Convergent Total Synthesis of Principinol D, a Rearranged Kaurane Diterpenoid

Total Synthesis of (+)-Kalmanol

Kalmanol, the flagship member of the kalmane diterpene family, possesses a complex and highly oxidized 5/5/8/5 tetracyclic skeleton with nine contiguous stereocenters and exhibits significant analgesic effects and cardiotoxic properties. We have achieved the efficient total synthesis of (+)-kalmanol in 22 steps with 2.3 % yield. The synthesis featured a Rh-catalyzed [5+2+1] cycloaddition reaction to construct 5/5/8 tricyclic skeleton, and a meticulously designed sequence of stereoselective oxidations of the 5/5/8/5 tetracyclic skeleton.

A concise and stereoselective synthesis of the BCDF tetracyclic ring system of C19-diterpenoid alkaloids

A new synthetic route for the BCDF tetracyclic ring system of C19-diterpenoid alkaloids (C19-DTAs) has been developed. The key step is a Pd-catalyzed transannular alkenylation that installs a functionalized bridged F ring. The overall strategy is concise and stereoselective, and it provides a valuable new tool for the synthesis of C19-DTAs. The synthesis begins with a bridged [3.2.1] ring system, which is converted to a key intermediate through a series of highly regio- and stereoselective processes. The introduction of an allylic side chain with high precision is accomplished, culminating in a Pd-catalyzed transannular alkenylation that installs a functionalized bridged F ring to yield the BCDF tetracyclic analog of C19-DTAs.

Forging the Tetracyclic Core Framework of Rhodomolleins XIV and XLII: A Ring-Distortion Approach

A ring distortion approach for the synthesis of an advanced intermediate en route to rhodomolleins XIV and XLII was described, which led to successful construction of the 5/8/5/5 tetracyclic core framework of the kalmane diterpenoids. Key steps of the strategy include an oxidative dearomatization-induced (ODI)-Diels-Alder cycloaddition, a Dowd-Beckwith rearrangement, and a bioinspired Wagner-Meerwein rearrangement.

Total Synthesis of Principinol B

We have accomplished the first and asymmetric total synthesis of principinol B, a grayanoid possessing an oxabicyclo[3.2.1] architecture. A functionalized 5/7/6/5 tetracyclic intermediate was assembled in a convergent manner by a diastereoselective intermolecular aldol reaction and subsequent carbonyl-olefin metathesis of two enantiomerically enriched fragments. The oxabicyclo[3.2.1] architecture containing a 6,10-ether bridge was constructed by the Williamson ether synthesis.

Total Synthesis of the Diterpenes (+)-Randainin D and (+)-Barekoxide via Photoredox-Catalyzed Deoxygenative Allylation

We report the first enantioselective total synthesis of diterpenoid randainin D, which possesses a hydroazulenone core with a β-substituted butenolide moiety on the cycloheptane ring. The trans-5/7 ring system was formed via a highly challenging ring-closing metathesis delivering the tetrasubstituted cycloheptenone. The butenolide moiety was installed via a novel deoxygenative allylation under Ir-photoredox catalysis, employing methyl oxalate as a red/ox tag. Moreover, the developed allylation was successfully utilized in the 7-step total synthesis of (+)-barekoxide. This study suggests that this deoxygenative allylation method is a promising strategy for the formation of Cq-C(sp3) bonds (Cq = quaternary center) in the context of natural product synthesis.

Total Syntheses of Rhodomollins A and B

The first and asymmetric total syntheses of rhodomollins A and B, two rhodomollane type grayanoids featuring a d-homograyanane carbon skeleton and an oxa-bicyclo[3.2.1] core, were accomplished via a convergent strategy. A Stille coupling and a lithium-halogen exchange/intramolecular nucleophilic addition to the aldehyde sequence were employed to assemble two enantioenriched fragments. The oxa-bicyclo[3.2.1] core was achieved through an intramolecular SN2 substitution of cyclic sulfate of 1,2-diols (Williamson ether synthesis). The A ring oxidation states were adjusted by a Payne/Meinwald rearrangement sequence and subsequent redox transformations.

