ABCD Ring System Research Articles

Construction of the tetracyclic ring system of diterpene alkaloids via cationic [5 + 2] cycloaddition.

The ABCD ring system of C18/C19 diterpene alkaloids was constructed via cationic [5 + 2] cycloaddition to forge a bicyclo[3.2.1]octane, an intramolecular aldol reaction to form a seven-membered ring, oxidation of a phenol at the para-position, and introduction of a one-carbon unit via Stille coupling followed by oxidative cleavage of a furan ring.

A Convergent Synthesis of Tetracyclic Indole Compounds by a ­Palladium-Catalyzed Cross-Coupling and Tandem Cyclization ­Reaction

AbstractA new strategy for the convergent synthesis of the ABCD ring system of indole terpene alkaloids has been developed based on a Sonogashira coupling of an o-iodoaniline (the A ring) with an alkyne bearing the D ring. After a tandem palladium-catalyzed cyclization, the tetracyclic ABCD ring structure found in the terpene indole alkaloids was obtained in good yield.

Toward the Total Synthesis of Alpkinidine: Synthesis of Haloquinone CE Ring System Synthons and Attempted Nucleophilic Bisannulation

Model chemistry involving the bisannulation of 2,3-dichloro-1,4-naphthoquinone with the ester enolate derived from ethyl o-nitrophenylacetic acid, which rapid assembled the ABCD ring system of a pentacyclic pyrroloacridine, has been applied to the attempted synthesis of the marine natural product alpkinidine. The reaction of ethyl o-nitrophenylacetic acid with 6,7-dichloro-2-methylisoquinoline-1,5,8(2H)-trione, required to extend the model strategy to alpkinidine, was unfruitful, giving only complex mixtures. Efforts to direct the regiochemistry of the key Michael substitution step using 6-bromo-2-methylisoquinoline-1,5,8(2H)-trione afforded an adduct sharing the complete carbon skeleton of alpkinidine, but this could not be elaborated to the natural product.

Diastereoselective Synthesis of the ABCD Ring System of Rubriflordilactone B

AbstractA novel nine-step diastereoselective route to the ABCD ring system of the natural product rubriflordilactone B is reported. Use of an α-substituted butenolide derived from maleic anhydride facilitated a 1,4-conjugate addition to provide a diene. The order in which a ring-closing metathesis and enolate oxidation were performed on this compound dictated the relative stereochemistry of the target. The final product exhibited anisotropic effects during room-temperature NMR studies, requiring elevated-temperature experiments to confirm its identity.

(-)-Tubifolidine as strychnos indole alkaloid: Spectroscopic charactarization (FT-IR, NMR, UV-Vis), antioxidant activity, molecular docking, and DFT studies

Strychnos indole alkaloids, which include tubifolidine (2) and are an important group of complex and interesting natural compounds as bio-actives, antioxidants as well as ABCDE ring system. Four different procedures have been employed for the tubifolidine: (i) spectroscopic characterization; (ii) antioxidant activity; (iii) molecular docking, and (iv) DFT studies. The spectroscopic characterization of tubifolidine was performed by FT-IR, NMR, and UV-Vis techniques, and elemental analysis. The antioxidant activity was explored by the DPPH method and molecular docking was used to enlighten the possible interaction of the tubifolidine with the topoisomerase receptor. The DFT/B3LYP/6–311++G(df,pd) level was used for geometry optimization, structure verification and to calculate the reactivity descriptors of tubifolidine. Besides, TD-DFT/B3LYP level was used to evaluate the electronic spectra and main excitations: the results revealed that the observed peak at 292 and 254 nm were mainly associated with n→ Π* and Π→ Π* transitions, and a bit σ → Π*. Besides, the n→ Π* and Π→ Π* transitions in the ground state contributed to the lowering stability of tubifolidine. FMO analysis displayed that the electro-donating power was dominant over the electroaccepting capability of tubifolidine.

Construction of Pentacyclic Limonoid Skeletons via Radical Cascade Reactions.

Limonoids 1 and 2 share a 6/6/6/5-membered ABCD-ring system and a six-membered oxacycle and differ in their C9-stereochemistries. A new radical-based strategy was devised to construct the pentacyclic skeletons of 1 and 2. An oxacycle-fused A-ring and enyne fragments were coupled to produce radical precursors 4a-4c with different C7-oxygen functionalities. The bridgehead tertiary bromide of 4a-4c participated in a radical cascade reaction with the three unsaturated bonds to cyclize the C9-diastereomeric BCD-rings.

Identification and characterization of sorgomol synthase in sorghum strigolactone biosynthesis.

