Total Synthesis of Four Classes of Daphniphyllum Alkaloids

Concise Total Synthesis of Ambiguine P

Ambiguine P (5) is a synthetically challenging representative of the hapalindole-type natural products. Inspired by the biosynthesis of this indole terpenoid family, we developed a Cope/Prins/Friedel-Crafts cascade, achieving a six-step synthesis of 5 from 2,2-dimethylcycloheptanone. Altered from the biosynthetic pathway, this cascade strategically commenced with a tricyclic indolenine derivative bearing a seven-membered ring, enabling efficient construction of the pentacyclic scaffold of 5. The oxidation state was elevated through chlorination and hydroxylation in the final stage.

Bioinspired Synthesis of Microstegiol and Biosynthetically Related Skeleton-Rearranged Abietanes

We report a bioinspired synthesis of microstegiol and biosynthetically related skeleton-rearranged abietane diterpenoids from commercially available carnosic acid. The strategy features a Wagner-Meerwein type methyl migration followed by several cascade transformations, enabling the divergent synthesis of five abietane diterpenoids, including viridoquinone, prattinin A, saprorthoquinone, microstegiol, and 1-deoxyviroxocin.

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.

Asymmetric Total synthesis of Asperones A and B through Organocatalyzed Quinone [5 + 2] Cycloaddition

The first asymmetric total synthesis of the anti-inflammatory polyketides asperones A (1) and B (2) has been accomplished. Key synthetic steps include a Diels-Alder and retro-Diels-Alder cascade to construct the poly substituted phenol, an Al-Salen-catalyzed asymmetric cyanosilylation to form the tertiary alcohol of gregatin A, and an organocatalyzed intermolecular [5 + 2] cycloaddition of p-quinone with electron-deficient alkenes to build the crucial [3.2.1] octane core of asperones A (1) and B (2).

Synthesis of drimanyl indole fragments of drimentine alkaloids and their antibacterial activities

Two types of drimanyl indole fragments of drimentine alkaloids were synthesized and evaluated their in vitro antibacterial activities using minimum inhibitory concentration. Analysis of structure-activity relationship against Ralstonia solanacearum revealed that fragment I exhibited superior inhibitory activity compared to fragment II. Notably, free NH of the indole motif was essential for antibacterial activity, while C12OH of the drimane skeleton was beneficial for enhancing the inhibitory effect. Compound 2, possessing these structural features, showed the highest activity to R. solanacearum among all the tested compounds with a MIC value of 8 µg/mL, indicating its potential as a promising lead for the development of novel antibiotics.

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.

Spokewise Total Syntheses of Four Erythrina Alkaloids and Telescoped Syntheses of Six Additional Alkaloids

Based on rich sulfur-involving chemical transformations, a novel spokewise synthetic strategy, a subclass of the collective strategies, has been developed to concisely synthesize four erythrina alkaloids through a single-step transformation from a common synthetic precursor. Moreover, six additional erythrina alkaloids have also been synthesized by subsequent 1-2 steps chemical transformations. The current synthetic approaches provide a valuable platform for collective total syntheses of erythrina alkaloids and pseudo-natural erythrina alkaloids.

Synthesis of the ABC Core of Daphniphyllum Alkaloids with a [5-6-7] Azatricyclic Scaffold via Ring Expansion of Azabicyclic and Azatricyclic Building Blocks

The [5-6-7] azatricyclic ABC core, found in several Daphniphyllum alkaloids, has been synthesized through a novel route involving ring expansion of a perhydroindolone to afford the AC ring system and a radical B ring closure as key steps. The level of functionalization of the reported octahydro-1,7-ethanocyclohepta[b]pyrroles suggests that they can serve as valuable building blocks in this alkaloid field. Also reported is the first synthesis of homomorphans by the ring enlargement of 2-azabicyclo[3.3.1]nonanes.

Asymmetric Synthesis and Biological Evaluation of Platensilin, Platensimycin, Platencin, and Their Analogs via a Bioinspired Skeletal Reconstruction Approach

Platensilin, platensimycin, and platencin are potent inhibitors of β-ketoacyl-acyl carrier protein synthase (FabF) in the bacterial and mammalian fatty acid synthesis system, presenting promising drug leads for both antibacterial and antidiabetic therapies. Herein, a bioinspired skeleton reconstruction approach is reported, which enables the unified synthesis of these three natural FabF inhibitors and their skeletally diverse analogs, all stemming from a common ent-pimarane core. The synthesis features a diastereoselective biocatalytic reduction and an intermolecular Diels-Alder reaction to prepare the common ent-pimarane core. From this intermediate, stereoselective Mn-catalyzed hydrogen atom-transfer hydrogenation and subsequent Cu-catalyzed carbenoid C-H insertion afford platensilin. Furthermore, the intramolecular Diels-Alder reaction succeeded by regioselective ring opening of the newly formed cyclopropane enables the construction of the bicyclo[3.2.1]-octane and bicyclo[2.2.2]-octane ring systems of platensimycin and platencin, respectively. This skeletal reconstruction approach of the ent-pimarane core facilitates the preparation of analogs bearing different polycyclic scaffolds. Among these analogs, the previously unexplored cyclopropyl analog 47 exhibits improved antibacterial activity (MIC80 = 0.0625 μg/mL) against S. aureus compared to platensimycin.

Controllable skeletal reorganizations in natural product synthesis

Covering: 2016 to 2023The synthetic chemistry community is always in pursuit of efficient routes to natural products. Among the many available general strategies, skeletal reorganization, which involves the formation, cleavage, and migration of C-C and C-heteroatom bonds, stands out as a particularly useful approach for the efficient assembly of molecular skeletons. In addition, it allows for late-stage modification of natural products for quick access to other family members or unnatural derivatives. This review summarizes efficient syntheses of steroid, terpenoid, and alkaloid natural products that have been achieved by means of this strategy in the past eight years. Our goal is to illustrate the strategy's potency and reveal the spectacular human ingenuity demonstrated in its use and development.

Navigating Excess Complexity: Total Synthesis of Daphenylline

Retrosynthetic analysis is a framework for designing synthetic routes to complex molecules that generally prioritizes disconnections which reduce molecular complexity. However, strict adherence to this principle can overlook pathways involving highly complex intermediates that can be easily prepared through powerful bond-forming transformations. Herein, we demonstrate this tactic of generating excess complexity, followed by strategic bond-cleavage, as a highly effective approach for the 11-step total synthesis of the Daphniphyllum alkaloid daphenylline. To implement this strategy, we accessed a bicyclo[4.1.0]heptane core through a dearomative Buchner cycloaddition, which enabled construction of the seven-membered ring after C-C bond cleavage. Installation of the synthetically challenging quaternary stereocenter methyl group was achieved through a thia-Paternò-Büchi [2 + 2] photocycloaddition followed by stereospecific thietane reduction, further illustrating how building excess complexity can enable desired synthetic outcomes after strategic bond-breaking events. This strategy leveraging bond cleavage transformations should serve as a complement to traditional bond-forming, complexity-generating synthetic strategies.

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.