Enantioselective Total Syntheses of Grayanane Diterpenoids and (+)-Kalmanol: Evolution of the Bridgehead Carbocation-Based Cyclization and Late-Stage Functional Group Manipulation Strategies

Grayanane diterpenoids contain over 300 highly oxidized and structurally complex members, many of which possess important biological activities. Full details are provided for the development of the concise, enantioselective and divergent total syntheses of grayanane diterpenoids and (+)-kalmanol. The unique 7-endo-trig cyclization based on a bridgehead carbocation was designed and implemented to construct the 5/7/6/5 tetracyclic skeleton, demonstrating the practical value of the bridgehead carbocation-based cyclization strategy. Extensive studies of late-stage functional group manipulation were performed to forge the C1 stereogenic center, during which a photoexcited intramolecular hydrogen atom transfer reaction was discovered and the mechanism was further studied through density functional theory (DFT) calculations. The biomimetic 1,2-rearrangement from the grayanoid skeleton provided a 5/8/5/5 tetracyclic framework and resulted in the first total synthesis of (+)-kalmanol.

Highly oxygenated grayanane diterpenoids with structural diversity from the flowers of Rhododendron dauricum and their analgesic activities

Twenty-eight grayanane diterpenoids (1-28) including 13 new ones, named daublossomins A-M (1-13), and two new natural products, 3-O-acetylgrayanotoxin II (14) and 10-epi-grayanotoxin III (15), were isolated from the flowers of Rhododendron dauricum L. (Ericaceae). Their structures were elucidated by means of comprehensive spectroscopic methods and quantum chemical calculations (¹³C NMR-DP4+ analysis and calculated ECD), and the absolute configurations of ten grayanane diterpenoids 1, 4, 5, 7, 8, 22, 23, 25, 27, and 28 were determined by X-ray crystallographic analysis. Daublossomin A (1) represents the first example of an 11,16-epoxygrayan-6-one diterpenoid. Daublossomins B (2) and C (3) are the first examples of 9β,10β-epoxygrayanane diterpenoids, and daublossomin I (9) is the second conjugated grayan-1(5),6(7),9(10)-triene diterpenoid. Compounds 1-11 and 13-27 were evaluated for their analgesic activities in the HOAc-induced writhing test in mice, and 1-8, 10, 11, 13, 15, 17, 18, 22-24, and 26 exhibited significant analgesic effects at a dose of 5.0 mg/kg (inhibition rates > 50%). Among them, daublossomins A (1) and F (6) still showed potent analgesic activity even at a lower dose of 0.2 mg/kg with the inhibition rates of 54.4% and 55.2%, respectively. Grayanotoxin III (20) showed more potent analgesic activities than the positive control, morphine, at a dose of 0.04 mg/kg. A preliminary structure-activity relationship for the analgesic grayanane diterpenoids was discussed, providing some useful clues to design and develop structurally novel potent analgesics.

Total synthesis of grayanane natural products

Grayananes are a broad family of diterpenoids found in Ericaceae plants, comprising more than 160 natural products. Most of them exhibit interesting biological activities, often representative of Ericaceae use in traditional medicine. Over the last 50 years, various strategies were described for the total synthesis of these diterpenoids. In this review, we survey the literature for synthetic approaches to access grayanane natural products. We will focus mainly on completed total syntheses, but will also mention unfinished synthetic efforts. This work aims at providing a critical perspective on grayanane synthesis, highlighting the advantages and downsides of each strategy, as well as the challenges remaining to be tackled.

Unified Total Synthesis of the Limonoid Alkaloids: Strategies for the De Novo Synthesis of Highly Substituted Pyridine Scaffolds

Highly substituted pyridine scaffolds are found in many biologically active natural products and therapeutics. Accordingly, numerous complementary de novo approaches to obtain differentially substituted pyridines have been disclosed. This article delineates the evolution of the synthetic strategies designed to assemble the demanding tetrasubstituted pyridine core present in the limonoid alkaloids isolated from Xylocarpus granatum, including xylogranatopyridine B, granatumine A and related congeners. In addition, NMR calculations suggested structural misassignment of several limonoid alkaloids, and predicted their C3-epimers as the correct structures, which was further validated unequivocally through chemical synthesis. The materials produced in this study were evaluated for cytotoxicity, anti-oxidant effects, anti-inflammatory action, PTP1B and Nlrp3 inflammasome inhibition, which led to compelling anti-inflammatory activity and anti-oxidant effects being discovered.

Total Synthesis of Mollanol A

Mollanol A is the first isolated member of the mollane-type grayanoids which possesses an unprecedented C-nor-D-homograyanane carbon skeleton and an 5,8-epoxide. Due to its transcriptional activation effects on the Xbp1 upstream promoters in different cell types, it has a potential therapeutic effect on inflammatory bowel disease. Here we report the first total synthesis of mollanol A, which constitutes a 15-step synthesis from commercially available materials via a convergent strategy. The synthesis involves an InCl₃-catalyzed Conia-ene cyclization reaction to construct the bicyclo[3.2.1]octane moiety and a vinylogous aldol reaction/intramolecular oxa-Michael addition sequence to rapidly assemble the oxa-bicyclo[3.2.1] core.

