Enantioselective palladium-catalyzed allylic alkylation reactions in the synthesis of Aspidosperma and structurally related monoterpene indole alkaloids

Enantioselective Synthesis of Indole-Fused Bicyclo[3.2.1]octanes via Palladium(II)-Catalyzed Cascade Reaction

Indole-fused bicyclo[3.2.1]octanes are highly important structural units in natural products and biologically active compounds. However, there has been limited success in the enantioselective synthesis of these skeletons due to the complexity of the structure and the control of the enantioselectivity. Herein an enantioselective construction of indole-fused bicyclo[3.2.1]octanes bearing an all-carbon quaternary bridgehead stereocenter was developed via an aminopalladition-triggered Heck-type reaction. The protocol features mild conditions and good tolerance for a wide range of functional groups. The transformation can also be scaled up to demonstrate its practicability. The mechanistic studies reveal that the formation of an intermediate indol-3-yl palladium species via C-H activation should be ruled out.

Synthetic Approaches to Tricyclic Aminoketones in the Total Synthesis of Aspidosperma and Kopsia Alkaloids

Aspidosperma and kopsia alkaloids are significant functional molecules because of their potent biological activities. Their intricate structures present an intrinsic synthetic challenge and thus attract significant attention from synthetic organic academic community. Over the past decades, a series of elegant strategies has been developed, in particular, the Stork's original Fischer indolization of tricyclic aminoketones 1. Herein, we report a comprehensive review on various synthetic approaches access to tricyclic aminoketones 1 and provide a practical guidance to readers whose are interested in employing tricyclic aminoketones 1 as versatile building blocks in the realm of total synthesis of aspidosperma, kopsia and structurally related alkaloids.

Pd-catalyzed arylation/aza-Michael addition cascade to C2-spiroindolines and azabicyclo[3.2.2]nonanones

A palladium-catalyzed arylation/aza-Michael addition cascade reaction of β-substituted cyclic enones and 2-haloanilines has been reported. Using 1 mol% Pd(PPh₃)₄ as a catalyst, C2-spiroindolines are accessed via an intermolecular vinylogous arylation of β-alkyl cyclic enones and 2-haloanilines followed by an intramolecular aza-Michael addition. The functional group tolerance of this transformation is examined by 18 examples in up to 93% yield. In the second part, we developed an α'-arylation/aza-Michael addition cascade strategy to construct azabicyclo[3.2.2]nonanones catalyzed by Pd(MeCN)₂Cl₂·PPh₃. This study provides a quick route to complex and useful spiro- and bridged-heterocycles from readily available starting materials in good yields with high regioselectivity.

Optically Active Flavaglines-Inspired Molecules by a Palladium-Catalyzed Decarboxylative Dearomative Asymmetric Allylic Alkylation

With the aid of a class of newly discovered Trost-type bisphosphine ligands bearing a chiral cycloalkane framework, the Pd-catalyzed decarboxylative dearomative asymmetric allylic alkylation (AAA) of benzofurans was achieved with high efficiency [0.2-1.0 mol% Pd₂(dba)₃/L], good generality, and high enantioselectivity (>30 examples, 82-99% yield and 90-96% ee). Moreover, a diversity-oriented synthesis (DOS) of previously unreachable flavaglines is disclosed. It features a reliable and scalable sequence of the freshly developed Tsuji-Trost-Stoltz AAA, a Wacker-Grubbs-Stoltz oxidation, an intra-benzoin condensation, and a conjugate addition, which allows the efficient construction of the challenging and compact cyclopenta[b]benzofuran scaffold with contiguous stereocenters. This strategy offers a new avenue for developing flavagline-based drugs.

Total Synthesis Provides Strong Evidence: Xestocyclamine A is the Enantiomer of Ingenamine

Xestocyclamine A ((−)-1) is featured prominently in a biosynthesis pathway leading to a large family of polycyclic alkaloids. The first total synthesis now proves that the structure of this compound had originally been misassigned. The route to (−)-1 is based on a double Michael addition for the formation of the bridged diazadecalin core and a palladium-catalyzed decarboxylative allylation to install the quaternary bridgehead center. Ring-closing alkyne metathesis allowed a 13-membered cycloalkyne to be forged, which was selectively reduced during an involved sequence of hydroboration/selective protodeborylation/alkyl-Suzuki coupling used to close the 11-membered ring. Crystallographic data prove the identity of synthetic (−)-1 with nominal xestocyclamine, but the spectra differ from those of the authentic alkaloid. To clarify the point, the synthesis was redirected toward ingenamine (3), which is supposedly a positional isomer of 1. The recorded data confirm the assignment of this particular natural product and strongly suggest that xestocyclamine A is in fact the enantiomer of ingenamine (+)-3.

