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At least 1,117 records · Page 62Linked to original sources

PubMed2000

Aryl radical cyclizations: one-Pot syntheses of protoberberine and pavine alkaloids.

Treatment of 2-(2'-bromo-beta-phenethyl)isocarbostyrils 7 with AIBN-Bu(3)SnH in boiling benzene gave 8-oxoberbines 3 in good yields. A similar treatment of 2-(2'-bromo-beta-phenethyl)isoquinolinium bromides 6 and their nor- and homoanalogues (10,11) induced 6-, 5-, and 7-exo radical closures in a one-pot manner to give protoberberines 2, dibenzo[b,g]indolizidine 14a and, dibenzo[a, h]-1-azabicyclo[5.4.0]undecane 15a, respectively. A one-pot radical cyclization of 1-(2'-bromobenzyl)isoquinoline methiodide 18a gave a pavine alkaloid, (+/-)-algemonine (19a).

Alkaloids↗

PubMed2000

Efficient synthesis of butenolide-medium ring ether hybrids by a [3 + 2] cyclization-ring-closing metathesis strategy.

A new strategy for the synthesis of bicyclic gamma-alkylidenebutenolides, butenolide-medium ring ether hybrids, is reported which involves Me(3)SiOTf-catalyzed cyclization of 1, 3-bis(trimethylsilyloxy)-1,3-butadienes with oxalyl chloride, Mitsunobu reaction, and subsequent ring-closing metathesis.

4-Butyrolactone↗

PubMed2000

Regioselective synthesis of medium-sized bicyclic butenolides by lewis acid catalyzed cyclization of cyclic 1, 3-Bis(trimethylsilyloxy)-1,3-butadienes with oxalyl chloride

A new strategy for the synthesis of medium-sized bicyclic gamma-alkylidenebutenolides is reported which involves Me(3)SiOTf-catalyzed cyclization of cyclic 1, 3-bis(trimethylsilyloxy)-1,3-butadienes with oxalyl chloride.

Journal Article↗

PubMed2000

Thermal cyclization of (2-Ethynylphenyl)triazenes: facile synthesis of substituted cinnolines and isoindazoles

High-temperature intramolecular cyclization of N, N-dialkyl-N'-(4-substituted-2-ethynylphenyl)triazenes provides under neutral conditions both 6-substituted cinnolines and 5-substituted isoindazoles in moderate to excellent yields.

Journal Article↗

PubMed2000

Sulfate radical anions (SO(4)(*)(-)) as donor of atomic oxygen in anionic transannular, self-terminating, oxidative radical cyclizations

SO(4)(*)(-) generated by the Fenton redox system S(2)O(8)(2)(-)- Fe(2+) induces a novel anionic, transannular, self-terminating, oxidative radical cyclization in the reactions with the ten-membered cycloalkyne 1 and the cycloalkynone 7, yielding the bicyclic ketones 5 and 6 or the alpha,beta-epoxy ketones 8 and 9, respectively. In these reactions SO(4)(*)(-) acts as an oxygen transfer reagent and can thus be considered as a donor of atomic oxygen in solution.

Journal Article↗

PubMed2000

A new alternative to the mannich reaction: tandem radical addition-cyclization reaction for asymmetric synthesis of gamma-butyrolactones and beta-amino acids

The radical addition-cyclization reaction of substrates having two different radical acceptors such as acrylate and aldoxime ether moieties was studied. This new free radical-mediated Mannich-type reaction proceeded smoothly via a tandem C-C bond-forming process. Furthermore, the diastereoselective tandem reaction provides the novel method for asymmetric synthesis of gamma-butyrolactones and beta-amino acid derivatives.

Journal Article↗

PubMed2001

Formation of benzo[b]fluorenes and the benzo[a]fluorene core of the fluostatins by cyclization of diaryldiynones.

[figure: see text] Thermal cyclization of 1-[2-(trimethylsilylethynyl)phenyl]-3-arylpropinones was expected to give benzo[b]fluorenones. However, benzo[a]-fluorenones were also formed as a result of a new rearrangement. These tetracycles possess the core structure of the fluostatins and isoprekinamycin.

Alkynes↗

PubMed2001

Optimization of palladium-catalyzed polyene cyclizations: suppression of competing hydride transfer from tertiary amines with Dabco and an unexpected hydride transfer from 1,4-dioxane.

[figure: see text] This paper demonstrates that both 1,2,2,6,6-pentamethylpiperidine (PMP) and 1,4-dioxane can act as hydride donors in palladium-catalyzed polyene cyclizations of 2 and 3. Studies using PMP-d3 and dioxane-d8 either incorporate a deuterium atom into the monosubstituted product or completely inhibit the hydride transfer so that the second ring closure occurs in high yield. Dabco is the best substitute for PMP.

