A Unified Strategy To Construct the Tetracyclic Ring of Calyciphylline A Alkaloids: Total Synthesis of Himalensine A

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 Synthesis of Four Classes of Daphniphyllum Alkaloids

The macrodaphniphyllamine-type, calyciphylline A-type, daphnilongeranin A-type, and daphnicyclidin D-type alkaloids are four structurally related classes of Daphniphyllum alkaloids. On the basis of a systematic analysis of the biogenetic network of these classes, we developed synthetic strategies centered on the C4-N and C1-C8 bonds of calyciphylline A, which took full advantage of the suitable substrates, reactions, and pathways that are altered from their counterparts in the postulated biogenetic network. Through this generalized biomimetic approach, we achieved the first synthesis of 14 Daphniphyllum alkaloids from the four subfamilies.

Streamlined Strategy for Scalable and Enantioselective Total Syntheses of the Eburnane Alkaloids

A general, concise, and efficient strategy for the enantioselective synthesis of the eburnane alkaloid family of natural products is disclosed. Specifically, 13 members of the natural product family were prepared from commercially available and inexpensive starting materials. The brevity and modularity of the route are largely on account of a two-phase synthesis logic and a key catalytic enantioconvergent cross-coupling to establish the C20 stereogenic center. The strategies described here are expected to facilitate in-depth biological studies and provide access to new anticancer eburnane analogues.

Simmons-Smith Cyclopropanation: A Multifaceted Synthetic Protocol toward the Synthesis of Natural Products and Drugs: A Review

Simmons-Smith cyclopropanation is a widely used reaction in organic synthesis for stereospecific conversion of alkenes into cyclopropane. The utility of this reaction can be realized by the fact that the cyclopropane motif is a privileged synthetic intermediate and a core structural unit of many biologically active natural compounds such as terpenoids, alkaloids, nucleosides, amino acids, fatty acids, polyketides and drugs. The modified form of Simmons-Smith cyclopropanation involves the employment of Et2Zn and CH2I2 (Furukawa reagent) toward the total synthesis of a variety of structurally complex natural products that possess broad range of biological activities including anticancer, antimicrobial and antiviral activities. This review aims to provide an intriguing glimpse of the Furukawa-modified Simmons-Smith cyclopropanation, within the year range of 2005 to 2022.

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.

Asymmetric Total Synthesis of Hasubanan Alkaloids: Periglaucines A-C, N,O-Dimethyloxostephine and Oxostephabenine

We report herein the asymmetric total synthesis of periglaucines A-C, N,O-dimethyloxostephine and oxostephabenine. The key strategies used include: 1) a Rh^I -catalyzed regio- and diastereoselective Hayashi-Miyaura reaction to connect two necessary fragments; 2) an intramolecular photoenolization/Diels-Alder (PEDA) reaction to construct the highly functionalized tricyclic core skeleton bearing a quaternary center; 3) a bio-inspired intramolecular Michael addition and transannular acetalization to generate the aza[4.4.3]propellane and the tetrahydrofuran ring.

Total Synthesis of Yuzurine-type Alkaloid Daphgraciline

The first total synthesis of daphgraciline has been achieved, which also represents the first example of the synthesis of Daphniphyllum yuzurine-type alkaloids (∼50 members). The unique bridged azabicyclo[4.3.1] ring system in the yuzurine-type subfamily was efficiently and diastereoselectively assembled via a mild type II [5+2] cycloaddition for the first time. The compact tetracyclic [6-7-5-5] skeleton was installed efficiently via an intramolecular Diels-Alder reaction, followed by a benzilic acid-type rearrangement. The synthetically challenging spiro tetrahydropyran moiety in the final product was installed diastereoselectively via a Ti^III-mediated reductive epoxide coupling reaction. Potential access to enantioenriched daphgraciline is presented.

Total Syntheses of Daphnezomine L-type and Secodaphniphylline-type Daphniphyllum Alkaloids via Late-Stage C-N Bond Activation

Here, we report the first total syntheses of daphnezomine L-type alkaloids daphnezomine L methyl ester and calyciphylline K via late-stage C-N bond activation. The first synthesis of secodaphniphylline-type alkaloid caldaphnidine D was also achieved via a similar strategy. Other key transformations employed in our synthesis were a facile vicinal diol olefination and an efficient radical cyclization cascade. Biological studies indicated two synthetic compounds possess promising neuroprotective activity.