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.

Enantioselective Total Syntheses of Grayanane Diterpenoids: (-)-Grayanotoxin III, (+)-Principinol E, and (-)-Rhodomollein XX

Enantioselective total syntheses of (-)-grayanotoxin III, (+)-principinol E, and (-)-rhodomollein XX were accomplished based on a convergent strategy. The left- and right-wing fragments were assembled via the diastereoselective Mukaiyama aldol reaction catalyzed by a chiral hydrogen bond donor. The unique 7-endo-trig cyclization based on a bridgehead carbocation forged the 5/7/6/5 tetracyclic skeleton that underwent redox manipulations and 1,2-migration to access different grayanane diterpenoids.

Total Synthesis of Isorosthin L and Isoadenolin I

Total syntheses of two Isodon diterpenes, isorosthin L and isoadenolin I, are reported. The synthetic strategy features a quick assembly of two simple building blocks through a diastereoselective intermolecular aldol reaction and a subsequent radical cyclization for efficient construction of a rather complex advanced intermediate bearing a quaternary stereocenter present in all Isodon diterpenes. Oxidative cleavage of the C-C bond in the cyclopentane enabled the conversion to a lactone moiety which is desired for the construction of the molecular skeleton through reductive coupling with an aldehyde carbonyl group.

Asymmetric Total Synthesis of Clionastatins A and B

Herein we report the first total synthesis of polychlorinated steroids clionastatins A and B, which was accomplished asymmetrically by means of a convergent, radical fragment coupling approach. Key features of the synthesis include an Ireland-Claisen rearrangement to introduce the C5 stereocenter (which was ultimately transferred to the C10 quaternary stereocenter of the clionastatins via a traceless stereochemical relay), a regioselective acyl radical conjugate addition to join the two fragments, an intramolecular Heck reaction to install the C10 quaternary stereocenter, and a diastereoselective olefin dichlorination to establish the synthetically challenging pseudoequatorial dichlorides. This work also enabled us to determine that the true structures of clionastatins A and B are in fact C14 epimers of the originally proposed structures.

Recent advances in the synthesis of ent-kaurane diterpenoids

Covering: 2015 to 2020The ent-kaurane diterpenoids are integral parts of tetracyclic natural products that are widely distributed in terrestrial plants. These compounds have been found to possess interesting bioactivities, ranging from antitumor, antifungal and antibacterial to anti-inflammatory activities. Structurally, the different tetracyclic moieties of ent-kauranes can be seen as the results of intramolecular cyclizations, oxidations, C-C bond cleavages, degradation, or rearrangements, starting from their parent skeleton. During the past decade, great efforts have been made to develop novel strategies for synthesizing these natural products. The purpose of this review is to describe the recent advances in the total synthesis of ent-kaurane diterpenoids covering the period from 2015 to date.

Synthetic efforts toward the bicyclo[3.2.1]octane fragment of rhodojaponin III

Rhodojaponin III is a grayanane-type diterpenoid natural product with a novel chemical scaffold. It shows potent antinociceptive activity and may represent a new class of natural non-opioid analgesics with a novel mode of action. We explored the Au(I)-catalyzed Conia-ene cyclization and the Mn(III)-mediated radical cyclization of alkynyl ketones for the synthesis of the bicyclo[3.2.1]octane fragment of rhodojaponin III. These strategies will be applicable in the synthesis of rhodojaponin III and analogs for future biological studies.

Total Synthesis of Ritterazine B

The first total synthesis of the cytotoxic alkaloid ritterazine B is reported. The synthesis features a unified approach to both steroid subunits, employing a titanium-mediated propargylation reaction to achieve divergence from a common precursor. Other key steps include gold-catalyzed cycloisomerizations that install both spiroketals and late stage C-H oxidation to incorporate the C7' alcohol.

