Concise Stereoselective Synthesis of Oxaspirocycles with 1-Tosyl-1,2,3-triazoles: Application to the Total Syntheses of (±)-Tuberostemospiroline and (±)-Stemona-lactam R

Total Synthesis of (-)-Rauvomine B via a Strain-Promoted Intramolecular Cyclopropanation

We describe the first total synthesis of the unusual cyclopropane-containing indole alkaloid (-)-rauvomine B via a strategy centered upon intramolecular cyclopropanation of a tetracyclic N-sulfonyltriazole. Preparation of this precursor evolved through two generations of synthesis, with the ultimately successful route involving a palladium-catalyzed stereospecific allylic amination, a cis-selective Pictet-Spengler reaction, and ring-closing metathesis as important bond-forming reactions. The key cyclopropanation step was found to be highly dependent on the structure and conformational strain of the indoloquinolizidine N-sulfonyltriazole precursor, the origins of which are explored computationally through DFT studies. Overall, our synthesis proceeds in 11 total steps and 2.4% yield from commercial materials.

Modular Synthesis of Highly Substituted 3-Azapyrroles by Rh(II)-Catalyzed N-H Bond Insertion and Cyclodehydration

A modular synthesis of highly substituted 3-azapyrroles has been developed using a three-step sequence comprising copper-catalyzed alkyne-azide cycloaddition (CuAAC), N-H bond insertion, and cyclodehydration. 1-Sulfonyl-1,2,3-triazoles (1-STs) can be accessed from common alkyne and sulfonyl azide building blocks by CuAAC using CuTC. Rhodium(II)-acetate-promoted 1-ST denitrogenation results in highly electrophilic rhodium azavinyl carbenes that, here, underwent insertion into the N-H bond of secondary α-aminoketones to form 1,2-aminoalkenes. These products were cyclized and dehydrated using BF₃·OEt₂ into highly substituted 3-azapyrroles. The three steps (CuAAC, N-H bond insertion, and cyclodehydration) could be telescoped into a one-pot process. The method proved to be highly efficient and tolerated a wide range of substituents.

Strategies for the synthesis of Stemona alkaloids: an update

Covering: 2009 to 2022The Stemona alkaloids, which are found in plant species from the family Stemonaceae, represent a tremendously large and structurally-diverse family of natural products. This review presents and discusses a selection of case studies, grouped by alkaloid class, that showcase the key strategies and overall progress that has been made in the synthesis of Stemona alkaloids and related compounds since 2009. Structural reassignments that have been reported over this period are also identified where necessary.

Annulation-Triggered Denitrogenative Transformations of 2-(5-Iodo-1,2,3-triazolyl)benzoic Acids

The ability of [1,2,3]triazolobenzoxazinones to act as a source of "hidden" diazo group was discovered. These diazo precursors can be easily prepared by the intramolecular cyclization of 2-(5-iodo-1,2,3-triazolyl)benzoic acids. The Cu-catalyzed capture of the hidden diazo group allows for further functionalization through the denitrogenative pathway. The transformations proceed via the formation of either diazoimine or diazoamide intermediates. Novel routes to various anthranilamides as well as thiolated benzoxazinones were developed using the one-pot cyclization/diazo capture procedure.

Asymmetric Total Syntheses of Five Pyrrole-Type Stemona Alkaloids

The asymmetric total syntheses of five pyrrole-type Stemona alkaloids and two stereoisomers were accomplished, among which 3-n-butylneostemonine and bisdehydroneostemonine were synthesized for the first time, and the NMR data of...

Synthesis of Various Bridged Ring Systems via Rhodium-Catalyzed Bridged (3+2) Cycloadditions

Here, we describe the rhodium-catalyzed bridged (3+2) cycloaddition cascade reactions of N-sulfonyl-1,2,3-triazoles, which allowed the efficient diastereoselective construction of various functionalized and synthetically challenging bridged ring systems. This simple, direct transformation had a broad substrate scope and excellent functional group tolerance. The highly strained polycyclic bicyclo[2.2.2]octa[b]indole core of fruticosine was synthesized efficiently using this methodology.

