Collective Syntheses of Icetexane Natural Products Based on Biogenetic Hypotheses

Total Synthesis of Brevitaxin

Brevitaxin was prepared in nine steps from commercially available carnosic acid. The construction of the 1,4-benzodioxin moiety involved an unique stepwise ortho-quinone-engaged [4+2] cycloaddition. Two strategic stages were employed to prepare the highly unsaturated cycloaddition precursor 3: (1) synthesizing the diene moiety (C1-C2 and C10-C20 double bonds) by regioselective ortho-quinone tautomerization, and (2) installing four sp2-hybridized carbon atoms (C3, C5, C6 and C7) in one step using a SeO2-promoted chemo- and regioselective oxidation reaction.

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).

Direct Two Carbon Ring Expansion of 1-Indanones with Ynones: An Eco-Friendly, One-Flask Approach to Functionally Enriched 5H-Benzo[7]annulenes

Direct 2C-ring expansion of 1-indanones with ynones to 5H-benzo[7]annulenes has been observed, and its generality has been gauged (19 examples). Overall, this simple and convenient cascade process to 5H-benzo[7]annulenes involves engagement of 1-indanone with two ynone moieties with formation of three new C-C σ-bonds, cleavage of C-C σ-bond, and concurrent functionality amplification. The resulting seven-membered ring, laced with an opportunistic disposition of four proximal functional groups, offers avenues for their further productive interplay. Our new approach embraces many green and eco-friendly features.

Formal Synthesis of Ecteinascidin 743 from N-Cbz-l-tyrosine

A formal total synthesis of ecteinascidin 743 and lurbinectedin is achieved. Key features involve a Pictet-Spengler cyclization coupling of the tetrahydroisoquinoline and phenylalaninol moieties prepared by a common route with high yield and selectivity, a Parikh-Doering oxidation with good chemoselectivity and functionality tolerance, and a light-mediated A-ring elaboration of pentacyclic methoxyquinone substrates. By the approach, the known advanced intermediate (4-step conversion to Et-743) can be obtained conveniently in 21 total steps from N-Cbz-l-tyrosine.

Total Syntheses of (±)-Dracocephalone A and (±)-Dracocequinones A and B

Described herein are the first total syntheses of (±)-dracocephalone A (1) and (±)-dracocequinones A (4) and B (5). The synthesis was initially envisioned as proceeding through an intramolecular isobenzofuran Diels-Alder reaction, a strategy that eventually evolved into a Lewis acid-promoted spirocyclization. This highly diastereoselective transformation set the stage for trans-decalin formation and a late-stage Suárez oxidation that produced a [3.2.1] oxabicycle suited for conversion to 1. Brønsted acid-mediated aromatization, followed by a series of carefully choreographed oxidations, allowed for rearrangement to a [2.2.2] oxabicycle poised for conversion to 4 and 5.

Recent Developments with Icetexane Natural Products

Icetexane diterpenoids are a diverse family of natural products sourced from several species of terrestrial plants. Icetexanes exhibit a broad array of biological activities and together with their complex 6-7-6 tricyclic scaffolds, they have piqued the interest of synthetic organic chemists, natural products chemists, and biological investigators over the past four decades and were reviewed 13 years ago. This review summarizes icetexane natural products isolated since 2009, provides an overview of new synthetic approaches to the icetexane problem, and proposes an additional classification of icetexanes based on novel structures that are unlike previously isolated materials.

Icetexane diterpenoids as Cav3.2 T-type calcium channel inhibitors from Salvia prattii and analgesic effect of their Semi-synthesized derivatives

Ten new icetexane diterpenoids, salpratins E-N (1-10) and a known analogue (11) were characterized from Salvia prattii Hemsl. Structurally, 1 is the first 19(4 → 3)-abeo-icetexane diterpenoid featuring with a 6/7/6 ring system. The structures of isolated compounds were determined by comprehensive analyses of spectroscopic data, ECD calculation, and single-crystal X-ray diffraction. Biological studies initially revealed that 1, 7, 10, and 11 are notable Ca_v3.2 T-type Ca²⁺ channel (TTCC) inhibitors with IC₅₀ values of 2.9, 5.1, 2.3, and 3.2 μM, respectively. Five icetexane related derivatives (13-17) were synthesized from an abietane type precursor, (+)-carnosic acid (12), for the purpose of overcoming the poor water solubility of aforementioned active compounds and further investigating diverse diterpenes with valuable activity. Among them, 13 and 14 showed potent inhibitions on Ca_v3.2, having IC₅₀ values of 6.7 and 2.4 μM, respectively. Significantly, they exhibited dose-dependent (1, 3, and 10 mg/kg) and comparable analgesic effects as that of Z944, a TTCCs inhibitor under clinical trial for pain management, in the mouse acetic acid writhing test. These findings further enrich structural diversity and bioactivity of Salvia diterpenoids, as well as provide promising structural templates for the development of Ca_v3.2 analgesics.

Programmed Polyene Cyclization Enabled by Chromophore Disruption

A new polyene cyclization strategy exploiting β-ionyl derivatives was developed. Photoinduced deconjugation of the extended π-system within these chromophores unveils a contrathermodynamic polyene that engages in a Heck bicyclization to afford [4.4.1]-propellanes. This cascade improves upon the limited regioselectivity achieved using existing biomimetic tactics and tolerates both electron-rich and electron-deficient (hetero)aryl groups. The utility of this approach was demonstrated with the diverted total synthesis of taxodione and salviasperanol, two isomeric abietane diterpenes that were previously inaccessible along the same synthetic pathway.

