Application of a thermal rearrangement reaction to questions of structure of condensed dihydrodiazepinones: The reaction of 2,3-diaminopyridine with ethyl benzoylacetate

The condensation of an unsymmetrical aromatic diamine with a β-ketoester can afford two theoretically possible dihydrodiazepinone products, differentiation between which is, at best, difficult and laborious. A recently described (3) rearrangement reaction, whereby dihydrodiazepinones are thermally converted into N-substituted imidazolones, has now been applied to the elucidation of structures of ambiguous diazepinone derivatives. By means of the thermal rearrangement method, the diazepinone derivative resulting from the reaction of 2,3-diamino-pyridine and ethyl benzoylacetate, for which structures 1a and 1b are equally plausible, has been definitively characterized as the 2-phenyl-4-one compound, 1a .     

Phosphoesterase activity and phosphate release from tributyl phosphate by aCitrobacter sp.

Tributyl phosphate (TBP) and other alkyl phosphates represent a class of persistent organophosphorus compounds of widespread use. Biodegradation of the phosphotriesters is postulated to occur through sequential hydrolytic cleavages via the phosphodiester and monoester intermediates to alcohol and inorganic phosphate (P_i). Immobilized cells of aCitrobacter sp. liberated P_i upon challenge with TBP but the reaction was short-lived. In vitro studies with purified phosphomonoesterase (phosphatase) used³¹P nuclear magnetic resonance to demonstrate P_i transfer onto ethanol (phosphotransferase activity). This suggested that in vivo the onset of a futile phosphohydrolytic and transphosphorylation cycle would limit the extent of phosphate production. A mutant deficient in the transphosphorylating phosphomonoesterase showed an extended release of P_i under challenge with TBP that was not subject to the complete and premature reaction termination that precluded application of the parent strain to possible industrial processes for alkyl phosphate biodegradation.     

The solubilities of zinc complexes of 8-quinolinol and some derivatives of 8-quinolinol

The solubilities of bis-8-quinolinolo-zinc^II, bis-2-methyl-8-quinolinolo-zinc^II and bis-5 : 7-dibromo-S-quinolinolo-zinc^II have been measured over the pH range of 4.5 to 9 at ionic strengths of 0.114 and 0.230 and at 25° and 40°. A mathematical treatment of the results gave nearly constant solubility products only when a dissolved but undissociated molecule, in addition to all of the dissociated and partly dissociated species, was considered. The relative abundance of each species of molecule in solution has been calculated.     

Thermal reactions of 8-methylbicyclo[4.2.0]oct-2-enes: competitive diradical-mediated [1,3] sigmatropic, stereomutation, and fragmentation processes

[reaction: see text] At 275 degrees C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the [1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.     

A new folding paradigm for repeat proteins

The folding/unfolding transitions of a series of designed consensus tetratricopeptide repeat proteins are quantitatively described by the classical one-dimensional Ising model, which thus represents a new folding paradigm for repeat proteins. Moreover, for the first time for any protein, a theoretical model predicts the folding/unfolding transition midpoint and the width of the transition.     

PASADENA hyperpolarization of succinic acid for MRI and NMR spectroscopy

We use the PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) method to achieve 13C polarization of approximately 20% in seconds in 1-13C-succinic-d2 acid. The high-field 13C multiplets are observed as a function of pH, and the line broadening of C1 is pronounced in the region of the pK values. The 2JCH, 3JCH, and 3JHH couplings needed for spin order transfer vary with pH and are best resolved at low pH leading to our use of pH approximately 3 for both the molecular addition of parahydrogen to 1-13C-fumaric acid-d2 and the subsequent transfer of spin order from the nascent protons to C1 of the succinic acid product. The methods described here may generalize to hyperpolarization of other carboxylic acids. The C1 spin-lattice relaxation time at neutral pH and 4.7 T is measured as 27 s in H2O and 56 s in D2O. Together with known rates of succinate uptake in kidneys, this allows an estimate of the prospects for the molecular spectroscopy of metabolism.     

Phase behavior of ranitidine HCl in the presence of degradants and atmospheric moisture--impact on chemical stability

For hydrophilic organic solids, it is well recognized that degradation is often promoted by exposure to humid conditions. Although this is an important issue for certain classes of materials, in particular pharmaceuticals, the factors which dictate the sensitivity of a given compound to moisture are not well understood. The goal of this work was to elucidate the synergistic influence of self-originating impurities and water vapor on the degradation kinetics of the histamine H2 receptor antagonist, ranitidine HCl. Physical mixtures of the drug and each of three major degradation products were subjected to conditions of elevated temperature and relative humidities. Pure samples showed a sigmoidal-shaped degradation profile for all storage conditions studied. During the lag time, the pure drug gained minimal quantities of moisture. Once degradation commenced, the samples started to absorb moisture. When mixed with the degradant, the lag period was eliminated for all storage conditions, even at low partial pressures of water. The extent of moisture gain by samples containing impurities could not be attributed to the presence of the impurity alone. It was found that the presence of impurities in contact with the surface of the drug, in combination with water vapor, promoted a phase transition of the crystalline material to the solution phase. A ternary phase diagram was constructed to visualize the proportion of the drug in the solid and solution phases as a function of impurity and moisture content. The increased mobility of molecules in solution presumably leads to enhanced reactivity relative to the crystalline material.     

Enhancement of TbIII–CuII Single‐Molecule Magnet Performance through Structural Modification

We report a series of 3d–4f complexes {Ln₂Cu₃(H₃L)₂X_n} (X=OAc^?, Ln=Gd, Tb or X=NO₃^?, Ln=Gd, Tb, Dy, Ho, Er) using the 2,2′‐(propane‐1,3‐diyldiimino)bis[2‐(hydroxylmethyl)propane‐1,3‐diol] (H₆L) pro‐ligand. All complexes, except that in which Ln=Gd, show slow magnetic relaxation in zero applied dc field. A remarkable improvement of the energy barrier to reorientation of the magnetisation in the {Tb₂Cu₃(H₃L)₂X_n} complexes is seen by changing the auxiliary ligands (X=OAc^? for NO₃^?). This leads to the largest reported relaxation barrier in zero applied dc field for a Tb/Cu‐based single‐molecule magnet. Ab initio CASSCF calculations performed on mononuclear Tb^III models are employed to understand the increase in energy barrier and the calculations suggest that the difference stems from a change in the Tb^III coordination environment (C_4v versus C_s).     

Incorporation of poly(2‐acrylamido‐2‐methyl‐N‐propanesulfonic acid) segments into block and brush copolymers by ATRP

2‐Acrylamido‐2‐methyl‐N‐propanesulfonic acid (AMPSA) was successfully polymerized via atom transfer radical polymerization (ATRP) using a copper chloride/2,2′‐bipyridine (bpy) catalyst complex after in situ neutralization of the acidic proton in AMPSA with tri(n‐butyl)amine (TBA). A 5 mol % excess of TBA was required to completely neutralize the acid and prevent protonation of the bpy ligand, as well as to avoid side reactions caused by large excess of TBA. The use of activators generated by electron transfer (AGET) ATRP with ascorbic acid as reducing agent resulted in both increased conversion of the AMPSA monomer during polymerization (up to 50% with a 0.8 [ascorbic acid]/[Cu(II)] ratio) and much shorter polymerization times (<30 min). Block copolymers and molecular brushes containing AMPSA side chains were prepared using this method, and the solution and surface behavior of these materials were investigated. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5386–5396, 2009