Ongoing Pursuit of Diterpenoid Alkaloids: A Synthetic View

Iridium Acylnitrenoid-Initiated Biomimetic Cascade Cyclizations: Stereodefined Access to Polycyclic δ-Lactams

Ring-fused azacyclic compounds are important building units in the synthesis of biorelevant natural products, pharmaceutical agents, and molecular materials. Herein, we present a new approach to these condensed azacycles by a biomimetic cascade cyclization of arylalkenyl dioxazolones. This cascade reaction was found to proceed with excellent stereoselectivity and a high functional group tolerance. The substrate scope of arylalkenyl dioxazolones turned out to be highly flexible and extendable to additional terminating subunits, such as heteroaryl and alkynyl moieties. This biomimetic cyclization was elucidated to be initiated by an intramolecular transfer of the in situ generated electrophilic Ir-acylnitrenoid to the tethered olefinic double bond, leading to a key N-acylaziridine intermediate, which is in turn reacted with pendant (hetero)arenes or alkynes in a highly regio- and stereoselective manner to produce ring-fused azacyclic compounds.

Total Synthesis of Vilmoraconitine

Vilmoraconitine belongs to one of the most complex skeleton types in the C19-diterpenoid alkaloids, which architecturally features an unprecedented heptacyclic core possessing a rigid cyclopropane unit. Here, we report the first total synthesis of vilmoraconitine relying on strategic use of efficient ring-forming reactions. Key steps include an oxidative dearomatization-induced Diels-Alder cycloaddition, a hydrodealkenylative fragmentation/Mannich sequence, and an intramolecular Diels-Alder cycloaddition.

Total Synthesis of Puberuline C

Puberuline C (1) is an architecturally complex C₁₉-diterpenoid alkaloid with a unique ring fusion pattern. The 6/7/5/6/6/6-membered rings (ABCDEF-rings) contain one tertiary amine and six oxygen functionalities, and possess 12 contiguously aligned stereocenters, three of which are quaternary. These structural features of 1 make its chemical construction exceptionally challenging. Here, we disclose the first total synthesis of 1. The synthesis was accomplished from 2-cyclohexenone (9) by integrating radical cascade and Mukaiyama aldol reactions as the key transformations. A double Mannich reaction fused the A- and E-rings, and Sonogashira coupling attached the C-ring, efficiently leading to ACE-rings with the requisite 19 carbons of 1. The chemically stable tertiary chloride of the ACE-ring structure was then transformed to the corresponding bridgehead radical, which participated in the simultaneous cyclization of the B- and F-rings via a highly organized radical cascade process. This unusual step installed five contiguous stereocenters, including two quaternary carbons, without damaging the preexisting multiple polar functionalities. Subsequently, the intramolecular Mukaiyama aldol reaction between silyl enol ether and acetal was realized by applying a combination of SnCl₄ and ZnCl₂, forging the last remaining D-ring of the hexacycle. Finally, 3 was elaborated into 1 through regio- and stereoselective functionalizations of the BCD-rings. Our novel radical-based strategy achieved the total synthesis of 1 in 32 total steps from simple 9, demonstrating the power of the radical cascade reaction to streamline the assembly of highly complex molecules.

Divergent Total Syntheses of Napelline-Type C20-Diterpenoid Alkaloids: (-)-Napelline, (+)-Dehydronapelline, (-)-Songorine, (-)-Songoramine, (-)-Acoapetaldine D, and (-)-Liangshanone

The napelline-type alkaloids possess an azabicyclo[3.2.1]octane moiety and an ent-kaurane-type tetracyclic skeleton (6/6/6/5) along with varied oxidation patterns embedded in the compact hexacyclic framework. Herein, we disclose a divergent entry to napelline-type alkaloids that hinges on convergent assembly of the ent-kaurane core using a diastereoselective intermolecular Cu-mediated conjugate addition and subsequent intramolecular Michael addition reaction as well as rapid construction of the azabicyclo[3.2.1]octane motif via an intramolecular Mannich cyclization. The power of this strategy has been demonstrated through efficient asymmetric total syntheses of eight napelline-type alkaloids, including (-)-napelline, (-)-12-epi-napelline, (+)-dehydronapelline, (+)-12-epi-dehydronapelline, (-)-songorine, (-)-songoramine, (-)-acoapetaldine D, and (-)-liangshanone.