Efficient Synthesis of Icetexane Diterpenes and Apoptosis Inducing Effect by Upregulating BiP-ATF4-CHOP Axis in Colorectal Cells

We herein present an efficient and robust synthetic strategy toward 12 icetexane diterpenes and their derivatives, which features a PPh₃/DIAD-mediated rearrangement of the reduced carnosic acid derivative (2) to give (-)-barbatusol (3) in a regioselective and scalable way. MTT assay led to the identification of (+)-grandione (11) and (-)-demethylsalvicanol o-quinone derivative (9) as highly cytotoxic agents against HCT-116, COLO-205, and Caco-2 cells. Interestingly, (+)-grandione (11) induced the HCT-116 cell apoptosis in a dose-dependent manner, which might be attributed to the upregulation of the BiP-ATF4-CHOP axis and promotion of the BiP-ATF4 interactions, thereby leading to endoplasmic reticulum (ER) stress. This work not only paves an efficient and scalable pathway to access icetexane diterpenes but also provides new leads for the development of anticolorectal agents with a unique mode of action.

Recent advances in total syntheses of natural products containing the benzocycloheptane motif

Covering: 2010 to 2020Benzocycloheptane is a fundamental and unique structural motif found in pharmaceuticals and natural products. The total syntheses of natural products bearing the benzocycloheptane subunit are challenging and there are only a few efficient approaches to access benzocycloheptane. Thus, new methods and innovative strategies for preparing such natural products need to be developed. In this review, recent progress in the total syntheses of natural products bearing the benzocycloheptane motif is presented, and key transformations for the construction of benzocycloheptane are highlighted. This review provides a useful guide for those engaged in the syntheses of natural products containing the benzocycloheptane motif.

Spirodesertols A and B, two highly modified spirocyclic diterpenoids with an unprecedented 6-isopropyl-3H-spiro[benzofuran-2,1′-cyclohexane] motif from Salvia deserta

Two highly modified spirocyclic diterpenoids with an unprecedented 6-isopropyl-3H-spiro[benzofuran-2,1′-cyclohexane] motif and four new icetexane diterpenoids were isolated from Salvia deserta. 1 showed more potent cytotoxicity than cis-platin.

Naturally occurring [4 + 2] type terpenoid dimers: sources, bioactivities and total syntheses

This review article highlights recent progress on their sources, bioactivities, biosynthetic hypotheses and total chemical syntheses of naturally occurring [4 + 2] type terpenoid dimers.

A Strategy toward Icetexane Natural Products

Icetexane diterpenoids are richly complex polycyclic natural products that have been described with a variety of biological activities. We report here a general synthetic approach toward the 6-7-6 tricyclic core structure of these interesting synthetic targets based on a two-step enolate alkylation and ring-closing metathesis reaction sequence.

Total Synthesis of the Sesquiterpenoid Periconianone A Based on a Postulated Biogenesis

The first enantioselective total synthesis of the complex tricarbocyclic sesquiterpenoid periconianone A based on a postulated biogenesis is reported. Key elements of the synthetic route include the use of an isopropenyl group as a removable directing group for stereoselective synthesis, a sequence featuring a Rh-mediated O-H insertion/[3,3]-sigmatropic rearrangement and subsequent α-ketol rearrangement, and a late stage aldol reaction to furnish the complex cage-like framework.

Structure, Absolute Configuration, and Antiproliferative Activity of Abietane and Icetexane Diterpenoids from Salvia ballotiflora

From the aerial parts of Salvia ballotiflora, eleven diterpenoids were isolated; among them, four icetexanes and one abietane (1-5) are reported for the first time. Their structures were established by spectroscopic means, mainly ¹H- and 13C-NMR, including 1D and 2D homo- and hetero-nuclear experiments. Most of the isolated diterpenoids were tested for their antiproliferative, anti-inflammatory, and radical scavenging activities using the sulforhodamine B assay on six cancer cell lines, the TPA-induced ear edema test in mice, and the reduction of the DPPH assay, respectively. Some diterpenoids showed anti-proliferative activity, these being icetexanes 6 and 3, which were the most active with IC50 (μM) = 0.27 ± 0.08 and 1.40 ± 0.03, respectively, for U251 (human glioblastoma) and IC50 (μM) = 0.0.46 ± 0.05 and 0.82 ± 0.06 for SKLU-1 (human lung adenocarcinoma), when compared with adriamycin (IC50 (μM) = 0.08 ± 0.003 and 0.05 ± 0.003, as the positive control), respectively. Compounds 3 and 10 showed significant reduction of the induced ear edema of 37.4 ± 2.8 and 25.4 ± 3.0% (at 1.0 μmol/ear), respectively. Compound 4 was the sole active diterpenoid in the antioxidant assay (IC50 = 98. 4 ± 3.3), using α-tocopherol as the positive control (IC50 (μM) = 31.7 ± 1.04). The diterpenoid profile found is of chemotaxonomic relevance and reinforces the evolutionary link of S. ballotiflora with other members of the section Tomentellae.