Strigolactones (SLs), first identified as germination stimulants for root parasitic weeds, act as endogenous phytohormones regulating shoot branching and as root-derived signal molecules mediating symbiotic communications in the rhizosphere. Canonical SLs typically have an ABCD ring system and can be classified into orobanchol- and strigol-type based on the C-ring stereochemistry. Their simplest structures are 4-deoxyorobanchol (4DO) and 5-deoxystrigol (5DS), respectively. Diverse canonical SLs are chemically modified with one or more hydroxy or acetoxy groups introduced into the A- and/or B-ring of these simplest structures, but the biochemical mechanisms behind this structural diversity remain largely unexplored. Sorgomol in sorghum (Sorghum bicolor [L.] Moench) is a strigol-type SL with a hydroxy group at C-9 of 5DS. In this study, we characterized sorgomol synthase. Microsomal fractions prepared from a high-sorgomol-producing cultivar of sorghum, Sudax, were shown to convert 5DS to sorgomol. A comparative transcriptome analysis identified SbCYP728B subfamily as candidate genes encoding sorgomol synthase. Recombinant SbCYP728B35 catalyzed the conversion of 5DS to sorgomol in vitro. Substrate specificity revealed that the C-8bS configuration in the C-ring of 5DS stereoisomers was essential for this reaction. The overexpression of SbCYP728B35 in Lotus japonicus hairy roots, which produce 5DS as an endogenous SL, also resulted in the conversion of 5DS to sorgomol. Furthermore, SbCYP728B35 expression was not detected in nonsorgomol-producing cultivar, Abu70, suggesting that this gene is responsible for sorgomol production in sorghum. Identification of the mechanism modifying parental 5DS of strigol-type SLs provides insights on how plants biosynthesize diverse SLs.

Asymmetric Total Synthesis of (-)-Spirochensilide A, Part 1: Diastereoselective Synthesis of the ABCD Ring and Stereoselective Total Synthesis of 13(R)-Demethyl Spirochensilide A.

A concise and diastereoselective construction of the ABCD ring system of spirochensilide A is described. The key steps of this synthesis are a semipinacol rearrangement reaction to stereoselectively construct the AB ring system bearing two vicinal quaternary chiral centers and a Co-mediated Pauson-Khand reaction to form the spiro-based bicyclic CD ring system. This chemistry leads to the stereoselective synthesis of 13(R)-demethyl spirochensilide A, paving the way for the first asymmetric total synthesis of (-)-spirochensilide A.

Synthetic Study on Lactonamycins, Part 2: Stereoselective Access to ABCD-Ring System

Toward a stereoselective total synthesis of the lactonamycins, we recently reported an approach to the DEF-ring system. Here we report a model study for constructing the ABCD-ring system, revealing a viable approach through (1) construction of the C-ring by asymmetric benzoin cyclization, (2) introduction of an angular hydroxy group through oxidation of an isoxazolium salt, and (3) construction of the AB rings through a ring-opening/closing sequence.

Novel Pathway for the Synthesis of 20-Deethyltubifolidine

A new and well planned approach has been accomplished for the total synthesis of 20-Deethyltubifolidine, which is effective for biological properties and fascinating molecules. This product embodies the ABCDE-ring system associated with the indole alkaloids. The synthesis was accomplished in six steps and produced a 38% yield.

Synthetic studies towards (–)-deserpidine: Total synthesis of the stereoisomer and derivative of (–)-deserpidine

The asymmetric synthesis of 16,17,20-epi-deserpidine and a derivative of (–)-deserpidine has been achieved. Key features in the assembly of the pentacyclic framework include a visible-light photocatalytic intra-/inter-/intramolecular radical cascade reaction to construct the tetracyclic ABCD ring system in one-pot and an intramolecularaldol reaction to forge the cyclohexane E ring.

Synthetic Approach to the ABCD Ring System of Anticancer Agent Fredericamycin A via Claisen Rearrangement and Ring-Closing Metathesis as Key Steps.

A new synthetic strategy to the ABCD ring system of the anticancer agent fredericamycin A (NSC-305263) was realized by the Diels–Alder reaction and olefin metathesis as key steps. The tactics developed here for the construction of the ABCD ring system also involve double Claisen rearrangement followed by a retro-Diels–Alder reaction and ring-closing metathesis. The metathesis approach performs a key role in the construction of A and D rings of the ABCD core unit. More importantly, ABCD fragment synthesis was accomplished without the involvement of protecting groups.

A Concise Approach for the Synthesis of the ABCD Ring System of Alpkinidine

AbstractA two step synthesis of a benzo analog of alpkinidine is described via Negishi coupling followed by a base mediated annulation reaction strategy. The organozinc compound prepared from ethyl 2‐iodobenzoate was coupled with 4‐chloro‐2‐methyl‐2,3‐dihydro‐1H‐pyrrolo[3,4‐c]quinolin‐1‐one to obtain ethyl 2‐(2‐methyl‐1‐oxo‐2,3‐dihydro‐1H‐pyrrolo[3,4‐c]quinolin‐4‐yl)‐benzoate which on further treatment with a base provided ABCD ring system of alpkinidine.

Which are the major players, canonical or non-canonical strigolactones?

