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At least 631 records · Page 35Linked to original sources
PubMed2002

Lewis acid promoted phenylseleno group transfer tandem radical cyclization reactions.

[reaction: see text] A new Lewis acid promoted phenylseleno group transfer tandem radical cyclization method was developed. In the presence of Lewis acids such as Yb(OTf)3 or Mg(ClO4)2, under photolysis condition at low temperature (-45 degrees C), various unsaturated alpha-phenylseleno beta-keto amides underwent radical cyclization reactions to give monocyclic or bicyclic products in a highly efficient, regioselective, and stereoselective manner.

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PubMed2002

Samarium(II)-mediated reactions of gamma,delta-unsaturated ketones. Cyclization and fragmentation processes.

[reaction: see text] On treatment with samarium(II) iodide, gamma,delta-unsaturated ketones undergo very different processes depending upon the nature of the reaction conditions. Employing samarium(II) iodide and MeOH, functionalized syn-cyclopentanol products are obtained stereoselectively. Mechanistic studies suggest that this cyclization occurs via a sequential reduction/intramolecular aldol reaction. With samarium(II) iodide and HMPA, products of a 4-exo-trig cyclization and of an interesting fragmentation reaction are observed.

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PubMed2002

Cyclization of 1,6-enynes with allylic chromate species.

[reaction: see text] Treatment of 1,6-enynes with tetramethallylchromate induces a cyclization reaction with concurrent introduction of a methallyl group. A catalytic amount of CrCl(3) combined with methallylmagnesium chloride also achieves this cyclization process. The resultant cyclic organometallic species undergo further functionalization upon reaction with electrophiles.

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PubMed2002

Synthesis of 3,4-disubstituted piperidines by carbonyl ene and Prins cyclizations: a switch in diastereoselectivity between Lewis and Brønsted acid catalysts.

[reaction: see text] A novel diastereoselective approach to cis and trans 3,4-disubstituted piperidines is described. Carbonyl ene cyclization of aldehydes 6a-e catalyzed by the Lewis acid methyl aluminum dichloride in refluxing chloroform affords trans piperidines 8a-e with diastereomeric ratios of up to 93:7. By contrast, Prins cyclization of 6a-e catalyzed by hydrochloric acid at low temperatures affords cis products 7a-e with diastereomeric ratios of up to 98:2.

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PubMed2003

Epoxide-initiated electrophilic cyclization of azides: a novel route for the stereoselective construction of azabicyclic ring systems and total synthesis of (+/-)-indolizidine 167B and 209D.

[reaction: see text] A novel and general method for the stereoselective construction of 5-hydroxymethyl azabicyclic ring skeletons based on epoxide initiated electrophilic cyclization of azides has been developed and applied in the synthesis of (+/-)-indolizidine 167B and 209D with an overall yield of 16.5% and 17.8%, respectively. The efficiency of this methodology is further exemplified in the synthesis of azepine skeleton via tandem cation-olefin-azide cyclization.

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PubMed2003

W(CO)5(L)-catalyzed endo-selective cyclization of allenyl silyl enol ethers: an efficient method for the cyclopentene annulation onto alpha,beta-unsaturated ketones.

[reaction: see text] Indium-mediated allenylation of alpha,beta-unsaturated ketones in the presence of tert-butyldimethylsilyl triflate and dimethyl sulfide gives 6-siloxy-1,2,5-trienes, which undergo W(CO)(5)(L)-catalyzed 5-endo cyclization to give the corresponding cyclopentene derivatives in good yield. Furthermore, this novel W(CO)(5)(L)-catalyzed cyclization of allenyl silyl enol ethers proceeds in a 6-endo manner when 5-siloxy-1,2,5-trienes are employed as a substrate. In these reactions, effective electrophilic activation of allenyl compounds for attack by silyl enol ethers is achieved using a catalytic amount of W(CO)(6).

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PubMed2003

Iron(III)-catalyzed Prins-type cyclization using homopropargylic alcohol: a method for the synthesis of 2-alkyl-4-halo-5,6-dihydro-2H-pyrans.

[reaction: see text] A new Prins-type cyclization between homopropargylic alcohol and aldehydes in the presence of FeX(3) to obtain 2-alkyl-4-halo-5,6-dihydro-2H-pyrans in good yield is described. Osmium-catalyzed cis dihydroxylation provided direct access to trans-2-alkyl-3-hydroxy-tetrahydro-pyran-4-ones. Anhydrous ferric halides are also shown to be excellent catalysts for the standard Prins cyclization using homoallylic alcohol. Isolation of an intermediate acetal provides substantiation of a proposed mechanism.

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PubMed2003

Lanthanide triflate-promoted palladium-catalyzed cyclization of alkenyl beta-keto esters and amides.

