Chemoselective Oxidation of Thiols with Oxoammonium Cations

The oxidation of various aryl and aliphatic thiols with the commercially available and environmentally benign reagent Bobbitt's salt (1) has been investigated. The reaction affords the corresponding disulfide products in good to excellent yields (71-99%) and can be accomplished in water, methanol, or acetonitrile solvent. Moreover, the process is highly chemoselective, tolerating traditionally oxidation-labile groups such as free amines and alcohols. Combined experimental and computational studies reveal that the oxidation takes place via a polar two-electron process with concomitant and unexpected deoxygenation of the oxoammonium cation through homolysis of the weak N-O bond, differing from prototypical radical-based thiol couplings. This unusual consumption of the oxidant has significant implications for the development of new nitroxide-based radical traps for probing S-centered radicals, the advancement of new electrochemical or catalytic processes involving nitroxide/oxoammonium salt redox couples, and applications to biological systems.

Importance of Noncovalent Interactions Involving Sulfur Atoms in Thiopeptide Antibiotics─Glycothiohexide α and Nocathiacin I

Noncovalent interactions involving sulfur atoms play essential roles in protein structure and function by significantly contributing to protein stability, folding, and biological activity. Sulfur is a highly polarizable atom that can participate in many types of noncovalent interactions including hydrogen bonding, sulfur-π interactions, and S-lone pair interactions, but the impact of these sulfur-based interactions on molecular recognition and drug design is still often underappreciated. Here, we examine, using quantum chemical calculations, the roles of sulfur-based noncovalent interactions in complex naturally occurring molecules representative of thiopeptide antibiotics: glycothiohexide α and its close structural analogue nocathiacin I. While donor-acceptor orbital interactions make only very small contributions, electrostatic and dispersion contributions are predicted to be significant in many cases. In pursuit of understanding the magnitudes and nature of these noncovalent interactions, we made potential structural modifications that could significantly expand the chemical space of effective thiopeptide antibiotics.

Intramolecular chalcogen bonding to tune molecular conformation of helical building block for supramolecular helix

We propose to employ intramolecular chalcogen bonding to make the helical building block take its otherwise unfavorable cis-conformation. 2,5-Thiophenediamide motif was taken to bridge two β-turn structures to lead to...

Experimental and theoretical insights into the trans influence of organo-sulfur and -selenium ligands in 5,6-membered palladium(ii) cationic pincer complexes based on iminophosphoranes

The trans influence of organochalcogen ancillary ligands was assessed by experimental and DFT studies in 5,6-membered SNS and SNSe Pd(ii) pincers.

Design, Synthesis, and Structure-Activity Relationships Study of N-Pyrimidyl/Pyridyl-2-thiazolamine Analogues as Novel Positive Allosteric Modulators of M3 Muscarinic Acetylcholine Receptor

The M₃ muscarinic acetylcholine receptor (mAChR) plays an essential pharmacological role in mediating a broad range of actions of acetylcholine (ACh) released throughout the periphery and central nerve system (CNS). Nevertheless, its agonistic functions remain unclear due to the lack of available subtype-selective agonists or positive allosteric modulators (PAMs). In the course of our extended structure-activity relationships (SARs) study on 2-acylaminothiazole derivative 1, a previously reported PAM of the M₃ mAChR, we successfully identified N-pyrimidyl/pyridyl-2-thiazolamine analogues as new scaffolds. The SARs study was rationalized using conformational analyses based on intramolecular interactions. A comprehensive study of a series of analogues described in this paper suggests that a unique sulfur-nitrogen nonbonding interaction in the N-pyrimidyl/pyridyl-2-thiazolamine moiety enable conformations that are essential for activity. Further, a SARs study around the N-pyrimidyl/pyridyl-2-thiazolamine core culminated in the discovery of compound 3g, which showed potent in vitro PAM activity for the M₃ mAChR with excellent subtype selectivity. Compound 3g also showed a distinct pharmacological effect on isolated smooth muscle tissue from rat bladder and favorable pharmacokinetics profiles, suggesting its potential as a chemical tool for probing the M₃ mAChR in further research.

