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It is now established that crystalline cellulose is held together not just by hydrogen bonding, but also by dispersion forces and by electrostatic attraction modulated by stereoelectronic factors such as the exo-anomeric effect. The surface chains of native cellulose microfibrils differ in C6 conformation from crystalline cellulose and therefore form different hydrogen bonds, both outward and inward. Dispersion and electrostatic forces, influenced by cellulose conformation, also operate at the microfibril surface. The surface conformation depends on whether cellulose interacts with water, with the surfaces of other microfibrils or with non-cellulosic polymers. Cellulose-water binding competes with other binding interactions, so that diverse surface interactions are finely balanced in free energy, difficult to simulate, and dependent on local details of water structuring about which little is known, especially in the presence of dispersed chains of hemicellulosic or pectic polymers. An example is the influence of hydration on the aggregation of microfibrils as measured by neutron scattering, which is large for primary-wall cellulose and small for hardwood microfibrils. There are many consequent uncertainties about the surface interactions of hydrated cellulose microfibrils, for example how pectins associate with cellulose or why cellulose-xylan interfaces resist hydration. Evidence from a range of experimental technologies, alongside simulations, will be needed to resolve these uncertainties. The practical implications are wide-ranging, from the mechanism of plant growth and the mechanical resilience of wood to the development of novel, wood-based building materials. [ABSTRACT FROM AUTHOR]

Sequential anionic intramolecular cyclizations and modelling were used for the first time to access unusual fused heterocyclic frameworks in excellent yields. 5‐Exo‐dig cyclizations yielded isoindolinone motifs and a subsequent 6‐exo‐ and 7‐endo‐dig cyclization was directed to provide either fused isoquinoline or azepine frameworks. Regioselectivities were controlled by exploiting stereoelectronic effects via nC−→π*(Ph) interactions, and modelling studies provided reaction scope. [ABSTRACT FROM AUTHOR]

Aminic radical-trapping antioxidants (RTAs), as one of the most important antioxidants, have not received sufficient attention yet. But, an increasing number of aminic RTAs have been identified as ferroptosis inhibitors in recent years, which can potentially mediate many pathological states including inflammation, cancer, neurodegenerative disease, as well as ocular and kidney degeneration. This highlights the importance of aminic RTAs in the field of medicine. Herein, we systematically explored the radical scavenging mechanism of aminic RTAs with a quantum chemical method, particularly emphasizing the role of stereoelectronic factors and resonance factors on the transfer of H-atom and the stability to one-electron oxidation. These theoretical results elucidate the diversity of free radical scavenging mechanisms for aminic RTAs, and has significant implications for the rational design of new aminic RTAs.

The stereoelectronic properties of the molecular structure of most stable conformers of gaboxadol and gaboxadol hydrochloride have been studied using DFT/B3P86-LANL2DZ methodology. The energies of stable conformers of gaboxadol and gaboxadol hydrochloride are −494.2689 and −510.0117 hartrees, respectively. The stability of the molecules arising from stereoelectronic interactions, leading to its bioactivity, has been confirmed using natural bond orbital analysis. The natural bond orbital analysis of donor-acceptor (σ→σ* and n→σ*) interactions showed that the stereoelectronic hyperconjugative and anomeric interactions are exhibited in gaboxadol hydrochloride and gaboxadol, respectively. Lengthening of the axial and equatorial C-H bond lengths and natural population analysis support these results. Spectral features of gaboxadol hydrochloride have been explored by the Fourier transform infrared, Raman and Nuclear magnetic resonance spectroscopic techniques combined with density functional theory computations. NH + … Cl − hydrogen bonding has been noticeable as a broad and strong absorption in the 2800-2400 cm −1 region. Broad peaks obtained by proton NMR are a result of the quadrupole effect of the N + atom. Docking studies using representative GABA receptor crystal structures revealed that molecules containing azinane and isoxazole cores fit within the ligand binding domains, and the gaboxadol hydrochloride molecule shows the best binding energy with the 3D32 GABA receptor. Also, gaboxadol hydrochloride has obtained a high value of HOMO energy and a narrow HOMO- LUMO energy gap, which enhances reactivity. [ABSTRACT FROM AUTHOR]