The diterpenoid alkaloids

The diterpenoid alkaloids are a family of extremely important natural products that have long been a research hotspot due to their myriad of intricate structures and diverse biological properties. This chapter systematically summarizes the past 11 years (2009-2019) of studies on the diterpenoid alkaloids, including the "so-called" atypical ones, covering the classification and biogenetic relationships, phytochemistry together with 444 new alkaloids covering 32 novel skeletons and the corrected structures, chemical reactions including conversion toward toxoids, synthetic studies, as well as biological activities. It should be noted that the synthetic studies, especially the total syntheses of various diterpenoid alkaloids, are for the first time reviewed in this treatise. This chapter, in combination with our four previous reviews in volumes 42, 59, 67, and 69, will present to the readers a more completed and updated profile of the diterpenoid alkaloids.

Asymmetric Total Syntheses of (+)-Davisinol and (+)-18-Benzoyldavisinol: A HAT-Initiated Transannular Redox Radical Approach

The first and asymmetric total syntheses of two C11-oxygenated hetisine-type diterpenoid alkaloids, namely, (+)-davisinol and (+)-18-benzoyldavisinol, is described. The concise synthetic approach features a HAT-initiated transannular redox radical cyclization, an ODI-Diels-Alder cycloaddition, and an acylative kinetic resolution. By incorporating an efficient late-stage assembly of the azabicycle, our strategy would streamline the synthetic design of C₂₀-diterpenoid alkaloids and pave the way for their modular syntheses.

Asymmetric Total Synthesis of Hetidine-Type C20-Diterpenoid Alkaloids: (+)-Talassimidine and (+)-Talassamine

Here, we report the first asymmetric total synthesis of (+)-talassimidine and (+)-talassamine, two hetidine-type C₂₀-diterpenoid alkaloids. A highly regio- and diastereoselective 1,3-dipolar cycloaddition of an azomethine ylide yielded a chiral tetracyclic intermediate in high enantiopurity, thus providing the structural basis for asymmetric assembly of the hexacyclic hetidine skeleton. In this key step, the introduction of a single chiral center induces four new continuous chiral centers. Another key transformation is the dearomative cyclopropanation of the benzene ring and subsequent S_N2-like ring opening of the resultant cyclopropane ring with water as a nucleophile, which not only establishes the B ring but also precisely installs the difficult-to-achieve equatorial C7-OH group.

Synthesis of Three-Dimensionally Fascinating Diterpenoid Alkaloids and Related Diterpenes

Three-dimensional cage-like natural products represent astounding and long-term challenges in the research endeavors of total synthesis. A central issue that synthetic chemists need to address lies in how to efficiently construct the polycyclic frameworks as well as to install the requisite substituent groups. The diterpenoid alkaloids that biogenetically originate from amination of diterpenes and diversify through late-stage skeletal reorganization belong to such a natural product category. As the characteristic components of the Aconitum and Delphinium species, these molecules display a rich array of biological activities, some of which are used as clinical drugs. More strikingly, their intricate and beautiful architectures have rendered the diterpenoid alkaloids elusive targets in the synthetic community. The successful preparation of these intriguing compounds relies on the development of innovative synthetic strategies.Our laboratory has explored the total synthesis of a variety of diterpenoid alkaloids and their biogenetically related diterpenes over the past decade. In doing so, we have accessed 6 different types of skeletons (atisine-, denudatine-, arcutane-, arcutine-, napelline-, and hetidine-type) and achieved the total synthesis of 6 natural products (isoazitine, dihydroajaconine, gymnandine, atropurpuran, arcutinine, and liangshanone). Strategically, an oxidative dearomatization/Diels-Alder (OD/DA) cycloaddition sequence was widely employed in our synthesis to form the ubiquitous [2.2.2]-bicyclic ring unit and its related ring-distorted derivatives in these complex target molecules. This protocol, in combination with additional bond-forming key steps, allowed us to prepare the corresponding polycyclic alkaloids and a biogenetically associated diterpene. For example, bioinspired C-H activation, aza-pinacol, and aza-Prins cyclizations were used toward a unified approach to the atisine-, denudatine-, and hetidine-type alkaloids via ajaconine intermediates in our first work. To pursue the synthesis of atropurpuran and related arcutine alkaloids, we harnessed a ketyl-olefin radical cyclization to assemble the carbocycle and an aza-Wacker cyclization to construct the unusual pyrrolidine ring. Furthermore, a one-pot alkene cleavage/Mannich cyclization tactic, sequential Robinson annulation, and intramolecular aldol addition were developed, which facilitated the formation of the napelline alkaloid scaffold and the first total synthesis of liangshanone. Finally, the utility of the Mannich cyclization and enyne cycloisomerization reactions allowed for access to the highly functionalized A/E and C/D ring fragments of aconitine (regarded as the "Holy Grail" of diterpenoid alkaloids). This Account provides insight into our synthetic designs and approaches used toward the synthesis of diterpenoid alkaloids and relevant diterpenes. These endeavors lay a foundation for uncovering the biological profiles of associated molecules and also serve as a reference for preparing other three-dimensionally fascinating natural products.

