Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs

Visible light-induced reductive aza-6π electrocyclization access to phenanthridines

Visible light-induced aza-6π electrocyclization was developed for the synthesis of aza-arenes from nitroarenes with diverse aldehydes. This protocol allows the reduction of nitroarenes by B2nep2 and subsequent 6π-electrocyclization of the in situ formed imine under visible light. An array of 6- and multi-substituted phenanthridines were constructed in moderate to good yields under purple LEDs at room temperature. A wide scope of substrates with diverse functional groups were well tolerated. In addition, the synthetic utility of this methodology was further demonstrated in the late-stage functionalization of celecoxib.

Oxidative Control of Photoinduced Cascade Electrocyclizations in Aromatic Azido Imines to Access Complex Fused Imidazoles or Pyrazoles

Photoinduced cascade of two 6π-electron six- and five-center electrocyclizations in aromatic azido imines is oxidatively controlled to yield complex fused benzimidazoles or indazoles. Formation of benzimidazoles occurs via an unprecedented carbon-to-nitrogen o-iminoaryl 1,2-shift.

Metal-free 2-isocyanobiaryl-based cyclization reactions: phenanthridine framework synthesis

The development of transition metal-free 2-isocyanobiaryl-based reactions has received much attention due to the widespread presence of phenanthidine frameworks as products in pharmacological chemistry and materials science. This review article focuses on the achievements from 2013 until now in various metal-free catalyzed reactions and discusses challenging mechanisms and features of the transformations.

Tuning the photophysical and chiroptical properties of [4]helicene-diketopyrrolopyrroles

Synthesis and functionalization of diketopyrrolo[3,4-c]pyrrole (DPP) derivatives containing chiral groups able to induce a strong chiral perturbation of the DPP core are still a challenging task. We report in this work the straightforward preparation of four bis([4]helicene)-DPP and bis([4]thiahelicene)-DPP dyes upon the condensation of 2-CN-[4](thia)helicene precursors, followed by their N-alkylation by nucleophilic substitution (compounds 9-11) or by a Mitsunobu-type strategy (compound 12). Compound 12, which contains sec-phenylethyl groups attached to the nitrogen atoms, has been obtained as (R,R) and (S,S) enantiomers. The four DPP-helicenes are luminescent in solution, while the N-benzyl (10) and N-sec-phenethyl (12) are emissive in the solid state as well. The chiroptical properties of compound 12 in solution and in the solid state indicate a strong chiral perturbation provided by the α-stereogenic centres, in spite of the stereodynamic nature of the [4]helicene flanking units.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

Effect of Activation Methods of Molecular Sieves on Ketimine Synthesis

Although the use of molecular sieves for imine synthesis is a common protocol, there have been no comprehensive studies on heat-drying methods. This can be crucial for reproducibility. It was found that molecular sieve 5A dried at 160 °C for 5 h under vacuum efficiently promoted the condensation of various ketones and amines to afford even relatively bulky ketimines. Several control experiments and analyses revealed that only a small amount of Brønsted acid sites was important for the activity, rather than dehydration ability. Other types of molecular sieves could be utilized for the reaction after treatment with water followed by heat drying. A continuous-flow acetalization reaction of alcohols using the activated molecular sieve 5A was also demonstrated.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Regioselective synthetic approach for key precursors of 6-aryl benzo[c]phenanthridin-10-ol derivatives: A useful compound for selective chromogenic recognition of fluoride

A regioselective synthetic strategy for 6-aryl-8,9-dihydro-benzo[c]phenanthridine-10(7H)-ones (4) is accomplished using a one-pot four-component reaction by fine-tuning of reaction temperature. DMSO is excellently used as a reactant-cum-solvent to introduce carbonyl functionality...

The Physical Chemistry and Chemical Physics (PCCP) Section of the International Journal of Molecular Sciences in Its Publications: The First 300 Thematic Articles in the First 3 Years

The Physical Chemistry and Chemical Physics Section (PCCP Section) is one of the youngest among the sections of the International Journal of Molecular Sciences (IJMS)-the year 2021 will only mark three years since its inception [...].

Synthesis of Aza[n]phenacenes (n = 4-6) via Photocyclodehydrochlorination of 2-Chloro-N-aryl-1-naphthamides

A novel methodology for the synthesis of aza[n]phenacenes was successfully developed utilizing photocyclodehydrochlorination reaction of 2-chloro-N-aryl-1-naphthamides. In these key intermediates, the factors influencing the photoreaction were studied. The target aza[n]phenacenes were obtained by triflation or chlorination from prepared phenanthridinones, followed by hydrogenation. The introduction of a nitrogen atom into a phenacene skeleton induced changes in the physicochemical properties. The important properties of prepared aza[n]phenacenes (n = 4-6) were studied experimentally and by density functional theory calculations and were compared to those of their carbo analogues. Furthermore, some important features of the crystalline aza[n]phenacenes were investigated, including intermolecular interaction in the crystal lattice and the increased solubility or decreased melting points.

Light-driven photoswitching of quinazoline analogues of combretastatin A-4 as an effective approach for targeting skin cancer cells

A novel quinazoline series of photoswitchable combretastatin A-4 (CA-4) analogues were synthesized and their photochemical properties and antiproliferative activity against A431 epidermoid carcinoma cells were studied. It was found that quinazoline analogues, in contrast to the majority of the known CA-4, exhibit high antiproliferative activity in the E-form as well. Photoswitching of the E-form to the Z-form resulted in a multiple (9-fold) increase in antiproliferative activity. 1H NMR monitoring showed that these compounds are very resistant to UV (λ = 365 nm) or sunlight irradiation and do not undergo photodegradation with a loss of antiproliferative activity that is inherent in heterocyclic analogues of CA-4. Similar photoswitching and an increase in antiproliferative activity are observed on exposure to sunlight. A selected compound (1a-Z51) in sub-micromolar concentrations induced apoptosis in A431 cells, while rad50/ATM/p53 were not involved in cell death. The growth of A431 cells was significantly inhibited after combination treatment with compound 1a-Z51 and chemotherapy drugs (cisplatin or 5-fluorouracil). In summary, the quinazoline analogues of CA-4 represent a promising strategy to achieve a photoswitchable potency for the treatment of cancers, including the development of combination therapies.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.