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At least 613 records · Page 34Linked to original sources
PubMed2000

Desymmetrization of 1,4-dien-3-ols and related compounds via Ueno-Stork radical cyclizations

[formula: see text] Desymmetrization of 1,4-dien-3-ols and related compounds via Ueno-Stork radical cyclizations is reported. The stereochemistry of the cyclization is controlled by the acetal center. Excellent stereocontrol at C(4) and C(5) of the newly formed tetrahydrofuran rings is observed. Use of a chiral auxiliary allows the preparation of enantiomerically pure material. The utility of this method has been demonstrated by achieving a short synthesis of (+)-eldanolide, the pheromone of the male African sugarcane stem borer Eldana saccharina.

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PubMed2000

Pyrrolidines from beta-aminoselenides via radical cyclization. Diastereoselectivity control by the N-substituent

N-Allyl-beta-aminoalkyl phenyl selenides-precursors of 3-aza-5-hexenyl radicals-were prepared by ring opening of N-allylaziridines with benzeneselenol under acidic conditions or by sodium cyanoborohydride reduction of N-allylimines of alpha-phenylselenenyl ketones. The effect of various N-protective groups (acyl, sulfonyl, or phosphinoyl) on diastereoselectivity in thermally or photochemically initiated 3-aza-5-hexenyl reductive radical cyclization was studied. Whereas N-unprotected derivatives afforded trans-2,4-disubstituded pyrrolidines with good selectivity, the diphenylphosphinoyl group directed cyclization to occur in a highly cis-selective manner.

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PubMed2000

Diastereoselective formation of 5-vinylcyclopentenes from 1, 6-enynes: cobalt-mediated C-H allylic activation and formal 5-endo-Dig cyclization

A novel intramolecular cyclization reaction mediated by dicobalt octacarbonyl (Co(2)(CO)(8)) is reported. Thermolysis in an argon atmosphere transforms the cobalt complex of 1-trimethylsilyl-6-hepten-1-ynes into 1-trimethylsilyl-5-vinylcyclopentenes in good yield and in a highly diastereoselective manner. This formal 5-endo-dig cyclization is proposed to proceed via an allylic C-H oxidative addition.

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PubMed2001

Highly diastereoselective 5-hexenyl radical cyclizations with Lewis acids and carbohydrate scaffolds.

[figure: see text] Carbohydrates as removable chiral scaffolds for free radical cyclizations were examined for the first time. This investigation illustrates the utility of two inexpensive carbohydrate derivatives as sources of asymmetry for 5-hexenyl radical cyclizations. Diastereomeric ratios as high as 100:1 were achieved with an ester-appended (+)-isosorbide hexose and 70:1 for a diol-protected D-xylose pentose. Temperature dependence, Lewis acids, and solvents were all examined. By correlation with known compounds, the newly generated chiral centers were of the (S)-configuration.

Alkenes↗
PubMed2000

Dearomatizing anionic cyclization of substituted N-cumyl-N-benzyl- benzamides on treatment with LDA: synthesis of partially saturated substituted isoindolones

On treatment with LDA, substituted N-benzylbenzamides (including those bearing electron-withdrawing, electron-donating, or conjugating groups) become lithiated and cyclize to give, after aqueous quench, a range of partially saturated isoindolones as single regio- and stereoisomers. In general, the isoindolones resist rearomatization. Reaction of N-cumyl-N-benzylbenzamides leads to cyclized products which may be deprotected to give N-unsubstituted isoindolones.

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PubMed2001

Regiocontrol of radical cyclization by Lewis acids. Efficient synthesis of optically active functionalized cyclopentanes and cyclohexanes.

[figure: see text] Treatment of alpha-alkylidenelactones 3a-d with Bu3SnH and a catalytic amount of Et3B effected a 5-exo radical cyclization preferentially to provide the corresponding 1 and 2 in a ratio of 70:30 to 100:0. Meanwhile, the reaction of 3a and 3b in the presence of Et2AlCl proceeded via a 6-endo cyclization pathway predominantly to afford 2a and 2b with 90% and 92% regioselectivity, respectively.

Journal Article↗
PubMed2001

Palladium(0)-catalyzed carbonylation-coupling-cyclization of allenic sulfonamides with aryl iodides and carbon monoxide.

