Nazarov cyclization reaction

Last updated

Contents

Nazarov cyclization
Named after Ivan Nikolaevich Nazarov
Reaction type Ring forming reaction
Identifiers
Organic Chemistry Portal nazarov-cyclization
RSC ontology ID RXNO:0000209

The Nazarov cyclization reaction (often referred to as simply the Nazarov cyclization) is a chemical reaction used in organic chemistry for the synthesis of cyclopentenones. The reaction is typically divided into classical and modern variants, depending on the reagents and substrates employed. It was originally discovered by Ivan Nikolaevich Nazarov (1906–1957) in 1941 while studying the rearrangements of allyl vinyl ketones. [1]

The Nazarov cyclization reaction Nazarov cyclization.png
The Nazarov cyclization reaction

As originally described, the Nazarov cyclization involves the activation of a divinyl ketone using a stoichiometric Lewis acid or protic acid promoter. The key step of the reaction mechanism involves a cationic 4π-electrocyclic ring closure which forms the cyclopentenone product (See Mechanism below). As the reaction has been developed, variants involving substrates other than divinyl ketones and promoters other than Lewis acids have been subsumed under the name Nazarov cyclization provided that they follow a similar mechanistic pathway.

The success of the Nazarov cyclization as a tool in organic synthesis stems from the utility and ubiquity of cyclopentenones as both motifs in natural products (including jasmone, the aflatoxins, and a subclass of prostaglandins) and as useful synthetic intermediates for total synthesis. The reaction has been used in several total syntheses and several reviews have been published. [2] [3] [4] [5] [6] [7]

Mechanism

The mechanism of the classical Nazarov cyclization reaction was first demonstrated experimentally by Charles Shoppee to be an intramolecular electrocyclization and is outlined below. Activation of the ketone by the acid catalyst generates a pentadienyl cation, which undergoes a thermally allowed 4π conrotatory electrocyclization as dictated by the Woodward-Hoffman rules. This generates an oxyallyl cation which undergoes an elimination reaction to lose a β-hydrogen. Subsequent tautomerization of the enolate produces the cyclopentenone product. [8] [9]

Mechanism of the classical Nazarov cyclization activated by Lewis acid catalyst. Note that a- and b-substitution are not required for the reaction to occur and that more complex a-substitution is also possible. Nazarov reaction mechanism.png
Mechanism of the classical Nazarov cyclization activated by Lewis acid catalyst. Note that α- and β-substitution are not required for the reaction to occur and that more complex α-substitution is also possible.

As noted above, variants that deviate from this template are known; what designates a Nazarov cyclization in particular is the generation of the pentadienyl cation followed by electrocyclic ring closure to an oxyallyl cation. In order to achieve this transformation, the molecule must be in the s-trans/s-trans conformation, placing the vinyl groups in an appropriate orientation. The propensity of the system to enter this conformation dramatically influences reaction rate, with α-substituted substrates having an increased population of the requisite conformer due to allylic strain. Coordination of an electron donating α-substituent by the catalyst can likewise increase the reaction rate by enforcing this conformation. [2]

Relevant conformations for the Nazarov cyclization; Lewis-acid chelation in the s-cis conformer Nazarov conformation.png
Relevant conformations for the Nazarov cyclization; Lewis-acid chelation in the s-cis conformer

Similarly, β-substitution directed inward restricts the s-trans conformation so severely that E-Z isomerization has been shown to occur in advance of cyclization on a wide range of substrates, yielding the trans cyclopentenone regardless of initial configuration. In this way, the Nazarov cyclization is a rare example of a stereoselective pericyclic reaction, whereas most electrocyclizations are stereospecific. The example below uses triethylsilane to trap the oxyallyl cation so that no elimination occurs. [2] (See Interrupted cyclizations below)

Stereoselectivity in the Nazarov cyclization evident via reductive trapping Nazarovselective.png
Stereoselectivity in the Nazarov cyclization evident via reductive trapping

Along this same vein, allenyl vinyl ketones of the type studied extensively by Marcus Tius of the University of Hawaii show dramatic rate acceleration due to the removal of β-hydrogens, obviating a large amount of steric strain in the s-cis conformer. [6]

Allene substrates for Nazarov cyclization with lowered steric interactions Tiusallene.png
Allene substrates for Nazarov cyclization with lowered steric interactions