Synthesis of tricyclo[7.2.1.09,10]dodecan-11-one core ring systems of norditerpenoid alkaloids and racemulosine

A synthetic strategy involving [2 + 2] enyne cycloaddition and pinacol rearrangement to construct common BCD ring systems of norditerpenoid alkaloids and racemulosine was developed.

Total Synthesis of Talatisamine

Talatisamine (1) is a member of the C₁₉ -diterpenoid alkaloid family, and exhibits K⁺ channel inhibitory and antiarrhythmic activities. The formidable synthetic challenge that 1 presents is due to its highly oxidized and intricately fused hexacyclic 6/7/5/6/6/5-membered-ring structure (ABCDEF-ring) with 12 contiguous stereocenters. Here we report an efficient synthetic route to 1 by the assembly of two structurally simple fragments, chiral 6/6-membered AE-ring 7 and aromatic 6-membered D-ring 6. AE-ring 7 was constructed from 2-cyclohexenone (8) through fusing an N-ethylpiperidine ring by a double Mannich reaction. After coupling 6 with 7, an oxidative dearomatization/Diels-Alder reaction sequence generated fused pentacycle 4 b. The newly formed 6/6-membered ring system was then stereospecifically reorganized into the 7/5-membered BC-ring of 3 via a Wagner-Meerwein rearrangement. Finally, Hg(OAc)₂ induced an oxidative aza-Prins cyclization of 2, thereby forging the remaining 5-membered F-ring. The total synthesis of 1 was thus accomplished by optimizing and orchestrating 33 transformations from 8.

Asymmetric Total Synthesis of Arcutinidine, Arcutinine, and Arcutine

We have accomplished the asymmetric total synthesis of arcutinidine, arcutinine, and arcutine, three arcutine-type C20-diterpenoid alkaloids. A pentacyclic intermediate was rapidly assembled by using two Diels-Alder reactions. We developed a cascade sequence of Prins cyclization and Wagner-Meerwein rearrangement to construct the core of arcutinidine, which was then elaborated into an oxygenated pentacycle through a scalable route. Chemoselective reductive amination followed by spontaneous imine formation furnished the pyrroline motif in the final stage. We clarified the S configuration of the α-carbon of the acyl group within arcutine through chemical synthesis and crystallographic analysis.

Progress towards the synthesis of aconitine: construction of the AE fragment and attempts to access the pentacyclic core

We reported the first successful preparation of fully functionalized aconitine AE fragment and attempts to access the pentacyclic skeleton of aconitine via radical cascade.

A gold-triggered dearomative spirocarbocyclization/Diels–Alder reaction cascade towards diverse bridged N-heterocycles

An efficient chemo- and diastereoselective gold-triggered post-Ugi non-oxidativeortho-dearomative spirocarbocyclization/Diels–Alder reaction cascade sequence has been developed to deliver diverse bridged polycyclic N-heterocycles.

A Benzyne Insertion Approach to Hetisine-Type Diterpenoid Alkaloids: Synthesis of Cossonidine (Davisine)

The hetisine-type natural products exhibit one of the most complex carbon skeletons within the diterpenoid alkaloid family. The use of network analysis has enabled a synthesis strategy to access alkaloids in this class with hydroxylation on the A-ring. Key transformations include a benzyne acyl-alkylation to construct a key fused 6-7-6 tricycle, a chemoselective nitrile reduction, and sequential C-N bond formations using a reductive cyclization and a photochemical hydroamination to construct an embedded azabicycle. Our strategy should enable access to myriad natural and unnatural products within the hetisine-type.

A Unified Approach for the Assembly of Atisine- and Hetidine-type Diterpenoid Alkaloids: Total Syntheses of Azitine and the Proposed Structure of Navirine C

A tetracyclic dinitrile was synthesized in twelve steps from cyclohex-2-en-1-one by using a chelation-triggered conjugate addition to a γ-hydroxy-substituted α,β-unsaturated nitrile and an oxidative dearomatization/Diels-Alder cycloaddition cascade as the key steps. The first total synthesis of azitine (in 17 steps) was achieved through a simple reductive cyclization of this intermediate and subsequent transformations while the total synthesis of the proposed structure of navirine C (in 19 steps) was accomplished by a hydrogen-atom-transfer reaction of the tetracyclic dinitrile, Pd/C-catalyzed reductive cyclization, and subsequent functional group manipulation.

Recent advances in the intramolecular Mannich reaction in natural products total synthesis

This review focuses on selected applications of the intramolecular Mannich reaction as a key step in the total synthesis of natural products (2000–2017).

