Divinyl Ketones Research Articles

Strong and Confined Acids Enable a Catalytic Asymmetric Nazarov Cyclization of Simple Divinyl Ketones.

We report a catalytic asymmetric Nazarov cyclization of simple, acylic, alkyl-substituted divinyl ketones using our recently disclosed strong and confined imidodiphosphorimidate Brønsted acids. The corresponding monocyclic cyclopentenones are formed in good yields and excellent regio-, diastereo-, and enantioselectivities. Further, the chemical utility of the obtained enantiopure cyclopentenones is demonstrated.

Phosphazene Base-Catalyzed Double Michael Addition: Stereoselective Synthesis of Cyclohexanones

Phosphazene bases have been utilized as efficient organocatalysts to catalyze the double Michael additions of divinyl ketones with active methylenes to afford functionalized cyclohexanones in 36-91% yields with >25:1 diastereoselectivity.

N‐Heterocyclic Carbene‐Catalyzed Double Michael Addition: Stereoselective Synthesis of Spirofluorenes and Multisubstituted Indanes

AbstractThe strong Brønsted basic character of N‐heterocyclic carbenes (NHCs) has been used to promote the cascade double Michael addition between fluorenes and dienones. Under catalyst loadings of 1–5 mol% of NHC, fluorene reacts with divinyl ketones (DVKs) to afford anti‐spirofluorene compounds in high yields. However, when benzenedi(enones) were employed as Michael acceptors in the presence of 10 mol% of NHC, fluorene undergoes a different inter‐ and intramolecular cascade double Michael addition, providing multi‐substituted indanes in high yields with excellent diastereoselectivity.magnified image

Triple Nucleophilic Attack of Nitromethane on (2-Iminoaryl)divinyl Ketones: A Domino Synthetic Strategy for Hexahydrophenanthridinones.

A novel domino reaction of (2-iminoaryl)divinyl ketones with nitromethane was developed for the efficient synthesis of hexahydrophenanthridin-9(5H)-ones. The reaction proceeded smoothly from readily available starting materials under mild reaction conditions to construct three new bonds and two rings with high diastereoselectivities in good to excellent yields in a single step. A mechanism is proposed, involving a stepwise double Michael addition/aza-Henry reaction cascade, and in this transformation, nitromethane acts as a trinucleophile.

Synthesis of 4-arylethyl-6-arylpyrimidine-2-thiols through aza-Michael addition/nucleophilic addition/aromatization tandem reactions

Abstract An efficient method for the synthesis of the title compounds by reactions of divinyl ketones with thiourea is described. This protocol has the advantages of high yields, mild reaction conditions and simple work-up procedure.

Asymmetric synthesis of multiple quaternary stereocentre-containing cyclopentyls by oxazolidinone-promoted Nazarov cyclizations.

Carbometalation of oxazolidinone (Ox)-substituted ynamides is used to generate highly substituted Ox-divinyl (and aryl vinyl) ketones for use in Nazarov cyclizations. The Ox-group serves as a remarkably effective chiral activating group, enabling the torquoselective Nazarov cyclization of these sterically congested substrates to be performed under mild conditions. It also serves as a charge-stabilizing group in the intermediate oxyallyl cation, suppressing undesired [1,2]-sigmatropic shifts of neighboring substituents and facilitating the regio- and stereoselective incorporation of nucleophiles to yield cyclopentanoids containing up to three contiguous all-carbon quaternary (4°) stereocentres.

A Gold(I)‐Catalyzed Oxidative Rearrangement of Heterocycle‐Derived 1,3‐Enynes Provides an Efficient and Selective Route to Divinyl Ketones

The gold‐catalyzed oxidation of N‐tosyl‐protected 6‐alkynyl‐3,4‐dihydro‐2H‐pyridines was studied in detail to obtain divinyl ketones in which one of the double bonds is embedded in a heterocyclic framework. The best reaction conditions were then extended to different types of substrates to assess the scope of the reaction. DFT calculations were exploited to gain insight into the regio‐ and chemoselectivity of the process too. The obtained divinyl ketones were then easily cyclized by a Nazarov process and the bi‐ or polycyclic compounds used as scaffolds in the synthesis of analogues of the plant hormones strigolactones.

Regioselective Mono‐aza‐Michael Additions of Divinyl Ketones with Benzotriazole and Other N‐Heterocycles

The mono‐aza‐Michael additions of divinyl ketones with benzotriazole and other N‐heterocycles in the presence of cesium carbonate were described. This protocol has advantages of high regioselectivity, high yield, and mild condition.

Multistereocenter-Containing Cyclopentanoids from Ynamides via Oxazolidinone-Controlled Nazarov Cyclization.