[reaction: see text] Lanthanide triflates were found to promote the palladium-catalyzed cyclization of alkenyl beta-keto esters and amides. In the presence of catalytic amounts of PdCl(2)(MeCN)(2) and Ln(OTf)(3), various alkenyl beta-keto esters and amides underwent regioselective cyclization reactions to give six-, seven-, or eight-membered-ring carbocycles in moderate to excellent yields.

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PubMed2003

Studies of intramolecular cyclizations of N-acyliminium ions derived from acyclic ketones: unanticipated stereochemical and structural results.

[reaction: see text] Intramolecular cyclizations of a series of (E)- and (Z)-olefinic acyclic ketone-derived N-acyliminium ions have been studied. It has been found that both the course of the reaction and the stereochemistry of the products are critically dependent upon the tether length and olefin geometry of the cyclization substrate.

Ketones↗
PubMed2003

Indium-mediated tandem radical addition-cyclization-trap reactions in aqueous media.

[reaction: see text] Tandem carbon-carbon bond-forming reactions were studied by using indium as a single-electron-transfer radical initiator. The radical addition-cyclization-trap reaction of a substrate having a vinyl sulfonamide group and an olefin moiety proceeded smoothly in aqueous media. The radical addition-cyclization reaction of hydrazone gave the functionalized cyclic products.

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PubMed2003

Beyond Schmittel and Myers-Saito cyclizations: rearrangements of 4-heteroatom-1,2-hexa-diene-5-ynes.

The thermal rearrangements of 4-heteroatom-1,2-hexadiene-5-ynes (2) were studied at the BLYP/6-311+G//BLYP/6-31G level of theory. Cyclization of 2 to heteroatom-containing cyclopentadienyl structures (6) competes with the Claisen-type rearrangement to acyclic, allenic structures. Cyclizations to cyclobutene (4)- and cyclohexadiene (8)-derived heterocycles are not feasible as a result of high reaction barriers and lower-lying alternative pathways. [structure: see text]

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PubMed2004

Cyclizations of N-stannylaminyl radicals onto nitriles.

[reaction: see text] Stannylaminyl radicals derived from radical reactions of Bu(3)SnH with azidoalkylmalononitriles exhibit highly efficient 5- and 6-exo cyclization onto either nitrile group to give aminoiminyl radicals that in turn are reduced to amidines or undergo successive 5-exo cyclization onto an internal alkene.

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PubMed2004

Six- versus five-membered ring formation in radical cyclizations of 7-bromo-substituted hexahydroindolinones.

[reaction: see text] Radical cyclization of N-allyl-7-bromo-3a-methyl-hexahydroindol-2-one affords a six-membered ring product that prevails over the isomeric five-membered compound. The former product is generated through two reaction pathways: (a) 6-endo-trig ring closure and (b) rearrangement of an intermediate methylenecyclopentyl radical obtained by 5-exo-trig cyclization.

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PubMed2004

Facile 5-endo amidyl radical cyclization promoted by vinylic iodine substitution.

[reaction: see text] BF(3).OEt(2)-catalyzed atom-transfer radical addition of iodoacetamides to alkynes yielded the corresponding vinyl iodides, which upon treatment with (t)BuOCl and I(2) afforded gamma-lactam derivatives in moderate to good yield. The mechanism was proposed to be 5-endo amidyl radical cyclization, and vinylic iodine substitution provided the driving force for the cyclization.

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PubMed2004

Tin(IV) chloride-chiral pyrogallol derivatives as new Lewis acid-assisted chiral Brønsted acids for enantioselective polyene cyclization.

[reaction: see text] New Lewis acid-assisted Brønsted acids (LBAs), tin(IV) chloride-2,6-dialkoxyphenols, serve as artificial cyclases for biomimetic polyene cyclization. For example, the enantioselective cyclization of 4-(homogeranyl)toluene using tin(IV) chloride-2,6-di[(1'R,2'R)-trans-2'-(3' ',5' '-xylyl)cyclohexanoxy]phenol gave a trans-fused tricyclic compound with 85% ee.

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PubMed2005

Cyclization or hydrogen migration: theoretical study and experimental evidence on the reactivities of unsaturated amidyl radicals.

[reaction: see text] Theoretical calculations (B3LYP/6-31G) backed up by deuterated experiments reveal that the N-substituents (R) play a crucial role in determining the reaction pathways of unsaturated amidyl radicals. With the increase of the bulkiness of N-alkyl group, the activation energy for 6-exo cyclization increases steadily, while the activation energy for 1,5-H abstraction remains almost unchanged. Therefore, cyclization occurs exclusively when R is H while 1,5-H migration occurs exclusively when R is t-Bu.

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