Tailored Synthesis of Skeletally Diverse Stemona Alkaloids through Chemoselective Dyotropic Rearrangements of β-Lactones

The collective synthesis of skeletally diverse Stemona alkaloids featuring tailored dyotropic rearrangements of β-lactones as key elements is described. Specifically, three typical 5/7/5 tricyclic skeletons associated with stemoamide, tuberostemospiroline and parvistemonine were first accessed through chemoselective dyotropic rearrangements of β-lactones involving alkyl, hydrogen, and aryl migration, respectively. By the rational manipulation of substrate structures and reaction conditions, these dyotropic rearrangements proceeded with excellent efficiency, good chemoselectivity and high stereospecificity. Furthermore, several polycyclic Stemona alkaloids, including saxorumamide, isosaxorumamide, stemonine and bisdehydroneostemoninine, were obtained from the aforementioned tricyclic skeletons through late-stage derivatizations. A novel visible-light photoredox-catalyzed formal [3+2] cycloaddition was also developed, which offers a valuable tool for accessing oxaspirobutenolide and related scaffolds.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Rhodium-catalyzed Sommelet–Hauser type rearrangement of α-diazoimines: synthesis of functionalized enamides

An efficient rhodium catalyzed Sommelet–Hauser type rearrangement of sulfur ylides derived from α-thioesters and N-sulfonyl-1,2,3-triazoles has been successfully accomplished for the synthesis of various functionalized enamides.

Cascade Rh(ii) and Yb(iii) catalyzed synthesis of substituted naphthofurans via transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols

Synthesis of substituted naphthofurans has been achieved via an efficient cascade Rh(ii)/Yb(iii) catalyzed transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols.

Rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones with 1-sulfonyl-1,2,3-triazole: synthesis of cyclopropa[cd]indole and benzofuran derivatives

A novel rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones and 1-sulfonyl-1,2,3-triazole is disclosed, affording cyclopropa[cd]indole and benzofuran derivatives in moderate to good yields.

The Expanding Utility of Rhodium-Iminocarbenes: Recent Advances in the Synthesis of Natural Products and Related Scaffolds

Rhodium-iminocarbenes that are derived from N-sulfonyl-1,2,3-triazoles have become an important class of reactive species and useful intermediates in organic synthesis. Over the last several years, many practical and versatile approaches involving rhodium-iminocarbene intermediates to synthetically challenging molecules (scaffolds) have been developed. This Minireview mainly summarizes the recent advance of rhodium-iminocarbene involved reactions in the synthesis of natural products and their related scaffolds by the end of 2017. Several applications in important pharmaceuticals are documented as well.

Stereoselective construction of sterically hindered oxaspirocycles via chiral bidentate directing group-mediated C(sp3)-O bond formation

The systematic investigation of chiral bidentate auxiliaries has resulted in the discovery of a chiral 2,2-dimethyl-1-(pyridin-2-yl)propan-1-amine-derived directing group that enables stereoselective palladium(ii)-catalyzed intramolecular C(sp3)-O bond formation. This new chiral directing group exhibited high reactivity in the activation of methylene C(sp3)-H bonds with excellent levels of stereoselectivity (a diastereomeric ratio of up to 39 : 1), which allowed the construction of a wide range of oxaspirocycles. Mechanistic investigations were also conducted to elucidate the reaction mechanism and understand the origin of the diastereoselectivity. DFT calculations suggest that only modest levels of diastereoselectivity are accomplished at the rate-determining C-H metalation-deprotonation step and the d.r. is further enriched at the reductive elimination step.

Diversity-oriented synthesis of heterocycles and macrocycles by controlled reactions of oxetanes with α-iminocarbenes

Using N-sulfonyl triazoles as substrates, compounds as diverse as 2-imino tetrahydrofurans, 13- and 15-membered ring aza-macrocycles can be prepared selectively via formal [1 + 4], [5 + 4 + 4] and [3 + 4 + 4 + 4] condensations of α-imino carbenes and oxetanes under Rh(ii)-catalysis or thermal activation. Spirocyclic N-heterocycles are also accessible by means of Buchwald-Hartwig and Pictet-Spengler cyclizations. By reaction control, substrate selection or further derivatization, a large variety of chemical structures is thus achievable. Finally, using triazoles reacting under thermal activation, interesting mechanistic insight was obtained.

