A direct retro-Reformatsky fragmentation: formal ring enlargement of cyclic ketones for novel and practical synthesis of heterocyclic enamines

A novel and practical synthesis of heterocyclic enamines has been developed from the formal ring enlargement of cyclic ketones, which comprised the retro-Reformatsky fragmentation reaction as a key step. Under alkaline bromination conditions, the Reformatsky adducts derived from five- to seven-membered cyclic ketones underwent efficiently a direct retro-Reformatsky fragmentation, followed by spontaneous alpha,alpha-dibromination, to produce alpha,alpha,omega-tribromo-beta-ketoester compounds in a one-pot reaction. Highly regioselective reduction of alpha,alpha,omega-tribromo-beta-ketoesters with Cu-Zn alloy under mild conditions afforded omega-bromo-beta-ketoesters in good to excellent yields. Treatment of omega-bromo-beta-ketoesters with sodium azide followed by intramolecular aza-Wittig reaction or catalytic hydrogenation furnished heterocyclic secondary enamines, while a straightforward cyclocondensation of omega-bromo-beta-ketoesters with amines led to the formation of heterocyclic tertiary enamines.     

Synthesis of alpha-hydrazino ketones via regio- and stereoselective electrophilic amination of manganese enolates and enamines

A straightforward procedure for the regio- and stereoselective synthesis of alpha-hydrazino ketones is described. Manganese enolates and manganese enamines derived from ketones and from the corresponding N-sulfinylimines react with azodicarboxylate esters (DTBAD and DEAD) in a regioselective fashion to afford in good to excellent yields the kinetic alpha-hydrazino ketones as sole or highly prevalent products. When enantiopure N-sulfinyl manganese enamines were used the stereoselectivity of these reactions ranged from 40% to 68% ee.     

The chemistry of cyclic enaminoketones. III. Synthesis of bi- and tricyclic enaminoketones from enamine esters and nitriles

The cyclization of enamines derived from β-aminoesters and β-aminonitriles into bi- and tricyclic enaminoketones ( 6,9,13 and 24 ) has been investigated. The enamines derived from aminonitriles cyclize smoothly with magnesium perchlorate in benzene or toluene, whereas the enamines derived from aminoesters cyclize spontaneously during their formation. The scope and limitation of this process is discussed.     

1,3,4-Oxadiazine derivatives from cyclohexanone enamines and asymmetric diimides : Possibility of ring-chain tautomerism in such heterocyclic system

1,3,4-oxadiazine derivatives, in equilibrium with their corresponding trisubstituted enamine isomers, are obtained in a regiospecific way from cyclohexanone enamines and ethoxycarbonyl aroyl diimides. Such an equilibrium is not present in analogous oxadiazine systems, derived from aryl aroyl diimides.     

The use of trifluoroacetaldehyde ethyl hemiacetal or hydrate in a simple and practical regioselective synthesis of beta-hydroxy-beta-trifluoromethyl ketones from enamines and imines

The reaction of trifluoroacetaldehyde ethyl hemiacetal or hydrate with an equimolar amount of enamines, derived from various methyl ketones, smoothly proceeded to give the corresponding beta-hydroxy-beta-trifluoromethyl ketones in high yields. An equimolar amount of imines derived from various methyl ketones with aliphatic, aromatic, and heteroaromatic substituents also readily reacted with trifluoroacetaldehyde ethyl hemiacetal or hydrate to afford the corresponding beta-hydroxy-beta-trifluoromethyl ketones in good to excellent yields. Difluoroacetaldehyde ethyl hemiacetal as well as pentafluoropropionaldehyde also participated in the reaction, affording good yields of the corresponding beta-hydroxy-beta-difluoromethyl or beta-pentafluoropropyl ketones.     

Stoichiometric Reactions of Enamines Derived from Diphenylprolinol Silyl Ethers with Nitro Olefins and Lessons for the Corresponding Organocatalytic Conversions – a Survey