Total Syntheses of Dihydroindole Aspidosperma Alkaloids: Reductive Interrupted Fischer Indolization Followed by Redox Diversification

We report a novel reductive interrupted Fischer indolization process for the concise assembly of the 20-oxoaspidospermidine framework. This rapid complexity generating route paves the way toward various dihydroindole Aspidosperma alkaloids with different C-5 side chain redox patterns. The end-game redox modulations were accomplished by modified Wolff-Kishner reaction and photo-Wolff rearrangement, enabling the total synthesis of (-)-aspidospermidine, (-)-limaspermidine, and (+)-17-demethoxy-N-acetylcylindrocarine and the formal total synthesis of (-)-1-acetylaspidoalbidine.

Probing Trends in Enantioinduction via Substrate Design: Palladium-Catalyzed Decarboxylative Allylic Alkylation of α-Enaminones

Herein, we report the palladium-catalyzed decarboxylative asymmetric allylic alkylation of α-enaminones. In addition to serving as valuable synthetic building blocks, we exploit the α-enaminone scaffold and its derivatives as probes to highlight structural and electronic factors that govern enantioselectivity in this asymmetric alkylation reaction. Utilizing the (S)-t-BuPHOX ligand in a variety of nonpolar solvents, the alkylated products are obtained in up to 99% yield and 99% enantiomeric excess.

Total Syntheses of (-)-Minovincine and (-)-Aspidofractinine through a Sequence of Cascade Reactions

We report 8‐step syntheses of (−)‐minovincine and (−)‐aspidofractinine using easily available and inexpensive reagents and catalyst. A key element of the strategy was the utilization of a sequence of cascade reactions to rapidly construct the penta‐ and hexacyclic frameworks. These cascade transformations included organocatalytic Michael‐aldol condensation, a multistep anionic Michael‐S_N2 cascade reaction, and Mannich reaction interrupted Fischer indolization. To streamline the synthetic routes, we also investigated the deliberate use of steric effect to secure various chemo‐ and regioselective transformations.

Palladium-catalysed dearomative aryl/cycloimidoylation of indoles

The first example of a dearomative palladium-catalysed isocyanide insertion reaction has been developed using functionalized isocyanides as the reaction partner of N-(2-bromobenzoyl)indoles. The imidoyl-palladium intermediate generated by tandem indole double bond/isocyanide insertion reactions could be trapped by intramolecular functional groups such as the C(sp2)-H bond and alkenes, affording diversified indoline derivatives bearing C3 imine-containing heterocycles. The dearomative aryl/cycloimidoylation of indoles proceeded smoothly in good to excellent yields with a wide functional group tolerance.

Heat- or light-induced acylarylation of unactivated alkenes towards 3-(α-acyl) indolines

A heat- or photoredox/iron dual catalysis-enabled rare example of radical acylation across unactivated alkenes with aldehydes.

Kinetic Resolution of Racemic Aldehydes through Asymmetric Allenoate γ-Addition: Synthesis of (+)-Xylogiblactone A

A synthesis of (+)-xylogiblactone A has been achieved from t-butyl 2-methylbuta-2,3-dienoate in a linear three-step sequence. The key elements of the synthesis include a kinetic resolution of racemic 2-silyoxyaldehyde through the allenoate γ-addition to yield the γ-adduct as a single isomer and the subsequent gold catalysis to form the butenolide core. For a general method, the kinetic resolution of several racemic 2-silyloxyaldehydes is also performed to provide products in high levels of stereoselectivity with unusual anti-Felkin-Anh addition fashion.

