Silicon-directed nazarov cyclizations VII

Denmark, Scott E.; Klix, Russell C.

doi:10.1016/s0040-4020(01)86655-2

Cited by 63 publications

(17 citation statements)

References 29 publications

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“…Finally, the use of the unique properties of allyl silane as nucleophile allowed the first synthesis of a fused carbocycle under catalytic conditions (Entry 8). As in the related work of Denmark for the Nazarov cyclization, [17] complete regioselectivity was observed for double bond formation via elimination of the silyl group.…”

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confidence: 61%

Cyclization of Cyclopropyl Carbonyls and the Homo-Nazarov Reaction

Simone¹,

Waser²

2009

Chimia

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Ring strain confers to the cyclopropane ring an exceptional reactivity. Nevertheless, activation of the cyclopropane ring is usually needed to allow ring-opening reactions under mild conditions. The introduction of one or several carbonyl functionalities on cyclopropanes is often used to enhance their electrophilic reactivity. When a donor group is also present at the vicinal position to the carbonyl, more reactive donor-acceptor systems are obtained, which are ideally suited for homo-conjugate addition or cycloaddition reactions. This short review focuses on the special case of intramolecular cyclization reactions. Particular emphasis is devoted to the cyclization of vinyl-and aryl-cyclopropyl ketones for the formation of 6 membered rings. This process corresponds to a formal homo-Nazarov cyclization and has been only used rarely in the past. Recently, the scope of the reaction was greatly expanded by the use of a silyl group as cation stabilizing group and by the first catalytic method for the synthesis of vinyl-cyclopropyl ketones. With these new developments, the bases are now set for the application of this special class of cationic cyclizations in the synthesis of more complex synthetic and natural products.

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confidence: 61%

Cyclization of Cyclopropyl Carbonyls and the Homo-Nazarov Reaction

Simone¹,

Waser²

2009

Chimia

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“…(7) The nucleophilicity of allylic silanes has been exploited in a large number of synthetic applications, so it should come as no surprise that the intermediate allylic cation from a Nazarov process can be intercepted efficiently as well. Exposure of the enone 25 [Equation (8)] and a ten-fold excess of allyl triisopropylsilane (26) to Lewis acid leads to a tandem Nazarov-[3+2] cycloaddition reaction. [16] The bridged product 27 with four rings and five stereocenters is isolated as a single isomer in high yield.…”

Section: Interrupted Nazarov Reactions and Other Tandem Processesmentioning

confidence: 99%

“…Similar use of a trimethylsilyl group has been made by Denmark and by others. [26][27][28][29][30] ( 14) Placement of the directing group need not be at the β carbon atom that is destined to become part of the fivemembered ring. Kang and co-workers have shown that ferric chloride effectively catalyzes the rearrangement of keto silane 42 to the exocyclic enone 43 [Equation (15)].…”

Section: Localizing the Double Bond In The Productmentioning

confidence: 99%

“…The stereochemistry of the β carbon atoms of 78 reflect the sense of conrotation, whereas the stereochemistry α to the carbonyl group is determined during a proton transfer step, and reflects a thermodynamic preference. (26) Transfer of asymmetry need not be from a sp 3 hybridized carbon atom. In the discussion that accompanies Scheme 1, mention was made of the preference for 73 to cyclize to Z-74.…”

Section: Control Of Absolute Stereochemistrymentioning

confidence: 99%

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Some New Nazarov Chemistry

Tius

2005

Eur J Org Chem

345

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The Nazarov cyclization is a 4π-electron conrotatory cyclization of a pentadienyl cation that leads to a five-membered ring, typically a cyclopentenone. Because the mechanism is well defined, it is often possible to make accurate and useful stereochemical predictions regarding the course of the reaction. Because the reaction intermediates are carbocations, it is also possible to devise tandem processes whereby the initial cyclization is followed by one or more C-C bond forming

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“…The situation in TFA has not been experimentally verified, however, and to our knowledge results confirming this are not readily found in the literature. We therefore examined the solvent isotope effect on the TFAcatalysed rates of ring closure reaction of 3'-methoxychalcones 16 to give 1-indanones 17, a reaction believed to go via an electrocyclic Nazarov reaction initiated by a preequilibrium carbonyl protonation (19,20). The results are given in Table 4.…”

Section: Resultsmentioning

confidence: 99%

Stereoelectronic effects in ring closure reactions: the 2′-hydroxychalcone – flavanone equilibrium, and related systems

Brennan¹,

Hunt²,

Jarvis³

et al. 1990

Can. J. Chem.