Crystal structure and bonding analysis of the first dinuclear calcium(II)-proton-pump inhibitor (PPI) `butterfly molecule': a combined microcrystal synchrotron and DFT study

Proton-pump inhibitors (PPI) are prodrugs used widely to treat acid-related diseases since the late 1980s. After an extensive research effort it has become clear that the fundamental interactions between metal atoms and PPIs are of paramount importance for both drug release and long-term therapeutic safety. Unfortunately, until now, very little information has been available on this topic. In this paper, we report the crystal structure analysis of a novel calcium-PPI compound incorporating bridging and terminal deprotonated (R)-rabeprazole tricyclic ligands (L), namely bis[μ-(R)-2-({[4-(3-methoxypropoxy)-3-methylpyridin-2-yl]methyl}sulfinyl)-6,7-dihydro-3H-benzofuro[5,6-d]imidazol-1-ido]bis{dimethanol[(R)-2-({[4-(3-methoxypropoxy)-3-methylpyridin-2-yl]methyl}sulfinyl)-6,7-dihydro-3H-benzofuro[5,6-d]imidazol-1-ido]calcium(II)} methanol hexasolvate, [Ca2(C20H22N3O4S)4(CH3OH)4]·6CH3OH or [Ca2(L)4(CH3OH)4]·6CH3OH, which crystallizes from methanol in the polar C2 space group. Using low-temperature microcrystal synchrotron radiation, we demonstrate that this compound is in the form of a beautiful `butterfly molecule', consisting of a C2-symmetric dinuclear (CH3OH)2LCa(II)(μ2-L)2Ca(II)L(HOCH3)2 framework. A large amount of disorder is found within the bridging L ligand and the conformation of the fused tetrahydrofuran ring exhibits great variety. All the sulfinyl groups remain intact and the nonbonded Ca...Ca distance is significantly longer than in other calcium dimers, indicating steric hindrance in the bridging ligands. Considerable hydrogen bonding and aromatic C-H...π interactions co-operate to stabilize the whole complex, as well as to facilitate supramolecular assembly. Additional investigations into the bond nature were made using density functional theory (DFT) methods at the B3LYP/6-31G(d) level; geometry optimization, Mulliken atomic charges, MEP (molecular electrostatic potential), HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital), TDOS (total density of states), PDOS (partial density of states), COOP (crystal orbital overlap population) and vibrational spectra were calculated/recorded and assessed carefully.

Stereoelectronic effects dictate molecular conformation and biological function of heterocyclic amides

Heterocycles adjacent to amides can have important influences on molecular conformation due to stereoelectronic effects exerted by the heteroatom. This was shown for imidazole- and thiazole-amides by comparing low energy conformations (ab initio MP2 and DFT calculations), charge distribution, dipole moments, and known crystal structures which support a general principle. Switching a heteroatom from nitrogen to sulfur altered the amide conformation, producing different three-dimensional electrostatic surfaces. Differences were attributed to different dipole and orbital alignments and spectacularly translated into opposing agonist vs antagonist functions in modulating a G-protein coupled receptor for inflammatory protein complement C3a on human macrophages. Influences of the heteroatom were confirmed by locking the amide conformation using fused bicyclic rings. These findings show that stereoelectronic effects of heterocycles modulate molecular conformation and can impart strikingly different biological properties.

1-Tosyl-2-[(1-tosyl-1H-benzimidazol-2-yl)methyl-sulfanyl]-1H-benzimidazole

In the title compound, C₂₉H₂₄N₄O₄S₃, the two N-tosyl­benzimidazolyl unit are connected through a —S—CH₂— fragment, the dihedral angle between the benzimidazole rings being 76.09 (5)°. The methyl­thio group is disordered with respect to exchange of the S and C atoms in a 0.547 (4):0.453 (4) ratio. In the crystal, C—H⋯O and C—H⋯π inter­actions connect adjacent mol­ecules into infinite layers parallel to the ab plane. The crystal packing is further stabilized by a π–π inter­action [centroid–centroid separation = 3.5187 (4) Å].