In the titled tricyclic orthocarbonate derivative, the three endocyclic C–O bonds are longer than the exocyclic C–O bond (∼1.40 Å vs. ∼1.37 Å). This indicates an anomeric-type interaction between the two electron lone pairs on the exocyclic oxygen atom and the antibonding orbitals of the two antiperiplanar endocyclic C–O bonds. The remaining endocyclic C–O bond – marginally shorter than the other two – apparently adds to this effect. Intriguingly, the antibonding orbital of the exocyclic C–O bond extends into the interior of the adamantyl cage, and is stereoelectronically prevented from overlapping with any of the six adjacent lone pairs. The results also seem to indicate a preference for interaction between a single donor oxygen atom and multiple acceptor antibonding orbitals rather than vice versa. The results add insightfully to the substantial body of evidence favouring the antiperiplanar lone pair hypothesis (ALPH). [ABSTRACT FROM AUTHOR]

The hexamethylenetetramine (HMT) framework displays interesting stereoelectronic interactions of the anomeric type. In the highly symmetrical parent system, the nitrogen centres act as both donors and acceptors. Protonation lowers symmetry and also leads to an enhancement of the anomeric interaction around the protonated centre. X-ray diffraction crystal structures of four derivatives of HMT – with succinic, (DL)-malic, phthalic and 4-hydroxybenzoic acids – reveal significant trends. (The first three form well-defined salts, 4-hydroxybenzoic acid forming a co-crystalline compound.) Each molecular structure is essentially characterised by a major anomeric interaction involving the protonated centre as acceptor. In two cases (succinic and 4-hydroxybenzoic), secondary protonation leads to a weaker anomeric interaction site that apparently competes with the dominant one. Bond length changes indicate that the anomeric interaction decreases as malic > phthalic > succinic > 4-hydroxybenzoic, which correlates with the degree of proton transfer to the nitrogen centre. Along with other bond length and angle changes, the results offer insight into the applicability of the antiperiplanar lone pair hypothesis (ALPH) in a rigid system. [ABSTRACT FROM AUTHOR]

For over forty years pteridinologists have studied the O2-mediated autoxidation of reduced folate derivatives. This effort has resulted in the identification of several transient intermediates and many products. Until now. however, no clear explanation has emerged of why certain reduced folate derivatives arc less susceptible to autoxidation than others. Presented below is a detailed theoretical model which readily explains the chemical behavior of reduced folate derivatives towards O2. Of note are the steric and electronic forces at work during the formation of the 4a-hydroperoxy intermediate and the stereoelectronic factors that predominate during its breakdown.

Abstract: Carbenes, thought of only as transient species for a long time, have become ubiquitous in organometallic chemistry. Their interaction with a metal center, which allows for their classification as a function of the nature of the carbene–metal bond, has inspired the investigations of many research groups in every area of chemistry, from physical chemistry to organic synthesis. In this Review, we intend to give a general overview of carbenes in a broad sense, discussing singlet and triplet carbenes with all variations within these families. More precisely, we describe here, for Fischer- and Schrock-type carbenes, N-heterocyclic carbenes and non-stabilized ones, the different synthetic routes to both the immediate precursors of carbenes and their metal complexes. Additionally, their steric and electronic properties are discussed in the light of both experimental and theoretical studies. [Copyright &y& Elsevier]

We have demonstrated experimentally by Raman analysis that 1a exists as a mixture of ttt/cct conformers at 22 °C in liquid or CCl4 solution, thus contrasting with the initial IR, NMR and X-ray analyses, which were strongly in favour of (ttt)-1a. The global stereoelectronic stabilization is ca. 4.0–4.2 kcal/mol for both parallel N lone pairs (lp) in (cct)-1a, based on a ΔH of 0.75 kcal/mol (±10%), as measured by Raman spectroscopy from 22 to 90 °C, as well as the roughly estimated MM2 MeNHEt and MeNHCH2NH2 gauche interactions. DFT calculations using B3LYP/6-31G* yield a standard ΔH value of 1.04 kcal/mol, in good agreement with the experiment, and predict a ttt/cct ratio of ca. 77:23 at 25 °C. Broadening of the 13C-NMR signals was observed in either CCl4 or CS2 or even CDCl3 solutions between -20 and -40 °C. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: A novel and efficient synthesis of (−)-8-epi-swainsonine 2 is reported. Stereocontrolled diol formation from the bicyclic alkene 3 followed by a stereoselective vinylation of the aldehyde and ring closing metathesis gave the indolizidine ring system, which was converted into (−)-8-epi-swainsonine 2. [Copyright &y& Elsevier]

