Synthesis of tetrazoles through a domino reaction: A molecular electron density theory study of energetics, selectivities, and molecular mechanistic aspects

This study corresponds to a molecular electron density theory (MEDT) investigation to shed light on the energetics, selectivities, and molecular mechanistic aspects of an experimental domino reaction. Theoretical evidences at the M06-2X/6-31G(d) level indicates that this domino reaction includes three different successive steps and is initialized by a stepwise HCl elimination from precursor chlorohydrazone NPCH to yield nitrile imine NI-2. A subsequent stepwise and highly regioselective [3 + 2] cycloaddition (32CA) reaction of NI-2 toward tetramethylguanidine TMG-3 affords corresponding formal [3 + 2] cycloadduct CA-1 as the sole product. Finally, a stepwise HNMe2 elimination experienced by CA-1 leads to amino triazole ATA as an aromatic five-membered target product. Computed rate constants reveal that the HCl elimination step should be considered as the bottleneck of this domino reaction. However, a topological analysis of electron localization function (ELF) of NI-2 demonstrates a zwitterionic type (zw-type) 32 C A reaction is expected between NI-2 and TMG-3. This 32CA reaction also displays an almost noticeable polar character arising from moderate electrophilicity and strong nucleophilicity of NI-2 and TMG-3, respectively. The regioselectivity of 32CA reaction can be explained via analysis of Parr functions values calculated at the reactive sites of reagents. The molecular mechanism of the 32CA reaction was explored through portraying bond forming/breaking patterns involved in this polar, stepwise, and zw-type reaction by means of the ELF analysis. Indeed, formation of both C-N single bonds along the first and second steps takes place through coupling of the corresponding pseudoradical centers.

C(alkyl)-C(vinyl) bond cleavage enabled by Retro-Pallada-Diels-Alder reaction

Activation and cleavage of carbon-carbon (C-C) bonds is a fundamental transformation in organic chemistry while inert C-C bonds cleavage remains a long-standing challenge. Retro-Diels-Alder (retro-DA) reaction is a well-known and important tool for C-C bonds cleavage but less been explored in methodology by contrast to other strategies. Herein, we report a selective C(alkyl)-C(vinyl) bond cleavage strategy realized through the transient directing group mediated retro-Diels-Alder reaction of a six-membered palladacycle, which is obtained from an in situ generated hydrazone and palladium hydride species. This unprecedented strategy exhibits good tolerances and thus offers new opportunities for late-stage modifications of complex molecules. DFT calculations revealed that an intriguing retro-Pd(IV)-Diels-Alder process is possibly involved in the catalytic cycle, thus bridging both Retro-Diels-Alder reaction and C-C bond cleavage. We anticipate that this strategy should prove instrumental for potential applications to achieve the modification of functional organic skeletons in synthetic chemistry and other fields involving in molecular editing.

A facile protocol for the preparation of 2-carboxylated thieno [2,3-b] indoles: a de novo access to alkaloid thienodolin

A metal-free strategy, alternative to the known complex cycloaddition reactions, towards 2-carboxylated thieno [2,3-b] indole derivatives has been successfully developed. The novel approach involves as starting materials easy accessible 1,2-diaza-1,3-dienes and indoline 2-thione and requires mild reaction conditions. Furthermore, the easy work-up required makes this method amenable for a one-pot approach as demonstrated in the preparation of thienodolin, a natural product isolated from Streptomyces albogriseolus that exhibits valuable biological properties.

Hetero-Diels-Alder Reactions of In Situ-Generated Azoalkenes with Thioketones; Experimental and Theoretical Studies

The hetero-Diels-Alder reactions of in situ-generated azoalkenes with thioketones were shown to offer a straightforward method for an efficient and regioselective synthesis of scarcely known N-substituted 1,3,4-thiadiazine derivatives. The scope of the method was fairly broad, allowing the use of a series of aryl-, ferrocenyl-, and alkyl-substituted thioketones. However, in the case of N-tosyl-substituted cycloadducts derived from 1-thioxo-2,2,4,4-tetramethylcyclobutan-3-one and the structurally analogous 1,3-dithione, a more complicated pathway was observed. By elimination of toluene sulfinic acid, the initially formed cycloadducts afforded 2H-1,3,4-thiadiazines as final products. Advanced DFT calculations revealed that the observed high regioselectivity was due to kinetic reaction control and that the (4 + 2)-cycloadditions of sterically less unhindered thiones occurred via highly unsymmetric transition states with shorter C..S-distances (2.27-2.58 Å) and longer N..C-distances (3.02-3.57 Å). In the extreme case of the sterically very hindered 2,2,4,4-tetramethylcyclobutan-1,3-dione-derived thioketones, a zwitterionic intermediate with a fully formed C‒S bond was detected, which underwent ring closure to the 1,3,4-thiadiazine derivative in a second step. For the hypothetical formation of the regioisomeric 1,2,3-thiadiazine derivatives, the DFT calculations proposed more symmetric transition states with considerably higher energies.

Molecular diversity of TEMPO-mediated cycloaddition of ketohydrazones and 3-phenacylideneoxindoles

This reaction selectively proceeded via aza-Diels–Alder reaction, [3+2] cycloaddition and ring-opening of oxindole to give diverse spirooxindoles and polysubstituted pyrazoles.

Evaluation the synergistic antitumor effect of methotrexate-camptothecin codelivery prodrug from self-assembly process to acid-catalyzed both drugs release: A comprehensive theoretical study

Therapeutic efficiency of amphiphilic methotrexate-camptothecin (MTX-CPT) prodrug compared to free drug mixture (MTX/CPT) has been investigated using all-atom molecular dynamics simulation and first principles density functional theory calculations. This comparison revealed that MTX-CPT prodrug tends to form spherical self-assembled nanoparticle (NP), while free MTX/CPT mixture forms rod-shape NP. These observations are attributed to a structural defect in the MTX-CPT prodrug and solvation free energies of MTX, CPT and MTX-CPT molecules. The results provided evidence that noncovalent interactions (NCIs) among the pharmaceutical drugs play a very important role in anticancer agents aggregation process, leading to enhanced stability of the self-assembled NPs. It is found that the stability of MTX-CPT self-assembled NP is greater than the MTX/CPT NP due to the synergistic effect of hydrogen bonding between monomers and solvent (water). Moreover, the noncatalyzed as well as catalyzed hydrolysis reactions of MTX-CPT prodrug are theoretically studied at the PCM(water)//M06-2X/6-31G(d,p) computational level to shed additional light on the role of acidic condition in tumor tissues. We found that the ester hydrolysis in mild acidic solutions is a concerted reaction. In an agreement between theory and experiment, we also confirmed that the activation energies of the catalyzed-hydrolysis steps are much lower than the activation energies of the corresponding steps in the noncatalyzed reaction. Thus, the MTX-CPT prodrug reveals very promising properties as a pH-controlled drug delivery system.