Synthesis of 6-Substituted Piperidin-3-ones via Rh(II)-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles with Electron-Rich Aromatic Nucleophiles

A highly diastereoselective rhodium(II)-catalyzed transannulation of aldehyde-tethered N-sulfonyl triazoles with electron-rich aromatic nucleophiles is reported for the first time to afford functionalized 6-substituted piperidin-3-ones. The reaction has a broad substrate scope including both aliphatic and aromatic N-sulfonyl-1,2,3-triazoles together with various aromatic nucleophiles. The addition of a catalytic amount of Lewis acid has proven to be crucial for the yield improvement. By employing this methodology, hardly accessible piperidin-3-ones bearing quaternary carbons could be obtained.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

A rhodium-catalyzed tandem reaction of N-sulfonyl triazoles with indoles: access to indole-substituted indanones

Rhodium(ii)-catalyzed tandem reaction of N-sulfonyl triazoles with indoles delivered structurally diverse indole-substituted indanones bearing vicinal quaternary carbon centers.

Straightforward regioselective construction of 3,4-dihydro-2H-1,4-thiazine by rhodium catalysed [3 + 3] cycloaddition of thiirane with 1-sulfonyl-1,2,3-triazole: a pronounced acid additive effect

A rhodium catalyzed regioselective coupling of thiirane with 1-sulfonyl-1,2,3-triazole has been realized to give a formal [3 + 3] cycloaddition product 3,4-dihydro-2H-1,4-thiazine via the 1,3-insertion model of azavinyl carbene.

Base-induced synthesis of N-dialkylaminomethyl-2H-1,2,3-triazoles from N-sulfonyl-1,2,3-triazoles

A facile synthetic method to access N-dialkylaminomethyl-2H-1,2,3-triazoles has been developed from N-sulfonyl-1,2,3-triazoles by a base induced reaction. Moreover, 4-phenyl-1H-1,2,3-triazoles can also give the same product even in the absence of a base at high temperature.

Facile synthesis of substituted 3-aminofurans through a tandem reaction of N-sulfonyl-1,2,3-triazoles with propargyl alcohols

A relay catalysis strategy for substituted 3-aminofurans synthesis has been developed.

Stereodivergent Intramolecular C(sp(3))-H Functionalization of Azavinyl Carbenes: Synthesis of Saturated Heterocycles and Fused N-Heterotricycles

A general approach for the formation of five-membered saturated heterocycles by intramolecular C(sp(3))-H functionalization is reported. Using N-sulfonyltriazoles as Rh(II) azavinyl carbene equivalents, a wide variety of stereodefined cis-2,3-disubstituted tetrahydrofurans were obtained with good to excellent diastereoselectivity from readily available acyclic precursors. The reaction is shown to be amenable to gram scale, and judicious choice of reaction conditions allowed for stereodivergence, providing selective access to the trans diastereomer in good yield. The resulting products were shown to be valuable intermediates for the direct preparation of fused N-heterotricycles in one step by intramolecular C-H amination or Pictet-Spengler cyclization.

Recent Advances in the Synthesis of Stemona Alkaloids

Stemona alkaloids, featuring polycyclic structures and interesting bioactivities, constitute a distinct class from the Stemonaceae family. In this review, recent advances in the synthesis of these unique alkaloids are briefly discussed, highlighting the application of novel synthetic strategies to access the core structures, as well as creative solutions to the installation of multiple stereogenic centers. The literature reviewed in this article covers the publications from 2010 to November 2014, a period that witnessed the prosperity of the synthesis of Stemona alkaloids.

A Rh-catalyzed 1,2-sulfur migration/aza-Diels–Alder cascade initiated by aza-vinyl carbenoids from sulfur-tethered N-sulfonyl-1,2,3-triazoles

A novel Rh(ii) catalyzed intramolecular 1,2-sulfur migration/intermolecular aza-Diels–Alder cascade of sulfur-tethered N-sulfonyl-1,2,3-triazoles has been developed, efficiently affording sulfur-containing tetrahydropyridine derivatives.

Rh(ii)-catalyzed cycloadditions of 1-tosyl 1,2,3-triazoles with 2H-azirines: switchable reactivity of Rh-azavinylcarbene as [2C]- or aza-[3C]-synthon

The Rh(ii)-catalyzed formal [3+2] and [3+3] cycloadditions of 1-tosyl 1,2,3-triazoles with 2H-azirines have been developed, which enable the efficient synthesis of polysubstituted 3-amino-pyrroles and 1,2-dihydropyrazines, respectively.

An efficient approach to 1,2,3-trisubstituted indole via rhodium catalyzed carbene Csp3–H bond insertion

A method for convenient synthesis of N-alkyl-2-aryl-indole-3-carbaldehyde via carbene C–H bond insertion.