Unified Total Synthesis of 3-epi -Ryanodol, Cinnzeylanol, Cinncassiols A and B, and Structural Revision of Natural Ryanodol and Cinnacasol

Total Syntheses of Phorbol and 11 Tigliane Diterpenoids and Their Evaluation as HIV Latency-Reversing Agents

Tigliane diterpenoids possess exceptionally complex structures comprising common 5/7/6/3-membered ABCD-rings and disparate oxygen functionalities. While tiglianes display a wide range of biological activities, compounds with HIV latency-reversing activity can eliminate viral reservoirs, thereby serving as promising leads for new anti-HIV agents. Herein, we report collective total syntheses of phorbol (13) and 11 tiglianes 14-24 with various acylation patterns and oxidation states, and their evaluation as HIV latency-reversing agents. The syntheses were strategically divided into five stages to increase the structural complexity. First, our previously established sequence enabled the expeditious preparation of ABC-tricycle 9 in 15 steps. Second, hydroxylation of 9 and ring-contractive D-ring formation furnished phorbol (13). Third, site-selective attachment of two acyl groups to 13 produced four phorbol diesters 14-17. Fourth, the oxygen functionalities were regio- and stereoselectively installed to yield five tiglianes 18-22. Fifth, further oxidation to the most densely oxygenated acerifolin A (23) and tigilanol tiglate (24) was realized through organizing a 3D shape of the B-ring. Assessment of the HIV latency-reversing activities of the 12 tiglianes revealed seven tiglianes (14-17 and 22-24) with 20- to 300-fold improved efficacy compared with prostratin (12), a representative latency-reversing agent. Therefore, the robust synthetic routes to a variety of tiglianes with promising activities devised in this study provide opportunities for advancing HIV eradication strategies.

Chemistry and Bioactivity of the Genus Persea - A Review

This review provides the first comprehensive appraisal of bioactive compounds and their biological activities in Persea species from 1950 to 2023. Relevant articles from reputable databases, including PubMed, Web of Science, Science Direct and Google Scholar were collected, leading to the isolation of about 141 metabolite compounds, mainly flavonoids, terpenoids, fatty alcohols, lignoids, and γ-lactone derivatives. These compounds exhibit diverse biological activities, including insecticidal, antifeedant, nematicidal, antibacterial, antifungal, antiviral, cytotoxic, anti-inflammatory, and antioxidant properties. The review emphasizes the significant chemical and pharmacological potential of different Persea species, encouraging further research in various fields and medicine. Valuable insights into potential applications of Persea plants are provided.

Size-Induced Inversion of Selectivity in the Acylation of 1,2-Diols

Relative rates for the Lewis base‐catalyzed acylation of aryl‐substituted 1,2‐diols with anhydrides differing in size have been determined by turnover‐limited competition experiments and absolute kinetics measurements. Depending on the structure of the anhydride reagent, the secondary hydroxyl group of the 1,2‐diol reacts faster than the primary one. This preference towards the secondary hydroxyl group is boosted in the second acylation step from the monoesters to the diester through size and additional steric effects. In absolute terms the first acylation step is found to be up to 35 times faster than the second one for the primary alcohols due to neighboring group effects.

Aromatic and Aliphatic Apiuronides from the Bark of Cinnamomum cassia

Cinnamomum cassia Presl (Cinnamon) has been widely cultivated in the tropical or subtropical areas, such as Yunnan, Fujian, Guandong, and Hainan in China, as well as India, Vietnam, Thailand, and Malaysia. Four new glycosides bearing apiuronic acid (1, 4, 6, and 7) and their sodium or potassium salts (2, 3, and 5), together with 31 known compounds, were isolated from a hot water extract of the bark of C. cassia via repeated chromatography. The structures of the new compounds (1-7) were determined by NMR, IR, MS, and ICP-AES data and by acid hydrolysis and sugar analysis. This is the first report of the presence of apiuronic acid glycosides. Some of the isolates were evaluated for their analgesic effects on a neuropathic pain animal model induced by paclitaxel. Cinnzeylanol (8), cinnacaside (9), kelampayoside A (10), and syringaresinol (11) showed analgesic effects against paclitaxel-induced cold allodynia.

