Mild and rational synthesis of palladium complexes comprising C(4)-bound N-heterocyclic carbenes

Oxidative addition of pyridyl-functionalised 4-iodoimidazolium salts to palladium(0) gives catalytically active complexes in which the N-heterocyclic carbene is bound to the palladium(II) centre in a non-classical bonding mode via C(4).     

A consistent correction for Redlich-Kwong-Soave volumes

If the volumetric and phase behaviour of a fluid mixture is calculated by means of an equation of state, certain translations along the volume axis may be effected that leave the predicted phase equilibrium conditions unchanged. This property may be exploited in the form at a consistent correction to improve volume estimations by the Redlich-Kwong-Soave method. Applications of this improved method to pure liquids, mixtures of liquids or gases, and petroleum fluids show that markedly superior volume estimations are obtained, except in the neighbourhood of the pure-component critical points; nonetheless, critical volumes for mixtures can be estimated correctly.     

Photoreaction of New Psoralen Analogs with DNA: Sequence and Mutation Specificity in the Escherichia coli lacZ Gene

New thio- and seleno-analogs of psoralen were synthesized and analyzed for their photoreactivity toward DNA. Using oligonucleotides of denned sequence, we first showed that these derivatives predominantly generated interstrand crosslinks at 5′-TpA sites. We also observed a surprisingly high reactivity of 7H-thiopyrano[3,2-f][l]benzofuran-7-one (PSO[0-S]) with the BamHl and PstI oligomers, giving rise to the formation of crosslinks at 5′-ApT sites and of the thymidine-psoralen-cytosine type. Next, the sequence specificity in the photochemical binding of all the compounds was investigated in two DNA fragments encompassing the lacZ gene of Escherichia coli, using the T₄ DNA polymerase sequencing methodology. Resulting maps demonstrated that thio-and seleno-analogs of psoralen preferentially photoreact-ed with thymine and cytosine residues. The AT-rich sequences proved to be particularly reactive sites as did adjacent thymines, especially at C-surrounding residues. Likewise, photoaddition at cytosines in CA/AC context was observed. It was highly significant that all of the derivatives exhibited similar sequence specificities with only minor differences. However, PSO(O-S) differed from the other heteropsoralens. Photoadducts occurred with a higher frequency at AC and CA dinucleotides, and new sites were detected. A comparison with 8-methoxypsor-alen photobinding is also reported. Finally, the mutagenic consequences of photoadducts induced in M13mp19 DNA by PSO(O-S) were determined in a forward system that detects all classes of mutagenic events. The high phototoxicity exhibited by PSO(O-S) could be attributed to crosslinks, and the comparison of the observed mutational specificity with the photoadduct distribution within the same gene showed that mutations were targeted at potential monoadduct sites where photolesions were detected in our footprinting experiments     

A versatile access to unsymmetrical and symmetrical α-diketones via organotin reagents

Unsymmetrical α -diketones have been obtained via cross coupling of acyltins with acyl halides under PdCl₂(PPh₃)₂ catalysis while symmetrical α -diketones have been readily obtained via “in situ” formation of acyltins using hexabutylditin and acyl chlorides under similar experimental conditions.     

Efficient synthesis of enantiopure pyrrolizidinone amino acid

Enantiopure (3S,5R,8S)-3-[N-(Boc)amino]-1-azabicyclo[3.3.0]octan-2-one 8-carboxylic acid (1) was synthesized in nine steps and 16% overall yield from aspartate beta-aldehyde 7. Carbene-catalyzed acyloin condensation of 7, followed by acetylation and samarium iodide reduction, gave linear precursor (2S,7S)-alpha,omega-diamino-4-oxosuberate 11, which was converted to N-(Boc)aminopyrrolizidin-2-one carboxylic acid 1 by a reductive amination/lactam cyclization sequence. X-ray analysis of (3S,5R,8S)-methyl N-(Boc)aminopyrrolizidin-2-one carboxylate 21 showed that its internal backbone dihedral angles (psi = -149 degrees, phi = -49 degrees ) were in good agreement with the ideal values for a type II' beta-turn. Proton NMR experiments on N'-methyl-N-(Boc)aminopyrrolizidin-2-one carboxamide 23 demonstrated significantly different NH chemical displacements and temperature coefficients suggestive of solvent shielded and exposed hydrogens indicative of a turn conformation. Because pyrrolizidinone amino acids can serve as conformationally rigid dipeptide surrogates, this synthesis should facilitate their application in the exploration of conformation-activity relationships of various biologically active peptides.     

Smooth and Algebraic Invariants of a Group Action: Local and Global Constructions

We provide an algebraic formulation of the moving frame method for constructing local smooth invariants on a manifold under an action of a Lie group. This formulation gives rise to algorithms for constructing rational and replacement invariants. The latter are algebraic over the field of rational invariants and play a role analogous to Cartan's normalized invariants in the smooth theory. The algebraic algorithms can be used for computing fundamental sets of differential invariants.