Total Syntheses of Calyciphylline A-Type Alkaloids (-)-10-Deoxydaphnipaxianine A, (+)-Daphlongamine E and (+)-Calyciphylline R via Late-Stage Divinyl Carbinol Rearrangements

Among the famous Daphniphyllum alkaloids family, the calyciphylline A-type subfamily has triggered particular interest from the organic synthesis community in recent years. Here, we report divergent total syntheses of three calyciphylline A-type alkaloids, namely, (-)-10-deoxydaphnipaxianine A, (+)-daphlongamine E, and (+)-calyciphylline R. Our work highlights an efficient, divergent strategy via late-stage divinyl carbinol rearrangements, including an unprecedented oxidative Nazarov electrocyclization using an unfunctionalized tertiary divinyl carbinol and an unusual allylic alcohol rearrangement. A highly efficient "donor-acceptor" platinum catalyst was used for a critical nitrile hydration step. Moreover, the power of selective amide reductions has also been showcased by novel and classic tactics.

Synthesis of the ABC Ring of Calyciphylline A-Type Alkaloids by a Stereocontrolled Aldol Cyclization: Formal Synthesis of (±)-Himalensine A

A synthetic approach to a functionalized ABC-tricyclic framework of calyciphilline A-type alkaloids, a building block toward this class of alkaloids, is reported. The key synthetic steps involve a radical cyclization to form the hydroindole system and piperidine ring closure through a stereocontrolled aldol cyclization. The resulting alcohol allows the methyl group to be installed in the bowl-shaped azatricyclic structure; this crucial reaction takes place with configuration retention. The synthesis of azatricyclic compound I constitutes a formal synthesis of himalensine A.

Concise Enantioselective Total Synthesis of Daphenylline Enabled by an Intramolecular Oxidative Dearomatization

Daphenylline is a structurally unique member of the triterpenoid Daphniphyllum natural alkaloids, which exhibit intriguing biological activities. Six total syntheses have been reported, five of which utilize aromatization approaches. Herein, we report a concise protecting-group-free total synthesis by means of a novel intramolecular oxidative dearomatization reaction, which concurrently generates the critical seven-membered ring and the quaternary-containing vicinal stereocenters. Other notable transformations include a tandem reductive amination/amidation double cyclization reaction, to assemble the cage-like architecture, and installation of the other two chiral stereocenters via a highly enantioselective rhodium-catalyzed challenging hydrogenation of the diene intermediate (90% e.e.) and an unprecedented remote acid-directed Mukaiyama-Michael reaction of the complex benzofused cyclohexanone (13:1 d.r.).

Five-membered carbocycle construction in the synthesis of Daphniphyllum alkaloids: recent strategic and methodological advances

In this review article, we summarize novel or non-standard strategies and methods for the five-membered carbocycle construction in recent Daphniphyllum alkaloid synthesis.

Studies toward the Total Syntheses of Calyciphylline D-Type Daphniphyllum Alkaloids

An efficient construction of an aza-[5.7.6.5] tetracyclic core structure of calyciphylline D-type Daphniphyllum alkaloids has been achieved. The synthetic route features a diastereoselective cyclopropanation, efficient construction of the core bridged 8-aza-[3.2.1]octane skeleton through a [3 + 2] IMCC strategy, oxidative dearomatization of phenol, and gram-scale preparation in each step.

A General Strategy for the Construction of Calyciphylline A-Type Alkaloids: Divergent Total Syntheses of (-)-Daphenylline and (-)-Himalensine A

An efficient general strategy for the synthesis of the Daphniphyllum alkaloids via the rapid construction of a common core intermediate has been established, based on which a divergent total synthesis of (-)-daphenylline and (-)-himalensine A has been accomplished in 16 and 19 steps, respectively. The present work features an enantioselective Mg(ClO₄ )₂ -catalyzed intramolecular amidocyclization to construct the aza-bridged core structure; a Cu-catalyzed intramolecular cyclopropanation and subsequent phosphine-catalyzed Cope-type rearrangement to furnish the himalensine A scaffold; and a one-pot Diels-Alder/aromatization method to assemble the aromatic skeleton of daphenylline.

Stereoselective Synthesis of the Core Structures of Pyrrocidines and Wortmannines through the Excited-State Nazarov Reactions

The reaction conditions and scope of the excited-state Nazarov reaction of dicyclicvinyl ketones were studied. The stereochemistry of this electrocyclization is consistent with the mechanism of the pericyclic reaction and Woodward-Hoffmann rule. UV-light-promoted excited-state Nazarov reactions gave hydrofluorenones bearing a syn-cis configuration via a disrotatory cyclization. The core tricyclic hydrofluorenones of pyrrocidines and wortmannines were constructed via the excited-state Nazarov reactions, which demonstrated their synthetic potential in complex natural product total synthesis.