Tetracyclic Diterpenoid Synthesis Facilitated by ODI-Cascade Approaches to Bicyclo[3.2.1]octane Skeletons

Tetracyclic diterpenoids (C₂₀) mainly refer to the plant terpenoids bearing biogenetically related carbon skeletons derived from copalyl diphosphates (ent-CPP and syn-CPP). This large family contains over 1600 known members that can be categorized into 11 major structural types. Among them, more than three-quarters share a bridged bicyclo[3.2.1]octane subunit, which is also an important branching point in biosynthesis en route to the other types of bicyclic scaffolds, such as bicyclo[2.2.2]-, bicyclo[3.3.0]-, and tricyclo[3.2.1.0]octanes. Combined with the significance of its stereochemical importance in biological activity, the assembly of the bicyclo[3.2.1]octane skeletons is critical to the success of the whole synthesis blueprint toward tetracyclic diterpenoids. Although a number of inspiring methodologies have been disclosed, general approaches by the incorporation of innovative cascade reactions permitting access to diverse structural types of tetracyclic diterpenoids remain limited and in urgent demand.Because of the long-standing interest in the synthesis of bridged diterpenoids, we have recently developed two complementary types of oxidative dearomatization induced (ODI) cascade approaches to the rapid and efficient construction of bicyclo[3.2.1]octane skeletons. In this Account, we summarize our original synthesis design, methodology development, and the application of these two strategies in tetracyclic diterpenoid synthesis during the past few years in our laboratory.First, we detail our preliminary investigation of the ODI-[5 + 2] cycloaddition/pinacol rearrangement cascade reaction, which showed a wide scope of vinylphenol substrates and led to cyclopentane and cyclohexane-fused bicyclo[3.2.1]octanes in good yields with excellent dr values. Next, we describe the utilization of this ODI-[5 + 2] cascade reaction which resulted in the asymmetric total syntheses of four highly oxygenated ent-kauranoids. The strategy concerning accurate stereochemical control in the ODI-[5 + 2] cycloaddition was then successfully transplanted to the total syntheses of three stemaranoids, thus providing a straightforward and diastereoselective route to C9-ethano-bridged tetracyclic diterpenoids. To access more complex diterpenoid rhodomollanol A, we exploited two additional biomimetic rearrangements, namely, the retro-Dieckmann fragmentation/vinylogous Dieckmann cyclization cascade and the photo-Nazarov cyclization/intramolecular cycloetherification cascade. Taken together with the ODI-[5 + 2] cascade, the asymmetric total synthesis of the target molecule was realized, which shed light on the biogenetic pathway of the unprecedented rhodomollane-type carbon framework. Finally, we describe an ODI-Diels-Alder/Beckwith-Dowd cascade approach as a valuable supplement to the ODI-[5 + 2] cascade for the fabrication of cycloheptane-fused bicyclo[3.2.1]octane skeletons. Its versatility was also demonstrated by the total syntheses of two challenging grayanane diterpenoids. In view of the high functional-group compatibility and scalability, we anticipate that the two novel cascade approaches will find further use in the field of complex natural product synthesis.

Enantioselective Total Synthesis of Andrographolide and 14-Hydroxy-Colladonin: Carbonyl Reductive Coupling and trans-Decalin Formation by Hydrogen Transfer

An enantioselective total synthesis of the labdane diterpene andrographolide, the bitter principle of the herb Andrographis paniculata (known as "King of Bitters"), was accomplished in 14 steps (LLS). Key transformations include iridium-catalyzed carbonyl reductive coupling to form the quaternary C4 stereocenter, diastereoselective alkene reduction to establish the trans-decalin ring, and carbonylative lactonization to install the α-alkylidene-β-hydroxy-γ-butyrolactone.

Enantioselective Total Synthesis of Mollebenzylanol A

Mollebenzylanol A is a tyrosine phosphatase 1B inhibitor isolated from the leaves of Rhododendron molle in 2018 that has a highly functionalized structure. The first enantioselective total synthesis of mollebenzylanol A was achieved in 13 steps from a known chiral starting material. An efficient and practical synthetic scheme was disclosed in a stereocontrolled manner, including stereo/regioselective epoxidation, Eschenmoser-Claisen rearrangement, and stereocontrolled dihydroxylation.

Crokonoids A-C, A Highly Rearranged and Dual-Bridged Spiro Diterpenoid and Two Other Diterpenoids from Croton kongensis

Crokonoid A (1), a highly rearranged diterpenoid featuring a dual-bridged tricyclo[4.4.1.1^1,4]dodecane-2,11-dione ring system and its two possible ent-kaurene diterpenoid precursors (2 and 3), was isolated and structurally characterized by solid data from Croton kongensis. Compound 1 exhibited significant cytotoxicity against HL-60 and A-549 cell lines with IC₅₀ values of 1.24 ± 0.56 and 1.92 ± 0.60 μM, respectively.