Efficient synthesis of 3H-imidazo[4,5-b]pyridines from malononitrile and 5-amino-4-(cyanoformimidoyl)imidazoles

1-Aryl-5-amino-4-(cyanoformimidoyl)imidazoles 2 were reacted with malononitrile under mild experimental conditions and led to 3-aryl-5,7-diamino-6-cyano-3H-imidazo[4,5-b]pyridines 5, when the reaction was carried out in the presence of DBU, or to 3-aryl-5-amino-6,7-dicyano-3H-imidazo[4,5-b]pyridines 3, in its absence. Both reactions evolved from the adduct formed by nucleophilic attack of the malononitrile anion to the carbon of the cyanoformimidoyl substituent. A 5-amino-1-aryl-4-(1-amino-2,2-dicyanovinyl)imidazole 4 was isolated when this reaction was carried out in the presence of DBU. The structure of compound 4 was confirmed by spectroscopic methods and by reaction with triethyl orthoformate and with acetic anhydride, leading respectively to 9-aryl-6-(cyanomethylidene)purines 11 and 12. Imidazole 2b was also reacted with ethyl acetoacetate, a carbon acid with a pK(a) comparable to that of malononitrile. Similar reaction conditions were used and the product isolated was a 6-carbamoyl-1,2-dihydropurine 10, showing that a different mechanism was operating in this case.     

Crystal engineering of urea alpha-network via I...O2N synthon and design of SHG active crystal N-4-iodophenyl-N'-4'-nitrophenylurea

Crystalline nitrodiphenyl ureas adopt the N-H...O tape alpha-network only when stabilization accrues from the I...O(2)N or C[triple bond]C-H...O(2)N synthon, otherwise the ureanitro motif is preferred; soft, weak interactions can direct polar self-assembly in strong N-HO hydrogen-bonded crystals.     

Evidence of wave front folding of sonic booms by a laboratory-scale deterministic experiment of shock waves in a heterogeneous medium

The influence of the planetary boundary layer on the sonic boom received at the ground level is known since the 1960s to be of major importance. Sonic boom propagation in a turbulent medium is characterized by an increase of the mean rise time and a huge variability. An experiment is conducted at a 1:100,000 scale in water to investigate ultrasonic shock wave interaction with a single heterogeneity. The experiment shows a very good scaling with sonic boom, concerning the size of the heterogeneities, the wave amplitude, and the rise time of the incident wave. The wave front folding associated with local focusing, and its link to the increase of the rise time, are evidenced by the experiment. The observed amplification of the peak pressure (by a factor up to 2), and increase of the rise time (by up to about one magnitude order), are in qualitative agreement with sonic boom observations. A nonlinear parabolic model is compared favorably to the experiment on axis, though the paraxial approximation turns out less precise off axis. Simulations are finally used to discriminate between nonlinear and linear propagations, showing nonlinearities affect mostly the higher harmonics that are in the audible range for sonic booms.     

Radiation effect studies in single crystal of Trifluoroacetyl-α-Aminoisobutyric acid

In this study, electron paramagnetic resonance of γ-irradiated single crystals of N-Trifluoroacetyl-α-amino isobutyric acid (F₃Ac-Aib-OH) was investigated at room temperature and analyzed for different orientations of the crystal in the magnetic field. The paramagnetic species in N-Trifluoroacetyl-α-aminoisobutyric acid was attributed to the ?F₂-R radical (R= CONHC(CH₃)COOH). Hyperfine coupling constants and g value were also determined. In addition, the single crystal of F₃Ac-Aib-OH was UV-irradiated and paramagnetic species formed was studied at room temperature. The effects of gamma irradiation on fluoroamino acid and stability were discussed.     

Experimental NMR and MS study of benzoylguanidines. Investigation of E/Z isomerism

Molecules containing the guanidinic nuclei possess several pharmacological applications, and knowing the preferred isomers of a potential drug is important to understand the way it operates pharmacologically. Benzoylguanidines were synthesized in satisfactory to good yields and characterized by NMR, Electrospray Ionization Mass Spectrometry (ESI‐MS) and Fourrier Transform InfraRed Spectroscopy techniques (FTIR). E/Z isomerism of the guanidines was studied and confirmed by NMR analysis in solution (¹H‐¹³C Heteronuclear Single Quantum Coherence (HSQC) and Heteronuclear Multiple‐Bond Correlation (HMBC), ¹H‐¹⁵N HMBC, ¹H‐¹H Correlation Spectroscopy (COSY) and Nuclear Overhauser Effect Spectroscopy (NOESY) experiments) at low temperatures. Compounds with p‐Cl and p‐Br aniline moiety exist mainly as Z isomer with a small proportion of E isomer, whereas compounds with p‐NO2 moiety showed a decrease in proportion of isomer Z. The results are important for the application of these molecules as enzymatic inhibitors. Copyright © 2013 John Wiley & Sons, Ltd.     

Toward reactant encapsulation for substrate-selectivity

A synthetic tris-(bis-(aminomethyl)pyridine) receptor was prepared in excellent yields via reversible imine condensation strategy. Catalytic activity in Henry reactions of the corresponding copper(II) complex was studied. Capitalizing on previous works by Anslyn with related receptors, the dramatic increase in basicity induced by this type of complex on diketo-derivatives was used to perform a nucleophilic addition of a deprotonated substrate onto an electrophile within the cavity. Hence, a Lewis acid stabilized nitronate was reacted with various aldehydes. A notable preference for small reactants easily accommodated in the cavity over encumbered ones was observed, thus representing an example of substrate-selectivity.     

Travelling wave mathematical analysis and efficient numerical resolution for a one-dimensional model of solid propellant combustion

We investigate a model of solid propellant combustion involving surface pyrolysis coupled to finite activation energy gas-phase combustion. Existence and uniqueness of a travelling wave solution are established by extending dynamical system tools classically used for premixed flames, dealing with the additional difficulty arising from the surface regression and pyrolysis. An efficient shooting method allows to solve the problem in phase space without resorting to space discretisation nor fixed-point Newton iterations. The results are compared to solutions from a CFD code developed at ONERA, assessing the efficiency and potential of the method, and the impact of the modelling assumptions is evaluated through parametric studies.     

Chalcogen Chiral Ylides for the Catalytic Asymmetric Epoxidation of Aldehydes: From Sulfur to Selenium and Tellurium

Abstract

The reaction of novel chiral selenonium and telluronium ylides was investigated with aldehydes and compared to the sulfur analogues. (2R,5R)-2,5-Dimethylselenolane was prepared and reacted as a catalyst for the benzylidenation of aldehydes. Disubstituted epoxides were readily prepared with a (surprising) absence of diastereoselectivity, and with enantiomeric excesses higher than 90%. The reaction of a tellurium analogue, (2S,5S)-2,5-diethyltellurolane, afforded the oxirane in very moderate yield and e.e.'s in the range of 62–82%. Though this was less productive, it is the first report of a chiral telluronium ylide leading to an asymmetric epoxidation of aldehydes.