Abstract: Evidence for the generalized anomeric effect (GAE) in the N-acyl-1,3-thiazolidines, an important structural motif in the penicillins, was sought in the crystal structures of N-(4-nitrobenzoyl)-1,3-thiazolidine and its (2:1) complex with mercuric chloride, N-acetyl-2-phenyl-1,3-thiazolidine, and the (2:1) complex of N-benzoyl-1,3-thiazolidine with mercuric bromide. An inverse relationship was generally observed between the C 2–N and C 2–S bond lengths of the thiazolidine ring, supporting the existence of the GAE. (Maximal bond length changes were ∼0.04Å for C 2–N 3, S 1–C 2, and ∼0.08Å for N 3–C 6.) Comparison with N-acylpyrrolidines and tetrahydrothiophenes indicates that both the nitrogen-to-sulphur and sulphur-to-nitrogen GAE’s operate simultaneously in the 1,3-thiazolidines, the former being dominant. (This is analogous to the normal and exo-anomeric effects in pyranoses, and also leads to an interesting application of Baldwin’s rules.) The nitrogen-to-sulphur GAE is generally enhanced in the mercury(II) complexes (presumably via coordination at the sulphur); a ‘competition’ between the GAE and the amide resonance of the N-acyl moiety is apparent. There is evidence for a ‘push–pull’ charge transfer between the thiazolidine moieties in the mercury(II) complexes, and for a ‘back-donation’ of charge from the bromine atoms to the thiazolidine moieties in the HgBr2 complex. (The sulphur atom appears to be sp2 hybridised in the mercury(II) complexes, possibly for stereoelectronic reasons.) These results are apparently relevant to the mode of action of the penicillins. [Copyright &y& Elsevier]

Abstract: The latest advances in N-heterocyclic carbene-based organometallic chemistry have drawn increasing attention to this class of ancillary ligands as an attractive alternative to tertiary phosphines. Studies focusing on the fundamental steric and electronic factors characterizing this family of compounds are therefore essential for the rationalization of the activity observed for such organometallic complexes in metal-mediated organic transformations. This knowledge is also of fundamental importance in catalyst design efforts. This review intends to provide a comprehensive overview of the progress in this area. [Copyright &y& Elsevier]

• 6- exo vs 7- endo radical cyclization of phenethylenamides. • M062X and PW6B95-D3 DFT calculations. • Explanation and prediction of regioselectivity through DFT computations. • Stereoelectronic effects govern 6- exo vs 7- endo regioselectivity. The 7- endo -trig and 6- exo -trig cyclizations of the N -formylphenethylenamide radicals have been computationally studied at the UM062X/6–31 + G** and UPW6B95-D3 DFT levels of theory. Computations show a regioselectivity preference for the 7- endo process over the 6- exo in agreement with known experimental results. This 7- endo process takes place preferentially through a transition structure of half-boat geometry which fully retains enamide planarity. On the other hand, the 6- exo -trig cyclization prefers a chair transition structure geometry which places the N -vinyl group in a pseudo-equatorial position. Computations also show that conformational locking in cyclic enamides greatly increases the preference for the 7- endo path in agreement with experimentally observed outcomes. [ABSTRACT FROM AUTHOR]

Abstract: Catalytic hydrogenation of dihydroindolizidinone occurred preferentially from the endo-face giving rapid entry to (8R,8aS)-8-methylhexahydroindolizin-5-one, a key intermediate in the synthesis of 5,8-disubstituted indolizidines and deoxypumiliotoxin 251H. The selectivity could be improved further by diimide reduction though this also resulted in some oxidation of the alkene to the diene. The basis of the unusual stereoselectivity in the diimide reduction is believed to be stereoelectronic in origin. [Copyright &y& Elsevier]

Aminic radical-trapping antioxidants (RTAs), as one of the most important antioxidants, have not received sufficient attention yet. But, an increasing number of aminic RTAs have been identified as ferroptosis inhibitors in recent years, which can potentially mediate many pathological states including inflammation, cancer, neurodegenerative disease, as well as ocular and kidney degeneration. This highlights the importance of aminic RTAs in the field of medicine. Herein, we systematically explored the radical scavenging mechanism of aminic RTAs with a quantum chemical method, particularly emphasizing the role of stereoelectronic factors and resonance factors on the transfer of H-atom and the stability to one-electron oxidation. These theoretical results elucidate the diversity of free radical scavenging mechanisms for aminic RTAs, and has significant implications for the rational design of new aminic RTAs., (Copyright © 2019 Li, Zhou, Huo, Chen, Liu and Sheng.)