Cyclizations silver® acetate

Catalysts. Silver and silver compounds are widely used in research and industry as catalysts for oxidation, reduction, and polymerization reactions. Silver nitrate has been reported as a catalyst for the preparation of propylene oxide (qv) from propylene (qv) (58), and silver acetate has been reported as being a suitable catalyst for the production of ethylene oxide (qv) from ethylene (qv) (59). The solubiUty of silver perchlorate in organic solvents makes it a possible catalyst for polymerization reactions, such as the production of butyl acrylate polymers in dimethylformamide (60) or the polymerization of methacrylamide (61). Similarly, the solubiUty of silver tetrafiuoroborate in organic solvents has enhanced its use in the synthesis of 3-pyrrolines by the cyclization of aHenic amines (62). 

Reaction of 2-pyrrolylbenzoic acid derivative 122 with PCI5 and subsequent reaction with diazomethane gave the diazoacetophenone 123 that upon treatment with silver oxide, sodium carbonate and sodium thiosulfate afforded acetic acid derivative 124, cyclization with acetic anhydride gave 125 (91JHC77) (Scheme 24). 

Very recently examples of tandem Michael-azomethine ylide cyclization reactions have been presented.626 Thus, divinyl sulfone reacted with imine (124) in the presence of lithium bromide and tri-ethylamine to give (126) in 40% yield (Scheme 38). Presumably formation of Michael adduct (125), tau-tomerization to an azomethine ylide and ensuing intramolecular [3 + 2] cycloaddition afforded (126). Indeed, (125) could be independently synthesized and converted to (126) under the reaction conditions. The preference for initial Michael addition, rather than cycloaddition, was variable. When (124) and divinyl sulfone were treated with silver acetate and triethylamine in DMSO, intermolecular azomethine cycloaddition occurred giving (127) in 27% yield. 

While virtually all of the research described above has focused on the inter-molecular cycloaddition of azomethine ylides, the intramolecular process holds considerable promise for the synthesis of polycyclic natural products. The Pfaltz group reported an intramolecular catalytic asymmetric cyclization of aryl iminoesters 112 using a complex of silver acetate with PHOX type ligand 100 (Scheme 2.29,... 

Cyclization of diazonium salts using iodide ion had been well developed by Beckwith s group, but the iodide corresponding to 50 is a secondary neopentyl iodide, which would be impossible to convert by S 2 chemistry into an alcohol. Our efforts to convert similar iodides to acetate esters with silver acetate by S l have also not met with success. Even if such conversions were possible, they would still be much less direct than with the new radical-polar crossover methodology. 

For primary and secondary bromides base-catalysis is required, while for tertiary bromides silver acetate or silver oxide are more effective cyclization catalysts. For tertiary substrates dehydrobromination leading to allylic hydroperoxides is a serious side reaction when base-catalysis is employed and, thus, silver ion catalysis is essential. Furthermore, the silver salts must be freshly prepared because metallic silver that might be present due to exposure to light causes decomposition of the dioxetane. The tetramethyl-l,2-dioxetane (7) was the first example prepared in this way (Eq. 13). For primary substrates, abstraction of the base-sensitive dioxetanyl hydrogens are probably responsible for the low yields. For secondary substrates, both side reactions might operate. 

Hexa hydro-3,4-d ioxobenzo[b] fura n 85 was obtained by intramolecular nucleophilic heterocyclization of 2-chloroacetyl-5,5-dimethylcyclohexane-1,3-dione 84 (85KGS1130). Cyclization was also achieved in the presence of sodium nitrite, sodium methoxide, sodium acetate, or silver acetate. The nucleophiles acted as basic deprotonating agents and did not interact with the chloroacetyl group (89ZOR1882). 

The electrophilic bromination of alkenes, e.g. with l,3-dibromo-5,5-dimethylhydantoin, in the presence of coned hydrogen peroxide, leads to y -bromo hydroperoxides. They are cyclized with bases or with silver acetate to give 1,2-dioxetanes (Kopecki 1973), e.g. ... 

Treatment of alkyne 5 with silver acetate induced an oxidative cyclization to provide N-arylpyrrole 6 in excellent yield. The reaction was applied successfully in the total synthesis of alkaloids ( )-harmicine and ( )-crispine A (13ARK(ii)6). 

Formation of Cyclic Ethers. Alcohols that have a hydrogen in the 5-position can be cyclized with silver acetate and bromine (eq 1). The ring closure occurs via an intermediate hy-pobromite. The reaction can be run under a variety of conditions (presence/absence of light, acidity, solvent, temperature). Because of this variety and the products formed, there has been some disagreement about the role silver plays in the hypobromite decomposition. ... 