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. 68, 1780 (1990).The 2'-hydroxychalcone (2-HOC6H4COCH4HC6H&)-flavanone equilibrium in trifluoroacetic acid (TFA) has been examined. The influence of substituents X on the rate of attainment of equilibrium shows that the 6-endo-trig modeof ring closure by Michael addition is disallowed, by demonstrating a negative p value for the reaction rate when X is varied. Reaction therefore proceeds either on the carbonyl-protonated form, which allows twisting about the 2,3 bond, its double bond character being reduced by resonance, or through direct rate-limiting protonation on the 2,3 double bond. Either pathway permits the allowed 6-exo-trig mode of ring closure to be followed. Alternative mechanisms involving intermolecular Michael addition of trifluoroacetate, followed by intramolecular 6-exo-tet displacement are considered. Such Michael adducts can be detected in the ring closures of 2-crotonyl-4-methylphenol and 4,4-dimethyl-1-(2-hydroxypheny1)-2-penten-1-one in TFA, but they do not appear to lie on the main pathway, because the reactions proceed with equal facility in methanesulphonic acid/chloroform medium, which does not contain a suitable nucleophile for such a mechanism. Further important mechanistic information isgiven by studying the reactions in TFA-d, together with measurements on the (E)-2-methyl-3-0~0-5-aryIpent-4-en-2-01 and 3'-methoxychalcone systems. These isotope effect studies, which yield kH/kD values of about 3, indicate that the proton is in flight during the rate-limiting step, and provide evidence against the mechanism involving a preequilibrium carbonyl protonation, such as in the Nazarov rearrangement of 3'-methoxychalcones, where kH/kD is ca. 0.7. Some results are also reported for ring closure of the 2-aminochalcones in TFA. Key words: 2'-hydroxychalcones, flavanones, 3'methoxychalcones, 3-aryl-6-methoxyindanones, 2'-aminochalcones, 2-aryl-1, 2,3,4-tetrahydroquinolines, Baldwin's rules. On a CtudiC l'tquilibre de la 2'-hydroxychalcone (2-HOC6H4COCH=CHC6H,X)-flavone dans I'acide trifluoroacttique (TFA). L'influence des substituants X sur le temps qu'il faut pour atteindre I'Cquilibre, exprimCe par la valeur nCgative de p sur la vitesse de la rCaction, dCmontre que le mode 6-endo-trig de fermeture du cycle par addition de Michael n'est pas permis. La rCaction se produit donc soit par une attaque sur la forme protonee du carbonyle, qui permet une torsion autour de la liaison 2,3 puisque soncaractere de liaison double est rCduit par resonance, soit par une protonation directe de la double liaison 2,3 qui limite la vitesse. On considbre aussi d'autres mCcanismes impliquant une addition de Michael intramolCculaire de I'ion trifluoroacktate, suivie par des dCplacements 6-exo-tet. On peut dCtecter de tels adduits de Michael dans les cyclisations du 2-crotonyl-4-mCthylph6nol et de la 4,4-dimCthyl-1-(2-hydroxyphCny1)-2-pentkn-I-one dans l'acide TFA; toutefois, ils ne semblent pas faire partie du mecanisme principal puisque les reactions se produisent avec une facilitC similaire dans un milieu comme l'...

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Silicon-directed nazarov cyclizations VII

Cited by 63 publications

References 29 publications

Cyclization of Cyclopropyl Carbonyls and the Homo-Nazarov Reaction

Cyclization of Cyclopropyl Carbonyls and the Homo-Nazarov Reaction

Some New Nazarov Chemistry

Stereoelectronic effects in ring closure reactions: the 2′-hydroxychalcone – flavanone equilibrium, and related systems

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