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1. Role of curvature in acridone for ¹O₂ oxidation of a natural product homoallylic alcohol: A novel iso ‐hydroperoxide intermediate

   https://doi.org/10.1111/php.13843  

   Lapoot, Lloyd; Jabeen, Shakeela; Durantini, Andrés M.; Greer, Alexander (August 2023, Photochemistry and Photobiology)

   Abstract A density functional theoretical (DFT) study is presented, implicating a¹O₂oxidation process to reach a dihydrobenzofuran from the reaction of the natural homoallylic alcohol, glycocitrine. Our results predict an interconversion between glycocitrine and aniso‐hydroperoxide intermediate [R(H)O⁺–O⁻] that provides a key path in the chemistry which then follows. Formations of allylic hydroperoxides are unlikely from a¹O₂‘ene’ reaction. Instead, the dihydrobenzofuran arises by¹O₂oxidation facilitated by a 16° curvature of the glycocitrine ring imposed by a pyramidalN‐methyl group. This curvature facilitates the formation of theiso‐hydroperoxide, which is analogous to theisospecies CH₂I⁺–I⁻and CHI₂⁺–I⁻formed by UV photolysis of CH₂I₂and CHI₃. Theiso‐hydroperoxide is also structurally reminiscent of carbonyl oxides (R₂C=O⁺–O⁻) formed in the reaction of carbenes and oxygen. Our DFT results point to intermolecular process, in which theiso‐hydroperoxide's fate relates to O‐transfer and H₂O dehydration reactions for new insight into the biosynthesis of dihydrobenzofuran natural products. 

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2. Synthesis and crystal structure of a bench-stable pyridinium ketene hemiaminal: 1-(1-ethoxyethenyl)-2-[methyl(phenyl)amino]pyridin-1-ium trifluoromethanesulfonate

   https://doi.org/10.1107/S2056989023005741  

   Krevlin, Z; Bote, I; Crespo, M; Lam, C; McMillen, C; Majireck, M (August 2023, Acta Crystallographica Section E Crystallographic Communications)

   The novel bench-stableN-quaternized keteneN,O-acetal, C₁₆H₁₉N₂O⁺·CF₃O₃S⁻, was synthesized and its structure determined. The title compound is a rare example of a pyridinium ketene hemiaminal for which a crystal structure has been determined, joining the 2-chloro-1-(1-ethyoxyethenyl)pyridin-1-ium trifluoromethanesulfonate salt from which it was synthesized. The cationic species of the title compound can be defined by three individually planar fragments assembling into a non-coplanar cation. The phenyl substituent extending from the amino nitrogen atom and the ethyoxyvinyl substituent extending from the pyridine N atom are oriented on the same side of the molecule and maintain the closest coplanar relationship of the three fragments. Supramolecular interactions are dominated by C—H...O interactions from the cation to the SO₃side of the trifluoromethanesulfonate anion, forming a two-dimensional substructure. 

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3. Methyl β-lactoside [methyl β- D -galactopyranosyl-(1→4)-β- D -glucopyranoside] monohydrate: a solvomorphism study

   https://doi.org/10.1107/S2053229621009499  

   Lin, Jieye; Oliver, Allen G.; Serianni, Anthony S. (October 2021, Acta Crystallographica Section C Structural Chemistry)

   Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate, C 13 H 24 O 11 ·H 2 O, (I), was obtained via spontaneous transformation of methyl β-lactoside methanol solvate, (II), during air-drying. Cremer–Pople puckering parameters indicate that the β-D-Gal p (β-D-galactopyranosyl) and β-D-Glc p (β-D-glucopyranosyl) rings in (I) adopt slightly distorted 4 C 1 chair conformations, with the former distorted towards a boat form ( B C1,C4 ) and the latter towards a twist-boat form ( O5 S C2 ). Puckering parameters for (I) and (II) indicate that the conformation of the βGal p ring is slightly more affected than the βGlc p ring by the solvomorphism. Conformations of the terminal O -glycosidic linkages in (I) and (II) are virtually identical, whereas those of the internal O -glycosidic linkage show torsion-angle changes of 6° in both C—O bonds. The exocyclic hydroxymethyl group in the βGal p residue adopts a gt conformation (C4′ anti to O6′) in both (I) and (II), whereas that in the βGlc p residue adopts a gg ( gauche – gauche ) conformation (H5 anti to O6) in (II) and a gt ( gauche – trans ) conformation (C4 anti to O6) in (I). The latter conformational change is critical to the solvomorphism in that it allows water to participate in three hydrogen bonds in (I) as opposed to only two hydrogen bonds in (II), potentially producing a more energetically stable structure for (I) than for (II). Visual inspection of the crystalline lattice of (II) reveals channels in which methanol solvent resides and through which solvent might exchange during solvomorphism. These channels are less apparent in the crystalline lattice of (I). 

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4. Crystal structure and characterization of a new one-dimensional copper(II) coordination polymer containing a 4-aminobenzoic acid ligand

   https://doi.org/10.1107/S2056989024001336  

   Gogia, Alisha; Novikov, Egor M.; Guzei, Ilia A.; Fonari, Marina S.; Timofeeva, Tatiana V. (March 2024, Acta Crystallographica Section E Crystallographic Communications)

   A Cu^IIcoordination polymer,catena-poly[[[aquacopper(II)]-bis(μ-4-aminobenzoato)-κ²N:O;κ²O:N] monohydrate], {[Cu(pABA)₂(H₂O)]·H₂O}_n(pABA =p-aminobenzoate, C₇H₄NO₂⁻), was synthesized and characterized. It exhibits a one-dimensional chain structure extended into a three-dimensional supramolecular assembly through hydrogen bonds and π–π interactions. While the twinned crystal shows a metrically orthorhombic lattice and an apparent space groupPbcm, the true symmetry is monoclinic (space groupP2/c), with disordered Cu atoms and mixed roles of water molecules (aqua ligand/crystallization water). The luminescence spectrum of the complex shows an emission at 345 nm,cf.349 nm forpABAH. 

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5. Crystal engineering of a 1:1 5-fluorocytosine–4-hydroxybenzaldehyde cocrystal: insights from X-ray crystallography and Hirshfeld analysis

   https://doi.org/10.1107/S2056989025004463  

   Sangavi, Marimuthu; Mohana, Marimuthu; Butcher, Ray J; McMillen, Colin D (June 2025, Acta Crystallographica Section E Crystallographic Communications)

   The 1:1 cocrystal of 5-fluorocytosine (5FC) and 4-hydroxybenzaldehyde (4HB), C₄H₄FN₃O·C₇H₆O₂has been synthesized and its structure characterized by single-crystal X-ray diffraction and Hirshfeld surface analysis. The compound crystallizes in the monoclinicP2₁/cspace group. A robust supramolecular architecture is stabilized by N—H...O, N—H...N, C—H...O and C—H...F hydrogen bonds, formingR₂²(8),R₄⁴(22),R₆⁶(32), andR₈⁸(34) ring motifs. The N—H...O and N—H...N hydrogen bonds form strong directional interactions, contributing to theR₂²(8) andR₈⁸(34) motifs through dimeric and extended ring structures. O—H...O interactions link 5FC and 4HB molecules, generating anR₆⁶(32) ring that enhances the packing. Weaker C—H...F bonds help form theR₄⁴(22) tetrameric motif, supporting the overall three-dimensional supramolecular framework. Additionally, C—F...π interactions between the fluorine atom and the aromatic ring add further to the crystal cohesion. Hirshfeld surface analysis and two-dimensional fingerprint plots confirm that O...H/H...O contacts are the most significant, highlighting the central role of hydrogen bonding in the stability and organization of the crystal structure. 

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