Classical Nazarov cyclizations

Though cyclizations following the general template above had been observed prior to Nazarov's involvement, it was his study of the rearrangements of allyl vinyl ketones that marked the first major examination of this process. Nazarov correctly reasoned that the allylic olefin isomerized in situ to form a divinyl ketone before ring closure to the cyclopentenone product. The reaction shown below involves an alkyne oxymercuration reaction to generate the requisite ketone. [10]

Early investigation into the Nazarov cyclization NazarovOriginal.png
Early investigation into the Nazarov cyclization

Research involving the reaction was relatively quiet in subsequent years, until in the mid-1980s when several syntheses employing the Nazarov cyclization were published. Shown below are key steps in the syntheses of Trichodiene and Nor-Sterepolide, the latter of which is thought to proceed via an unusual alkyne-allene isomerization that generates the divinyl ketone. [11] [12]

Synthesis of Trichodiene using a classical Nazarov cyclization NazarovTrichodiene.png
Synthesis of Trichodiene using a classical Nazarov cyclization
Synthesis of Nor-Sterepolide using a classical Nazarov cyclization Nazarovnorsterepolide.png
Synthesis of Nor-Sterepolide using a classical Nazarov cyclization

Shortcomings

The classical version of the Nazarov cyclization suffers from several drawbacks which modern variants attempt to circumvent. The first two are not evident from the mechanism alone, but are indicative of the barriers to cyclization; the last three stem from selectivity issues relating to elimination and protonation of the intermediate. [2]

  1. Strong Lewis or protic acids are typically required for the reaction (e.g. TiCl4, BF3, MeSO3H). These promoters are not compatible with sensitive functional groups, limiting the substrate scope.
  2. Despite the mechanistic possibility for catalysis, multiple equivalents of the promoter are often required in order to effect the reaction. This limits the atom economy of the reaction.
  3. The elimination step is not regioselective; if multiple β-hydrogens are available for elimination, various products are often observed as mixtures. This is highly undesirable from an efficiency standpoint as arduous separation is typically required.
  4. Elimination destroys a potential stereocenter, decreasing the potential usefulness of the reaction.
  5. Protonation of the enolate is sometimes not stereoselective, meaning that products can be formed as mixtures of epimers.
Shortcomings of the Nazarov cyclization reaction Nazarovunselective.png
Shortcomings of the Nazarov cyclization reaction

Modern variants

The shortcomings noted above limit the usefulness of the Nazarov cyclization reaction in its canonical form. However, modifications to the reaction focused on remedying its issues continue to be an active area of academic research. In particular, the research has focused on a few key areas: rendering the reaction catalytic in the promoter, effecting the reaction with more mild promoters to improve functional group tolerance, directing the regioselectivity of the elimination step, and improving the overall stereoselectivity. These have been successful to varying degrees.

Additionally, modifications focused on altering the progress of the reaction, either by generating the pentadienyl cation in an unorthodox fashion or by having the oxyallyl cation "intercepted" in various ways. Furthermore, enantioselective variants of various kinds have been developed. The sheer volume of literature on the subject prevents a comprehensive examination of this field; key examples are given below.

Silicon-directed cyclization

The earliest efforts to improve the selectivity of the Nazarov cyclization took advantage of the β-silicon effect in order to direct the regioselectivity of the elimination step. This chemistry was developed most extensively by Professor Scott Denmark of the University of Illinois, Urbana-Champaign in the mid-1980s and utilizes stoichiometric amounts of iron trichloride to promote the reaction. With bicyclic products, the cis isomer was selected for to varying degrees. [13]

Silicon-directed Nazarov cyclization Siliconnazarov.png
Silicon-directed Nazarov cyclization

The silicon-directed Nazarov cyclization reaction was subsequently employed in the synthesis of the natural product Silphinene, shown below. The cyclization takes place before elimination of the benzyl alcohol moiety, so that the resulting stereochemistry of the newly formed ring arises from approach of the silyl alkene anti to the ether. [10]

Synthesis of Silphinene using Nazarov cyclization Silphinenenazarov.png
Synthesis of Silphinene using Nazarov cyclization

Polarization

Drawing on the substituent effects compiled over various trials of the reaction, Professor Alison Frontier of the University of Rochester developed a paradigm for "polarized" Nazarov cyclizations in which electron donating and electron withdrawing groups are used to improve the overall selectivity of the reaction. Creation of an effective vinyl nucleophile and vinyl electrophile in the substrate allows catalytic activation with copper triflate and regioselective elimination. In addition, the electron withdrawing group increases the acidity of the α-proton, allowing selective formation of the trans-α-epimer via equilibration. [14]

Polarized Nazarov cyclization Polarized Nazarov.png
Polarized Nazarov cyclization

It is often possible to achieve catalytic activation using a donating or withdrawing group alone, although the efficiency of the reaction (yield, reaction time, etc.) is typically lower.