Access to Benzylic Quaternary Carbons from Aromatic Ketones

The construction of benzylic all-carbon quaternary stereocenters, which are ubiquitous in biomolecules and drugs, is a task of high practical significance. Herein, we disclose a highly efficient one-pot method of constructing all-carbon quaternary structural units from aryl ketones, revealing that the entire process involves three consecutive chemical events, namely nucleophilic addition, Meinwald 1,2-hydrogen migration, and alkylation. Interestingly, dimerization of acetophenones results in formation of 2,4-diarylfurans under the employed conditions rather than the quaternary carbon products.

Indole Alkaloid Synthesis Facilitated by Photoredox Catalytic Radical Cascade Reactions

The monoterpene indole alkaloids, containing over 3000 known members and more than 40 structural types, represent one of the largest natural product families that have proven to be an important drug source. Their complex chemical structures and significant biological activities have rendered these alkaloids attractive targets in the synthetic community for decades. While chemists have developed many synthetic methodologies and tactics toward this end, general strategies allowing divergent access to a large variety of structural types and members of monoterpene indole alkaloids are still limited and highly desirable. Photoredox catalysis has emerged in recent years as a powerful tool to realize chemical transformations via single electron transfer (SET) processes that would otherwise be inaccessible. In particular, when the radical species generated by the visible light photoinduced approach is involved in well-designed cascade reactions, the formation of multiple chemical bonds and the assembly of structurally complex molecules would be secured in a green and economic manner. This protocol might serve to remodel the way of thinking for the preparation of useful pharmaceuticals and complex natural products. Due to a long-standing interest in the synthesis of diverse indole alkaloids, our group previously developed a cyclopropanation strategy ( Qin , Y. Acc. Chem. Res. 2011 , 44 , 447 ) that was versatile to access several intriguing indole alkaloid molecules. With an idea of developing more general synthetic approaches to as many members of various indole alkaloids as possible, we recently disclosed new radical cascade reactions enabled by photoredox catalysis, leading to the collective asymmetric total synthesis of 42 monoterpene indole alkaloids belonging to 7 structural types. Several important discoveries deserve to be highlighted. First, the use of photocatalytic technology allowed us to achieve an unusual reaction pathway that reversed the conventional reactivity between two nucleophilic amine and enamine groups. Second, a crucial nitrogen-centered radical, directly generated from a sulfonamide N-H bond, triggered three types of cascade reactions to deliver indole alkaloid cores with manifold functionalities and controllable diastereoselectivities. Moreover, expansion of this catalytic, scalable, and general methodology permitted the total synthesis of a large collection of indole alkaloids. In this Account, we wish to provide a complete picture of our studies concerning the original synthetic design, method development, and applications in total synthesis. It is anticipated that the visible-light-driven cascade strategy will find further utility in the realm of natural product synthesis.

Palladium-Catalyzed Decarboxylative Alkynylation of α-Acyloxyketones by C(sp3 )-O Bond Cleavage

Palladium-catalyzed decarboxylative alkynylation of α-acyloxyketones triggered by C(sp³ )-O bond cleavage is disclosed. The decarboxylation strategy featuring a neutral reaction condition enabled an unprecedent catalytic alkynylation of a ketone enolate. The reaction was applied to a variety of substrates, giving desired products in good yields. We successfully obtained X-ray crystallography of a new palladium-enolate intermediate that was synthesized by a reaction of [Pd(cod)(CH₂ TMS)₂ ] with XPhos and α-acyloxyketone at room temperature, indicating facile C(sp³ )-O bond disconnection.

Spiro Indane-Based Phosphine-Oxazolines as Highly Efficient P,N Ligands for Enantioselective Pd-Catalyzed Allylic Alkylation of Indoles and Allylic Etherification

A series of indane-based phosphine-oxazoline ligands with a spirocarbon stereogenic center were examined for palladium-catalyzed asymmetric allylic alkylation of indoles. Under optimized conditions, high yields (up to 98%) and enantioselectivities (up to 98% ee) were obtained with a broad scope of indole derivatives. The ligand was determined to be the most efficient P,N-ligand for this reaction. Moreover, the ligand was also efficient for Pd-catalyzed asymmetric allylic etherification with hard aliphatic alcohols as nucleophiles.