Synthesis of Complex Diterpenes: Strategies Guided by Oxidation Pattern Analysis

With complex molecular architectures, intriguing oxidation patterns, and wide-ranging biological activities, diterpene natural products have greatly impacted research in organic chemistry and drug discovery. Our laboratory has completed total syntheses of several highly oxidized diterpenes, including the ent-kauranoids maoecrystal Z, trichorabdal A, and longikaurin E; the antibiotic pleuromutilin; and the insecticides ryanodol, ryanodine, and perseanol. In this Account, we show how analysis of oxidation patterns and inherent functional group relationships can inform key C-C bond disconnections that greatly simplify the complexity of polycyclic structures and streamline their total syntheses. In articulating these concepts, we draw heavily from the approaches to synthetic strategy that were codified by Evans, Corey, Seebach, and others, based on the formalism that heteroatoms impose an alternating acceptor and donor reactivity pattern upon a carbon skeleton. We find these ideas particularly useful when considering oxidized diterpenes as synthetic targets.In the first part of the Account, we describe the use of reductive cyclizations as strategic tactics for building polycyclic systems with γ-hydroxyketone motifs. We have leveraged Sm-ketyl radical cyclizations as "reactivity umpolungs" to generate γ-hydroxyketones in our total syntheses of the Isodon ent-kauranoid diterpenes (-)-maoecrystal Z, (-)-longikaurin E, and (-)-trichorabdal A. Following this work, we identified the same γ-hydroxyketone pattern in the diterpene antibiotic (+)-pleuromutilin, which again inspired the use of a SmI₂-mediated reductive cyclization, this time to construct a bridging eight-membered ring. This collection of four total syntheses highlights how reductive cyclizations are particularly effective umpolung tactics when used to simultaneously form rings and introduce 1,4-dioxygenation patterns.In the second part of the Account, we detail the syntheses of the complex and highly oxidized ryanodane and isoryanodane diterpenes and present the oxidation pattern analysis that guided our synthetic designs. We first discuss our 15-step total synthesis of (+)-ryanodol, which incorporated five of the eight oxygen atoms in just two transformations: a dihydroxylation of (S)-pulegone and a SeO₂-mediated trioxidation of the A-ring cyclopentenone. This latter transformation gave rise to an independent investigation of SeO₂-mediated peroxidations of simple bicyclic cyclopent-2-en-1-ones. The syntheses of (+)-ryanodine and (+)-20-deoxyspiganthine are also presented, which required modified end-game strategies to selectively incorporate the key pyrrole-2-carboxylate ester. Finally, we describe our fragment coupling approach to prepare the isoryanodane diterpene (+)-perseanol. Using a similar oxidation pattern analysis to that developed in the synthesis of ryanodol, we again identified a two-stage strategy to install the five hydroxyl groups. This strategy was enabled by a Pd-mediated carbopalladation/carbonylation cascade and leveraged unexpected, emergent reactivity to sequence a series of late-stage oxidations.While each of the diterpene natural products discussed in this Account present unique synthetic questions, we hope that through their collective discussion, we provide a conceptual framework that condenses and summarizes the chemical knowledge we have learned and inspires future discourse and innovations in strategy design and methodology development.

The Total Synthesis of Diquinane-Containing Natural Products

Diquinane or bicyclo[3.3.0]octane is a conspicuous structural unit existing in the carbo-frameworks of a wide range of natural products such as alkaloids and terpenoids. These diquinane-containing molecules not merely exhibit intriguing architectures, but also showcase a broad spectrum of significant bioactivities, which draw widespread attention from the global synthetic community. During the past decade, with an aim to accomplish the total syntheses of such specified cornucopias of natural products, a variety of elegant strategies for construction of the diquinane ring system have been disclosed. In this Minireview, the achievements on this subject in the timeline from 2010 to June 2020 are demonstrated and it is discussed how the diquinane unit is strategically forged in the context of the specific target structure. In addition, impacts of the selected works to the field of natural product total synthesis is highlighted and the particular outlook of diquinane-containing natural product synthesis is provided.

Size-Driven Inversion of Selectivity in Esterification Reactions: Secondary Beat Primary Alcohols

Relative rates for the Lewis base-mediated acylation of secondary and primary alcohols carrying large aromatic side chains with anhydrides differing in size and electronic structure have been measured. While primary alcohols react faster than secondary ones in transformations with monosubstituted benzoic anhydride derivatives, relative reactivities are inverted in reactions with sterically biased 1-naphthyl anhydrides. Further analysis of reaction rates shows that increasing substrate size leads to an actual acceleration of the acylation process, the effect being larger for secondary as compared to primary alcohols. Computational results indicate that acylation rates are guided by noncovalent interactions (NCIs) between the catalyst ring system and the DED substituents in the alcohol and anhydride reactants. Thereby stronger NCIs are formed for secondary alcohols than for primary alcohols.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Alkane-, alkene-, alkyne-γ-lactones and ryanodane diterpenes from aeroponically grown Persea indica roots