Alternative cation generation

By extension, any pentadienyl cation regardless of its origin is capable of undergoing a Nazarov cyclization. There have been a large number of examples published where the requisite cation is arrived at by a variety of rearrangements. [2] One such example involves the silver catalyzed cationic ring opening of allylic dichloro cylopropanes. The silver salt facilitates loss of chloride via precipitation of insoluble silver chloride. [15]

Nazarov cyclization initiated by dichlorocyclopropane rearrangement Westchloro.png
Nazarov cyclization initiated by dichlorocyclopropane rearrangement

In the total synthesis of rocaglamide, epoxidation of a vinyl alkoxyallenyl stannane likewise generates a pentadienyl cation via ring opening of the resultant epoxide. [16]

Nazarov cyclization initiated by oxidation Rocaglamidenazarov.png
Nazarov cyclization initiated by oxidation

Interrupted cyclization

Once the cyclization has occurred, an oxyallyl cation is formed. As discussed extensively above, the typical course for this intermediate is elimination followed by enolate tautomerization. However, these two steps can be interrupted by various nucleophiles and electrophiles, respectively. Oxyallyl cation trapping has been developed extensively by Fredrick G. West of the University of Alberta and his review covers the field. [17] The oxyallyl cation can be trapped with heteroatom and carbon nucleophiles and can also undergo cationic cycloadditions with various tethered partners. Shown below is a cascade reaction in which successive cation trapping generates a pentacyclic core in one step with complete diastereoselectivity. [18]

Nazarov cyclization cationic cascade Westcascade.png
Nazarov cyclization cationic cascade

Enolate trapping with various electrophiles is decidedly less common. In one study, the Nazarov cyclization is paired with a Michael reaction using an iridium catalyst to initiate nucleophilic conjugate addition of the enolate to β-nitrostyrene. In this tandem reaction the iridium catalyst is required for both conversions: it acts as the Lewis acid in the Nazarov cyclization and in the next step the nitro group of nitrostyrene first coordinates to iridium in a ligand exchange with the carbonyl ester oxygen atom before the actual Michael addition takes place to the opposite face of the R-group. [19]

Tandem Nazarov cyclization and Michael reaction NazarovMichaelTandem.png
Tandem Nazarov cyclization and Michael reaction

Enantioselective variants

The development of an enantioselective Nazarov cyclization is a desirable addition to the repertoire of Nazarov cyclization reactions. To that end, several variations have been developed utilizing chiral auxiliaries and chiral catalysts. Diastereoselective cyclizations are also known, in which extant stereocenters direct the cyclization. Almost all of the attempts are based on the idea of torquoselectivity; selecting one direction for the vinyl groups to "rotate" in turn sets the stereochemistry as shown below. [2]

Torquoselectivity in the Nazarov cyclization Torquoselectivity.png
Torquoselectivity in the Nazarov cyclization

Silicon-directed Nazarov cyclizations can exhibit induced diastereoselectivity in this way. In the example below, the silyl-group acts to direct the cyclization by preventing the distant alkene from rotating "towards" it via unfavorable steric interaction. In this way the silicon acts as a traceless auxiliary. (The starting material is not enantiopure but the retention of enantiomeric excess suggests that the auxiliary directs the cyclization.) [2]

Silicon as a chiral auxiliary for the Nazarov cyclization NazarovSiliconauxilliary.png
Silicon as a chiral auxiliary for the Nazarov cyclization

Tius's allenyl substrates can exhibit axial to tetrahedral chirality transfer if enantiopure allenes are used. The example below generates a chiral diosphenpol in 64% yield and 95% enantiomeric excess. [2]

Axial to tetrahedral chirality transfer in the Nazarov cyclization of allenyl vinyl ketones Axialtotetrahedral transfer.png
Axial to tetrahedral chirality transfer in the Nazarov cyclization of allenyl vinyl ketones

