A True Reverse Anomeric Effect Does Exist After All: A Hydrogen Bonding Stereocontrolling Effect in 2-Iminoaldoses.

The reverse anomeric effect is usually associated with the equatorial preference of nitrogen substituents at the anomeric center. Once postulated as another anomeric effect with explanations ranging from electrostatic interactions to delocalization effects, it is now firmly considered to be essentially steric in nature. Through an extensive research on aryl imines from 2-amino-2-deoxyaldoses, spanning nearly two decades, we realized that such substances often show an anomalous anomeric behavior that cannot easily be rationalized on the basis of purely steric grounds. The apparent preference, or stabilization, of the ß-anomer takes place to an extent that not only neutralizes but also overcomes the normal anomeric effect. Calculations indicate that there is no stereoelectronic effect opposing the anomeric effect, resulting from the repulsion between electron lone pairs on the imine nitrogen and the endocyclic oxygen. Such data and compelling structural evidence unravel why the exoanomeric effect is largely inhibited. We are now confident, as witnessed by 2-iminoaldoses, that elimination of the exo-anomeric effect in the α-anomer is due to the formation of an intramolecular hydrogen bond between the anomeric hydroxyl and the iminic nitrogen, thereby accounting for a true electronic effect. In addition, discrete solvation may help justify the observed preference for the ß-anomer.

From Potential Prebiotic Synthons to Useful Chiral Scaffolds: A Synthetic and Structural Reinvestigation of 2-Amino-Aldononitriles.

This paper explores and revisits in detail the formation and characterization of sugar-based aminonitriles, whose ultimate origin can be traced to the interaction of biomolecules with cyanide. Although the synthesis and spectroscopic data of 2-amino-aldononitriles were reported long ago, there are both contradictory and confusing results among the published data. We have now addressed this concern through an exhaustive structural elucidation of acylated 2-amino- and 2-alkyl(aryl)amino-2-deoxyaldonitriles using mass spectrometry and FT-IR, FT-Raman, and NMR spectroscopies. Several structures could be unambiguously determined through single-crystal X-ray diffraction, which allowed us to correct other misassignments. Moreover, this study unveils how steric and electronic effects influence the acylation outcome of the amino, (alkyl, aryl)amino, or acetamido group at C-2. The chirality at the latter, which was assigned tentatively through optical rotation correlation, and hence the preferential threo stereochemistry generated during the cyanohydrin synthesis of 2-amino-2-deoxy aldononitriles have now been established with confidence.

Schiff Bases and Stereocontrolled Formation of Fused 1,3-Oxazolidines from 1-Amino-2-Indanol: A Systematic Study on Structure and Mechanism.

This paper thoroughly explores the formation of Schiff bases derived from salicylaldehydes and a conformationally restricted amino alcohol (1-amino-2-indanol), as well as the generation of 1,3-oxazolidines, a key heterocyclic core present in numerous bioactive compounds. We provide enough evidences, both experimental-including crystallographic analyses and DFT-based calculations on imine/enamine tautomerism in the solid state and solution. In the course of imine formation, a pentacyclic oxazolidine-oxazine structure could be isolated with complete stereocontrol, whose configuration has been determined by merging theory and experiment. Mechanistic studies reveal that, although oxazolidines can be obtained under kinetic conditions, the prevalence of imines obeys to thermodynamic control as they are the most stable structures. The stereochemical outcome of imine cyclization under acylating conditions leads to formation of 2,4-trans-oxazolidines.

Turner syndrome associated with Down syndrome: about a case / Síndrome Turner asociado a síndrome Down: a propósito de un caso

The coexistence of double aneuploidy of Down and Turner syndromes is rare; most cases have been due to double mitotic errors. The objective of the study was to report a case with monosomy of the X chromosome and trisomy of chromosome 21, in mosaic variety, highlighting the phenotypic effect that the presence of different chromosomal abnormalities can produce and compare with those reported in the literature. A 10-year-old Ecuadorian female, born to a multipregnant mother with 46 years at conception, is seen in consultation with a predominant clinical phenotype of Down syndrome, associated with menarche, presence of pubic and axillary villu, where a karyotype is verified 45 X[7]/47XX+ 21 [3]/46, X, der (X)(: p11.1-> q11.1)[1]/46,XX [1]. The present case is a double Turner-Down aneuploidy, with predominantly X monosomy cell line, who shows important mental retardation and some signs of puberal development not usually in Turner syndrome. These features highlight the clinical importance of doing a karyotype in mental retardation cases and searching low mosaics of another aneuploidies in atypical cases. Its complex chromosomal formula and support with molecular cytogenetics allowed diagnostic confirmation and genetic counseling.

