Isolation, biological activity, and synthesis of isoquinoline alkaloids

Covering: 2019 to 2023Isoquinoline alkaloids, an important class of N-based heterocyclic compounds, have attracted considerable attention from researchers worldwide. To follow up on our prior review (covering 2014-2018) and present the progress of this class of compounds, this review summarizes and provides updated literature on novel isoquinoline alkaloids isolated during the period of 2019-2023, together with their biological activity and underlying mechanisms of action. Moreover, with the rapid development of synthetic modification strategies, the synthesis strategies of isoquinoline alkaloids have been continuously optimized, and the total synthesis of these classes of natural products is reviewed critically herein. Over 250 molecules with a broad range of bioactivities, including antitumor, antibacterial, cardioprotective, anti-inflammatory, neuroprotective and other activities, are isolated and discussed. The total synthesis of more than nine classes of isoquinoline alkaloids is presented, and thirteen compounds constitute the first total synthesis. This survey provides new indications or possibilities for the discovery of new drugs from the original naturally occurring isoquinoline alkaloids.

Organocatalytic Asymmetric Construction of 2,6-Diazabicyclo-[2.2.2]octanes by Harnessing the Potential of an 3-Oxindolium Ion Intermediate

Due to its structural complexity and intrinsic sensitivity of bridged aminal junction, 2,6-diazabicyclo[2.2.2]octane (2,6-DABCO) has remained a highly desirable target in synthetic chemistry. However, the asymmetric access to this unit is still insufficient and hampered by the need for meticulously created functionalities for intricate double aza-cyclizations. Herein, we have developed a novel enantio- and diastereoselective protocol to access polycyclic chiral 2,6-DABCOs under metal-free conditions. This domino process involves the amine-catalyzed [4+2] annulation between glutaraldehyde and 2-arylindol-3-ones, followed by an acid-mediated Pictet-Spengler reaction/intramolecular aza-cyclization cascade sequence with tryptamine by trapping of in situ generated 3-oxindolium ion intermediate for the first time. Overall, 2,6-DABCOs fused with medicinally relevant scaffolds were isolated with good yield and excellent stereoselectivity by constructing five new bonds and four stereocenters in a one-pot operation.

Stereodivergent Total Synthesis of Tacaman Alkaloids

This paper describes a concise, asymmetric and stereodivergent total synthesis of tacaman alkaloids. A key step in this synthesis is the biocatalytic Baeyer-Villiger oxidation of cyclohexanone, which was developed to produce seven-membered lactones and establish the required stereochemistry at the C14 position (92 % yield, 99 % ee, 500 mg scale). Cis- and trans-tetracyclic indoloquinolizidine scaffolds were rapidly synthesized through an acid-triggered, tunable acyl-Pictet-Spengler type cyclization cascade, serving as the pivotal reaction for building the alkaloid skeleton. Computational results revealed that hydrogen bonding was crucial in stabilizing intermediates and inducing different addition reactions during the acyl-Pictet-Spengler cyclization cascade. By strategically using these two reactions and the late-stage diversification of the functionalized indoloquinolizidine core, the asymmetric total syntheses of eight tacaman alkaloids were achieved. This study may potentially advance research related to the medicinal chemistry of tacaman alkaloids.

BiBr3 -Mediated Intramolecular Aza-Prins Cyclization of Aza-Achmatowicz Rearrangement Products: Asymmetric Total Synthesis of Suaveoline and Sarpagine Alkaloids

An intramolecular aza-Prins cyclization of aza-Achmatowicz rearrangement products was developed in which bismuth tribromide (BiBr3 ) plays a dual role as an efficient Lewis acid and source of the bromide nucleophile. This approach enables the facile construction of highly functionalized 9-azabicyclo[3.3.1]nonanes (9-ABNs), which are valuable synthetic building blocks and a powerful platform for the synthesis of a variety of alkaloid natural products and drug molecules. Suitable substrates for the aza-Prins cyclization include 1,1-disubstituted alkenes, 1,2-disubstituted alkenes, alkynes, and allenes, with good to excellent yields observed. Finally, we showcase the application of this new approach to the enantioselective total synthesis of six indole alkaloids: (-)-suaveoline (1), (-)-norsuaveoline (2), (-)-macrophylline (3), (+)-normacusine B (4), (+)-Na -methyl-16-epipericyclivine (5) and (+)-affinisine (6) in a total of 9-14 steps. This study significantly expands the synthetic utility of the aza-Achmatowicz rearrangement, and the strategy (aza-Achmatowicz/aza-Prins) is expected to be applicable to the total synthesis of other members of the big family of macroline and sarpagine indole alkaloids.

