Efficient synthetic access to the hetisine C20-diterpenoid alkaloids. A concise synthesis of nominine via oxidoisoquinolinium-1,3-dipolar and dienamine-Diels-Alder cycloadditions

Enantioselective total synthesis of atisane diterpenoids: (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one

Enantioselective total synthesis of (+)-sapinsigin H, (+)-agallochaol C, and (+)-16α, 17-dihydroxy-atisan-3-one has been accomplished starting from enantiopure Wieland-Miescher ketone. Key features of the syntheses include a benzannulation step to construct the tricyclic core, an oxidative dearomatization step to generate the diene, and a Diels-Alder reaction with ethylene gas to establish the bicyclo[2.2.2]octane framework. Efficient late-stage functionalisation of the A-ring by aerobic oxidation and Baeyer-Villiger oxidation completed the atisane target molecules.

Catalytic Asymmetric Total Synthesis of (-)-Garryine via an Enantioselective Heck Reaction

The first asymmetric total synthesis of the hexacyclic veatchine-type C20-diterpenoid alkaloid (-)-garryine is presented. Key steps include a Pd-catalyzed enantioselective Heck reaction, a radical cyclization, and a photoinduced C-H activation/oxazolidine formation sequence. Of note, a highly enantioselective Heck reaction developed in this work provides efficient access to 6/6/6 tricyclic compounds, in particular, containing a C19-functionalitiy, which is useful for diverse transformations.

Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

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.

Sulfur-Mediated Reactions through Sulfonium Salts and Ylides

This Account discusses several new reaction methods developed in our group that utilize sulfur-mediated reactions through sulfonium salts and ylides, highlighting the interplay of rational design and serendipity. Our initial goal was to convert aliphatic C–H bonds into C–C bonds site-selectively, and without the use of transition-metal catalysts. While a proof-of-concept has been achieved, this target is far from being ideally realized. The unexpected discovery of an anti-Markovnikov rearrangement and subsequent studies on difunctionalization of alkynes were much more straightforward, and eventually led to the new possibility of asymmetric N–H insertion of sulfonium ylides through Brønsted acid catalysis.

1 Introduction

2 Allylic/Propargylic C–H Functionalization

3 Anti-Markovnikov Rearrangement

4 Difunctionalization of Alkynes

5 Asymmetric N–H Insertion of Sulfonium Ylides

6 Conclusion

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.

Comparison of (5 + 2) Cycloadditions Involving Oxidopyrylium and Oxidopyridinium Ions: Relative Reactivities

A variety of (5 + 2) cycloaddition reactions involving oxidopyridinium and oxidopyrylium zwitterions are compared to investigate the effects of nitrogen-for-oxygen substitution on reactivity. Activation barriers for nitrogen-containing systems are predicted to be larger than those for analogous oxygen-containing systems. Correlations between barrier heights and synchronicity of C-C bond formation, changes to aromaticity, reactant distortion, and interaction energies between zwitterions and alkenes were assessed, leading to the conclusion that reactivity depends more on distortion effects (including aromaticity loss) than on interaction effects (such as those associated with highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interactions).

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.

Intramolecular Pyridinium Oxide Cycloadditions: Systematic Study of Substitution, Diastereoselectivity, and Regioselectivity

Intramolecular pyridinium oxide cycloadditions form complex polycyclic nitrogenous architectures. The diastereoselectivity and regioselectivity of pyridinium oxide cycloadditions was systematically investigated for the first time using complex substrates. Predictably high levels of diastereoselectivity and regioselectivity are observed, which can be attributed to minimization of steric (syn-pentane) and torsional strain in the products. The reaction is reversible under the reaction conditions, and it is stereospecific with respect to the dipolarophile geometry.

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.

The Future of Retrosynthesis and Synthetic Planning: Algorithmic, Humanistic or the Interplay?

The practice of deploying and teaching retrosynthesis is on the cusp of considerable change, which in turn forces practitioners and educators to contemplate whether this impending change will advance or erode the efficiency and elegance of organic synthesis in the future. A short treatise is presented herein that covers the concept of retrosynthesis, along with exemplified methods and theories, and an attempt to comprehend the impact of artificial intelligence in an era when freely and commercially available retrosynthetic and forward synthesis planning programs are increasingly prevalent. Will the computer ever compete with human retrosynthetic design and the art of organic synthesis?

Shaping Molecular Landscapes: Recent Advances, Opportunities, and Challenges in Dearomatization

Dearomatization is a fundamental chemical transformation, and it underlies some of the most efficient tactics for generating three-dimensional complexity from basic two-dimensional precursors. The dearomative toolbox, once restricted to only a handful of reactions, has begun to grow more sophisticated as novel methods are added, introducing more functionality under milder conditions and with more control over chemo-, regio-, and stereoselectivity than ever before. Over the past two decades, major developments in dearomative processes have bolstered significant total-synthesis endeavors and greatly expanded the scope and complexity of chemical building blocks accessible from feedstock arenes. In this Perspective, we highlight some of the recent advances and key challenges that remain in this vibrant area of organic chemistry.

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.

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.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

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.

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.

A practical deca-gram scale ring expansion of (R)-(−)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau–Demjanov reaction.