Total Synthesis of Rameswaralide Utilizing a Pharmacophore-Directed Retrosynthetic Strategy

Total Syntheses of Scabrolide B, Ineleganolide, and Related Norcembranoids

Concise total syntheses of several 5/7/6 norcembranoids, including ineleganolide, scabrolide B, sinuscalide C, and fragilolide A have been achieved through a fragment coupling/ring closure approach. The central seven-membered ring was forged through sequential Mukaiyama-Michael/aldol reactions using norcarvone and a decorated bicyclic lactone incorporating a latent electrophile. Subsequent manipulations installed the reactive enedione motif and delivered scabrolide B in 11 steps from a chiral pool-derived enone. Finally, ineleganolide, sinuscalide C, and fragilolide A were each accessed in one additional step.

Theoretical Study on the Biosynthesis of the Mandapamates: Mechanistic Insights Using Density Functional Theory

Density functional theory (B3LYP-D3(BJ) and ωB97XD) calculations have been used to assess the stereochemical outcomes of the proposed transannular [4 + 2] cycloaddition pathway for the biosynthesis of mandapamate and isomandapamate from macrocyclic intermediates. Calculations reveal that the topological shift between macrocyclic conformers is vital in controlling the stereoselectivity of the downstream steps toward the isomeric mandapamates. A stepwise 4 + 2 type process is energetically favored over a concerted [4 + 2] pathway at room temperature, and is consistent with the stereochemistries found in the natural products.

Total Synthesis of the Norcembranoid Scabrolide B and Its Transformation into Sinuscalide C, Ineleganolide, and Horiolide

It was recognized only recently that the sister norcembranoids scabrolides A and B have notably different carbotricyclic scaffolds. Therefore, our synthesis route leading to scabrolide A could not be extended to its sibling. Rather, a conceptually new approach had to be devised that relied on a challenging intramolecular alkenylation of a ketone to forge the congested central cycloheptene ring at the bridgehead enone site; the required cyclization precursor was attained by a lanthanide-catalyzed Mukaiyama-Michael addition. The dissonant 1,4-oxygenation pattern was then installed by allylic rearrangement/oxidation of the enone, followed by suprafacial 1,3-transposition. Synthetic scabrolide B was transformed into sinuscalide C by dehydration and into ineleganolide by base-mediated isomerization/oxa-Michael addition, which has potential biosynthetic implications; under basic conditions, the latter compound converts into horiolide by an intricate biomimetic cascade.

Systematic Route to Construct the 5-5-6 Tricyclic Core of Furanobutenolide-Derived Cembranoids and Norcembranoids

Herein, we present a highly efficient method for constructing the intricate 5-5-6 fused ring system commonly found in the polycyclic furanobutenolide-derived cembranoid and norcembranoid natural product family with remarkable diastereoselectivity, utilizing an intramolecular Diels-Alder reaction as the cornerstone. Notably, employing a propargyl ether tether as the dienophile yields significant enhancements in the transformation process compared to its propargyl ester counterpart, as demonstrated in our previous total synthesis of havellockate. This advancement holds promising implications for future investigations, offering a streamlined pathway for rapidly assembling the tricyclic core characteristic of this diverse family of natural products.

Divergent Synthesis of Scabrolide A and Havellockate via an exo-exo-endo Radical Cascade

Here we report a concise and divergent synthesis of scabrolide A and havellockate, representative members of polycyclic marine natural product furano(nor)cembranoids. The synthesis features a highly efficient exo-exo-endo radical cascade. Through the generation of two rings, three C-C bonds, and three contiguous stereocenters in one step, this remarkable transformation not only assembles the bowl-shaped, common 6-5-5 fused ring system from simple building blocks but also precisely installs the functionalities at desired positions and sets the stage for further divergent preparation of both target molecules. Further studies reveal that the robust and unusual 6-endo radical addition in the cascade is likely facilitated by the rigidity of the substrate.

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

Isatogenols as Precursors for the Synthesis of Fully Substituted Indolines through Regio- and Stereoselective [3 + 2] Cycloaddition Using Various Olefins

Here, we describe a unique reactivity of isatogen derivatives bearing a hydroxy group at the C3-position (isatogenol) and their synthetic application to highly regio- and stereoselective [3 + 2] cycloaddition reactions. This method provides facile access to polyfused and highly functionalized heterocycles including consecutive stereocenters. Furthermore, DFT calculations revealed that hydrogen bonding is a key to controlling the regio- and stereoselectivity in the cycloaddition using acrylates.