La coexistencia de doble aneuploidía de los síndromes de Down y Turner es rara; la mayoría de los casos se han debido a dobles errores mitóticos. Reportar un caso con trisomía del cromosoma 21 y monosomía del cromosoma en X, en variedad mosaico, que curiosamente presenta un despertar puberal precoz y comparar con los reportados en la literatura. Paciente ecuatoriana de sexo femenino, de 10 años de edad, nacida de madre multigesta con 46 años a la concepción, que es vista en consulta con fenotipo clínico predominante de Síndrome Down, asociado a menarquia y telarquia, donde se constata un cariotipo. El presente caso es el primero informado de mosaicismo de doble aneuploidía de Turner-Down asociado con un despertar puberal precoz. Su fórmula cromosómica compleja y el apoyo con la citogenética molecular permitió la confirmación diagnostica y la asesoría genética.

A synthetically benign one-pot construction of enamino-xanthene dyes.

Polyhydroxylated phenols are components of biomass and precursors of pigments in plants. This paper reports a novel entry to xanthene dyes, involving the reaction of 2,4,6-trihydroxybenzaldehyde with primary aliphatic amines. This catalyst-free synthesis exhibits a high atom economy and can be conducted under eco-friendly conditions and operational simplicity.

Expanding amphiphilic architectures by ring-opening of epoxides and polyepoxides with N-methyl-d-glucamine: Structure, chiral bias and gelation.

The facile reaction of a readily available aminopolyol from the chiral pool, N-methyl-d-glucamine, which avoids the side reactions usually associated to anomers of amino sugars, with epoxide and polyepoxide derivatives, enables the preparation of new non-ionic surfactant-like structures combining hydrophilic and hydrophobic moieties. The molecular architectures thus obtained range from linear to tripodal and pyramidal structures. The resulting substances containing multiple chiral centers exist as diastereomeric mixtures, for which various conformations are likewise possible by virtue of inter-chain interactions. The stability and chirality preferences of all possible stereoisomers have been evaluated in detail by DFT methods. Given the amphiphilic structure of both protected and O-protected derivatives obtained by acetylation, self-aggregation could eventually lead to solvent entrapment. Unfortunately, only one compound behaves as efficient hydrogelator and DMSO-gelator at low concentrations. The issue is also discussed in terms of the different molecular arrangements.

Imine or Enamine? Insights and Predictive Guidelines from the Electronic Effect of Substituents in H-Bonded Salicylimines.

Imine and enamine bonds decorate the skeleton of numerous reagents, catalysts, and organic materials. However, it is difficult to isolate at will a single tautomer, as dynamic equilibria occur easily, even in the solid state, and are sensitive to electronic and steric effect, including π-conjugation and H-bonding. Here, using as model Schiff bases generated from salicylaldehydes and TRIS in a set of linear free energy relationships (LFER), we disclose how the formation of either imines or enamines can be controlled and provide a comprehensive framework that captures the structural underpinning of this prediction. This work highlights the potentiality of tailor-made designs en route to compounds with desirable functionality.

Electronic effects in tautomeric equilibria: the case of chiral imines from d-glucamine and 2-hydroxyacetophenones.

A one-pot procedure for preparing a series of chiral imines by direct condensation of d-glucamine with 2-hydroxyacetophenones is described. Under conventional acetylation an unexpected mixture of two different peracetylated molecules is obtained, one with an open enamine structure, and the other incorporating an N-acetyl-1,3-oxazolidine into the acyclic skeleton. Surprisingly, both molecules coexist within the crystal's unit cell, as inferred from single-crystal X-ray analysis of a 5-bromo-substituted aryl derivative. Moreover, the 1,3-oxazolidine ring exists as rotational conformers (E,Z) owing to the restricted rotation around the N-acetyl bond. The equilibrium involving imine and enamine structures has been assessed in detail, providing in addition linear free-energy relationships between the tautomerization constants (KT) and the electronic effect of the substituents.

On the Plausibility of Pseudosugar Formation in Cometary Ices and Oxygen-rich Tholins.

We revisit herein the formation and structure of dihydroxy dioxanes, which can be obtained from prebiotically available precursors and can be regarded as primeval sugar surrogates. Previous studies dealing with the heterogeneous composition of interstellar bodies point to the existence of significant amounts of small polyalcohols along with oxygen-containing oligomers. Even though such derivatives did not give rise to nucleosides and oligonucleotides, nor they were incorporated into subsequent metabolic routes, molecular chimeras based on sugar-like species could be opportunistic scaffolds in pre-evolutionary scenarios. We could figure out that pseudosugars, assembled by hemiacetalic bonds from available precursors in both interstellar and terrestrial scenarios, were presumably more abundant than thought. Moreover, these species share some key features with naturally-occurring sugar rings, such as anomeric preferences, coordinating ability, and the prevalent occurrence of racemic compounds.