The chemistry of Daphniphyllum alkaloids

The triterpenoids Daphniphyllum alkaloids share the unique fused hexacyclic ring framework are isolated from the genus Daphniphyllum. These natural products possess comprehensive biological activities and exhibit excellent potential medicinal appliment. This review covers the reported isolation studies and biological activities of Daphniphyllum alkaloids spanning the period from 1966 to the beginning of 2020, In the meantime, the total synthesis of Daphniphyllum alkaloids will be emphatically summarized for supplement over this review series.

Total Synthesis of Daphniphyllum Alkaloids: From Bicycles to Diversified Caged Structures

Native to the Asia-Pacific region and widely applied in traditional Chinese medicine, the genus Daphniphyllum has produced over 330 known Daphniphyllum alkaloids. Investigations into these alkaloids have shown an exceptional range of interesting bioactivities. Challenging and caged polycyclic architectures and the promising biological profiles make Daphniphyllum alkaloids intriguing synthetic targets. Based on their backbones, these alkaloids can be categorized into 13-35 structurally distinct subfamilies. In addition to our work, almost 30 impressive total syntheses of Daphniphyllum alkaloids from seven subfamilies, namely, daphniphylline-type, secodaphniphylline-type, daphnilactone A-type, bukittinggine-type, daphmanidin A-type, calyciphylline A-type, and calyciphylline B-type alkaloids, have been reported by 11 research groups. However, many Daphniphyllum alkaloid subfamilies remain inaccessible by chemical synthesis.In this Account, we summarize our recent endeavors in the total synthesis of Daphniphyllum alkaloids commencing from simple chiral bicyclic synthons. Daphniphyllum alkaloids with diversified skeletons from four different subfamilies, namely, calyciphylline A-type, daphnezomine A-type, bukittinggine-type, and yuzurimine-type alkaloids, have been achieved. Furthermore, the tricyclic core structure of daphniglaucin C-type alkaloids daphnimacropodines was also synthesized. First, we describe a 14-step synthesis of calyciphylline A-type alkaloid (-)-himalensine A, which features a mild Cu-mediated nitrile hydration, an intramolecular Heck reaction to assemble the pivotal 2-azabicyclo[3.3.1]nonane moiety, and a Meinwald rearrangement to introduce the critical oxidative state into the skeleton. We then introduce the synthesis of daphnezomine A-type alkaloid dapholdhamine B, which possesses a unique aza-adamantane core. This target molecule was fabricated using key reactions including Huang's amide-activation-annulation. An unexpected radical detosylation during the synthesis of dapholdhamine B further inspired an ambitious radical cyclization cascade strategy, which eventually led to an efficient total synthesis of bukittinggine-type alkaloid (-)-caldaphnidine O. This highly chemo-, regio-, and stereoselective radical reaction cascade also shed light on the synthetic strategy of other alkaloids with caged structures. We next describe the first total synthesis of yuzurimine-type alkaloid (+)-caldaphnidine J. The key steps in our approach include a Pd-catalyzed regioselective hydroformylation and a novel Swern oxidation/ketene dithioacetal Prins reaction cascade. The work has achieved the first synthesis of a member of the largest subfamily of Daphniphyllum alkaloids. Finally, we show our efforts toward the total synthesis of daphniglaucin C-type alkaloids. Overall, we hope that the interesting strategies and synthetic methods demonstrated in our efforts could inspire a wide variety of additional applications to natural product synthesis.

Total Synthesis of (-)-Daphnezomines A and B

Daphnezomines A and B are structurally unusual Daphniphyllum alkaloids that contain a unique aza-adamantane core skeleton. Herein, a modular approach to these alkaloids is presented that exploits a diverse array of reaction strategies. Commencing from a chiral pool terpene-(S)-carvone, the azabicyclo[3.3.1]nonane backbone, which occurs widely in Daphniphyllum alkaloids, was easily accessed through a Sharpless allylic amination and a palladium-catalyzed oxidative cyclization. A protecting group enabled a stereoselective B-alkyl Suzuki-Miyaura coupling sequence and an Fe-mediated hydrogen atom transfer (HAT)-based radical cyclization were then applied to construct C6 and C8 stereocenters. A final epimer locking strategy enabled the assembly of the highly congested aza-adamantane core, thereby achieving the first total synthesis of (-)-daphnezomines A and B in 14 steps.

