Scaffold Diversity through a Branching Double-Annulation Cascade Strategy: Iminium-Induced One-Pot Synthesis of Diverse Fused Tetrahydroisoquinoline Scaffolds

Recent Progress in the Synthesis of Benzoxazin-4-Ones, Applications in N-Directed Ortho-Functionalizations, and Biological Significance

The development of efficient synthetic procedures to access fused N, O-heterocyclic skeletons has been a pivotal research topic in organic synthesis for several years. Owing to the applications of N, O-fused heterocycles in organic synthesis, material sciences, and medicinal chemistry, significant efforts have been dedicated to design novel methods for their construction. To this end, 1,3-benzoxazin-4-ones are privileged candidates for N, O-heterocyclic molecules often found in natural products, agrochemicals, and materials science applications. In this review, we aim to summarize the existing literature on the synthesis of 1,3-benzoxazin-4-ones from 2010 onwards. Moreover, 1,3-benzoxazin-4-ones have also been identified as an excellent native directing group for the ortho-functionalization via C-H activation, which is often a strenuous task requiring pre-functionalized substrates. In the latter part of this report, we compiled several interesting examples of N-directed functionalizations of 1,3-benzoxazin-4-ones. Additionally, to emphasize biological importance, recent developments on the anticancer evaluations of benzoxazine-4-one core are included. We believe that by harnessing the methodologies discussed herein, new possibilities could be unlocked for the synthesis of fused N, O-heterocycles, leading to the development of novel biologically active compounds and functional materials.

Aryne and CO2-based formal [2 + 2 + 2] annulation to access tetrahydroisoquinoline-fused benzoxazinones

Tetrahydroisoquinoline and its fused polyheterocycles are prevalent structural motifs found in numerous natural products. In this study, we report a highly efficient and convergent synthetic approach for the construction of tetrahydroisoquinoline-fused polyheterocycles through a three-component formal [2 + 2 + 2] annulation process by combining 3,4-dihydroisoquinolines, CO2, and benzynes. Notably, electron-rich 3,4-dihydroisoquinolines and electron-deficient benzynes exhibit greater reactivity in this annulation. Moreover, this method benefits from the convergent synthesis and the utilization of carbon dioxide, providing a valuable strategy for the facile synthesis of tetrahydroisoquinoline-fused polyheterocycles, with potential applications in the discovery and development of novel organic molecules.

Design, synthesis and mechanism study of novel natural-derived isoquinoline derivatives as antifungal agents

In screening for natural fungicidal leads, two series of novel 3-aryl-isoquinoline derivatives 8 and 9 were designed and synthesized based on sanguinarine, chelerythrine and berberine. Their structures were confirmed by 1D, 2D NMR and HRMS. Most of the title compounds showed medium to excellent antifungal activity in vitro at 50 mg/L, which were much more active than the lead of sanguinarine. Especially, 9f possessed the best effective against Alternaria solani (80.4%), Alternaria alternata (88.2%) and Physalospora piricola (93.8%). Furthermore, the EC₅₀ of 9f (3.651 mg/L) against P. piricola was marginally better than chlorothalonil (3.869 mg/L). In vivo antifungal activity of 9f against P. piricola was studied on apples. The curative and protection results at the dosage of 50 and 100 mg/L showed as 70.45 ~ 81.67% and 64.96 ~ 80.34%, respectively, which were equal to that of chlorothalonil (80.30 ~ 86.67%, 73.08 ~ 76.92%). Molecular electrostatic potential and molecular docking analysis revealed that 9f was fully covered by positive potential contour, which was easier to interact with the negative amino acid resides of succinate dehydrogenase than 8f. 9f could be used as a novel antifungal lead compound for further study. Two series of novel isoquinoline derivatives 8, 9 containing 3-aryl were rational designed and synthesized based on quaternary isoquinoline alkaloids. The bioassay and interaction mechanism studies indicated that 9f should be considered as potential antifungal lead.

Photocatalytic Visible-Light-Induced Nitrogen Insertion via Dual C(sp3)-H and C(sp2)-H Bond Functionalization: Access to Privileged Imidazole-based Scaffolds

Here we have demonstrated a visible-light-mediated metal-free organic-dye-catalyzed dehydrogenative N-insertion leading to highly substituted imidazoles and privileged dihydroisoquinoline-based imidazole derivatives via C(sp³)-H and C(sp²)-H bond functionalization. A sustainable, convenient, metal-free azidation/C-H aminative cyclization approach in the absence of stoichiometric oxidants is presented. This protocol involves a rare photoinduced iminyl radical as a key intermediate for the "N" insertion.

Scaffold Diversity Synthesis Delivers Complex, Structurally, and Functionally Distinct Tetracyclic Benzopyrones

Complexity-generating chemical transformations that afford novel molecular scaffolds enriched in sp3 character are highly desired. Here, we present a highly stereoselective scaffold diversity synthesis approach that utilizes cascade double-annulation reactions of diverse pairs of zwitterionic and non-zwitterionic partners with 3-formylchromones to generate highly complex tetracyclic benzopyrones. Each pair of annulation partners adds to the common chroman-4-one scaffold to build two new rings, supporting up to four contiguous chiral centers that include an all-carbon quaternary center. Differently ring-fused benzopyrones display different biological activities, thus demonstrating their immense potential in medicinal chemistry and chemical biology research.

Quantum chemical approaches to [NiFe] hydrogenase

The mechanism by which [NiFe] hydrogenase catalyses the oxidation of molecular hydrogen is a significant yet challenging topic in bioinorganic chemistry. With far-reaching applications in renewable energy and carbon mitigation, significant effort has been invested in the study of these complexes. In particular, computational approaches offer a unique perspective on how this enzyme functions at an electronic and atomistic level. In this article, we discuss state-of-the art quantum chemical methods and how they have helped deepen our comprehension of [NiFe] hydrogenase. We outline the key strategies that can be used to compute the (i) geometry, (ii) electronic structure, (iii) thermodynamics and (iv) kinetic properties associated with the enzymatic activity of [NiFe] hydrogenase and other bioinorganic complexes.

A metal-free cyclic iminium induced one-pot double annulation cascade: access to dihydroisoquinolinium (DHIQ) salts

A reactive cyclic iminium induced one-pot Groebke–Blackburn–Bienayme (GBB) double annulation cascade (DAC) for the synthesis of skeletally diverse DHIQ salts has been described. The key features of this protocol are solvent-free, robust method, clean reaction profile and operational simplicity.