Rethinking aromaticity in H-bonded systems. Caveats for transition structures involving hydrogen transfer and π-delocalization.

Monoaza- and diaza-derivatives of malondialdehydes, in short aminoacroleins and vinamidines, are prototypical examples of open-chain structures prone to π-electron delocalization, for which intramolecular hydrogen bonding enhances (or diminishes) their pseudoaromaticity depending on the substitution pattern. This interplay is illustrated herein by DFT-based calculations of aromaticity indices in the gas phase and polar solvents. Elucidation of transition structures involved in tautomeric conversions helps to solve how the intramolecular hydrogen transfer occurs. While TSs exhibit a high degree of aromaticity, the dichotomy between forward and backward pathways points to a complex trajectory. Addition of thermal corrections to the electronic energy decreases both the enthalpy and free energy leading to negative ΔH(‡) and ΔG(‡) values. This variational effect accounts for the otherwise elusive distinction between transition structures and saddle points (usually overlooked for high electronic barriers). Also, this rationale fits well within the framework of Marcus' theory.

Pseudo-cyclic structures of mono- and di-azaderivatives of malondialdehydes. Synthesis and conformational disentanglement by computational analyses.

Mono- and diaza-derivatives of malondialdehydes, namely 3-alkyl(aryl)amino-2-arylacroleins and 1,5-dialkyl(aryl)-3-arylvinamidines are open-chain systems in which extended electron delocalization and pseudoaromaticity can be envisaged. A set of diversely functionalized compounds has been synthesized and characterized by spectroscopic data and X-ray diffractometry. Quantum-chemical calculations were performed for all possible neutral tautomers and conformers in the gas phase and compared to those in polar solvents (CHCl3, DMSO, and EtOH) at the M06-2X/6-311++G(d,p) level. Tautomeric equilibria and conformational preferences can be rationalized in terms of structural factors, which can be roughly estimated as summation or subtractions of intramolecular interactions. As expected, a key role is played by intramolecular hydrogen bonds whose strength varies from the gas phase to polar ethanol. This issue also delves into the concept of resonance-assisted H-bond, where the donor and acceptor atoms are connected by a π-conjugated system. The most stable conformers (structures a and c) possess a high degree of pseudoaromaticity as inferred from HOMA indexes and other delocalization parameters.

Stepwise formation of 1,3-diazolium-4-thiolates by münchnone cycloadditions: promising candidates for nonlinear optics.

An improved preparation of mesoionic heterocycles 1,3-diazolium-4-thiolates by [3 + 2] cycloadditions of münchnones with aryl isothiocyanates is reported. The process takes place with high or complete regioselectivity, and fast and clean transformations are observed under microwave heating in DMF. DFT calculations support that this cycloaddition proceeds preferably through a stepwise mechanism. Given the pattern substitution around the mesoionic ring resulting in a push-pull system, theoretical estimations predict large hyperpolarizabilities in some cases, which is typical of molecules exhibiting nonlinear optical responses.

On the prebiotic synthesis of D-sugars catalyzed by L-peptides: assessments from first-principles calculations.

What accounts for a particular chiral selection in the case of a few sugars of prebiotic relevance, thereby mirroring the asymmetry observed in nature? By using first-principles calculations, the generation of pentoses from glycolaldehyde (the initial product of the autocatalytic formose reaction), which has been detected in outer space), has been modeled by using L-Val-L-Val as a primeval catalyst. Our theoretical study provides insight into the mechanism of this reaction and satisfactorily explains a few key molecular events. Our rationale agrees with the reported experimental data and shows that the D-configuration is only favored for ribose. L-pentoses are usually favored in the presence of L-configured dipeptides, as observed experimentally, although no chiral selection could be observed in the case of xylose. These results confirm that a prebiotic sugar soup could be fine-tuned in the presence of shorter peptides as catalysts and that D-ribose would have also resulted in an advantageous imbalance for further amplification and chemical evolution.

Tautomerism in Schiff bases. The cases of 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-naphthaldehyde investigated in solution and the solid state.

