Rigid and concave, 2,4-cis-substituted azetidine derivatives: A platform for asymmetric catalysis

Synthesis of chiral Cu(II) complexes from pro-chiral Schiff base ligand and investigation of their catalytic activity in the asymmetric synthesis of 1,2,3-triazoles

A pro-chiral Schiff base ligand (HL) was synthesized by the reaction of 2-amino-2-ethyl-1,3-propanediol and pyridine-2-carbaldehyde in methanol. The reaction of HL with CuCl2·2H2O and CuBr2 in methanol gave neutral mononuclear Cu(II) complexes with general formula of [Cu(HL)Cl2] (1) and [Cu(HL)Br2] (2), respectively. By slow evaporation of the methanolic solutions of 1 and 2, their enantiomers were isolated in crystalline format. The formation of pure chiral crystals in the racemic mixture was amply authenticated by single crystal X-ray analysis, which indicated that S-[Cu(HL)Cl2], R-[Cu(HL)Cl2], and S-[Cu(HL)Br2] are crystallized in chiral P212121 space group of orthorhombic system. Preferential crystallization was used to isolate the R and S enantiomers as single crystals and the isolated compounds were also studied by CD analysis. Structural studies indicated that the origin of the chirality in these compounds is related to the coordination mode of the employed pro-chiral ligand (HL) because one of its carbon atoms has been converted to a chiral center in the synthesized complexes. Subsequently, these complexes were used in click synthesis of a β-hydroxy-1,2,3-triazole and the results of catalytic studies indicated that 1 and 2 can act as enantioselective catalysts for the asymmetric synthesis of β-hydroxy-1,2,3-triazole product under mild condition. This study illustrates the significant capacity of the use of pro-chiral ligands in preparing chiral catalysts based on complexes which can also be considered as an effective approach to cheap chiral catalysts from achiral reagents.

Azetidines Kill Multidrug-Resistant Mycobacterium tuberculosis without Detectable Resistance by Blocking Mycolate Assembly

Tuberculosis (TB) is the leading cause of global morbidity and mortality resulting from infectious disease, with over 10.6 million new cases and 1.4 million deaths in 2021. This global emergency is exacerbated by the emergence of multidrug-resistant MDR-TB and extensively drug-resistant XDR-TB; therefore, new drugs and new drug targets are urgently required. From a whole cell phenotypic screen, a series of azetidines derivatives termed BGAz, which elicit potent bactericidal activity with MIC99 values <10 μM against drug-sensitive Mycobacterium tuberculosis and MDR-TB, were identified. These compounds demonstrate no detectable drug resistance. The mode of action and target deconvolution studies suggest that these compounds inhibit mycobacterial growth by interfering with cell envelope biogenesis, specifically late-stage mycolic acid biosynthesis. Transcriptomic analysis demonstrates that the BGAz compounds tested display a mode of action distinct from the existing mycobacterial cell wall inhibitors. In addition, the compounds tested exhibit toxicological and PK/PD profiles that pave the way for their development as antitubercular chemotherapies.

Acid Gas Capture by Nitrogen Heterocycle Ring Expansion

Acid gases including CO2, OCS, CS2, and SO2 are emitted by industrial processes such as natural gas production or power plants, leading to the formation of acid rain and contributing to global warming as greenhouse gases. An important technological challenge is to capture acid gases and transform them into useful products. The capture of CO2, CS2, SO2, and OCS by ring expansion of saturated and unsaturated substituted nitrogen-strained ring heterocycles was computationally investigated at the G3(MP2) level. The effects of fluorine, methyl, and phenyl substituents on N and/or C were explored. The reactions for the capture CO2, CS2, SO2, and OCS by 3- and 4-membered N-heterocycles are exothermic, whereas ring expansion reactions with 5-membered rings are thermodynamically unfavorable. Incorporation of an OCS into the ring leads to the amide product being thermodynamically favored over the thioamide. CS2 and OCS capture reactions are more exothermic and exergonic than the corresponding CO2 and SO2 capture reactions due to bond dissociation enthalpy differences. Selected reaction energy barriers were calculated and correlated with the reaction thermodynamics for a given acid gas. The barriers are highest for CO2 and OCS and lowest for CS2 and SO2. The ability of a ring to participate in acid gas capture via ring expansion is correlated to ring strain energy but is not wholly dependent upon it. The expanded N-heterocycles produced by acid gas capture should be polymerizable, allowing for upcycling of these materials.

Preparation of Optically Active Biphenyl Compounds via an Albumin- Mediated Asymmetric Nitroaldol Reaction

Abstract:

Human serum albumin (HSA) was found to catalyze the asymmetric nitroaldol reaction of biphenyl aldehydes with nitromethane to afford the corresponding optically active 2-nitro alcohols. Careful optimization of the conditions for the reaction of 4-phenylbenzaldehyde with nitromethane in water at a neutral pH improved both the reactivity and the enantioselectivity. Finally, the reaction of 4-phenylbenzaldehyde (56 mg, 0.30 mmol) in nitromethane (2.8 mL) and water (1.1 mL) using HSA (68 mg) at 5 °C for 240 h gave (R)-1-([1,1'-biphenyl]-4-yl)-2-nitroethanol in 71% yield (52 mg), with an ee up to 85% ee. Subsequent recrystallization improved the ee up to 95%. The reaction was useful in a preparative-scale operation, and the biocatalyst could be reused several times. The procedure was also applicable to other substrates with different substitution patterns. Although the nitroaldol reaction of 2-phenylbenzaldehyde with nitromethane proceeded with low enantioselectivity to afford the corresponding (R)-2-nitroalcohols (35% ee), the reactions of the substrates bearing Br, Me, OMe, or CN group at the 4'-position of the benzene ring gave the corresponding optically active compounds with high enantioselectivities (80-88% ee).

Facile synthesis of 2-azaspiro[3.4]octane

Our annulation strategy utilized for the synthesis of 2-azaspiro[3.4]octane is explained. Three successful routes for the synthesis were developed. One of the approaches involved annulation of the cyclopentane ring and the remaining two approaches involved annulation of the four membered ring. All three approaches employ readily available starting materials with conventional chemical transformations and minimal chromatographic purifications to afford the title compound. The merits and limitations of the three approaches are also discussed.

Palladium and Platinum 2,4-cis-amino Azetidine and Related Complexes

Seven N,N'-palladium(II) chloride complexes, one N,N'-palladium(II) acetate complex of 2,4-cis-azetidines where prepared and analyzed by single crystal XRD. Two platinum(II) chloride N,N'-complexes of 2,4-cis-azetidines where prepared and analyzed by single crystal XRD. Computational analysis and determination of the %Vbur was examined conducted. A CNN' metallocyclic complex was prepared by oxidative addition of palladium(0) to an ortho bromo 2,4-cis-disubstituted azetidine and its crystal structure displays a slightly pyramidalized metal-ligand orientation.