Tius has additionally developed a camphor-based auxiliary for achiral allenes that was employed in the first asymmetric synthesis of roseophilin. The key step employs an unusual mixture of hexafluoro-2-propanol and trifluoroethanol as solvent. [2] [20]

Synthesis of roseophilin using asymmetric Nazarov cyclization Tiusroseophilin.png
Synthesis of roseophilin using asymmetric Nazarov cyclization

The first chiral Lewis acid promoted asymmetric Nazarov cyclization was reported by Varinder Aggarwal and utilized copper (II) bisoxazoline ligand complexes with up to 98% ee. The enantiomeric excess was unaffected by use of 50 mol% of the copper complex but the yield was significantly decreased. [2]

Lewis-acid mediated asymmetric Nazarov cyclization Aggarwalnazarov.png
Lewis-acid mediated asymmetric Nazarov cyclization

Extensions of the Nazarov cyclization are generally also subsumed under the same name. For example, an α-β, γ-δ unsaturated ketone can undergo a similar cationic conrotatory cyclization that is typically referred to as an iso-Nazarov cyclization reaction. [21] Other such extensions have been given similar names, including homo-Nazarov cyclizations and vinylogous Nazarov cyclizations. [22] [23]

Retro-Nazarov reaction

Because they overstabilize the pentadienyl cation, β-electron donating substituents often severely impede Nazarov cyclization. Building from this, several electrocyclic ring openings of β-alkoxy cyclopentanes have been reported. These are typically referred to as retro-Nazarov cyclization reactions. [2]

Retro-Nazarov reaction Retronazarov.png
Retro-Nazarov reaction

Imino-Nazarov reaction

Nitrogen analogues of the Nazarov cyclization reaction (known as imino-Nazarov cyclization reactions) have few instances; there is one example of a generalized imino-Nazarov cyclization reported (shown below), [24] and several iso-imino-Nazarov reactions in the literature. [25] [26] Even these tend to suffer from poor stereoselectivity, poor yields, or narrow scope. The difficulty stems from the relative over-stabilization of the pentadienyl cation by electron donation, impeding cyclization. [27]

Imino-Nazarov cyclization Iminonazarov.png
Imino-Nazarov cyclization

See also

Related Research Articles

The aldol reaction is a reaction that combines two carbonyl compounds to form a new β-hydroxy carbonyl compound.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are usually categorized by the following criteria:

<span class="mw-page-title-main">Michael addition reaction</span> Reaction in organic chemistry

In organic chemistry, the Michael reaction or Michael 1,4 addition is a reaction between a Michael donor and a Michael acceptor to produce a Michael adduct by creating a carbon-carbon bond at the acceptor's β-carbon. It belongs to the larger class of conjugate additions and is widely used for the mild formation of carbon-carbon bonds.

The Robinson annulation is a chemical reaction used in organic chemistry for ring formation. It was discovered by Robert Robinson in 1935 as a method to create a six membered ring by forming three new carbon–carbon bonds. The method uses a ketone and a methyl vinyl ketone to form an α,β-unsaturated ketone in a cyclohexane ring by a Michael addition followed by an aldol condensation. This procedure is one of the key methods to form fused ring systems.

<span class="mw-page-title-main">Chiral auxiliary</span> Stereogenic group placed on a molecule to encourage stereoselectivity in reactions

In stereochemistry, a chiral auxiliary is a stereogenic group or unit that is temporarily incorporated into an organic compound in order to control the stereochemical outcome of the synthesis. The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions. The auxiliary can then be typically recovered for future use.

<span class="mw-page-title-main">Favorskii rearrangement</span>

The Favorskii rearrangement is principally a rearrangement of cyclopropanones and α-halo ketones that leads to carboxylic acid derivatives. In the case of cyclic α-halo ketones, the Favorskii rearrangement constitutes a ring contraction. This rearrangement takes place in the presence of a base, sometimes hydroxide, to yield a carboxylic acid but most of the time either an alkoxide base or an amine to yield an ester or an amide, respectively. α,α'-Dihaloketones eliminate HX under the reaction conditions to give α,β-unsaturated carbonyl compounds.

<span class="mw-page-title-main">Baldwin's rules</span>

Baldwin's rules in organic chemistry are a series of guidelines outlining the relative favorabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976.