This work presents the study of the roots of the Macaronesian paleoendemism Persea indica (L.) Spreng. The root biomass of this protected tree species has been produced by soil-less aeroponic culture under controlled environment. This system has important advantages over traditional plant production techniques because it provides opportunities to optimize the yield of metabolite production under well-controlled conditions, thereby facilitating commercial-scale production of bioactive compounds. Thus, for the first time a study of this type has permitted the isolation from the roots of seven undescribed dextrorotatory lactones: the alkane-γ-lactones (+)-majoranolide and (+)-dihydromajorenolide, the alkene-γ-lactones (+)-majorenolide and (+)-majorenolide acetate, and the alkyne-γ-lactones, (+)-majorynolide, (+)-majorynolide acetate and (+)-isomajorynolide. In addition, thirteen known compounds were also isolated including two possible avocadofurane precursors, avocadynone acetate and avocadenone acetate, the monoterpene esters cis- and trans-p-coumarate of (-)-borneol, and the ryanoid diterpenes cinnzeylanone, anhidrocinnzeylanine, cinnzeylanine, cinnzeylanol, epiryanodol, perseanol, cinncassiol E, perseaindicol and secoperseanol. The configuration at C-14 de two ryanodane diterpenes has also been revised in this work. Furthermore, (-)-borneol cis-p-coumarate has showed to be insecticidal to S. littoralis and cytotoxic to insect (Sf9) cells, (+)-majorenolide antifeedant to aphids and cytotoxic to Sf9, cinnceylanol antifeedant and insecticidal to S. littoralis, and (+)-majorynolide (2), insecticidal against S. littoralis, cytotoxic to Sf9 and nematicidal, suggesting a defensive role for these compounds.

Cassiabudanols A and B, Immunostimulative Diterpenoids with a Cassiabudane Carbon Skeleton Featuring a 3-Oxatetracyclo[6.6.1.02,6.010,14]pentadecane Scaffold from Cassia Buds

Two novel diterpenoids, cassiabudanols A (1) and B (2), were isolated from cassia buds. Their structures were determined by comprehensive spectroscopic analysis and single-crystal X-ray diffraction. Compounds 1 and 2 possess an unprecedented 11,14- cyclo-8,14:12,13-di- seco-isoryanodane (cassiabudane) carbon skeleton featuring a unique 3-oxatetracyclo[6.6.1.0^2,6.0^10,14]pentadecane bridged system, and their biosynthetic pathways are proposed. Compounds 1 and 2 exhibited significant immunostimulative activity, and the mode of action of 2 involves upregulating CD4⁺ and CD8⁺ T cells and downregulating Tregs.

Evolution of Radical-Based Convergent Strategies for Total Syntheses of Densely Oxygenated Natural Products

Densely oxygenated natural products often exhibit potent bioactivities and are expected to function as selective cellular probes and novel drug leads. Here we describe our efforts to perfect radical-based convergent strategies for generic total syntheses of these exceedingly challenging structures.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial as opposed to marine sources and includes labdanes, clerodanes, abietanes, pimaranes, kauranes, cembranes and their cyclization products. The literature from January to December, 2016 is reviewed.

A 15-step synthesis of (+)-ryanodol

(+)-Ryanodine and (+)-ryanodol are complex diterpenoids that modulate intracellular calcium-ion release at ryanodine receptors, ion channels critical for skeletal and cardiac muscle excitation-contraction coupling and synaptic transmission. Chemical derivatization of these diterpenoids has demonstrated that certain peripheral structural modifications can alter binding affinity and selectivity among ryanodine receptor isoforms. Here, we report a short chemical synthesis of (+)-ryanodol that proceeds in only 15 steps from the commercially available terpene (S)-pulegone. The efficiency of the synthesis derives from the use of a Pauson-Khand reaction to rapidly build the carbon framework and a SeO2-mediated oxidation to install three oxygen atoms in a single step. This work highlights how strategic C-O bond constructions can streamline the synthesis of polyhydroxylated terpenes by minimizing protecting group and redox adjustments.

Chemical Conversion of Ryanodol to Ryanodine

Ryanodine (1) is a plant-derived natural product with powerful pharmacological and insecticidal action, and is a potent modulator of intracellular calcium release channels. Compound 1 possesses a 1H-pyrrole-2-carboxylate ester at the C3-position of heptahydroxylated terpenoid ryanodol (2). Whereas 2 was readily obtained from 1 by basic hydrolysis, 1 has never been synthesized from 2, due to the extreme difficulty in selectively introducing the bulky pyrrole moiety at the severely hindered C3-hydroxyl group of heptaol 2. Here we report chemical conversion of 2 to 1 for the first time. The derivatization was realized through the use of a new protective group strategy and the application of on-site construction of the pyrrole-2-carboxylate ester from the glycine ester and 1,3-bis(dimethylamino)allylium tetrafluoroborate.