Achieving ready-enantioselective access to multistereocenter-containing cyclopentyl rings is an area of great significance to organic synthesis. In this work, we describe a general protocol for accessing multistereocenter-containing cyclopentanoids from simple N-alkynyloxazolidinones (Ox-ynamides). This protocol involves conversion of Ox-ynamides into Ox-activated divinyl and aryl vinyl ketones that undergo facile Nazarov cyclization with excellent chemo-, regio-, and stereocontrol. The Ox auxiliary directs all aspects of reactivity and selectivity, both in the electrocyclization and in the subsequent transformations of the resulting oxyallyl intermediate. Stereoinduction in the electrocyclization results from a "coupled-torque" mechanism in which rotation of the Ox group, driven by increasing orbital overlap of the nitrogen lone pair with the incipient oxyallyl cation, is coupled with the rotation of the termini of the pentadienyl cation, favoring a particular direction of conrotatory ring closure (torquoselectivity). The associated lone-pair stabilization of the transition state by Ox promotes cyclization of traditionally resistant substrates, broadening the scope of this asymmetric Nazarov cyclization. The Ox group also facilitates the stereo- and regioselective incorporation of nucleophiles (Nu) and dienes, giving more complex, multistereocenter containing cyclopentanoids. Finally, the Ox group is readily removed and recovered or can be converted into other amine functionalities.

Mechanisms of Carbonyl Activation by BINOL N-Triflylphosphoramides: Enantioselective Nazarov Cyclizations

BINOL N-triflylphosphoramides are versatile organocatalysts for reactions of carbonyl compounds. Upon activation by BINOL N-triflylphosphoramides, divinyl ketones undergo rapid and highly enantioselective (torquoselective) Nazarov cyclizations, making BINOL N-triflylphosphoramides one of the most important classes of catalysts for the Nazarov cyclization. However, the activation mechanism and the factors that determine enantioselectivity have not been established until now. Theoretical calculations with ONIOM and M06-2X are reported which examine how BINOL N-triflylphosphoramides activate divinyl ketones and control the torquoselectivity of the cyclization. Unexpectedly, the computations reveal that the traditionally accepted mechanisms for these catalysts (i.e., NH···O═C hydrogen bonding or proton transfer) are not the dominant activation mechanisms. Instead, the active catalyst is a less-stable tautomer of the phosphoramide containing a P(═NTf)OH group. Proton transfer from the catalyst to the substrate...

Triaryl-Substituted Divinyl Ketones Cyclization: Nazarov Reaction versus Friedel–Crafts Electrophilic Substitution

The acid-catalyzed cyclization of a wide range of triaryl-substituted divinyl ketones has been studied. It was found that the reaction pathway strongly depends on the nature of the aryl substituent at the α-position to the carbonyl group. An electron-rich aromatic substituent promotes the reaction to proceed through the intramolecular Friedel-Crafts electrophilic substitution giving dihydronaphthalene derivatives. In contrast, the presence of an electron-deficient substituent is favorable for the Nazarov 4π-conrotatory cyclization yielding triaryl-substituted cyclopentenones. The electrophilic substitution reaction was applied to thiophene and thiazole derivatives.

Ruthenium Lewis Acid‐Catalyzed Asymmetric Diels–Alder Reactions: Reverse‐Face Selectivity for α,β‐Unsaturated Aldehydes and Ketones

Acrolein, methacrolein, methyl vinyl ketone, ethyl vinyl ketone, 3‐methyl‐3‐en‐2‐one, and divinyl ketone were coordinated to a cationic cyclopentadienyl ruthenium(II) Lewis acid incorporating the electron‐poor bidentate BIPHOP–F ligand. Analysis by NOESY and ROESY NMR techniques allowed the determination of conformations of enals and enones present in solution in CD2Cl2. The results were compared to solid‐state structures and to the facial selectivities of catalytic asymmetric Diels–Alder reactions with cyclopentadiene. X‐Ray structures of four Ru‐enal and Ru‐enone complexes show the α,β‐unsaturated C=O compounds to adopt an anti‐s‐trans conformation. In solution, enals assume both anti‐s‐trans and anti‐s‐cis conformations. An additional conformation, syn‐s‐trans, is present in enone complexes. Enantioface selectivity in the cycloaddition reactions differs for enals and enones. Reaction products indicate enals to react exclusively in the anti‐s‐trans conformation, whereas with enones, the major product results from the syn‐s‐trans conformation. The alkene in s‐cis conformations, while present in solution, is shielded and cannot undergo cycloaddition. A syn‐s‐trans conformation is found in the solid state of the bulky 6,6‐dimethyl cyclohexanone‐Ru(II) complex. The X‐ray structure of divinyl ketone is unique in that the Ru(II) center binds the enone via a η2 bond to one of the alkene moieties. In solution, coordination to Ru–C=O oxygen is adopted. A comparison of facial preference is also made to the corresponding indenyl Lewis acids.

ChemInform Abstract: Nazarov Cyclization of Divinyl Ketones Bearing an Ester Group at the β‐Position: A Remarkable Effect of α‐Substitution and Alkene Geometry on Regioselectivity.