The Rubottom oxidation is a useful, high-yielding chemical reaction between silyl enol ethers and peroxyacids to give the corresponding α-hydroxy carbonyl product. The mechanism of the reaction was proposed in its original disclosure by A.G. Brook with further evidence later supplied by George M. Rubottom. After a Prilezhaev-type oxidation of the silyl enol ether with the peroxyacid to form the siloxy oxirane intermediate, acid-catalyzed ring-opening yields an oxocarbenium ion. This intermediate then participates in a 1,4-silyl migration to give an α-siloxy carbonyl derivative that can be readily converted to the α-hydroxy carbonyl compound in the presence of acid, base, or a fluoride source.

In organic chemistry, aldol reactions are acid- or base-catalyzed reactions of aldehydes or ketones.

A (4+3) cycloaddition is a cycloaddition between a four-atom π-system and a three-atom π-system to form a seven-membered ring. Allyl or oxyallyl cations (propenylium-2-olate) are commonly used three-atom π-systems, while a diene plays the role of the four-atom π-system. It represents one of the relatively few synthetic methods available to form seven-membered rings stereoselectively in high yield.

In organic chemistry, α-halo ketones can be reduced with loss of the halogen atom to form enolates. The α-halo ketones are readily prepared from ketones by various ketone halogenation reactions, and the products are reactive intermediates that can be used for a variety of other chemical reactions.

The Saegusa–Ito oxidation is a chemical reaction used in organic chemistry. It was discovered in 1978 by Takeo Saegusa and Yoshihiko Ito as a method to introduce α-β unsaturation in carbonyl compounds. The reaction as originally reported involved formation of a silyl enol ether followed by treatment with palladium(II) acetate and benzoquinone to yield the corresponding enone. The original publication noted its utility for regeneration of unsaturation following 1,4-addition with nucleophiles such as organocuprates.

2-Cyclopentenone is a ketone with chemical formula C5H6O and CAS number 930-30-3. It is structurally similar to cyclopentanone, with the additional feature of α-β unsaturation in the ring system. 2-Cyclopentenone contains two functional groups, a ketone and an alkene. It is a colorless liquid.

In organic chemistry, the Baylis–Hillman, Morita–Baylis–Hillman, or MBH reaction is a carbon-carbon bond-forming reaction between an activated alkene and a carbon electrophile in the presence of a nucleophilic catalyst, such as a tertiary amine or phosphine. The product is densely functionalized, joining the alkene at the α-position to a reduced form of the electrophile.

<span class="mw-page-title-main">Torquoselectivity</span>

Torquoselectivity is a special kind of stereoselectivity observed in electrocyclic reactions in organic chemistry, defined as "the preference for inward or outward rotation of substituents in conrotatory or disrotatory electrocyclic reactions." Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic reactions, as it represents a further level of selectivity beyond the Woodward-Hoffman rules. The name derives from the idea that the substituents in an electrocyclization appear to rotate over the course of the reaction, and thus selection of a single product is equivalent to selection of one direction of rotation. The concept was originally developed by Kendall N. Houk.

<span class="mw-page-title-main">Hydrogen-bond catalysis</span>

Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic reactions. In biological systems, hydrogen bonding plays a key role in many enzymatic reactions, both in orienting the substrate molecules and lowering barriers to reaction. However, chemists have only recently attempted to harness the power of using hydrogen bonds to perform catalysis, and the field is relatively undeveloped compared to research in Lewis acid catalysis.

Rearrangements, especially those that can participate in cascade reactions, such as the aza-Cope rearrangements, are of high practical as well as conceptual importance in organic chemistry, due to their ability to quickly build structural complexity out of simple starting materials. The aza-Cope rearrangements are examples of heteroatom versions of the Cope rearrangement, which is a [3,3]-sigmatropic rearrangement that shifts single and double bonds between two allylic components. In accordance with the Woodward-Hoffman rules, thermal aza-Cope rearrangements proceed suprafacially. Aza-Cope rearrangements are generally classified by the position of the nitrogen in the molecule :

α,β-Unsaturated carbonyl compound Functional group of organic compounds

α,β-Unsaturated carbonyl compounds are organic compounds with the general structure (O=CR)−Cα=Cβ-R. Such compounds include enones and enals. In these compounds the carbonyl group is conjugated with an alkene. Unlike the case for carbonyls without a flanking alkene group, α,β-unsaturated carbonyl compounds are susceptible to attack by nucleophiles at the β-carbon. This pattern of reactivity is called vinylogous. Examples of unsaturated carbonyls are acrolein (propenal), mesityl oxide, acrylic acid, and maleic acid. Unsaturated carbonyls can be prepared in the laboratory in an aldol reaction and in the Perkin reaction.