[reaction: see text] alpha-Allenic sulfonamides undergo carbonylation-coupling-endo-cyclization with aryl iodides in the presence of Pd(PPh(3))(4) (5 mol %), K(2)CO(3), and CO (20 atm) to form 3-aroyl-2- or 3-pyrrolines. Alternatively the carbonylation-coupling-exo-cyclization of gamma- and delta-substituted sulfonamides under the same conditions afforded pyrrolidine- or piperidine-substituted enones.

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PubMed2001

Use of a conformational radical clock for evaluating alkyllithium-mediated cyclization reactions.

The reductive lithiation of nitrile 9 led to the cyclic product 11 as a single diastereomer in 42% ee. The intermediate radical is a conformational radical clock. The radical lifetime can be determined from the optical purity of the product 11. We show that the lifetime of the intermediate radical is too brief to allow a radical cyclization, and thus the cyclization proceeds through an alkyllithium intermediate.

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PubMed2001

Cyclization of methyl-substituted 6-heptenyl radicals.

[reaction: see text] The behavior of a series of methyl-substituted 6-heptenyl radicals, generated from the corresponding iodides ((Me(3)Si)(3)SiH, AIBN in benzene at 80 degrees C), has been investigated. The stereoselectivity of the 6-exo cyclizations, affording dimethylcyclohexanes, is low, and sizable quantities of methylcycloheptane, generated via 7-endo cyclization, are also produced.

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PubMed2001

A novel reaction for annulation onto alpha,beta-unsaturated ketones: W(CO)(5).L promoted exo- and endo-selective cyclizations of omega-acetylenic silyl enol ethers prepared by 1,4-addition of propargyl malonate to enones.

A highly useful method for five- and six-membered ring annulation onto alpha,beta-unsaturated ketones is described. 1,4-Addition of propargylmalonate to alpha,beta-unsaturated ketones in the presence of silyl triflate gives 7-siloxy-6-en-1-yne derivatives in good yield. W(CO)(5).L-catalyzed cyclization of these substrates can be induced to give preferentially either exo- or endo-cyclized products in good yield simply by changing the reaction solvent. [reaction: see text]

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PubMed2001

Highly diastereoselective radical cyclizations on soluble ring opening metathesis supports.

This study represents the first examples of stereoselective radical cyclizations on soluble supports. A stereocontrol element consisting of a polymer-imbedded (+)-isosorbide chiral auxiliary was used in each monomer subunit. A survey of various Lewis acids was also examined. The best results gave very high distereoselectivites of >100:1 in a hepa-1,3-dienenyl radical cyclization using zinc chloride as a Lewis acid. [reaction: see text]

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PubMed2001

Aerobic organocatalytic photoinitiated arene oxidations: application to electron transfer initiated cyclization reactions.

[reaction: see text] Electron transfer initiated oxidative cyclization reactions can be effected by catalytic amounts of N-methylquinolinium hexafluorophosphate through photoirradiation in the presence of dioxygen. Solid sodium thiosulfate serves as an effective reducing agent to remove the reactive oxygen species formed from catalyst regeneration and radical coupling reactions with O(2), allowing these cyclizations to proceed efficiently on preparatively useful scales.

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PubMed2002

A new paradigm for cationic cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes.

A new example of stereospecific cationic cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes is provided. Protonation of a double bond vicinal to the iron tricarbonyl diene moiety is employed to trigger the cyclization, rather than the previously reported Lewis/protic acid dehydroxylation of diastereomeric alcohols, eliminating one step of separation and avoiding some reactivity problems previously encountered for one of the alcohol diastereoisomers. [reaction: see text]

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PubMed2002

Formation of bicyclic ethers from Lewis acid promoted cyclizations of cyclic oxonium ions.

Oxacycles carrying a vinyl group and an acetal were extended with a cyclization terminator (vinyl silane or vinyl sulfide) by Suzuki coupling. Treatment with Lewis acid induced cyclization to provide bicyclic ethers in reasonable yields. In the case of the vinyl silane, an ene-like mechanism is preferred, whereas the thioenol ether enters into a Prins-type reaction channel. In one instance, the bicyclic compound was opened to give the ten-membered functionalized enone 24. [reaction: see text]

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