The successful implementation of this strategy is shown in Scheme 4. In the central double cyclization step, the combined action of palladium(n) acetate (10 mol %), triphenylphosphine (20 mol %), and silver carbonate (2 equiv.) on trienyl iodide 16 in refluxing THF results in the formation of tricycle 20 (ca. 83 % yield). Compound 20 is the only product formed in this spectacular transformation. It is noteworthy that the stereochemical course of the initial insertion (see 17—>18) is guided by an equatorially disposed /-butyldimethylsilyl ether at C-6 in a transition state having a preferred eclipsed orientation of the C-Pd a bond and the exocyclic double bond (see 17). Insertion of the trisubstituted cycloheptene double bond into the C-Pd bond in 18 then gives a new organopal-... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

Kikugawa and coworkers utilized silver carbonate in TFA and zinc acetate in nitro-methane in the synthesis of a variety of benzolactams (39, n = 1,2,3), in excellent yields, by cyclization onto aryl rings on the acyl side chain of 38 (Scheme 6) . AgBp4 was also effective but typically afforded lower yields . ... 

D-Camphor-lO-sulfonic acid (CSA) has been utilized to form aj-fused bicyclic acetals from hexopyranosides (Scheme 20) <1997TL849>. A silver terrafluoroborate-promoted cyclization of a series of 3-hydroxyalkyl-2-cyanopiperidines has been used to prepare the octahydropyrano[2,3- ]pyridine of upenamide (Scheme 21) <2004EJ01057>. Related pyranopyr-idines have been prepared using an intramolecular Wadsworth-Emmons cyclization <2003TL8545>. 

The Heck reaction of dihydroindoles produces a mixture of cyclized products, the ratio of which can be controlled by varying the reaction conditions < 1998CEJ1554>. Hence, the < a ti-product is produced exclusively using palladium acetate as catalyst in the presence of silver carbonate and is isomerized to the endoA oms. i on exposure to camphor-sulfonic acid (CSA) at room temperature (Scheme 11). 

The equivalence of sulfur and oxygen in this ring system carries over to NSAIDs as well. Preparation of the sulfur analogue of isoxepac (6-4) starts with the alkylation of thiophenol (27-1) with benzyl chloride (26-1). Cyclization of the intermediate thioether (27-2) then affords the homothioxanthone (27-3). The carboxyl side chain is then extended by means of the Amdt-Eistert homologation reaction. The acid is thus hrst converted to its acid chloride by means of thionyl chloride. Reaction with excess diazomethane leads to the diazoketone (27-4). Treatment of that intermediate with silver benzoate and triethylamine leads the ketone to rearrange to an acetic acid. There is thus obtained tiopinac (27-5) [28]. 

The development of methods to effect nucleophilic addition to carbon-carbon double bonds by prior activation with metal cations has been applied, at least in a preliminary way, as a method of pyrrole ring closure. The conversion of butadienes to N-substituted pyrroles can be accomplished in two stages. In acetic add, 1,4-dienes react with PdnCl2 to give tr-allyl complexes with introduction of acetate at C-4. The ir-allyl complexes then react with amines to give a l-amino-4-acetoxy-2-butene (equation 70). When the addition of the amine is carried out in the presence of a silver salt and triphenylphosphine, a pyrrole is isolated, probably by cyclization of the amino-substituted allyl-Pd complex (equation 71) (81CC59). Although this procedure is attractive in terms of the simplicity of the... 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

Gol dfarb and Litvinov first formylated a thienothiophene. In Vilsmeier formylation of 2-ethylthieno[2,3-6]thiophene (20) the formyl group enters the vacant a-position, producing 5-ethyl-2-formylthieno-[2,3-6]thiophene (76%).44 Oxidation of the latter with silver oxide gives 5-ethylthieno[2,3-6]thiophene-2-carboxylic acid (55) identical with that formed by cyclizing the ester of (5-ethyl-3-formyl-2-thienylthio)acetic... 

The A-chloro compound 176, obtained from the corresponding meth-oxyamide with f-butyl hypochlorite, was cyclized to l-methoxy-2-indolone 163 by silver carbonate in trifluoroacetic acid (87% yield) (84JA5728), or by other silver or mercury salts (87T2577), and more conveniently by anhydrous zinc acetate in 1,2-dichloroethane (91% yield) (87CL1771). The reaction proceeds through the spiro intermediate 177, followed by both possible 1,2-shifts. 

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