In organic chemistry, the Conia-ene reaction is an intramolecular cyclization reaction between an enolizable carbonyl such as an ester or ketone and an alkyne or alkene, giving a cyclic product with a new carbon-carbon bond. As initially reported by J. M. Conia and P. Le Perchec, the Conia-ene reaction is a heteroatom analog of the ene reaction that uses an enol as the ene component. Like other pericyclic reactions, the original Conia-ene reaction required high temperatures to proceed, limiting its wider application. However, subsequent improvements, particularly in metal catalysis, have led to significant expansion of reaction scope. Consequently, various forms of the Conia-ene reaction have been employed in the synthesis of complex molecules and natural products.

References

  1. ↑ Nazarov, I.N.; Zaretskaya, I.I. (1941), Izv. Akad. Nauk. SSSR, Ser. Khim: 211–224{{citation}}: Missing or empty |title= (help)
  2. 1 2 3 4 5 6 7 8 9 10 11 Frontier, A. J.; Collison, C. (2005), "The Nazarov cyclization in organic synthesis. Recent advances.", Tetrahedron , 61 (32): 7577–7606, doi:10.1016/j.tet.2005.05.019
  3. ↑ Santelli-Rouvier, C.; Santelli, M. (1983), "The Nazarov Cyclization", Synthesis , 1983 (6): 429–442, doi:10.1055/s-1983-30367
  4. ↑ Denmark, S.E.; Habermas, K.L.; Jones, T. K. (1994), "The Nazarov Cyclization", Organic Reactions , 45: 1–158
  5. ↑ Denmark, S.E. (1991), Paquette, L.A. (ed.), "The Nazarov and Related Cationic Cyclizations", Comprehensive Organic Synthesis, Oxford: Pergamon Press, 5: 751–784, doi:10.1016/b978-0-08-052349-1.00138-4, ISBN   9780080523491
  6. 1 2 Tius, M. A. (2005), "Some New Nazarov Chemistry", Eur. J. Org. Chem. , 2005 (11): 2193–2206, doi:10.1002/ejoc.200500005
  7. ↑ Pellissier, Hélène (2005), "Recent developments in the Nazarov process", Tetrahedron , 61 (27): 6479–6517, doi:10.1016/j.tet.2005.04.014
  8. ↑ Shoppee, C. W.; Lack, R. E. (1969), "Intramolecular electrocyclic reactions. Part I. Structure of 'bromohydroxyphorone': 3-bromo-5-hydroxy-4,4,5,5-tetramethylcyclopent-2-enone", Journal of the Chemical Society C: Organic (10): 1346–1349, doi:10.1039/J39690001346
  9. ↑ Shoppee, C. W.; Cooke, B. J. A. (1972), "Intramolecular electrocyclic reactions. Part II. Reactions of 1,5-di-phenylpenta-1,4-dien-3-one", Journal of the Chemical Society, Perkin Transactions 1 : 2271, doi:10.1039/p19720002271
  10. 1 2 Kürti, L.; Czakó, B. (2005). Strategic Applications of Named Reactions in Organic Synthesis (1 ed.). Burlington, MA: Elsevier Academic Press. pp. 304–305. ISBN   9780124297852.
  11. ↑ Harding, K. E.; Clement, K. S. (1984), "A Highly Stereoselective, Convergent Synthesis of (±)-Trichodiene", The Journal of Organic Chemistry , 49 (20): 3870–3871, doi:10.1021/jo00194a054
  12. ↑ Arai, Y.; Takeda, K.; Masuda, K.; Koizumi, T. (1985), "Synthesis of(±)-Nor-Sterepolide", Chemistry Letters , 14 (10): 1531–1534, doi:10.1246/cl.1985.1531
  13. ↑ Denmark, S.E.; Jones, T.K. (1982), "Silicon-Directed Nazarov Cyclization", J. Am. Chem. Soc. , 104 (9): 2642–2645, doi:10.1021/ja00373a055
