Juan C. Palacios

From parity to chirality: chemical implications revisited

Abstract Parity violation represents an essential property of particle and atomic handedness used to cope with the complex phenomenon of asymmetry in the universe. At the molecular level, however, numerous experiments suggest that parity-violating energy differences have not determined the amplification and propagation of homochirality. Asymmetric transformations conducted under far-from-equilibrium conditions reveal the existence of non-linear autocatalysis which is stochastic in nature. In any event and, globally considered, chirality appears as a unifying characteristic of our visible environment with evolutionary implications, thereby suggesting areas for productive research.

Nonlinear stereochemical effects in asymmetric reactions

Abstract A brief survey discussing the conceptual considerations of nonlinear effects in asymmetric reactions as well as their recent applications in catalysis and autocatalysis.

Chiral autocatalysis: where stereochemistry meets the origin of life

This article summarizes a series of recent and simple experiments to produce optically active substances from achiral precursors. These symmetry-breaking processes include either autocatalytic crystallization or asymmetric autocatalysis, and provide new insights into the origin of biomolecular homochirality. In addition, support from an extraterrestrial origin of chiral molecules has also come from recent findings.

Synthetic variations based on low-valent chromium: new developments

This article presents an overview of the synthetic chemistry of low-valent chromium, presumably Cr(II) species, and highlights most of the recent developments. These organometallic reactions represent a convenient strategy for the construction of carbon–carbon bonds displaying unique elements of stereocontrol, chemoselectivity, and functional-group compatibility. The introduction of milder protocols, catalytic versions, and the discovery of ligand tuning effects as a means of controlling carbanion selectivity, are the most salient improvements in this expansive field.

Symmetry Breaking by Spontaneous Crystallization – Is it the Most Plausible Source of Terrestrial Handedness we have Long Been Looking for? – A Reappraisal

In the light of recent and controversial findings on spontaneous resolution of racemates and their implications in the origin of homochirality on earth, we present here a detailed review of this important topic. Although spontaneous resolution cannot at this moment be reliably predicted, there has also been considerable progress in crystal structure prediction and, not only thermodynamic factors, but also kinetic ones, play important roles in the efficiency of packing and crystallization. In addition, self-association and supramolecular control phenomena may be identified in cases where spontaneous resolution of enantiomers is actually occurring. While this contribution summarizes our current understanding of this intriguing phenomena, it is hoped that future work on crystalline conglomerates (or homochiral crystals) of prebiotic importance will be of further help to understand the general problem of terrestrial chirogenesis.

The reaction of 2-amino-2-deoxyhexopyranoses with isocyanates. Synthesis of ureas and their transformation into heterocyclic derivatives

Abstract The reactions of 2-amino-2-deoxyglycopyranoses with aryl isocyanates have been investigated in detail and ureas and heterocyclic derivatives are obtained. The mechanism of formation of glycofurano[2,1- d ]imidazolidin-2-ones 62 has now become visible, while previous reports and the classical literature dealing with the subject in question proposed alternative structures for the reaction products. The reactions are pH-dependent and only furanoid bicycles are smoothly obtained at acidic pH values, whereas in neutral or basic media 5-hydroxyimidazolidin-2-one derivatives 66 can be isolated. These monocyclic structures appear to be the true intermediates of the reaction and, under appropriate conditions, can be converted exclusively into the corresponding cis -fused five-membered ring systems. Likewise, the first cis -fused glycopyrano[2,1- d ]imidazolidin-2-ones 75 have been also prepared.

A Family of Hydrogels Based on Ureido-Linked Aminopolyol-Derived Amphiphiles and Bolaamphiphiles: Synthesis, Gelation under Thermal and Sonochemical Stimuli, and Mesomorphic Characterization

This article describes the systematic preparation of a novel family of carbohydrate amphiphiles and bolaamphiphiles in which hydrophilic and hydrophobic units are connected via a ureido or bis(ureido) moiety. The sugar core is derived from aminopolyols such as D-glucamine (1), N-methyl-D-glucamine (2), or the sugar-like species tris(hydroxymethyl)aminomethane (3). The O-unprotected derivatives behave as self-organizing nonionic surfactants with good water gelation ability, which can be induced under thermal or ultrasound-driven stimuli. In addition, some derivatives of 1 and 2, and rarely 3 also formed lyotropic liquid crystals with lamellar or hexagonal structures that exhibit low-temperature transitions.

Can we predict the conformational preference of amides

To what extent, if any, is the conformation of secondary amides revealed by theory? This question has now been addressed by computational methods using calculations at the B3LYP/6-31G level of theory and (1)H NMR spectroscopy. Both gas-phase and solvent studies predict a Z-anti conformation to be the lowest in energy for an evaluated series of acetamides. Moreover, Z-anti conformations may also be inferred from the chemical shifts of the N-CH alpha protons determined by NMR spectroscopy. Thus, a proton situated anti to the N-H proton consistently appears approximately 0.8 ppm further downfield than a proton situated gauche to the N-H proton. This finding, which could only be derived by using the DFT calculations of conformational preference as a guide to interpret the NMR data, might prove to be useful as a simple and convenient methodology for establishing amide conformation experimentally.

Reactions of thioamides with metal carboxylates in organic media

Abstract Reactions of thioamides with metal carboxylates in organic solvents are described. These processes enable the selective preparation of nitriles, imides or amides depending on the substitution pattern of the starting material. Mechanistic hypotheses supported by experimental evidences, including the unequivocal synthesis of bis(thioacetanilide)mercury(II) as a key reaction intermediate, are also proposed.

Clay-catalyzed solventless addition reactions of furan with α,β-unsaturated carbonyl compounds

Abstract The reaction of furan with α,β-unsaturated carbonyl dienophiles catalyzed by K10 montmorillonite in the absence of organic solvents produces the corresponding Diels-Alder adducts and, in the case of methyl vinyl ketone, Michael-type products, under much milder conditions than the conventional protocols. The results are consistent with acid catalysis on the clay surface. Acrylates gave lower yields and/or decomposition products. The reaction can be extended to alkynic substrates such as DMAD to afford cycloadducts in good yields.