight different reaction paths have been considered which are based on four different conformers of the TMS-protected prolinol-enamine intermediate. Optimisation of the enamine structures gave two intermediates with nearly the same energy. These intermediates both have an E configuration at the C==C bond and the double bond is positioned anti or syn, relative to the 2-substituent in the pyrrolidine ring. For the four intermediates, the chiral TMS-protected-diaryl substituent effectively shields one of the faces of the reacting C==C bond in the enamine intermediate. A number of transition states have been calculated for the enantioselective fluorination by N-fluorobenzenesulfonimide (NFSI) and based on the transition-state energies it has been found that the enantioselectivity depends on the orientation of the C==C bond, being anti or syn, relative to the 2-substituent on the pyrrolidine ring, rather than the approach of the electrophilic fluorine to the face of the reacting carbon atom in the enamine which is less shielded relative to the face with the highest shielding. The calculated enantiomeric excess of 96 % ee (ee=enantiomeric excess) for the fluorination reaction corresponds well with the experimentally found enantiomeric excess-97 % ee. The transition state for the alpha-amination reaction with the same type of intermediate has also been calculated by using diethyl azodicarboxylate as the amination reagent. The implication of the intermediate structures on the stereoselection of alpha-functionalisation of aldehydes is discussed.

Organocatalysed Asymmetric β-Amination and Multicomponentsyn-Selective Diamination of α,β-Unsaturated Aldehydes

An easy and affordable route for obtaining chiral beta-aminated- and alpha,beta-diaminated aldehydes, 1,3-aminoalcohols, and related compounds by using organocatalysis is presented. The chiral secondary amine (S)-2-[bis(3,5-bistrifluoromethylphenyl)trimethylsilanyloxymethyl]pyrrolidine is used as the catalyst to activate alpha,beta-unsaturated aldehydes, which allows succinimide to add in a 1,4-regio- and stereoselective fashion thereby forming N-protected 1,3-aminoaldehydes in good yields and enantioselectivities. This is followed by two easy transformations giving rise to optically active 1,3-aminoalcohols, a common motif in many biologically active compounds, for example, fibrinogen receptor antagonists. Furthermore, optically active alpha,beta-syn-diaminated aldehydes were obtained by the addition of diethyl azodicarboxylate in a one-pot reaction.

Intriguing Behavior of Cinchona Alkaloids in the Enantioselective Organocatalytic Hydroxyamination of α-Substituted-α-cyanoacetates

The cinchona alkaloid quinine promotes the enantioselective nitroso-aldol reaction between alpha-aryl-alpha-cyanoacetates and nitrosobenzene to give the hydroxyamination products with total chemoselectivity. Treatment of the reaction mixture with Zn/AcOH affords the corresponding amines in high yield and moderate enantioselectivity. An unusual effect on the enantioselectivity was observed with the catalyst loading and solvent. A reductive protocol allows the construction of an optically active 1,2-diamine moiety bearing a quaternary center.