  14. ↑ He, W.; Sun, X.; Frontier, A.J. (2003), "Polarizing the Nazarov Cyclization: Efficient Catalysis under Mild Conditions", J. Am. Chem. Soc. , 125 (47): 14278–14279, doi:10.1021/ja037910b, PMID   14624567
  15. ↑ Grant, T.N. & West, F.G. (2006), "A New Approach to the Nazarov Reaction via Sequential Electrocyclic Ring Opening and Ring Closure", J. Am. Chem. Soc. , 128 (29): 9348–9349, doi:10.1021/ja063421a, PMID   16848467
  16. ↑ Malona, J.A.; Cariou, K.; Frontier, A.J. (2009), "Nazarov Cyclization Initiated by Peracid Oxidation: The Total Synthesis of (±)-Rocaglamide", J. Am. Chem. Soc., 131 (22): 7560–7561, doi:10.1021/ja9029736, PMC   2732401 , PMID   19445456
  17. ↑ Grant, T.N.; Rieder, C.J.; West, F.G. (2009), "Interrupting the Nazarov reaction: domino and cascade processes utilizing cyclopentenyl cations", Chem. Commun. (38): 5676–5688, doi:10.1039/B908515G, PMID   19774236
  18. ↑ Bender, J.A.; Arif, A.M.; West, F.G. (1999), "Nazarov-Initiated Diastereoselective Cascade Polycyclization of Aryltrienones", J. Am. Chem. Soc. , 121 (32): 7443–7444, doi:10.1021/ja991215f
  19. ↑ Janka, M.; He, W.; Haedicke, I.E.; Fronczek, F.R.; Frontier, A.J.; Eisenberg, R. (2006), "Tandem Nazarov Cyclization-Michael Addition Sequence Catalyzed by an Ir(III) Complex", J. Am. Chem. Soc. , 128 (16): 5312–5313, doi:10.1021/ja058772o, PMID   16620081
  20. ↑ Harrington, P.E.; Tius, M.A. (1999), "A Formal Total Synthesis of Roseophilin: Cyclopentannelation Approach to the Macrocyclic Core", Org. Lett. , 1 (4): 649–652, doi:10.1021/ol990124k, PMID   10823195
  21. ↑ Jung, M. E.; Yoo, D. (2007), "Unprecedented Rearrangement of a 4-Alkoxy-5-bromoalk-2-en-1-ol to a Cyclopentenone via an Iso-Nazarov Cyclization Process", The Journal of Organic Chemistry , 72 (22): 8565–8568, doi:10.1021/jo071104w, PMID   17910498
  22. ↑ De Simone, F.; Andrès, J.; Torosantucci, R.; Waser, J. r. m. (2009), "Catalytic Formal Homo-Nazarov Cyclization", Organic Letters , 11 (4): 1023–1026, doi:10.1021/ol802970g, PMID   19199774
  23. ↑ Rieder, C. J.; Winberg, K. J.; West, F. G. (2009), "Cyclization of Cross-Conjugated Trienes: The Vinylogous Nazarov Reaction", Journal of the American Chemical Society , 131 (22): 7504–7505, doi:10.1021/ja9023226, PMID   19435345
  24. ↑ Tius, M. A.; Chu, C. C.; Nieves-Colberg, R. (2001), "An imino Nazarov cyclization", Tetrahedron Letters , 42 (13): 2419–2422, doi:10.1016/s0040-4039(01)00201-5
  25. ↑ Kim, S.-H.; Cha, J. K. (2000), "Synthetic Studies Toward caphalotaxine: Functionalization of Tertiary N-Acylhemiaminals by Nazarov Cyclization", Synthesis , 2000 (14): 2113–2116, doi:10.1055/s-2000-8711
  26. ↑ Larini, P.; Guarna, A.; Occhiato, E. G. (2006), "The Lewis Acid-Catalyzed Nazarov Reaction of 2-(N-Methoxycarbonylamino)-1,4-pentadien-3-ones", Org. Lett. , 8 (4): 781–784, doi:10.1021/ol053071h, PMID   16468766
  27. ↑ Smith, D. A.; Ulmer, C. W. (1997), "Effects of Substituents in the 3-Position on the [2 + 2] Pentadienyl Cation Electrocyclization", The Journal of Organic Chemistry , 62 (15): 5110–5115, doi:10.1021/jo9703313
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.