Silicon-directed nazarov cyclizations-IV

Denmark, Scott E.; Habermas, K. L.; Hite, G. A.; Jones, Todd K.

doi:10.1016/s0040-4020(01)90571-x

Cited by 59 publications

(9 citation statements)

References 37 publications

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“…In accordance with Denmark's results about silicon-directed Nazarov cyclizations, our findings suggest that steric interactions play a role in the selection between the two possible conrotatory pathways and lead to the formation of the cis diastereomer for both 2-methyl and 4-methyl substituted derivatives, as depicted in Scheme . In the case of C4 substituted trienes, we may assume that the more stable conformer is that in which the R group on C4 is equatorial.…”

Section: Resultssupporting

confidence: 91%

“…In the case of a counterclockwise mode of conrotation (path B), not only does the bond formation involve the more hindered face of the endocyclic double bond but it leads to an unfavorable transition structure II in which the six-membered ring is a twist-boat conformation. The selectivity obtained in the cyclization of 4-methyl substituted trienes 62 and 63 is in accordance with that found in the silicon-directed cyclization of carbacyclic dienones in which ratios up to 94:6 were measured in favor of the cis diastereomer . In the hydrolysis of 4- tert -butyl substituted derivative 64 , the steric repulsion between the two alkyl groups in transition structure I could hamper cyclization.…”

Section: Resultssupporting

confidence: 85%

“…It has been shown that the factors that control the sense of torquoselectivity in the Nazarov reaction are primarily steric in origin, 2a, which favor an approach to the less-hindered face of the endocyclic olefin and lead to a prevailing clockwise ( R ) or counterclockwise ( S ) rotation when viewed down the C−O bond. In our case, the problem of the diastereoselection in the ring closure was dealt with by using 2-, 3-, and 4-alkyl substituted lactam derivatives 44 , 45 (Scheme ), and 61 − 64 (Scheme ).…”

Section: Resultsmentioning

confidence: 99%

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New Synthetic Approach to Cyclopenta-Fused Heterocycles Based upon a Mild Nazarov Reaction

et al. 2003

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The Pd-catalyzed coupling reaction of lactam or lactone-derived vinyl triflates and phosphates with alpha-alkoxydienylboronates gives conjugated alkoxytrienes in which one of the double bonds is embedded in a heterocyclic moiety. If subjected to mild acidic hydrolysis, these compounds undergo a 4pi electrocyclization process (Nazarov reaction) which furnishes cyclopenta-fused O- and N-heterocycles in good yields. The scope of the work has been that of closely examining the role and effect of both the heteroatom and the heterocycle ring size on the outcome of the electrocyclization, as well as the torquoselectivity of this process. The presence of the heteroatom was essential in stabilizing the oxyallyl cation intermediate, thus allowing the reaction to occur. The ring size was also a basic parameter in the cyclization step: five-membered azacycles required more drastic conditions to give 5-5 fused systems and did so only after an initial hydrolysis to the corresponding divinyl ketones. As for the torquoselectivity, with both 2-methyl and 4-methyl substituted lactam derivatives steric interactions seem to have a role in forcing the conrotatory process to take place in one sense only: allowing the synthesis of diastereomerically pure compounds to be realized. Because different patterns of substitution on the heterocycle are compatible with the reaction conditions, the methodology developed could be very useful for the synthesis of natural products and biologically active compounds containing cyclopenta-fused O- and N-heterocycle moieties.

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Section: Resultssupporting

confidence: 91%

Section: Resultssupporting

confidence: 85%

Section: Resultsmentioning

confidence: 99%

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New Synthetic Approach to Cyclopenta-Fused Heterocycles Based upon a Mild Nazarov Reaction

et al. 2003

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“…(1 R *,5 R *)-1-(( E )-2-(Trimethylsilyl)ethenyl)-2-oxabicyclo[3.3.0]octan-3-one (29) was prepared according to the following general procedure . A solution of trans -(2-bromovinyl)trimethylsilane (2.15 g, 12.0 mmol) in 10 mL of dry THF was added slowly to a slurry of magnetically stirred Mg turnings (0.35 g, 14.4 mmol) in 5 mL of dry THF.…”

Section: Methodsmentioning

confidence: 99%

Sequenced Reactions with Samarium(II) Iodide. Tandem Nucleophilic Acyl Substitution/Ketyl−Olefin Coupling Reactions

Molander

Harris

1996

J. Am. Chem. Soc.

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Samarium(II) iodide has been employed to promote a tandem intramolecular nucleophilic acyl substitution/intramolecular ketyl−olefin coupling cyclization sequence, generating bicyclic, tricyclic, and spirocyclic ring systems in excellent yield and with high diastereoselectivity. This versatile reaction sequence allows entry to several different naturally occurring tricyclic systems containing the angular and linear triquinane framework.

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“…8 In compound 3b the methyl group in position 3 is too far from the reacting centers to affect the sense of electrocyclic conrotation so that the two faces of the endocyclic double bond are not differentiated and both modes of conrotation are thus possible. 10 The here described synthetic sequence could be applicable to the construction of new derivatives structurally connected to some attractive natural products, in particular to biologically active substances endowed with a ring system in which a substituted aromatic nucleus is fused to a hydrindane framework, 11 as in the core of the hamigerans (Fig. 1).…”

Section: Pd-catalyzed Cross-coupling Reaction With Lactamand Lactone-derived Vinyl Triflates or Phosphatesmentioning

confidence: 99%

LIC-KOR promoted formation of conjugated dienes as useful building blocks for palladium-catalyzed syntheses

et al. 2005

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It is demonstrated that a,b-unsaturated acetals can be considered a synthetic tool for transforming carbonyl derivatives into cheap and easily accessible starting materials for the construction of various and more complex structures. The lithium-potassium mixed superbase LIC-KOR induces a conjugate elimination reaction that converts a,b-unsaturated acetals into 1E-1-alkoxybuta-1,3-dienes. These derivatives can be readily metalated in situ and functionalized by reaction with electrophiles. The results can be grouped in two sections: (1) the palladium-catalyzed cross-coupling reaction between alkoxydienylboronates and tetralone-or isochromanone-derived vinyl triflates; (2) the regio-and stereoselective cross coupling reaction with aryl derivatives in the presence of a palladium catalyst (Heck conditions).

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Silicon-directed nazarov cyclizations-IV

Cited by 59 publications

References 37 publications

New Synthetic Approach to Cyclopenta-Fused Heterocycles Based upon a Mild Nazarov Reaction

New Synthetic Approach to Cyclopenta-Fused Heterocycles Based upon a Mild Nazarov Reaction

Sequenced Reactions with Samarium(II) Iodide. Tandem Nucleophilic Acyl Substitution/Ketyl−Olefin Coupling Reactions

LIC-KOR promoted formation of conjugated dienes as useful building blocks for palladium-catalyzed syntheses

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