Construction of the highly oxidized bicyclo[3.2.1]octane CD ring system of aconitine via a late stage enyne cycloisomerization

An enyne cycloisomerization enables assembly of the highly oxidized bicyclo[3.2.1]octane CD rings of the complex diterpenoid alkaloid aconitine.

Stereoselective Total Synthesis of Hetisine-type C20 -Diterpenoid Alkaloids: Spirasine IV and XI

The first total synthesis of the architecturally complex hetisine-type heptacyclic C₂₀ -diterpenoid alkaloids (±)-spirasine IV and XI is reported. The A/F/G/C tetracyclic skeleton with the challenging N-C6 and C14-C20 linkages was efficiently constructed by an intramolecular azomethine-ylide-based 1,3-dipolar cycloaddition with unusual regioselectivity. SmI₂ -mediated free-radical addition to the arene moiety without prior dearomatization and a stereoselective intramolecular aldol reaction further enabled rapid access to the hetisine core, providing a bicyclo[2.2.2]octane ring with a new oxygen substitution pattern.

Magnesiate Addition/Ring-Expansion Strategy To Access the 6-7-6 Tricyclic Core of Hetisine-Type C20-Diterpenoid Alkaloids

A synthetic strategy to access the fused 6-7-6 tricyclic core of hetisine-type C20-diterpenoid alkaloids is reported. This strategy employs a Diels-Alder cycloaddition to assemble a fused bicyclic anhydride intermediate, which is elaborated to a vinyl lactone-acetal bearing an aromatic ring in five steps. Aromatic iodination is followed by magnesium-halogen exchange with a trialkyl magnesiate species, which undergoes intramolecular cyclization. Subsequent oxidation provides the desired 6-7-6 tricyclic diketoaldehyde, with carbonyl groups at all three positions for eventual C-N bond formation and subsequent elaboration.

Enabling syntheses of diterpenoid alkaloids and related diterpenes by an oxidative dearomatization/Diels–Alder cycloaddition strategy

Recent accomplishments in the total syntheses of diterpenoid alkaloids and their biosynthetically associated diterpenes have highly relied on an oxidative dearomatization/Diels–Alder cycloaddition strategy, and are highlighted in this article.

Syntheses of Denudatine Diterpenoid Alkaloids: Cochlearenine, N-Ethyl-1α-hydroxy-17-veratroyldictyzine, and Paniculamine

The denudatine-type diterpenoid alkaloids cochlearenine, N-ethyl-1α-hydroxy-17-veratroyldictyzine, and paniculamine have been synthesized for the first time (25, 26, and 26 steps from 16, respectively). These syntheses take advantage of a common intermediate (8) that we have previously employed in preparing aconitine-type natural products. The syntheses reported herein complete the realization of a unified strategy for the preparation of C20, C19, and C18 diterpenoid alkaloids.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Total synthesis of atropurpuran

Due to their architectural intricacy and biological significance, the synthesis of polycyclic diterpenes and their biogenetically related alkaloids have been the subject of considerable interest over the last few decades, with progress including the impressive synthesis of several elusive targets. Despite tremendous efforts, conquering the unique structural types of this large natural product family remains a long-term challenge. The arcutane diterpenes and related alkaloids, bearing a congested tetracyclo[5.3.3.0(4,9).0(4,12)]tridecane unit, are included in these unsolved enigmas. Here we report a concise approach to the construction of the core structure of these molecules and the first total synthesis of (±)-atropurpuran. Pivotal features of the synthesis include an oxidative dearomatization/intramolecular Diels-Alder cycloaddition cascade, sequential aldol and ketyl-olefin cyclizations to assemble the highly caged framework, and a chemoselective and stereoselective reduction to install the requisite allylic hydroxyl group in the target molecule.

Expeditious synthesis of the fused hexacycle of puberuline C via a radical-based cyclization/translocation/cyclization process

The fused 6/7/5/6/6/6-hexacyclic ring system of puberuline C was assembled in 18 steps from 2-(ethoxycarbonyl)cyclohexanone. After the azabicyclo[3.3.1]nonane derivative was sequentially coupled with propargyl magnesium bromide, 2-iodo cyclopentenone and allyl bromide, the pentacycle was constructed in a single step via a radical-based cyclization/translocation/cyclization process. The C11-bridgehead radical generated via C-Br homolysis participated in a 7-endo cyclization, and the 1,5-hydrogen translocation of the resultant radical was followed by transannular 6-exo cyclization to simultaneously realize the construction of the two rings and the introduction of the five contiguous stereocenters. The last 6-exo cyclization was induced by the Mukaiyama aldol reaction, and the C16-ketone was stereoselectively reduced by the action of SmI2/t-BuOH, leading for the first time to the synthesis of the entire hexacycle of puberuline C.