Total Synthesis of Yuzurine-type Alkaloid Daphgraciline

Single-atom molecular editing: transformative advances in carbocyclic and heterocyclic frameworks

Single-atom editing has emerged as a transformative strategy in organic synthesis, enabling precise modification of carbocyclic and heterocyclic frameworks by selectively targeting single atoms. These frameworks are crucial backbones of pharmaceuticals, agrochemicals, and advanced materials, making this approach powerful for organic chemists. In drug discovery and natural product synthesis, single-atom editing diversifies molecular scaffolds and tailors molecular properties to enhance pharmacological activity. In heterocyclic synthesis, this approach enables controlled heteroatom substitution, addition or deletion in an unprecedented and highly selective manner compared to traditional methods. Recent advances in transition-metal catalysis, organocatalysis, photoredox catalysis, and heterocycle-to-heterocycle metamorphosis have expanded the versatility of single-atom editing, enabling the synthesis of various carbocyclic and heterocyclic compounds. Principally, this approach has been exploited to design new architectures that are not easily accessible by other methods and to establish major improvements in the synthesis of known scaffolds, providing more efficient and sustainable routes towards large-scale chemical synthesis. This review overviews recent advances, focusing on carbocyclic and heterocyclic frameworks, and is organized by key single-atom editing strategies, such as ring contractions, atom deletions, ring expansions, and atom insertions. The review highlights key transformations like Favorskii and Wolff rearrangements, alongside modern photochemical and transition-metal-catalyzed processes, to provide a broad overview of synthetic applications and inspire further advancements in targeted molecular edits.

Forging the Tetracyclic Core Framework of Daphlongamine B Enabled by a Peripheral Derivation of the Aza-Angular Triquinane Strategy

A synthetic route to the highly functionalized tetracyclic core framework of daphlongamine B is described. Key features of the strategy involve an oxidative dearomatization-induced [4+2] cycloaddition, a di-π-methane rearrangement, and a ring-closing metathesis reaction. Our approach enables the reliable construction of a fully elaborated tetracyclic precursor, which, in turn, provides valuable functional handles for further elaboration to the target molecule.

Recent advancements in the chemistry of Diels-Alder reaction for total synthesis of natural products: a comprehensive review (2020-2023)

Despite being discovered nearly a century ago, the Diels-Alder (DA) reaction remains a crucial tool in the total synthesis of natural products. It accommodates a broad range of building blocks with varying complexity and levels of derivatization, allowing the formation of six-membered rings with precise stereochemistry. This, in turn, simplifies the synthesis of core structures found in many natural products. In recent years, modifications to the traditional Diels-Alder reaction have expanded its scope. These modifications include the inverse electron demand Diels-Alder reaction, dehydro Diels-Alder reaction, hetero-Diels-Alder reaction, photoenolization Diels-Alder reaction, asymmetric Diels-Alder reaction, and domino Diels-Alder reaction have been employed to extend the scope of this process in the synthesis of natural products. This review discusses the application of the Diels-Alder reaction in the total synthesis of natural products from 2020 to 2023, along with select methodologies that are inspired by or can be used to synthesize natural products.

Synthetic Study toward Daphnimacropodines

Daphnimacropodines A-C are members of a small but structurally distinct subfamily of Daphniphyllum alkaloids. Their congested polycyclic skeletons, and two vicinal quaternary stereocenters, present significant synthetic challenges. This paper describes two stereoselective approaches to constructing the tricyclic core structures of daphnimacropodines, achieved through a straightforward Rh-catalyzed [4 + 3] cycloaddition using simple building blocks. This work also highlights an intramolecular Heck reaction that rapidly assembles the cyclohexane ring moiety, a Tsuji-Trost allylation that forged the critical C-8 quaternary stereocenter, an efficient hetero-Diels-Alder reaction, and an intramolecular nucleophilic addition, which paved the way to the key cyclopentane ring. The assembly of the tetrahydropyrrole motif was also investigated.