Strigolactones (SLs) can be classified into two structurally distinct groups: canonical and non-canonical SLs. Canonical SLs contain the ABCD ring system, and non-canonical SLs lack the A, B, or C ring but have the enol ether-D ring moiety, which is essential for biological activities. The simplest non-canonical SL is the SL biosynthetic intermediate carlactone. In plants, carlactone and its oxidized metabolites, such as carlactonoic acid and methyl carlactonoate, are present in root and shoot tissues. In some plant species, including black oat (Avena strigosa), sunflower (Helianthus annuus), and maize (Zea mays), non-canonical SLs in the root exudates are major germination stimulants. Various plant species, such as tomato (Solanum lycopersicum), Arabidopsis, and poplar (Populus spp.), release carlactonoic acid into the rhizosphere. These observations suggest that both canonical and non-canonical SLs act as host-recognition signals in the rhizosphere. In contrast, the limited distribution of canonical SLs in the plant kingdom, and the structure-specific and stereospecific transportation of canonical SLs from roots to shoots, suggest that plant hormones inhibiting shoot branching are not canonical SLs but, rather, are non-canonical SLs.

Total synthesis of C19-diterpenoid alkaloid: construction of a functionalized ABCDE-ring system

An efficient synthetic approach for the ABCDE ring system of methyllycaconitine starting from a BCD tricycle precursor was developed.

Synthesis of the Tetracyclic Structure of Batrachotoxin Enabled by Bridgehead Radical Coupling and Pd/Ni-Promoted Ullmann Reaction.

The steroidal ABCD-ring system of the potent neurotoxin batrachotoxin was efficiently assembled in a convergent fashion. Bridgehead radical coupling between the simple AB-ring and D-ring fragments (3 and 4) formed the sterically congested linkage at the C9-oxygen-attached tetrasubstituted carbon. The C-ring was then cyclized by the Pd/Ni-promoted Ullmann reaction of the vinyl triflate and vinyl bromide of 19, giving rise to tetracyclic structure 1.

Synthetic Study on Acremoxanthone A: Construction of Bicyclo [3.2.2]nonane CD Skeleton and Fusion of AB Rings

Toward the total synthesis of acremoxanthone A, a model study has revealed a convergent approach to construct the ABCDE ring system. The key steps include: (1) an effective construction of the bicyclo[3.2.2]nonane skeleton, (2) protocol for generating the bridgehead anion and trapping, and (3) 1,3-dipolar cycloaddition of a nitrile oxide to the internal alkene.

Synthetic Study of Rubriflordilactone B: Highly Stereoselective Construction of the C-5-epi ABCDE Ring System.

A highly stereocontrolled construction of the C-5-epi ABCDE-ring system of rubriflordilactone B has been developed. The present synthesis features a convergent strategy to construct the C-5-epi AB-ring utilizing Mukaiyama-Michael reaction and forge the CDE ring in one step using intramolecular [2 + 2 + 2] cycloaddition of triynes.

Asymmetric Total Synthesis of (+)-Ryanodol and (+)-Ryanodine.

(+)-Ryanodine (1) is the ester derivative of 1H-pyrrole-2-carboxylic acid and the complex terpenoid (+)-ryanodol (2), which possesses eleven contiguous stereogenic centers on the ABCDE-ring system. Compound 1 is known to be a potent modulator of intracellular calcium release channels, whereas the activity of 2 is significantly weaker. To chemically construct 1, the multiple oxygen functional groups must be installed on the fused pentacycle in stereoselective fashions and the extremely hindered C3-hydroxy group must be acylated in a site-selective manner. First, the total synthesis of 2 was accomplished by introducing the five stereocenters from the previously prepared enantiopure ABDE-ring 7. Stereoselective construction of the C3-secondary, C2- and C6-tertiary alcohols was achieved by three nucleophilic reactions. The C9- and C10-trisubstituted carbon centers were regio- and stereoselectively introduced by hydroboration/oxidation of the six-membered C-ring, which was formed by the ring-closing metathesis reaction. Direct esterification of the C3-alcohol with pyrrole-2-carboxylic acid proved unsuccessful; therefore, we developed a new, two-step protocol for attachment of the pyrrole moiety. The C3-hydroxy group was first converted into the less sterically cumbersome glycine ester, which was then transformed into the pyrrole ring through condensation with 1,3-bis(dimethylamino)allylium tetrafluoroborate. This procedure resulted in the first total synthesis of 1.

Pectenotoxin's ABCDE Ring System: A Complex Target to Test the Potential of Singlet Oxygen Super Cascades as Tools for Synthesis

A difuran substrate was synthesized that then participated in a singlet oxygen mediated super cascade reaction sequence to give the ABCDE ring skeleton of certain pectenotoxins. During this super cascade, two separate reaction sequences occurred in parallel to create both the [6,6]-spiroketal and the 2,8-dioxabicyclo[3.2.1]octane motifs in one synthetic operation.