Schiff bases derived from hydroxyl naphthaldehydes and o-substituted anilines have been prepared and their tautomerism assessed by spectroscopic, crystallographic, and computational methods. Tautomeric equilibria have also been studied and reveal in most cases a slight preference of imine tautomers in solution; a fact supported by DFT calculations in the gas phase as well as incorporating solvent effects through the SMD model. To simulate the effect exerted by the crystal lattice on tautomer stability, we have developed a computational protocol in the case of 1-tert-butyl-2-(2-hydroxy-1-naphthylmethylene)aminobenzene whose data have been obtained experimentally at 120 K. Although a rapid imine-enamine interconversion may be occurring in the solid state, the imine tautomer becomes the most stable form and the energy difference should be related to the difference in the packing of the molecules.

On the enhanced reactivity and selectivity of triazole formation in molecular flasks. A theoretical rationale.

The azide-alkyne cycloaddition assisted by a self-assembled molecular flask developed by Rebek and coworkers (Org. Lett., 2002, 4, 327) has been simulated by means of the ONIOM methodology, thereby evidencing the reliability of this theoretical approach to model such large encapsulated systems. Experimental evidences accounting for this transformation within the supramolecular assembly such as the significant rate enhancement, complete regioselectivity, and product inhibition as the reaction proceeds have been qualitatively disentangled through estimation of the energy barriers and the structural characteristics of the corresponding host-guest complexes.

Assessing the whole range of CuAAC mechanisms by DFT calculations--on the intermediacy of copper acetylides.

The archetypal Cu(I)-catalyzed alkyne-azide click cycloaddition (CuAAC) has been explored thoroughly via density functional calculations, modeling copper nuclei with the LANL2DZ basis set and aqueous environments with CPCM solvation. All the mechanistic proposals, ranging from the intermediacy of copper acetylides to π-complexes and multinuclear clusters have been compared. The known features of the CuAAC reaction such as the observed second order kinetics for the Cu(I) species and the marked regioselectivity have been taken into account. The calculated energy barriers point to the intermediacy of copper(i) acetylides with two metal centers, in agreement with the observed kinetics, which exhibit barriers of 10.1 kcal mol(-1) and 13.7 kcal mol(-1) for the 1,4- and 1,5-regiochemistries, respectively, thus accounting for the marked regioselectivity of the copper catalyzed azide-alkyne cycloaddition. The copper acetylide versus π-complexes dilemma has also been experimentally addressed through the click reaction of benzyl azide and isotopically labeled phenylacetylene. The total proton/deuterium exchange in the afforded triazole demonstrates the formation of a copper acetylide intermediate during the transformation.

A quantitative structure-reactivity relationship in N-acetyl oxazolidines: an electrostatic interaction controls rotamer population.

The conformational population of Z and E isomers of the amide bond in N-acetyl oxazolidines is dictated by the electronic nature of the vicinal aryl ring. Experimental and theoretical data support a rationale based on a strong and stereodirecting charge-charge interaction that should be added to the arsenal of non-covalent interactions and whose influence can be more important than once thought.

Push-pull 1,3-thiazolium-5-thiolates. Formation via concerted and stepwise pathways, and theoretical evaluation of NLO properties.

The transformation of münchnones (mesoionic rings featuring the 1,3-oxazolium-5-olate core) into their sulfur counterparts (1,3-thiazolium-5-thiolates) by reaction with CS(2), pioneered by Huisgen and his group in the early 1970s, has been re-investigated in detail by means of both experimental and theoretical methods. The synthetic strategy can be tuned to incorporate donor and acceptor groups in appropriate positions. Calculations of molecular hyperpolarizabilities together with orbital topologies evidence that these sulfur-containing heterocycles exhibit nonlinear optical responses, thereby pointing to potential applications of mesoionic structures in the NLO field. From a mechanistic viewpoint, modeling of the whole systems at the B3LYP/6-31G(d) level reveals that concerted and stepwise pathways are operative depending on the substitution pattern of the parent münchnone, which also account for the experimental results.

Unusual aryl migration in a mesomeric betaine in the solid and liquid state: mechanistic insights into the SNAr reaction.

An intramolecular S(N)Ar mechanism has been identified in the unexpected aryl migration observed in a mesomeric betaine. The process changes drastically the optical and spectroscopic properties and should be a valuable model for related heteroaromatic systems.

Schiff bases from D-glucosamine and aliphatic ketones.

Despite the comprehensive literature and enormous versatility of chiral imines derived from aminosugars and aldehydes, the corresponding counterparts generated from ketones remain an underestimated research subject. Filling in the gap, this manuscript sheds light on the synthetic and structural aspects of such substances and updates the few antecedents reported so far.