Molecular Complexity-Inspired Synthetic Strategies toward the Calyciphylline A-Type Daphniphyllum Alkaloids Himalensine A and Daphenylline

In this report, we detail two distinct synthetic approaches to calyciphylline A-type Daphniphyllum alkaloids himalensine A and daphenylline, which are inspired by our analysis of the structural complexity of these compounds. Using MolComplex, a Python-based web application that we have developed, we quantified the structural complexity of all possible precursors resulting from one-bond retrosynthetic disconnections. This led to the identification of transannular bonds as especially simplifying to the molecular graph, and, based on this analysis, we pursued a total synthesis of himalensine A from macrocyclic intermediates with planned late-stage transannular ring formations. Despite initial setbacks in accessing an originally designed macrocycle, targeting a simplified macrocycle ultimately enabled investigation of this intermediate's unique transannular reactivity. Given the lack of success to access himalensine A based solely on molecular graph analysis, we revised our approach to the related alkaloid, daphenylline. Herein, we also provide the details of the various synthetic challenges that we encountered and overcame en route to a total synthesis of daphenylline. First, optimization of a Rh-mediated intramolecular Buchner/6π-electrocyclic ring-opening sequence enabled construction of the pentacyclic core. We then describe various attempts to install a key quaternary methyl group and, ultimately, our solution to leverage a [2 + 2] photocycloaddition/bond cleavage sequence to achieve this elusive goal. Finally, a late-stage Friedel-Crafts cyclization and deoxygenation facilitated the 11-step total synthesis, which was made formally enantioselective by a Rh-mediated dihydropyridone conjugate arylation. Complexity analysis of the daphenylline synthesis highlights how complexity-building/C-C cleavage combinations can be uniquely effective in achieving synthetic outcomes.

The Latest Progress in the Chemistry of Daphniphyllum Alkaloids

Daphniphyllum alkaloids (DAs) are interesting molecules with rich molecular skeletons and diverse biological activities. Since their discovery, phytochemists have isolated, purified, and identified more than 350 DAs. Synthetic chemists, attracted by the structure and activity of DAs, have accomplished many elegant synthetic jobs. Herein, we summarize work on the isolation, structural identification, bioactivity testing, and synthesis of DAs from 2018 to 2023, with the aim of providing a reference for future studies.

Asymmetric total synthesis of polycyclic xanthenes and discovery of a WalK activator active against MRSA

The development of new antibiotics continues to pose challenges, particularly considering the growing threat of multidrug-resistant Staphylococcus aureus. Structurally diverse natural products provide a promising source of antibiotics. Herein, we outline a concise approach for the collective asymmetric total synthesis of polycyclic xanthene myrtucommulone D and five related congeners. The strategy involves rapid assembly of the challenging benzopyrano[2,3-a]xanthene core, highly diastereoselective establishment of three contiguous stereocenters through a retro-hemiketalization/double Michael cascade reaction, and a Mitsunobu-mediated chiral resolution approach with high optical purity and broad substrate scope. Quantum mechanical calculations provide insight into stereoselective construction mechanism of the three contiguous stereocenters. Additionally, this work leads to the discovery of an antibacterial agent against both drug-sensitive and drug-resistant S. aureus. This compound operates through a unique mechanism that promotes bacterial autolysis by activating the two-component sensory histidine kinase WalK. Our research holds potential for future antibacterial drug development.

Enantioselective Total Synthesis of (-)-Daphenylline

A concise enantioselective total synthesis of (-)-daphenylline, a hexacyclic Daphniphyllum alkaloid with a unique benzene ring, was achieved in 14 steps. The synthesis commences with two chiral stereocenters, C2 and C18, readily installed via Carreira's Ir/amine dual-catalyzed allylation. The allylic bridgehead amine 6 was rapidly prepared through Wickens' photoredox-catalyzed hydrocarboxylation of olefin and CuBr2-catalyzed α-amination of ketone. The tetracycle 4 was formed via Pd-catalyzed reductive Heck reaction or, more concisely, by Krische's Rh-catalyzed reductive 1,6-enyne cyclization. In this synthesis, newly reported Wickens' photoredox-catalyzed hydrocarboxylation was used twice, and Friedel-Crafts acylation thrice.

