A Concise Enantioselective Total Synthesis of (-)-Deoxoapodine

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Benzo-Fused-Ring Toolbox Based on Palladium/Norbornene Cooperative Catalysis: Methodology Development and Applications in Natural Product Synthesis

ConspectusBenzo-fused skeletons are ubiquitous in agrochemicals, medicines, natural products, catalysts, and other organic function materials. The assembly of these skeletons in an efficient manner is an actively explored field in organic synthesis. Palladium/norbornene (Pd/NBE) cooperative catalysis is a powerful tool for the expeditious assembly of polysubstituted arenes through bis-functionalization of the ortho and ipso positions of aryl iodides in one operation. Owing to the efforts of Lautens, Catellani, and others, an array of Pd/NBE-promoted annulations for the syntheses of diversified benzo-fused rings have been developed. However, these methods have not been broadly applied in total synthesis yet.Our group is interested in efficient and practical total synthesis of biologically active molecules. In the past 7 years, we have been devoted to the development of new annulation strategies for the assembly of common benzo-fused skeletons through Pd/NBE-promoted reactions of aryl iodides with novel bifunctional reagents. In this Account, we summarize our laboratory's systematic efforts in this direction. First, readily available epoxides and aziridines were exploited as versatile bifunctional alkylating reagents, which enables quick assembly of a series of valuable benzo-fused heterocycles, including isochromans, dihydrobenzofurans, 1,3-cis-tetrahydroisoquinolines (THIQs), 1,3-trans-THIQs, etc. Second, a convergent access to 5-7-membered benzo-fused carbocycles (including indanes and tetrahydronaphthalenes) was developed by Pd/NBE-promoted annulation of aryl iodides with simple olefinic alcohol-containing alkylating reagents. Third, a Pd/NBE-promoted annulation between aryl iodides and cyclohexanone-containing amination reagents was developed for the construction of benzo-fused N-containing bridged scaffolds. Thus, we have established a practical and versatile toolbox for the quick assembly of diversified benzo-fused skeletons. These new annulation reactions are of high chemo-, regio-, and stereoselectivities with good step and atom economy. Moreover, they are able to rapidly increase molecular complexity from simple building blocks. Finally, their synthetic value has been demonstrated by immediate adoption in several efficient total syntheses of medicines and complex natural products. Compared to conventional synthetic logics, the Pd/NBE-promoted annulation toolbox allows the development of highly convergent strategies, which significantly improves the overall synthetic efficiency.We believe the results presented in this Account will have significant implications beyond our research. It can be envisaged that new Pd/NBE-promoted annulations as well as new applications in complex total synthesis will be revealed in the near future.

Selective Oxidations in the Synthesis of Complex Natural ent-Kauranes and ent-Beyeranes

Syntheses of two natural products derived from the ent-kaurene kaurenoic acid are described for the first time using regio- and diastereoselective oxidations. Palladium- and manganese-mediated oxidations were used to accomplish the syntheses of two ent-beyerane metabolites. The use of the White-Gormisky-Zhao catalyst Mn(CF₃-PDP) enabled the first application of a nondirected metal-catalyzed oxidation in an unactivated C-H bond in a total synthesis.

Total Synthesis of (-)-Voacinol and (-)-Voacandimine C

We describe the first total synthesis of complex aspidosperma alkaloids (-)-voacinol and (-)-voacandimine C via a late-stage C7-methylenation strategy inspired by a biogenetic hypothesis. We envisioned rapid access to these natural alkaloids from a common, symmetrical precursor assembled by methylenation of a D-ring-oxidized variant of the structurally related natural product (-)-deoxoapodine. Chemoselective N9-oxidation of a pentacyclic deoxoapodine precursor enabled the synthesis of the corresponding hexacyclic C8-aminonitrile. Stereocontrolled methylenation of a C8-enamine derivative of deoxoapodine, accessed by ionization of the C8-aminonitrile, afforded a symmetrical dodecacyclic bisaminonitrile as a versatile precursor to these bisindole alkaloids. The final-stage, biosynthesis-inspired, controlled reductive opening of the oxolane substructures of this dodecacyclic intermediate provided a unified approach to (-)-voacinol and (-)-voacandimine C, while direct reduction of the same intermediate afforded the structurally related (-)-methylenebisdeoxoapodine.

