Study of the Total Synthesis of (−)-Exiguolide

In this article, we disclose the various routes and strategies we had to explore before finally achieving the total synthesis of (−)-exiguolide ((−)-<b>1</b>). Two first types of approaches were set, both relying on the Trost’s domino ene–yne coupling/oxa-Michael reaction that we choose for its ability to control the geometry of the methylacrylate-bearing tetrahydropyrane ring <i>B</i>. In our first approach, we expected to assemble the two main fragments (C14–C21 and C1–C13) by creating the C13–C14 bond through a palladium(0)-catalyzed cross-coupling, but this step failed, unfortunately. In the second approach, which was more linear, we created the C16–C17 bond through condensation of a lithium acetylide on a Weinreb amide, and we assembled the C1–C5 and C6–C21 subunits through Trost’s domino ene–yne coupling/oxa-Michael reaction. These two approaches served us to design an ameliorated third strategy, which finally led to the total synthesis of (−)-exiguolide

Structural Tuning of Energetic Material Bis(1<i>H</i>‑tetrazol-5-yl)amine Monohydrate under Pressures Probed by Vibrational Spectroscopy and X‑ray Diffraction

As a high-energy density material, bis­(1<i>H</i>-tetrazol-5-yl)­amine monohydrate (BTA·H₂O) was investigated at high pressures up to 25 GPa using in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, and ab initio simulations. Upon compression, both the Raman and IR vibrational bands were found to undergo continuous and gradual broadening without significant change of the profile, indicating pressure-induced structural disordering rather than phase transition. X-ray diffraction patterns confirmed the pressure effect on the structural evolutions of BTA·H₂O. Upon decompression, the back transformation was observed with almost identical Raman and IR spectra and X-ray pattern of the recovered material, indicating the complete reversibility of the pressure-induced disordering of BTA·H₂O and thus the high chemical stability of the aromatic rings in BTA·H₂O. Interestingly, in contrast with all of other Raman and IR modes of BTA·H₂O, which exhibit blue shifts, the N–H stretching mode shows a prominent red shift upon compression to ∼8 GPa, strongly suggesting pressure-enhanced hydrogen bonding between BTA and H₂O. The analysis of X-ray diffraction patterns of BTA·H₂O indicates that the unit-cell parameters undergo anisotropic compression rate. The pressure dependence of the unit-cell parameters and volumes coincides with the behavior of the hydrogen-bonding enhancement. Aided with first-principles simulations, these pressure-mediated structural modifications consistently suggest that hydrogen bonding played an important role in the compression behavior and structural stability of BTA·H₂O under high pressures

High-Temperature Spin Crossover Behavior in a Nitrogen-Rich Fe^III-Based System

A nitrogen-rich ligand <i>bis</i>(1<i>H</i>-tetrazol-5-yl)­amine (H₃bta) was employed to isolate a new Fe^III complex, Na₂NH₄[Fe^III(Hbta)₃]·3DMF·2H₂O (<b>1</b>). Single crystal X-ray diffraction revealed that complex <b>1</b> consists of Fe^III ions in an octahedral environment where each metal ion is coordinated by three Hbta^2– ligands forming the [Fe^III(Hbta)₃]^3– core. Each unit is linked to two one-dimensional (1-D) Na⁺/solvent chains creating a two-dimensional (2-D) network. In addition, the presence of multiple hydrogen bonds in all directions between ammonium cation and ligands of different [Fe^III(Hbta)₃]^3– units generates a three-dimensional (3-D) network. Magnetic measurements confirmed that the Fe^III center undergoes a Spin Crossover (SCO) at high temperature (<i>T</i>_1/2 = 460(10) K)

Structural Tuning of Energetic Material Bis(1<i>H</i>‑tetrazol-5-yl)amine Monohydrate under Pressures Probed by Vibrational Spectroscopy and X‑ray Diffraction

As a high-energy density material, bis­(1<i>H</i>-tetrazol-5-yl)­amine monohydrate (BTA·H₂O) was investigated at high pressures up to 25 GPa using in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, and ab initio simulations. Upon compression, both the Raman and IR vibrational bands were found to undergo continuous and gradual broadening without significant change of the profile, indicating pressure-induced structural disordering rather than phase transition. X-ray diffraction patterns confirmed the pressure effect on the structural evolutions of BTA·H₂O. Upon decompression, the back transformation was observed with almost identical Raman and IR spectra and X-ray pattern of the recovered material, indicating the complete reversibility of the pressure-induced disordering of BTA·H₂O and thus the high chemical stability of the aromatic rings in BTA·H₂O. Interestingly, in contrast with all of other Raman and IR modes of BTA·H₂O, which exhibit blue shifts, the N–H stretching mode shows a prominent red shift upon compression to ∼8 GPa, strongly suggesting pressure-enhanced hydrogen bonding between BTA and H₂O. The analysis of X-ray diffraction patterns of BTA·H₂O indicates that the unit-cell parameters undergo anisotropic compression rate. The pressure dependence of the unit-cell parameters and volumes coincides with the behavior of the hydrogen-bonding enhancement. Aided with first-principles simulations, these pressure-mediated structural modifications consistently suggest that hydrogen bonding played an important role in the compression behavior and structural stability of BTA·H₂O under high pressures

Substituted Diphenylamine Antioxidants and Benzotriazole UV Stabilizers in Aquatic Organisms in the Great Lakes of North America: Terrestrial Exposure and Biodilution

Substituted diphenylamine antioxidants (SDPAs) and benzotriazole UV stabilizers (BZT-UVs) are industrial additives of emerging environmental concern. However, the bioaccumulation, biomagnification, and spatial distribution of these contaminants in the Great Lakes of North America are unknown. The present study addresses these knowledge gaps by reporting SDPAs and BZT-UVs in herring gull (<i>Larus argentatus</i>) eggs, lake trout (<i>Salvelinus namaycush</i>), and their food web in the Great Lakes for the first time. Herring gull eggs showed much higher detection frequency and concentrations of target SDPAs and 2-(2H-benzotriazol-2-yl)-4,6-di-<i>tert</i>-pentylphenol (UV328) than that of the whole body fish homogenate. For herring gull eggs, the samples from upper Great Lakes contained significantly greater levels of SDPAs than those eggs from lower lakes, possibly due to the differences in terrestrial food in diet. Interestingly, the predominant SDPAs in herring gull eggs were dinonyl- (C9C9) and monononyl-diphenylamine (C9) which were previously shown to be less bioaccumulative than other SDPAs in fish. In contrast, dioctyl-diphenylamine (C8C8) was the major SDPA in lake trout, and biodilution of C8C8 was observed in a Lake Superior lake trout food web. Such variations in herring gull eggs and fish indicate the differences in accumulation and elimination pathways of SDPAs and BZT-UVs and require further elucidation of these mechanisms