Arynes as synthetic linchpins towards the construction of diversely functionalized natural product skeletons

Arynes are a privileged class of reactive intermediates in synthetic organic chemistry, and their unusual reactivities have been the subject of engrossing research interest. Recently, there are many reports on novel aryne-based synthetic innovations as a linchpin approach to accomplish the total synthesis of structurally diverse natural products or their derivatives in a racemic and enantiopure fashion. This review provides an overview of the literature on synthetic strategies, employing arynes as crucial intermediates to construct architecturally intriguing bioactive natural products/derivatives in a period of 2019 to 2022. This study highlights the need to investigate the effective synthesis and search for new biological uses of highly functionalized natural product skeletons.

Natural and synthetic β-carboline as a privileged antifungal scaffolds

The discovery of antifungal agents with novel structure, broad-spectrum, low toxicity, and high efficiency has been the focus of medicinal chemists. Over the past decades, β-carboline scaffold has attracted extensive attention in the scientific community due to its potent and diverse biological activities with nine successfully marketed β-carboline-based drugs. In this review, we summarized the current states and advances in the antifungal activity of natural and synthetic β-carbolines. Additionally, the structure-activity relationships and their antifungal mechanisms targeting biofilm, cell wall, cell membrane, and fungal intracellular targets were also systematically discussed. In summary, β-carbolines have the great potential to develop new efficient scaffolds to combat fungal infections.

Redox-Neutral and Atom-Economic Route to β-Carbolines via Gold-Catalyzed [4 + 2] Cycloaddition of Indolylynamides and Cyanamides

Gold(I)-catalyzed [4 + 2] cycloaddition of indolylynamides and cyanamides (aminonitriles) is an efficient redox-neutral and atom-economic route to diversely substituted 1,3-diamino-β-carbolines. The protocol operates under mild conditions (Ph₃PAuNTf₂ 5 mol %, DCE, 60 °C) with a good tolerance to functional groups (23 examples and yields up to 98%). The obtained β-carboline systems represent a versatile synthetic platform with modifiable substituents for successive functionalizations. Control experiments indicate the crucial role of both the nature of reactants and the identity of employed catalysts in the developed cycloaddition.

Enantioselective synthesis of 1-aminoindene derivatives via asymmetric Brønsted acid catalysis

We describe a catalytic asymmetric iminium ion cyclization reaction of simple 2-alkenylbenzaldimines using a BINOL-derived chiral N-triflyl phosphoramide. The corresponding 1-aminoindenes and tetracyclic 1-aminoindanes are formed in good yields and high enantioselectivities. Further, the chemical utility of the obtained enantiopure 1-aminoindene is demonstrated for the asymmetric synthesis of (S)-rasagiline.

On the application of 3d metals for C-H activation toward bioactive compounds: The key step for the synthesis of silver bullets

Several valuable biologically active molecules can be obtained through C-H activation processes. However, the use of expensive and not readily accessible catalysts complicates the process of pharmacological application of these compounds. A plausible way to overcome this issue is developing and using cheaper, more accessible, and equally effective catalysts. First-row transition (3d) metals have shown to be important catalysts in this matter. This review summarizes the use of 3d metal catalysts in C-H activation processes to obtain potentially (or proved) biologically active compounds.

Synthesis and Assessment of Fused β-Carboline Derivatives as Kappa Opioid Receptor Agonists

The synthesis of 5-formyl-6-aryl-6H-indolo[3,2,1-de][1,5] naphthyridine-2-carboxylates by reaction between 1-formyl-9H-β-carbolines and cinnamaldehydes in the presence of pyrrolidine in water with microwave irradiation is described. Pharmacophoric modification of the formyl group offered several new fused β-carboline derivatives, which were investigated for their κ-opioid receptor (KOR) agonistic activity. Two compounds 4 a and 4 c produced appreciable agonist activity on KOR with EC₅₀ values of 46±19 and 134±9 nM, respectively. Moreover, compound-induced KOR signaling studies suggested both compounds to be extremely G-protein-biased agonists. The analgesic effect of 4 a was validated by the increase in tail flick latency in mice in a time-dependent manner, which was completely blocked by the KOR-selective antagonist norBNI. Moreover, unlike U50488, an unbiased full KOR agonist, 4 a did not induce sedation. The docking of 4 a with the human KOR was studied to rationalize the result.

Benzannulation strategies for the synthesis of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines

This review presents a critical and authoritative analysis of several exciting benzannulation approaches developed in the past decade for the construction of carbazoles, indolocarbazoles, benzocarbazoles, and carbolines.