Total Syntheses of (−)- and (+)-Goniomitine

Advances and Strategies towards Synthesis of Aspidosperma Indole Alkaloids Goniomitine

Goniomitine is of the aspidosperma alkaloid family, with an angularly fused tetracyclic skeleton housing an all-carbon quaternary carbon chiral center alongside an aminal functional group. This natural product has garnered attention as a synthetic target due to its intriguing molecular architecture and anti-proliferative activity in recent years. Following the first synthesis of (-)-goniomitine by Takano in 1991, synthetic chemists have developed various methods. This review provides an overview of the methodologies used in the synthesis of goniomitine in racemic and enantiopure forms via divergent construction indole framework, indole functionalization, and the integrated oxidation/reduction/cyclization (iORC) sequence from 1991 to 2023.

Total Synthesis of Quebrachamine and Kopsiyunnanine D

The total syntheses of (±)-quebrachamine and (±)-kopsiyunnanine D are reported. Key transformations include an intermolecular Horner-Wadsworth-Emmons olefination to merge the two fragments convergently and an intramolecular Mitsunobu reaction to introduce the synthetically challenging nine-membered azonane ring efficiently.

Chiral Aminoalcohol-Derived δ-Lactams Provide Easy Access to Piperidines and Acyclic Five-Carbon Building Blocks Bearing a Tertiary and a Quaternary Stereocenter

A procedure for the synthesis of enantiopure piperidines and acyclic building blocks (5-aminopentanols, O-protected 5-hydroxypentanenitriles) containing a tertiary and a quaternary stereocenter has been developed. Starting from a phenylglycinol- or aminoindanol-derived δ-lactam bearing an alkyl substituent at the α-position of the N,O-acetal carbon, easily accessible by a cyclocondensation reaction, the stereoselective dialkylation at the carbonyl α-position generates the quaternary stereocenter and the subsequent two-step reductive removal of the chiral inductor provides enantiopure 3,3,5-trisubstituted piperidines. Alternatively, the simultaneous reductive opening of the oxazolidine and piperidone rings of the dialkylated lactams followed by reductive or oxidative cleavage of the chiral inductor opens access to chiral 2,2,4-trisubstituted 5-amino-1-pentanols or 2,4,4-trisubstituted 5-hydroxypentanenitriles.

Lithium Enolate with a Lithium-Alkyne Interaction in the Enantioselective Construction of Quaternary Carbon Centers: Concise Synthesis of (+)-Goniomitine

We report a method for direct enantioselective alkylation of 3-alkynoic and 2,3-alkendioic acids that form quaternary stereogenic centers, and application of this method to the total enantioselective synthesis of a complex alkaloid (+)-goniomitine. The methods were effective in the alkylation of both 3-alkynoic acids, 2,3-alkendioic acids substrates with a broad range of heterocyclic and functionalized alkyl group substituents. Accompanying crystallographic studies provide mechanistic insight into the structure of well-defined chiral aggregates, highlighting cation-π interactions between lithium and alkyne groups.

Rapid construction of indole-fused 8-10 membered lactones via a tandem reaction

An intramolecular anaerobic Mukaiyama hydration-initiated tandem reduction/condensation/acyl migration/aromatization reaction was developed, which enabled the rapid construction of indole-fused 8-10 membered lactones starting from cyclic 2-allyl-2-(2-nitrophenyl)-1,3-diketones. A nitro substituent in the substrates acted as both an oxygen source in the Mukaiyama hydration step and a nitrogen source in a tandem indole ring construction step. Our reaction features mild conditions, atom economy, and inexpensive reagents and it can be conveniently scaled up to a gram scale in modest yields. A rational reaction mechanism was also proposed based on previous reports and control experiments.

Annulative π-Extension (APEX) of Indoles to Pyrido[1,2-a]indoles Using 4-Oxo Peroxides as C4 Units

Annulative π-extension (APEX) of 3-substituted indoles to pyrido[1,2-a]indoles is developed by using 4-oxo peroxides as π-extending reagents, which are employed as versatile C4 building blocks. This transformation is initiated by Brønsted acid-mediated Hock rearrangement of the peroxyl group. Notably, the pyrido[1,2-a]indole products are obtained by elimination of the indole moiety from the corresponding dihydropyrido[1,2-a]indoles, which could be selectively formed at room temperature.

Asymmetric construction of pyrido[1,2-a]-1H-indole derivatives via a gold-catalyzed cycloisomerization

Pyrido[1,2-a]-1H-indoles are important scaffolds found in many biologically active compounds. Herein, we first developed an IPrAuCl/AgSbF₆-catalyzed cycloisomerization of N-1,3-disubstituted allenyl indoles affording pyrido[1,2-a]-1H-indoles. Then the axial-to-central chirality transfer starting from enantio-enriched N-1,3-disubstituted allenylindoles affording optically active pyrido[1,2-a]-1H-indoles has been realized in excellent yields and enantioselectivities. A mechanism has been proposed based on mechanistic studies. Synthetic applications have also been demonstrated.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

A unified synthesis of topologically diverse Aspidosperma alkaloids through divergent iminium-trapping

Aspidospermidine, vincadifformine, 1,2-dehydroaspidospermidine, goniomitine, and quebrachamine, five Aspidosperma alkaloids distributed within three structurally diverse topologies, were synthesized from a single molecular scaffold, namely indole-valerolactam 6. This common intermediate can be divergently manipulated, through the incorporation of conformational and electronic constraints that influence the chemo-selectivity of the iminium ion derived therefrom, between three different reaction paths: N(1) vs. C(3) cyclization (indole numbering) vs. over-reduction. Moreover, a catalytic carbene insertion for direct C(3)-H indole functionalization is reported for the first time in an approach to goniomitine (4), and a following tandem ester reduction/iminium generation/cyclization secured its tetracyclic system. The development of a highly diastereoselective one-pot hemi-reduction/cyclization/deprotection process to obtain a cis-pyridocarbazole directly allowed the synthesis of pentacyclic Aspidosperma alkaloids 1, 2, and 3.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Creation of Novel Cyclization Methods Using sp-Hybridized Carbon Units and Syntheses of Bioactive Compounds

