Hydrogen-Bonded Crystalline Molecular Machines with Ultrafast Rotation and Displacive Phase Transitions

Two new crystalline rotors 1 and 2 assembled through N−H⋅⋅⋅N hydrogen bonds by using halogenated carbazole as stators and 1,4-diaza[2.2.2]bicyclooctane (DABCO) as the rotator, are described. The dynamic characterization through ¹H T₁ relaxometry experiments indicate very low rotational activation barriers (E_a) of 0.67 kcal mol⁻¹ for 1 and 0.26 kcal mol⁻¹ for 2 , indicating that DABCO can reach a THz frequency at room temperature in the latter. These E_a values are supported by solid-state density functional theory computations. Interestingly, both supramolecular rotors show a phase transition between 298 and 250 K, revealed by differential scanning calorimetry and single-crystal X-ray diffraction. The subtle changes in the crystalline environment of these rotors that can alter the motion of an almost barrierless DABCO are discussed here.     

Catalyst free three-component synthesis of (±)-pyrazolylpyrrolopyrroles by 1,3-dipolar cycloaddition reaction

Novel pyrazolylpyrrolopyrroles were prepared by the three-component reaction of pyrazole-4-carbaldehydes, α-amino acid esters and 1-arylmaleimides in toluene and in the absence of a catalyst. This protocol provided a simple one-step procedure for the synthesis of the title compounds with the advantage of an easy work-up, mild reaction conditions and endo-selectivity.     

Hydrogen‐bonding patterns in 3‐alkyl‐3‐hydroxyindolin‐2‐ones

The molecules of racemic 3‐benzoylmethyl‐3‐hydroxyindolin‐2‐one, C₁₆H₁₃NO₃, (I), are linked by a combination of N—H...O and O—H...O hydrogen bonds into a chain of centrosymmetric edge‐fused R₂²(10) and R₄⁴(12) rings. Five monosubstituted analogues of (I), namely racemic 3‐hydroxy‐3‐[(4‐methylbenzoyl)methyl]indolin‐2‐one, C₁₇H₁₅NO₃, (II), racemic 3‐[(4‐fluorobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂FNO₃, (III), racemic 3‐[(4‐chlorobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂ClNO₃, (IV), racemic 3‐[(4‐bromobenzoyl)methyl]‐3‐hydroxyindolin‐2‐one, C₁₆H₁₂BrNO₃, (V), and racemic 3‐hydroxy‐3‐[(4‐nitrobenzoyl)methyl]indolin‐2‐one, C₁₆H₁₂N₂O₅, (VI), are isomorphous in space group P. In each of compounds (II)–(VI), a combination of N—H...O and O—H...O hydrogen bonds generates a chain of centrosymmetric edge‐fused R₂²(8) and R₂²(10) rings, and these chains are linked into sheets by an aromatic π–π stacking interaction. No two of the structures of (II)–(VI) exhibit the same combination of weak hydrogen bonds of C—H...O and C—H...π(arene) types. The molecules of racemic 3‐hydroxy‐3‐(2‐thienylcarbonylmethyl)indolin‐2‐one, C₁₄H₁₁NO₃S, (VII), form hydrogen‐bonded chains very similar to those in (II)–(VI), but here the sheet formation depends upon a weak π–π stacking interaction between thienyl rings. Comparisons are drawn between the crystal structures of compounds (I)–(VII) and those of some recently reported analogues having no aromatic group in the side chain.     

Residual methods for the large-scale matrix pth root and some related problems

The problem of finding the pth root of a matrix has received special attention in the last few years. Standard approaches for this problem include and combine some variations of Newton’s method, which in turn involve matrix factorizations that, in general, are not suitable for large-scale problems. Motivated by some recently developed low-cost iterative schemes for nonlinear problems, we consider and analyze specialized residual methods that only require a few matrix-matrix products per iteration, and hence are suitable for the large-scale case. As a by-product we also discuss the advantages of residual methods for general nonlinear problems whose variables separate. Preliminary and encouraging numerical results are presented for computing pth roots of large-scale symmetric and positive definite matrices, for different values of p.     

Convex but not Strictly Convex Central Configurations

Central configurations of the n-body problem have been studied for more than 200 years since the pioneer works of Euler and Lagrange. In this article we study convex central configurations which are not strictly convex. We give explicit examples of such configurations in both planar and spatial n-body problems. Particularly, in the spatial case, we consider regular polyhedra with bodies of same mass m at the vertices and bodies of same mass M at the middle points of each edge. In this setting we prove that the cube is the unique regular polyhedron such that this construction leads to a convex central configuration which is not strictly convex.     

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2003 is reviewed, and 389 references are cited.     

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2002 is reviewed, and 388 references are cited.     

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. 421 references are cited.     

Synthesis of new 6-Aroylpyrido[2,3-d]pyrimidines

The synthesis of 6-dimethylaminomethylenaminopyrimidin-4(3H)-ones 2 and its reaction with β-dimethyl-aminopropiophenone hydrochloride 3 is discussed in this work. The reaction of 6-aminopyrimidin-4(3H)-ones 1 with an excess of dimethylformamide dimethyl acetal gives rise to the formation of 6-dimethylaminomethyleneaminopyrimidines 2. The heating of equimolecular quantities of 2 and 3 in dimethylformamide leads to the 6-aroylpyrido[2,3-d]pyrimidines derivatives 4. The structures of compounds 2 and 4 were determined on the basis of nmr measurements.