Synthesis of Natural Products by C-H Functionalization of Heterocycless

Total synthesis is considered by many as the finest combination of art and science. During the last decades, several concepts were proposed for achieving the perfect vision of total synthesis, such as atom economy, step economy, or redox economy. In this context, C-H functionalization represents the most powerful platform that has emerged in the last years, empowering rapid synthesis of complex natural products and enabling diversification of bioactive scaffolds based on natural product architectures. In this review, we present an overview of the recent strategies towards the total synthesis of heterocyclic natural products enabled by C-H functionalization. Heterocycles represent the most common motifs in drug discovery and marketed drugs. The implementation of C-H functionalization of heterocycles enables novel tactics in the construction of core architectures, but also changes the logic design of retrosynthetic strategies and permits access to natural product scaffolds with novel and enhanced biological activities.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

The application of C–H bond functionalization in the total syntheses of indole natural products

The recent advances in total synthesis of indole natural products focusing on the application of C–H bond functionalization are summarized.

Aspidosperma and Strychnos alkaloids: Chemistry and biology

Of Nature's nearly 3000 unique monoterpene indole alkaloids derived from tryptophan, those members belonging to the Aspidosperma and Strychnos families continue to impact the fields of natural products (i.e., isolation, structure determination, biosynthesis) and organic chemistry (i.e., chemical synthesis, methodology development) among others. This review covers the biological activity (Section 2), biosynthesis (Section 3), and synthesis of both classical and novel Aspidosperma (Section 4), Strychnos (Section 5), and selected bis-indole (Section 6) alkaloids. Technological advancements in genetic sequencing and bioinformatics have deepened our understanding of how Nature assembles these intriguing molecules. The proliferation of innovative synthetic strategies and tactics for the synthesis of the alkaloids covered in this review, which include contributions from over fifty research groups from around the world, are a testament to the creative power and technical skills of synthetic organic chemists. To be sure, Nature-the Supreme molecular architect and source of a dazzling array of irresistible chemical logic puzzles-continues to inspire scientists across multiple disciplines and will certainly continue to do so for the foreseeable future.

Synthesis and reactivity of hydroindole enelactams leading to densely functionalized scaffolds

The 5-endo-trig radical cyclization of N-benzyl-N-[(2-substituted)cycloalkenyl] trichloroacetamides (tetrasubstituted enamides) using Bu3SnH and AIBN is a reliable synthetic procedure giving access to 3a-methyl- and 3a-methoxycarbonyl enelactams. The substrate-controlled diastereoselective enolate alkylation of these enelactams resulted in the synthesis of a set of 3-substituted derivatives that upon reduction furnished polyfunctionalized cis-octahydroindoles. The latter building blocks, which embody three consecutive stereocenters at C-3, C-3a, and C-7a, were also synthesized through an initial reductive radical cyclization using (carbo-substituted)dichloroacetamides.

A Concise Total Synthesis of (-)-Berkelic Acid

Reported here is a concise total synthesis of (-)-berkelic acid in eight linear steps. This synthesis features a Catellani reaction/oxa-Michael cascade for the construction of the isochroman scaffold, a one-pot deprotection/spiroacetalization operation for the formation of the tetracyclic core structure, and a late-stage Ni-catalyzed reductive coupling for the introduction of the lateral chain. Notably, four stereocenters are established from a single existing chiral center with excellent stereocontrol during the deprotection/spiroacetalization process. Stereocontrol of the intriguing deprotection/spiroacetalization process is supported by DFT calculations.