Some recent results on the development of new and reliable procedures for the construction of diverse ring systems based on the chemistry of sp-hybridized species, especially allene functionality, are described. This review includes: (i) synthesis of the multi-cyclic skeletons by combination of the π-component of allene with suitable other π-components such as alkyne, alkene, or additional allene under Rh-catalyzed conditions; (ii) synthesis of heterocycles as well as carbocycles by reaction of the sp-hybridized center of allene with some nucleophiles in an endo-mode manner; and (iii) total syntheses of natural products and related compounds from the sp-hybridized starting materials.

A FISCHER INDOLIZATION STRATEGY TOWARD THE TOTAL SYNTHESIS OF (-)-GONIOMITINE

A Fischer indolization strategy toward the core of (-)-goniomitine is reported. Initial investigations into the Pd-catalyzed asymmetric allylic alkylation of dihydropyrido[1,2-a]indolone (DHPI) substrates are also discussed.

Palladium-catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes with O2: a facile protocol to selectively synthesize 2- and 3-vinylindoles

A facile and versatile palladium catalyzed oxidative cyclization of aniline-tethered alkylidenecyclopropanes with O₂ to selectively construct important synthetic intermediates 2- and 3-vinylindoles in moderate to good yields has been developed.

Enantioselective Pd-Catalyzed Allylic Alkylation Reactions of Dihydropyrido[1,2-a]indolone Substrates: Efficient Syntheses of (-)-Goniomitine, (+)-Aspidospermidine, and (-)-Quebrachamine

The successful application of dihydropyrido[1,2-a]indolone (DHPI) substrates in Pd-catalyzed asymmetric allylic alkylation chemistry facilitates rapid access to multiple alkaloid frameworks in an enantioselective fashion. Strategic bromination at the indole C3 position greatly improved the allylic alkylation chemistry and enabled a highly efficient Negishi cross-coupling downstream. The first catalytic enantioselective total synthesis of (-)-goniomitine, along with divergent formal syntheses of (+)-aspidospermidine and (-)-quebrachamine, are reported herein.

Rhodium-catalyzed pyridannulation of indoles with diazoenals: a direct approach to pyrido[1,2-a]indoles

A new rhodium-catalyzed [4 + 2]-pyridannulation of 3-substituted indoles with diazoenals resulted in the biologically important pyrido[1,2-a]indoles.

Total Synthesis of (±)-Goniomitine via Radical Translocation

The aspidosperma alkaloid goniomitine was synthesized in six steps from 2-ethyl-δ-valerolactam. The convergent strategy features an Ullman coupling to assemble the required carbon atoms. A complexity-generating radical translocation reaction was used to build the indole architecture.

Synthesis of a carbon analogue of scytonemin

The synthesis of a carbon analogue of scytonemin was accomplished on the basis of molybdenum-mediated intramolecular double Pauson-Khand type reaction of bis(allenyne), followed by the double aldol condensation of the formed double Pauson-Khand type adduct.

Rh/Cu-catalyzed multiple C–H, C–C, and C–N bond cleavage: facile synthesis of pyrido[2,1-a]indoles from 1-(pyridin-2-yl)-1H-indoles and γ-substituted propargyl alcohols

The Rh/Cu-catalyzed synthesis of pyrido[2,1-a]indoles from 1-(pyridin-2-yl)-1H-indoles and γ-substituted propargyl alcohols involving multiple C–H, C–C, and C–N bond cleavage and construction is presented.

Total synthesis of (-)-goniomitine

The total synthesis of (-)-goniomitine has been accomplished in 11 steps starting from commercially available diethyl l-malate. The synthesis features a chiral pool approach to prepare the chiral C-9 unit containing a quaternary carbon center, an Ir-catalyzed C-H borylation to synthesize the 2-indoleboronic acid pinacol ester, and a Suzuki reaction to couple together the two key intermediates. Notably, the high degree of convergence of this strategy makes it particularly amenable to the total synthesis of other aspidosperma family natural products.

Propargyltrimethylsilanes as allene equivalents in transition metal-catalyzed [5 + 2] cycloadditions

Conventional allenes have not been effective π-reactive 2-carbon components in many intermolecular cycloadditions including metal-catalyzed [5 + 2] cycloadditions. We report herein that rhodium-catalyzed [5 + 2] cycloadditions of propargyltrimethylsilanes and vinylcyclopropanes provide, after in situ protodesilylation, a highly efficient route to formal allene cycloadducts. Propargyltrimethylsilanes function as safe, easily handled synthetic equivalents of gaseous allenes and hard-to-access monosubstituted allenes. In this one-flask procedure, they provide cycloadducts of what is formally addition to the more sterically encumbered allene double bond.

Cyclization and annulation reactions of nitrogen-substituted cyclopropanes and cyclobutanes

This feature article covers recent progress in cyclization and annulation reactions of aminocyclopropanes and aminocyclobutanes to access nitrogen-rich building blocks.

Goniomitine: an overview on the chemistry of this indole alkaloid

This paper reports an overview on the chemistry of the indole alkaloid goniomitine focusing, mainly, on the methods of synthesis related to this natural product and analogs.