Asymmetric total synthesis of yuzurimine-type Daphniphyllum alkaloid (+)-caldaphnidine J

Ever since Hirata’s report of yuzurimine in 1966, nearly fifty yuzurimine-type alkaloids have been isolated, which formed the largest subfamily of the Daphniphyllum alkaloids. Despite extensive synthetic studies towards this synthetically challenging and biologically intriguing family, no total synthesis of any yuzurimine-type alkaloids has been achieved to date. Here, the first enantioselective total synthesis of (+)-caldaphnidine J, a highly complex yuzurimine-type Daphniphyllum alkaloid, is described. Key transformations of this approach include a highly regioselective Pd-catalyzed hydroformylation, a samarium(II)-mediated pinacol coupling, and a one-pot Swern oxidation/ketene dithioacetal Prins reaction. Our approach paves the way for the synthesis of other yuzurimine-type alkaloids and related natural products.

Despite being known for more than 50 years, yuzurimine-type alkaloids have not been accessed by total synthesis. Here, the authors report the first enantioselective total synthesis of (+)-Caldaphnidine J, a highly complex yuzurimine-type Daphniphyllum alkaloid.

Daphnillonins A and B: Alkaloids Representing Two Unknown Carbon Skeletons from Daphniphyllum longeracemosum

Two highly rearranged daphniphyllum alkaloids, daphnillonins A (1) and B (2), were isolated from Daphniphyllum longeracemosum and structurally characterized by a combination of diverse methods, including the calculation of electronic circular dichroism. Compound 1 possesses an unprecedented carbon architecture with a very unique 8-methyl-6-azabicyclo[3.2.1]octane moiety, and compound 2 represents a new carbon skeleton with an uncommon 7/6/5/7/5/5-fused ring system. The biosynthetic pathways for the two alkaloids were proposed with the concurrent major alkaloids as the precursors.

Construction of the Tetracyclic Core of Calyciphylline B-Type Daphniphyllum Alkaloids

A double cyclization strategy was developed to construct the common tetracyclic core of calyciphylline B-type alkaloids. Key features of the synthesis included asymmetric Evans alkylation, ring-closing metathesis reaction, intermolecular amidation, intramolecular aza-Michael addition, and aldol condensation reactions. This strategy may be applied to the total syntheses of this type of natural product.

Toward the Synthesis of Yuzurimine-Type Alkaloids: Stereoselective Construction of the Heterocyclic Portions of Deoxyyuzurimine and Macrodaphnine

The heterocyclic portions of yuzurimine-type alkaloids, such as deoxyyuzurimine and macrodaphnine, were synthesized by using a stereoselective hydroboration-oxidation reaction to install the C20 methyl group, the intramolecular Mitsunobu reaction to construct the E-ring portion, and the intramolecular S_N2 reaction to construct the F-ring portion as key steps.

Enantioselective Total Synthesis of (-)-Caldaphnidine O via a Radical Cyclization Cascade

The synthetically challenging, diverse chemical skeletons and promising biological profiles of the Daphniphyllum alkaloids have generated intense interest from the synthetic chemistry community. Herein, the first and enantioselective total synthesis of (-)-caldaphnidine O, a complex bukittinggine-type Daphniphyllum alkaloid, is described. The key transformations in this concise approach included an intramolecular aza-Michael addition, a ring expansion reaction sequence, a Sm(II)/Fe(III)-mediated Kagan-Molander coupling, and the rapid formation of the entire hexacyclic ring skeleton of the target molecule via a radical cyclization cascade reaction, which was inspired by an unexpected radical detosylation observed in our recent dapholdhamine B synthesis.

Synthesis of the Core Structure of Daphnimacropodines

Daphniphyllum alkaloids daphnimacropodines A-C possess a highly congested ring system and share a common tetracyclic ring skeleton. To access the challenging chemical structure of daphnimacropodines, a divergent synthetic approach toward their total synthesis is described. A stereoselective synthesis of the core structure of daphnimacropodines has been achieved from a simple diketone building block. Our approach features an intramolecular carbamate aza-Michael addition and a hydropyrrole synthesis via a Au-catalyzed alkyne hydration followed by an aldol condensation, whereas all the other attempts failed.

Exploration of 1,3-dipolar cycloaddition reactions to construct the core skeleton of Calyciphylline A-type alkaloids

Nitrone induced 1,3-dipolar [3 + 2] cycloadditions were studied to construct the core structure of Calyciphylline A-type Daphniphyllum alkaloids.