Synergistic Experimental and Computational Investigation of the Bioorthogonal Reactivity of Substituted Aryltetrazines

Tetrazines (Tz) have been applied as bioorthogonal agents for various biomedical applications including pretargeted imaging approaches. In radioimmunoimaging, pretargeting increases the target-to-background ratio while simultaneously reducing the radiation burden. We have recently reported a strategy to directly 18F-label highly reactive tetrazines based on a 3-(3-fluorophenyl)-Tz core structure. Herein, we report a kinetic study on this versatile scaffold. A library of 40 different tetrazines was prepared, fully characterized, and investigated with emphasis on second order rate constants for the reaction with trans-cyclooctene (TCO). Our results reveal the effects of various substitution patterns and moreover demonstrate the importance of measuring reactivities in the solvent of interest, as click rates in different solvents do not necessarily correlate well. In particular, we report that tetrazines modified in 2-position of the phenyl substituent show high intrinsic reactivity towards TCO, which is diminished in aqueous systems by unfavorable solvent effects. The obtained results enable the prediction of the bioorthogonal reactivity and thereby facilitate the development of the next-generation of substituted aryltetrazines for in vivo application.

Structural study and Hirshfeld surface analysis of (Z)-4-(2-methoxybenzylidene)-3-phenylisoxazol-5(4H)-one

The title compound, C17H13NO3, adopts a Z configuration about the C=C bond. The isoxazole and methoxybenzylidene rings are almost coplanar with a dihedral angle of 9.63 (7)° between them. In contrast, the phenyl substituent is twisted significantly out of the plane of the oxazole ring, with the two rings inclined to each other by 46.22 (4)°. The crystal structure features C—H...O, C—H...N and C—H...π hydrogen bonds and π–π contacts. An analysis of the Hirshfeld surfaces points to the importance of H...H, H...C/C...H and H...O/O...H contacts. The included surface areas of the title compound were compared to those of the isomeric structure (Z)-4-(4-methoxybenzylidene)-3-phenylisoxazol-5(4H)-one [Zhang et al. (2015). CrystEngComm, 17, 7316–7322].

Benzobicyclo[3.2.1]octene Derivatives as a New Class of Cholinesterase Inhibitors

A library of amine, oxime, ether, epoxy and acyl derivatives of the benzobicyclo[3.2.1]octene were synthesized and evaluated as inhibitors of both human acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). The majority of the tested compounds exhibited higher selectivity for BChE. Structural adjustment for AChE seems to have been achieved by acylation, and the furan ring opening of furo-benzobicyclo[3.2.1]octadiene results for compound 51 with the highest AChE affinity (IC50 = 8.3 µM). Interestingly, its analogue, an oxime ether with a benzobicyclo[3.2.1]-skeleton, compound 32 was one of the most potent BChE inhibitors in this study (IC50 = 31 µM), but not as potent as endo-43, an ether derivative of the benzobicyclo[3.2.1]octene with an additional phenyl substituent (IC50 = 17 µM). Therefore, we identified several cholinesterase inhibitors with a potential for further development as potential drugs for the treatment of neurodegenerative diseases.

7a-Phenyltetrahydropyrrolo[2,1-b]oxazol-5(6H)-one

In the title compound, C12H13NO2, the pyrrolidinone moiety is almost flat while the oxazole ring adopts an envelope conformation with the carbon atom bearing the phenyl substituent as the flap: the angle between the mean planes of the fused heterocyclic rings is 45.47 (19)°. In the crystal, C—H...O and C—H...π contacts link the molecules into infinite [010] chains.

Distinctive reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under photoredox conditions

A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the unprecedented reductive coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of a phenyl substituent in the aniline moiety of the substrate was critical for the reactivity. The reaction proceeded via radical–radical cross-coupling of α-amino radicals generated by proton-coupled single-electron transfer in the presence of an Ir photocatalyst. On the other hand, symmetrical 1,2-diamines were selectively produced from the same starting materials by the judicious choice of the reaction conditions, showcasing the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines. The developed method can be employed for the synthesis of various bulky vicinal diamines, which are potential ligands in stereoselective synthesis.

Design, synthesis, molecular modelling and in vitro screening of monoamine oxidase inhibitory activities of novel quinazolyl hydrazine derivatives

A new series of N'-substituted benzylidene-2-(4-oxo-2-phenyl-1,4-dihydroquinazolin-3(2H)-yl)acetohydrazide ( 5a–5h ) has been synthesized, characterized by FT-IR, NMR spectroscopy and mass spectrometry and tested against human monoamine oxidase (MAO) A and B. Only (4-hydroxy-3-methoxybenzylidene) substituted compounds gave submicromolar inhibition of MAO-A and MAO-B. Changing the phenyl substituent to methyl on the unsaturated quinazoline ring ( 12a–12d ) decreased inhibition, but a less flexible linker ( 14a–14d ) resulted in selective micromolar inhibition of hMAO-B providing insight for ongoing design.

Distinctive Reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under Photoredox Catalysis

A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the unprecedented reductive coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of phenyl substituent in the aniline moiety of the substrate was critical for the reactivity. The reaction proceeded via radical-radical cross-coupling of a-amino radicals generated by proton-coupled single-electron transfer in the presence of an Ir-photocatalyst. On the other hand, symmetrical 1,2-diamines were selectively produced from the same starting materials by judicious choice of the reaction conditions, showcasing the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines. The developed method can be employed for the synthesis of various bulky vicinal diamines, which are potential ligands in stereoselective synthesis.

Developing Further Versatility in Benzoxazine Synthesis via Hydrolytic Ring-Opening

In this study, 2-(aminomethyl)phenol and its derivatives, the reactants for 2-substituted 1,3-benzoxazines, are synthesized by HCl hydrolysis from the typical benzoxazines. The phenol/aniline-based mono-oxazine benzoxazine, PH-a, and the bisphenol A/aniline-based bis-oxazine benzoxazine, BA-a, are used as examples to demonstrate the feasibility of this new approach. Their chemical structures are characterized by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and Raman spectroscopies, and are further verified by elementary analysis. Their thermal properties are studied by differential scanning calorimetry (DSC). These two 2-(aminomethyl) phenolic derivatives are reacted with paraformaldehyde to close the oxazine rings. A benzoxazine with a phenyl substituent at the 2-position of the oxazine ring is obtained from the 2-((phenylamino)methyl)phenol (hPH-a) and benzaldehyde. All these results highlight the success of the HCl hydrolysis and the formation of stable intermediates, namely 2-(aminomethyl) phenolic derivatives, from readily available benzoxazine monomers. This further demonstrates the feasibility of using these intermediates as reactants for a novel benzoxazine synthesis.