Journal Article↗

PubMed2000

Diastereoselective vinyl addition to chiral hydrazones via tandem thiyl radical addition and silicon-tethered cyclization

A diastereoselective method for addition of a vinyl group to alpha-hydroxy hydrazones under neutral tin-free radical cyclization conditions, leading to substituted vinylglycinols, is presented. Tandem thiyl radical addition/cyclization upon a silicon-tethered vinyl group followed by treatment with potassium fluoride accomplishes a one-pot neutral vinyl addition process to afford acyclic allylic anti-hydrazino alcohols in good yield.

Journal Article↗

PubMed2001

A novel and efficient methodology for the C-C bond forming radical cyclization of hydrophobic substrates in water.

[reaction: see text]. The combination of water-soluble radical initiator 2,2'-azobis[2-(2-imidazolin-2-yl)propane] (VA-061), water-soluble chain carrier 1-ethylpiperidine hypophosphite (EPHP), and surfactant cetyltrimethylammonium bromide (CTAB) was found to be the most suitable condition for effective radical cyclization in water for a variety of hydrophobic substrates.

Azo Compounds↗

PubMed2001

Synthesis of dixanthones and poly(dixanthone)s by cyclization of 2-aryloxybenzonitriles in trifluoromethanesulfonic acid.

[reaction: see text] Condensation of 2-fluorobenzonitriles with phenoxides affords 2-aryloxybenzonitriles that cyclize cleanly in trifluoromethanesulfonic acid at room temperature to give xanthone-iminium triflates. The C=N bond in these compounds is remarkably resistant to hydrolysis, but prolonged reaction with strong aqueous acid under vigorous conditions affords xanthones in good yield. The synthesis is exemplified for a novel series of polynuclear dixanthones and for a high molar mass polyxanthone derived from the previously unreported monomer 3,3'-difluoro-4,4'-biphenyldicarbonitrile.

Journal Article↗

PubMed2001

A concise synthesis of turneforcidine via a metalloiminium ion cyclization terminated by the 2-(methylthio)-3-(trimethylsilyl)-1-propenyl moiety.

A concise synthetic route to racemic turneforcidine (1) is described that relies on the stereocontrolled cyclization of the 2-(methylthio)-3-(trimethylsilyl)-1-propenyl bearing imine 5 in the presence of TiCl(4). Reaction: see text.

Imines↗

PubMed2001

Chemo-, regio-, and diastereoselective synthesis of functionalized cyclopropanes by cyclization of dilithiated nitriles with epibromohydrin.

The cyclization of 1,1-dianions with epibromohydrin results in chemo-, regio-, and diastereoselective formation of functionalized hydroxymethyl cyclopropanes. [reaction: see text]

Journal Article↗

PubMed2002

Synthesis of 3-substituted 4-aroylisoquinolines via Pd-catalyzed carbonylative cyclization of o-(1-alkynyl)benzaldimines.

[reaction: see text] o-(1-Alkynyl)benzaldimines react with aryl iodides and 1 atm of CO in the presence of tri-n-butylamine and a Pd(PPh(3))(4) catalyst to afford good yields of 3-substituted 4-aroylisoquinolines by acylpalladation of the carbon-carbon triple bond and cyclization.

Journal Article↗

PubMed2002

Oxonia-cope rearrangement and side-chain exchange in the Prins cyclization.

[reaction: see text] Evidence is presented here for the mechanism of the Prins cyclization of benzylic homoallylic alcohols, which shows that the outcome of the reaction is dependent upon the substituents on the aromatic ring. The presence of an electron-rich aromatic ring favors an oxonia-Cope rearrangement yielding a symmetrical tetrahydropyran as the major product formed via a side-chain exchange process. In contrast, with electron-deficient aromatic rings the expected 2,4,6-trisubstituted tetrahydropyran is formed.

Journal Article↗

PubMed2001

Biradical and zwitterionic cyclizations of oxy-substituted enyne-allenes.

[see reaction]. Oxyanion substitution of enyne-allenes causes both Myers-Saito and Schmittel cyclizations to switch their product formation preferences from diradicals to polar, closed-shell singlets. The oxyanion stabilization is larger for the Schmittel products than the Myers-Saito products because the latter must sacrifice aromaticity to maximize interaction. The changing character of the different reaction paths is reflected in their activation energies.

Journal Article↗

PubMed2001

Tandem conjugate reduction-aldol cyclization using Stryker's reagent.

[see reaction]. Conjugate reduction by Stryker's reagent to form copper enolates, followed by intramolecular aldol cyclization, successfully generated five- and six-membered carbocycles in one pot efficiently. This tandem reaction is generally diastereoselective and provides good yields of the beta-hydroxyketones without any dehydration at low temperatures.

Journal Article↗

PubMed2001

Formal synthesis of uvaricin via palladium-mediated double cyclization.

[see reaction]. A novel palladium-mediated, ligand-controlled double cyclization is reported. The C2-symmetric diene produced was desymmetrized via Sharpless asymmetric dihydroxylation and was further transformed into a known intermediate in the synthesis of uvaricin.

Furans↗