Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis

Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β^2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.     

Prediction of H-Bonding Motifs for Pyrazoles and Oximes Using the Cambridge Structural Database

H-bonding motifs for pyrazoles and oximes have been examined in the Cambridge Structural Database. The accessible surface of the N atoms has been found to be useful as a discriminator to divide structures into dimer and catemer motifs for both pyrazoles and oximes. Low accessibility favors dimers and tetramers and high values favor catemers and trimers. Total molecular volume shows some correlation for oximes, while high values favor dimers. Empirical rules were successfully applied to predict the motifs of eight new structures in the subsequent release of the CSD.     

Flow injection determination of oxalate in urine based on an inhibitory effect

Summary Two methods based on the use of the normal and stopped-flow injection modes were developed for the determination of oxalate based on its inhibitory effect on the catalytic action of Fe(III) on the 2,4-diaminophenol/hydrogen peroxide system. The linear determination ranges achieved were between 0.2 and 12.0 g ml^–1 and between 0.2 and 40.0 g ml^–1, the precision was ±5.4%, and ±3.5%, and the sampling rate was 30 and 20 samples h^–1 for the normal and stopped-flow method, respectively. Both methods have been applied to the determination of oxalate in urine with excellent results.     

1,2,4,5‐Tetrazines vs. Carboxylic Acid Dimers: Molecular Chemistry vs. Supramolecular Chemistry

The structures of six new tetrazines have been determined and their molecular packing has been compared to the supermolecular architecture observed in related carboxylic acid dimers. In the tetrazines, covalent N? N bonds are considered to replace the intermolecular O? H???O hydrogen bonds of the carboxylic acids. In the systems investigated, it is apparent that, in the majority of cases, the covalent six‐membered ring of the tetrazine is an appropriate replacement for the carboxylic acid synthon. This apparent interplay between molecular and supramolecular units may have applications in the crystal engineering of new materials.     

The Unusual Transformation of an Aromatic 1<Emphasis Type="Italic">H</Emphasis>-Imidazole into a Non-Aromatic 2<Emphasis Type="Italic">H</Emphasis>-Imidazole

2H-Imidazole derivative 5 has been synthesized and characterized by the X-ray diffraction (XRD) method. The compound crystallizes in the monoclinic space group Cc with cell parameters a = 19.398(1), b = 8.890(1), c = 10.247(1), β = 110.76(1), Z = 4. The molecules are inter-linked through C–H⋅sO and C–H⋅sπ interactions forming infinite chains.     

A structural study of new tetrakis(1H-pyrazol-1-yl)methanes

Tetrakis(1H-pyrazol-1-yl)methanes are very rare compounds of which only two are known: the unsubstituted 1 obtained classically by Hückel in 1937 from carbon tetrachloride and prepared again several times and the 3,5-dimethyl substituted 2 obtained serendipitously by Pombeiro in 2009. We have now extended this group to include four new derivatives 8, 9, 11 and 12 bearing methyl groups. The X-ray crystal structure of the four compounds has been determined. They have been studied by NMR both in solution (¹H, ¹³C, ¹⁵N) and in the solid state (¹³C and ¹⁵N). DFT calculations of the six compounds (geometries, energies and absolute shieldings) have been used to discuss the experimental observations.     

Synthesis, photophysical properties, and laser behavior of 3-amino and 3-acetamido BODIPY dyes

The asymmetrically substituted BODIPY dyes 9a and 9b have been synthesized through a key redox step involving the alpha-nitroso derivative of the starting pyrrol. Both dyes emit fluorescence with quantum yields of ca. 0.7, but only 8b behaves as a good laser dye, with an efficiency of 48% in ethanol solution.     

Aminopyrazoles and their conjugated acids. An X-ray study of 3,5-dimethyl-4-aminopyrazole and the Picrate of 3(5)-aminopyrazole

The crystal structure determination of 3,5-dimethyl-4-aminopyrazole ( 3 ) and of 3(5)-aminopyrazolium picrate salt ( 5a ) have been undertaken. The packing in 3 is characterized by the formation of sheets in which the N-H of the pyrazole and one H atom of the amino group are involved. The remaining HB donor interconnects sheets. In 5a , the anions and the cations are linked by a two-dimensional network of hydrogen bonds in the ab plane. Each N-H and C-H in the cation are involved in hydrogen interactions with the O atoms of the anion. Ab initio molecular orbital methods at the HF and B3LYP levels using a 6–31G** basis set have been carried out to compare the relative stability of the tautomeric forms of the 3(5)-aminopyrazolium and 4-aminopyrazolium cations.