Construction of a pentacyclic ring system of isoryanodane diterpenoids by SmI2-mediated transannular cyclization

Recent Developments in Transannular Reactions

Transannular reactions have shown a remarkable performance for the construction of polycyclic scaffolds from medium- or large sized cyclic molecules in an unconventional manner. Recent examples of transannular reactions reported from 2011 have been reviewed, emphasizing the excellent performance of this approach when accessing the target compounds. This review also highlights how this methodology provides an alternative approach to other commonly used methodologies for the construction of cyclic entities such as cyclization or cycloaddition reactions

Highly oxygenated isoryanodane diterpenoids from the leaves of Cinnamomum cassia and their immunomodulatory activities

A total of twelve highly oxygenated isoryanodane (also known as cinncassiol D-type) diterpenoids including nine undescribed ones, named cinnacassins A-I, were isolated from the leaves of Cinnamomum cassia. Their chemical structures were elucidated by extensive spectrometric and spectroscopic techniques including HRESIMS, 1D and 2D NMR, single-crystal X-ray diffraction analysis, calculated ¹³C-NMR DP4+ analysis, and chemical methods. The absolute configuration of cinnacassin A was unambiguously delineated by single-crystal X-ray diffraction analysis. Cinnacassin H represents the first example of 16-O-glucosylated isoryanodane diterpenoid, and cinnacassin I is the first isoryanod-13(18)-ene diterpenoid. The relationship of the configuration C-18 and the chemical shifts of H₂-19 and C-20 in the 19-hydroxy-isoryanodane diterpenoids was discussed, and the 18S-configuration of three known 19-hydroxy-isoryanodane diterpenoids, cinncassiol D₁, 19-O-β-D-glucopyranosyl-cinncassiol D₁, and cinncassiol D₃ was assigned. All the isolated isoryanodane diterpenoids were evaluated for their immunomodulatory effects in vitro, and cinnacassin A and cinncassiol D₁ enhanced the proliferation of Con A-induced murine T cells with enhancement rates ranging from 17.9% to 45.4%, which were more potent than the positive control, thymosin α1. In addition, cinncassiol D₁ significantly promoted the proliferation of LPS-induced murine B cells with an enhancement rate up to 116.1%, two-fold more potent than thymosin α1 at a concentration of 1.5625 μM.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI₂, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI₂ in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI₂-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated.

Itol A May Affect the Growth and Development of Spodoptera frugiperda through Hijacking JHBP and Impeding JH Transport

Isoryanodane and ryanodane diterpenes have a carbon skeleton correlation in structures, and their natural product-oxidized diterpenes show antifeedant and insecticidal activities against Hemiptera and Lepidoptera. While ryanodine mainly acts on the ryanodine receptor (RyR), isoryanodane does not. In this study, we demonstrated that itol A, an isoryanodane diterpenoid, could significantly downregulate the expression level of juvenile hormone-binding protein (JHBP), which plays a vital role in JH transport. RNAi bioassay indicated that silencing the Spodoptera frugipreda JHBP (SfJHBP) gene decreased itol A activity, which confirmed the developmental phenotypic observation. Parallel reaction monitoring (PRM) further confirmed that itol A affected JHBP's expression abundance. Although JHBP is not proven as the direct or only target of itol A, we confirmed that itol A's action effect depends largely on JHBP and that JHBP is a potential target of itol A. We present foundational evidence that itol A inhibits the growth and development of Spodoptera frugiperda mainly through hijacking JHBP.

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.

A 16-step synthesis of the isoryanodane diterpene (+)-perseanol

(+)-Perseanol is an isoryanodane diterpene with potent antifeedant and insecticidal properties isolated from the tropical shrub Persea indica.¹ It is structurally related to (+)-ryanodine, a high affinity ligand and modulator of ryanodine receptors (RyRs)—ligand-gated ion channels critical for intracellular Ca²⁺ signaling in vertebrates and invertebrates.² Whereas ryanodine modulates RyR-dependent Ca²⁺ release across many organisms, including mammals, preliminary data indicate that ryanodane and isoryanodane congeners that lack the pyrrole-2-carboxylate ester, such as perseanol, may have selective activity in insects.³ Here we report the first chemical synthesis of (+)-perseanol, which proceeds in 16 steps from commercially available (R)-pulegone. The synthesis features a two-step annulation process that rapidly assembles the tetracyclic core from readily accessible cyclopentyl building blocks. This work demonstrates how convergent fragment coupling, when combined with strategic oxidation tactics, can enable the concise synthesis of complex and highly oxidized diterpene natural products.