The Nazarov cyclization of divinyl ketones with an ester at the β-position was examined with particular reference to where the cyclic double bond forms. We observed unprecedented regioselectivity, dictated by the subtle substitution patterns at the α-position and alkene geometry of α,β and mostly, this selectivity is regardless of substitutions at α′- and β′-positions. The major implications of these observations are an aromatic group at the α-position with E-olefin geometry provides a cyclopentenone in which the double bond is not in conjugation with an ester, whereas Z-olefin provides a cyclopentenone in which the double bond is in conjugation with an ester; and divinyl ketones bearing an ester group at the β-position and an alkyl group at the α-position with E-olefin geometry provide a cyclopentenone in which the double bond is in conjugation with the ester.

ChemInform Abstract: Diastereoselective Synthesis of 3,4‐Benzomorphan Derivatives via Tandem [5 + 1]/Hemiaminalization of (2‐Aminoaryl)divinyl Ketones.

AbstractThe single step metal‐free title reaction between (2‐aminoaryl)divinyl ketones and nitromethane as well as other nucleophilic active methylene compounds tolerates various functional groups.

ChemInform Abstract: Hydroxylamine Catalyzed Nazarov Cyclizations of Divinyl Ketones.

Abstract The first examples of iminium catalyzed Nazarov cyclizations of divinyl ketones are presented. Experiments describing hydroxylamine catalysis of the cyclization of eight α-alkoxy divinyl ketones (60–79% yield) and one unactivated divinyl ketone (38% yield) are reported. Phenyl substitution at the β-position of the divinyl ketone inhibits cyclization, whereas β-alkyl substituted α-alkoxy divinyl ketones readily cyclize.

Diastereoselective Synthesis of 3,4-Benzomorphan Derivatives via Tandem [5 + 1]/Hemiaminalization of (2-Aminoaryl)divinyl Ketones.

A novel tandem formal [5 + 1]/hemiaminalization reaction based on the readily available (2-aminoaryl)divinyl ketones and various nucleophiles has been developed. The reaction represents a highly efficient and convenient methodology for the synthesis of 3,4-benzomorphan derivatives with high diastereoselectivity, and three new bonds and two rings are successively formed in one step under mild, metal-free conditions.

Nazarov cyclization of divinyl ketones bearing an ester group at the β-position: a remarkable effect of α-substitution and alkene geometry on regioselectivity.

The Nazarov cyclization of divinyl ketones with an ester at the β-position was examined with particular reference to where the cyclic double bond forms. We observed unprecedented regioselectivity, dictated by the subtle substitution patterns at the α-position and alkene geometry of α,β and mostly, this selectivity is regardless of substitutions at α'- and β'-positions. The major implications of these observations are an aromatic group at the α-position with E-olefin geometry provides a cyclopentenone in which the double bond is not in conjugation with an ester, whereas Z-olefin provides a cyclopentenone in which the double bond is in conjugation with an ester; and divinyl ketones bearing an ester group at the β-position and an alkyl group at the α-position with E-olefin geometry provide a cyclopentenone in which the double bond is in conjugation with the ester.

Hydroxylamine catalyzed Nazarov cyclizations of divinyl ketones

The first examples of iminium catalyzed Nazarov cyclizations of divinyl ketones are presented. Experiments describing hydroxylamine catalysis of the cyclization of eight α-alkoxy divinyl ketones (60–79% yield) and one unactivated divinyl ketone (38% yield) are reported. Phenyl substitution at the β-position of the divinyl ketone inhibits cyclization, whereas β-alkyl substituted α-alkoxy divinyl ketones readily cyclize.

Synthesis of new photochromic diarylethenes of cyclopentenone series by Nazarov reaction

An effective method is proposed for the synthesis of photochromic diarylethenes of cyclopentenone series with SnCl4-catalyzed 4π-electron cyclization of divinyl ketones by Nazarov reaction as the key step. It was shown that the introduction of an aryl group at position 4 of the cyclopentenone ring significantly improved the quantum yield of diarylethene photocyclization reaction.

Reactivity and stereoselectivity of 6π and nazarov electrocyclizations of bridged bicyclic trienes and divinyl ketones.

The 6π electrocyclizations and Nazarov cyclizations of a series of bridged bicyclic substrates were modeled with the M06-2X density functional and the def2-TZVPP basis set, and the factors responsible for the reactivities of these substrates and the stereoselectivities of their ring closures were identified. The ring closures of these bridged bicyclic trienes are up to a million-fold faster (ΔΔG(⧧) = 10 kcal mol(-1)) than that of 1,3,5-hexatriene, despite the absence of any activating functional groups. Three effects, preorganization, predistortion, and a CH π interaction, are responsible for this sizable difference in reactivity. Stereoselectivity is partially controlled by torsional effects, but for highly exo selective electrocyclizations, it is reinforced by a second effect (either a CH π interaction or a steric clash). The absence of this second effect in the ring closures of several divinyl ketones explains the reduced selectivity of these ring closures. In one case, a divinyl ketone (ketone 6) undergoes Nazarov cyclization to yield the endo product preferentially. For this example, through-space interaction of a nonconjugated alkene with the divinyl ketone π system in the endo transition state and a steric effect override the intrinsic exo selectivity.