Total Syntheses of Rhodomollins A and B

The first and asymmetric total syntheses of rhodomollins A and B, two rhodomollane type grayanoids featuring a d-homograyanane carbon skeleton and an oxa-bicyclo[3.2.1] core, were accomplished via a convergent strategy. A Stille coupling and a lithium-halogen exchange/intramolecular nucleophilic addition to the aldehyde sequence were employed to assemble two enantioenriched fragments. The oxa-bicyclo[3.2.1] core was achieved through an intramolecular SN2 substitution of cyclic sulfate of 1,2-diols (Williamson ether synthesis). The A ring oxidation states were adjusted by a Payne/Meinwald rearrangement sequence and subsequent redox transformations.

Total Synthesis of (±)- and (-)-Daphnillonin B

The first total synthesis of (±)- and (-)-daphnillonin B, a daphnicyclidin-type alkaloid with a new [7-6-5-7-5-5] A/B/C/D/E/F hexacyclic core, has been achieved. The [6-5-7] B/C/D ring system was efficiently and diastereoselectively constructed via a mild type I intramolecular [5+2] cycloaddition, followed by a Grubbs II catalyst-catalyzed radical cyclization. The [5-5] fused E/F ring system was synthesized via a diastereoselective intramolecular Pauson-Khand reaction. Notably, the synthetically challenging [7-6-5-7-5-5] hexacyclic core was reassembled by a unique Wagner-Meerwein-type rearrangement from the [6-6-5-7-5-5] hexacyclic framework found in calyciphylline A-type Daphniphyllum alkaloids.

Development of water-soluble phenazine-2,3-diol-based photosensitizers for singlet oxygen generation

Phenazine-2,3-diol-based dyes, KY-1Na and KY-2Na bearing one and two carboxylic acid sodium salts, respectively, have been newly developed as water-soluble photosensitizers (PSs) possessing the ability to generate singlet oxygen (¹O₂). In order to evaluate the solubility of KY-1Na and KY-2Na in water, the hydrophobicity/hydrophilicity of the two PSs was investigated by experimental measurement of the logarithms (log P_o/w) of the 1-octanol/water partition coefficient (P_o/w) for the PS. The log P_o/w values of both KY-1Na and KY-2Na were determined to be -0.9, indicating that both the PSs are more hydrophilic than Rose Bengal (-0.6) and have hydrophilicity equivalent to methylene blue (-0.9). Both the PSs in water show a broad photoabsorption band in the range of 500 to 600 nm. Thus, we estimated the ¹O₂ quantum yields (Φ_Δ) of KY-1Na and KY-2Na in water by using 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) as a water-soluble ¹O₂ scavenger. It was found that in water the Φ_Δ value (0.19) of KY-2Na is higher than that of KY-1Na (0.06). Density functional theory (DFT) calculations suggested that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions for the molecular structure of KY-2Na are adequately separated, leading to a decrease in the energy gap (ΔE_ST) between the singlet state (S₁) and the triplet state (T₁) that causes efficient intersystem crossing (ISC), compared to that for the molecular structure of KY-1Na. Indeed, time-dependent DFT (TD-DFT) calculations demonstrated that the ΔE_ST(S₁-T₁) value (0.82 eV) of KY-2Na is smaller than that (0.98 eV) of KY-1Na, resulting in a relatively high Φ_Δ value of KY-2Na. Consequently, we demonstrate that phenazine-2,3-diol-based PSs bearing carboxylic acid salts possess high solubility and moderate ¹O₂ generation ability in water.

Construction of the [7-5-5] Tricyclic Core of Daphniphyllum Alkaloids via a Cationic Cascade Reaction

The [7-5-5] tricyclic core of daphniphyllum alkaloids, containing contiguous stereogenic centers at C14 and C15 and a tetrasubstituted alkene moiety between C9 and C10, was constructed via a cascade reaction that involved an electrocyclic reaction of a pentadienyl cation and intramolecular interception of the resultant cyclopentenyl cation.