New Cyclization Route to the Skeletons of Lennoxamine and Chilenine

A new cyclization route, triggered by epoxide opening, has been performed to provide the key intermediates for isoindolobenzapine alkaloids, lennoxamine and chilenine. The epoxide was prepared by the Stille reaction using vinyltributylstannane and the following dioxirane treatment. Cyclization under the treatment of BF₃ · OEt₂ provided an azepine moiety, and the oxidative cyclization toward the known precursor for the alkaloids has been achieved by reaction with a stoichiometric amount of Pd(OAc)₂. This formal synthesis suggests a new route to the alkaloids.     

Synthesis of cis-Fused Bicyclic Systems by Radical Cyclization Approach: Formal Synthesis of Ethisolide and iso-Avenaciolide

Formal synthesis of ethisolide and iso-avenaciolide was achieved using furanoid glycal-vinyl radical intermediates. The vinyl radical cyclization by 5-exo-dig mode gave the cis-fused bicyclic systems with an efficient introduction of the exo-methylene group, besides helping in the inversion of the adjacent stereocentre. Further, the study describes the synthesis of cis-fused bicyclic systems from L-arabinose and D-xylose for the creation of diverse natural and synthetic products of this class.     

Free Radical Cyclization of Acyclic Sugar Dithioacetals: An Approach to Mannostatin A Analogues

Abstract

The tributyltin hydride + AIBN mediated free radical cyclization of oxime ethers tethered dithioacetals 6 and 12, obtained from D-ribose or D-glucose, respectively, is reported. The desired carbocycles 7, 8 and 14 have been obtained in good yield and moderate diastereoselectivity. These products are new mannostatin A analogues.     

Ugi/原子转移自由基环合反应合成γ-丁内酰胺的研究

将碘乙酸、烯丙基胺、芳香醛和异腈室温下在甲醇中进行多组分Ugi反应得到加合物,进一步在二氯甲烷中以三乙基硼为引发剂,进行α-酰胺基自由基原子转移5-exo环合反应,可以得到γ-丁内酰胺,操作简便,产率中等.     

Synthesis of 4''''-Spironucleoside via Radical Translocation Cyclization Reaction

A novel synthetic strategy for the construction of 4＇-spironucleosides via cascade radical translocation cyclization of N-allyl-N-（2＇-bromophenyl） amide moiety of 5＇-carboxylic nucleosides and the N-propynyl analogues is described. The conformationally restricted nucleosides herein synthesized are of potentially biological interests.     

Synthesis of 3-Arylselanyl Benzothiophenes through Visible-Light-Mediated Radical Cyclization

A novel method for synthesizing 3-arylselanyl benzothiophenes through visible-light-mediated radical cyclization was developed. Diaryl selenides were used as aryl selenium sources under the irradiation of white-light-emitting diodes (5 W). The reactions were conducted under mild conditions, where the desired products were obtained at room temperature (23 °C) without the need for catalysts or additives. This simple method allows for an efficient route for synthesizing benzothiophene derivatives, which are crucial scaffolds in organic synthesis.     

α-Carbonyl Radical Cyclization for the Total Synthesis of Natural Products

Intramolecular radical cyclization reactions are now used routinely to synthesize carbocyclic and heterocyclicstructures. We have reported that α-carbonyl radicals 1, generated from the corresponding iodo ketones or enones,underwent intramolecular radical cyclization smoothly to afford products 2. [1,2]     

The Tricyclooctanone Approach to the Total Synthesis of Steroids. The Cyclization of the A-CD Unit

The first steps of a novel approach to the total synthesis of 9, 11-dehydroestrone via tricyclo[3.3.0.0^2,8]octan-3-one (2) are described. One route involves a tandem-type transformation of the key intermediate 3 (A-CD unit) consisting of cyclopropane cleavage and ring B closure to afford C, 18-bisnor-13 α, 17 α-estradiol derivatives. E.g. the 3-methoxy-9 ζ-hydroxy-17 α-methanesulfonyloxy derivative (-)- 6 has been synthesized in 8 steps and 10% overall yield from 1,3-cyclohexadiene. As an alternative, the A-CD type intermediate 4b has been prepared and could be used for a ring C enlargement prior to cyclization.     

A Novel Cyclization of Squalene Initiated by Radical Cation

Squalene 1 is recognized as the precursor of sterols and polycyclic triterpenoids that play important roles in numerous life processes¹. In view of the fact that the cyclization of hexa-1, 5-diene by γ-irradiation² and some isoprenoid polyenes by photochemical method³ through radical cation intermediates have been reported and that tris (p-bromophenyl)ammoniumyl hexachloroantimonate (TBPA⁺SbCl₆, 2) could oxidize olfines to the corresponding radical cations that underwent pericyclic reactions⁴, we thought it might be worthwhile to try the biomimetic cyclization of squalene initiated by radical cations. Previously we⁵ have reported the cyclization of geraniol and nerol to cis-p-mentha-2,8-dien-l-ol by reaction with 2. Herein, we report the cyclization of 1 initiated by 2 in dry dichloromethane to give 20-R-protosta-5,24-diene 3a and 20-S-protosta-5,24-diene 3b in the ratio of 1:1 with the total yield of 20%(Scheme 1).     

Intramolecular Ketone-Olefin Radical Cyclization with Low-Valent Titanium Reagent: Synthesis of Benzopyrans

A novel protocol for intramolecular ketyl-olefin radical cyclization with low-valent titanium reagent is outlined. It allows the formation of the benzopyran nucleus from ortho-allyloxy propiophenones as the sole product in moderate yields via intramolecular radical cyclization.     

通过自由基串联环化反应合成4-喹诺酮类化合物

4-喹诺酮结构广泛存在于天然产物和药物分子中. 本工作以易得的N-芳基-O-炔丙基氨基甲酸酯为原料, CO为羰基源, 发展了一种高效、模块化合成4-喹诺酮结构的方法. 氨基甲酸酯底物经2-碘酰基苯甲酸(IBX)氧化产生酰胺氮自由基, 进而发生自由基串联环化反应得到4-喹诺酮结构.     

Organic Photocatalytic Cyclization of Polyenes: A Visible‐Light‐Mediated Radical Cascade Approach

A visible‐light‐mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro‐2‐propanol. The catalyst system is also suitable for 1,3‐dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.     

Tandem Electrocyclic Ring Opening/Radical Cyclization: Application to the Total Synthesis of Cribrostatin 6

A concise total synthesis of cribrostatin 6 (1), an antimicrobial and antineoplastic agent, was accomplished using a tandem electrocyclic ring opening/radical cyclization sequence. Specifically, intermediate 4 underwent a 4π-electrocyclic ring opening, radical cyclization, and homolytic aromatic substitution sequence followed by an oxidation to afford the natural product 1 in one pot. Owing to the rapid buildup of complexity in the key step, 1 could be synthesized from commercially available starting materials in only four linear steps. Application of this chemistry to the concise syntheses of analogs of cribrostatin 6 (1) is also reported.     

A Free Radical Cyclization Catalyzed by Ruthenium Hydride Species

A photolytically generated ruthenium hydride species catalyzing a free radical cyclization reaction was developed. As the new methodology ensures reproducibility of the free radical reaction of trialkyltin hydrides and a fast hydrogen transfer to the radical intermediates, the methodology provides fast quenching of radical intermediates and thus suppresses rearrangement of radical intermediates before the hydride quench. By offering new reactivity and selectivity to the trialkyltin hydride mediated free radical cyclization reactions, the methodology will find wide range of applications in organic synthesis.