Collected mass spectrometry data on monoterpene indole alkaloids from natural product chemistry research

This Data Descriptor announces the submission to public repositories of the monoterpene indole alkaloid database (MIADB), a cumulative collection of 172 tandem mass spectrometry (MS/MS) spectra from multiple research projects conducted in eight natural product chemistry laboratories since the 1960s. All data have been annotated and organized to promote reuse by the community. Being a unique collection of these complex natural products, these data can be used to guide the dereplication and targeting of new related monoterpene indole alkaloids within complex mixtures when applying computer-based approaches, such as molecular networking. Each spectrum has its own accession number from CCMSLIB00004679916 to CCMSLIB00004680087 on the GNPS. The MIADB is available for download from MetaboLights under the identifier: MTBLS142 (https://www.ebi.ac.uk/metabolights/MTBLS142).

Design Type(s)	mass spectrometry data transformation objective • mass spectrometry data analysis objective • data integration objective
Measurement Type(s)	mass spectrum
Technology Type(s)	liquid chromatography-tandem mass spectrometry
Factor Type(s)
Sample Characteristic(s)	Strychnos usambarensis • Picralima nitida • Geissospermum laeve • Pleiocarpa mutica • Alstonia • Callichilia inaequalis • Chimarris cymosa • Mostuea brunonis • Gonioma < moth > • Cinchona • Catharanthus roseus • Voacanga grandifolia

Machine-accessible metadata file describing the reported data (ISA-Tab format)

Enantioselective Divergent Synthesis of C19-Oxo Eburnane Alkaloids via Palladium-Catalyzed Asymmetric Allylic Alkylation of an N-Alkyl-α,β-unsaturated Lactam

The C19-oxo-functionalized eburnane alkaloids display unique chemical structure and interesting biological activity. Herein, we report a divergent enantioselective strategy to access these alkaloids by use of a challenging palladium-catalyzed asymmetric allylic alkylation of an N-alkyl-α,β-unsaturated lactam. 19-( S)-OH-Δ¹⁴-vincamone (phutdonginin), (-)-19-OH-eburnamine, (+)-19-oxoeburnamine, and (+)-19-OH-eburnamonine (1-4) have been concisely synthesized for the first time in 11 to 13 steps.

Formal Aza-Wacker Cyclization by Tandem Electrochemical Oxidation and Copper Catalysis

In oxidative electrochemical organic synthesis, radical intermediates are often oxidized to cations on the way to final product formation. Herein, we describe an approach to transform electrochemically generated organic radical intermediates into neutral products by reaction with a metal catalyst. This approach combines electrochemical oxidation with Cu catalysis to effect formal aza-Wacker cyclization of internal alkenes. The Cu catalyst is essential for transforming secondary and primary alkyl radical intermediates into alkenes. A wide range of 5-membered N-heterocycles including oxazolidinone, imidazolidinone, thiazolidinone, pyrrolidinone, and isoindolinone can be prepared under mild conditions.

Catalytic asymmetric dearomatization (CADA) reaction-enabled total synthesis of indole-based natural products

The recent enantioselective total syntheses of natural products enabled by catalytic asymmetric dearomatization reactions of indole derivatives are presented.

Iodospirocyclization of Tryptamine-Derived Isocyanides: Formal Total Synthesis of Aspidofractinine

The N-iodosuccinimide-mediated spirocyclization of tryptamine-derived isocyanides to generate spiroindolenines is reported. The products contain both an imine and an imidoyl iodide as flexible handles for follow-up chemistry. Nucleophilic addition typically occurs chemoselectively on the imine moiety with complete diastereoselectivity, providing opportunities for the construction of complex molecular frameworks. The synthetic potential of the method was showcased in the formal total synthesis of (±)-aspidofractinine.

Synthesis of 8-Hydroxygeraniol

An operationally simple protocol for the conversion of geranyl acetate to 8-hydroxygeraniol is reported. The convenient two-step procedure relies on an efficient, chemo- and regioselective SeO₂-promoted oxidation, followed by straightforward deacetylation. This facile means to prepare 8-hydroxygeraniol is expected to enable biosynthetic studies pertaining to thousands of monoterpene indole alkaloids.

Synthesis of highly rigid phosphine–oxazoline ligands for palladium-catalyzed asymmetric allylic alkylation

Rigid phosphine–oxazoline ligands with a spirocarbon stereogenic center were developed, which exhibited excellent catalytic performance for Pd-catalyzed allylic alkylation reactions.