Marine natural products

Covering: January to the end of December 2022This review covers the literature published in 2022 for marine natural products (MNPs), with 645 citations (633 for the period January to December 2022) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, the submerged parts of mangroves and other intertidal plants. The emphasis is on new compounds (1417 in 384 papers for 2022), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of NP structure class diversity in relation to biota source and biome is discussed.

Structure-Unit-Based Total Synthesis of (-)-Sinulochmodin C

Herein we report a structure-unit-based asymmetric total synthesis of sinulochmodin C, a norcembranoid diterpenoid bearing a transannular strained ether bridge β-keto tetrahydrofuran moiety. Our synthetic route features an intramolecular double Michael addition to construct stereospecifically the [7,6,5,5] tetracyclic skeleton, a vinylogous hydroxylation/oxidation procedure or a stereospecific epoxide opening/oxidation sequence to establish the γ-keto enone intermediate, a Lewis acid/Brønsted acid mediated transannular oxa-Michael addition to fuse the β-keto tetrahydrofuran moiety, a Mukaiyama hydration/Pd-C hydrogenation to reverse the C1-configuration of the isopropenyl unit, and a bioinspired transformation of sinulochmodin C into scabrolide A.

Merging Manganese and Iminium Catalysis: Selective Hydroalkenylation of Unsaturated Aldehydes and Ketones

The use of synergistic catalytic strategy can usually circumvent the intrinsic limitations of one catalytic system. In this communication, we disclose a cooperative catalysis strategy of manganese and iminium catalysis to realize selective hydroalkenylation of unsaturated aldehydes and ketones. Its success stems from the LUMO activation of unsaturated carbonyl compounds with secondary amines as the organocatalyst and the synergistic HOMO activation of alkenylboronic acids with Mn2 (CO)8 Br2 . This protocol exhibits several synthetic advances, e.g., simple operation, good functional group compatibility and good regioselectivity. The theoretical calculation indicates the migratory insertion followed by demetallation-isomerization process is kinetically more favorable than Michael-like nucleophilic addition. The use of proline-derived organocatalyst can deliver the desired products in moderate enantioselectivity.

Total Syntheses of Scabrolide A and Yonarolide

The concise total syntheses of oxidized norcembranoid terpenoids (-)-scabrolide A and (-)-yonarolide have been accomplished in 10 and 11 steps, respectively. The carbocyclic skeleton was efficiently constructed from two chiral-pool-derived fragments, including a [5,5]-bicyclic lactone accessed through a powerful Ni-catalyzed pentannulation of functionalized cyclopentenone with methylenecyclopropane and subsequent fragmentation. Additional features included a Liebeskind-Srogl coupling, induction of a cyclization/elimination cascade by a zinc-amido base, and installation of a sensitive enedione motif by late-stage γ-oxidation.

A Convergent Total Synthesis of (+)-Ineleganolide

We report the total synthesis of the furanobutenolide-derived diterpenoid (+)-ineleganolide. The synthetic approach relies on a convergent strategy based on the coupling of two enantioenriched fragments, which are derived from (-)-linalool and (+)-norcarvone, respectively. A high-yielding, one-step Michael addition and aldol cascade furnishes a pentacyclic framework as a single diastereomer, thereby overcoming previous challenges in controlling stereochemistry. The endgame features an O2-facilitated C-H oxidation and a samarium diiodide-induced semipinacol rearrangement to furnish the highly rigid central seven-membered ring.

Asymmetric Total Synthesis of Havellockate

The first total synthesis of the furanobutenolide-derived cembranoid diterpenoid havellockate is disclosed. Our convergent strategy employs a Julia-Kocienski olefination to join two enantioenriched fragments to produce a diene that is subsequently used in a propiolic acid esterification/Diels-Alder cascade. This sequence generates the fused carbocyclic core of the natural product in short order. A challenging Zn-mediated Barbier allylation then forges the final C-C bond and also establishes two vicinal stereogenic centers. Finally, a Cu-catalyzed aerobic oxidation facilitates the formation of the β-hydroxybutanolide to complete the total synthesis.