The stoichiometric reactions of enamines prepared from aldehydes and diphenyl‐prolinol silyl ethers (intermediates of numerous organocatalytic processes) with nitro olefins have been investigated. As reported in the last century for simple achiral and chiral enamines, the products are cyclobutanes ( 4 with monosubstituted nitro‐ethenes), dihydro‐oxazine N‐oxide derivatives ( 5 with disubstituted nitro‐ethenes), and nitro enamines derived from γ‐nitro aldehydes ( 6 , often formed after longer reaction times). The same types of products were shown to be formed, when the reactions were carried out with peptides H‐Pro‐Pro‐Xaa‐OMe that lack an acidic H‐atom. Functionalized components such as alkoxy enamines, nitro‐acrylates, acetamido‐nitro‐ethylene, or hydroxylated nitro olefins also form products carrying the diphenyl‐prolinol silyl ether as a substituent. All of these products must be considered intermediates in the corresponding catalytic reactions; the investigation of their chemical properties provided useful hints about the rates, the conditions, the catalyst resting states or irreversible traps, and/or the limitations of the corresponding organocatalytic processes. High‐level DFT and MP2 computations of the structures of alkoxy enamines and thermodynamic data of a cyclobutane dissociation are also described. Some results obtained with the stoichiometrically prepared intermediates are not compatible with previous mechanistic proposals and assumptions.     

Structural Studies of Crystalline Enamines

Crystal structure analyses of five crystalline enamines together with recently published structural data for two other enamines reveal varying degree of pyramidality at the enamine nitrogen atom. The pyramidality appears to be most pronounced in enamines from piperidine and morpholine, less so in enamines derived from prolinoid amines. A pyrolidine enamine obtained from a derivative of cyclohexane-1,4-dione seems to have a virtually planar enamine group. The implications of these findings for current stereochemical problems in enamine chemistry are discussed.     

υ-Triazolines. Part XIII. Reactions of α-ketoenamines with toluene-P-sulphonylazide

Some enamines derived from α-diketones have been reacted with tosylazide yielding unstable 5-amino-υ-triazolines, which were found to undergo cyeloreversion to diazo compounds and arnidines. In the case of the enamines derived from 4-aryl-2,3-butanediones, a competitive rearrangement to (Z)-4-aryl-4-arnino-3-tosylamino-3-butene-2-ones was observed.     

Relative Stability of the Key Enamine,Oxazolidinone, and Imine Intermediates in Some Proline-catalyzed Asymmetric Reactions

Many proline-catalyzed asymmetric addition reactions with ketones as substrates were assumed to involve a key intermediate, an enamine, produced by the condensation of proline and ketone. In this paper, the key intermediate enamines derived from L-proline and cyclohexanone （or acetone） as well as the corresponding oxazolidinone and imine tautomers have been investigated by means of density functional calculations at the B3LYP/6-311＋G^＊＊ level. The predicted order of stability for these tautomers is oxazolidinones 〉 enamines 〉 imines in gas phase and oxazolidinones 〉 imines 〉 enamines in aprotic THF solvent. This prediction explains why enamine intermediate can not be observed experimentally. The predicted energy/enthalpy difference between the formal oxazolidinone structure and the zwitterionic imine structures is very small in THF solvent, suggesting the oxazolidinone-to-imine tautomerization can be readily induced in solvent. ^13C NMR chemical shifts of the oxazolidinone and imine structures have been computed and used to explain the experimental NMR spectra observed in oxazolidinone-to-imine tautomerization induced by protic solvent.     

Synthesis of 6H-dibenzo[b,d]pyran-6-ones using the inverse electron demand Diels-Alder reaction

A set of coumarin-fused electron-deficient 1,3-dienes was synthesized, which differ in the nature of the electron-withdrawing group (EWG) at the terminus of the diene unit and (when EWG = CO(2)Me) the nature and position of substituents. These dienes reacted with the enamine derived from cyclopentanone and pyrrolidine to afford the corresponding cyclopenteno-fused 6H-dibenzo[b,d]pyran-6-ones, most likely via a domino inverse electron demand Diels-Alder (IEDDA)/elimination/transfer hydrogenation sequence. The parent diene (EWG = CO(2)Me, no substituents) was reacted with a range of electron-rich dienophiles (mostly enamines) to afford the corresponding 6H-dibenzo[b,d]pyran-6-ones or their nondehydrogenated precursors, which were aromatized upon treatment with a suitable oxidant. The enamines could either be synthesized prior to the reaction or generated in situ. The syntheses of 30 dibenzopyranones are reported.     

1,2- and 1,4-addition reactions of cycloalkanones with levoglucosenone

The reactions of levoglucosenone with 2-acetylcyclohexanone and 2-acetyldodecanone occurred as tandem Michael addition and 1: 1 aldol condensation to give the corresponding spiro derivatives. Conditions ensuring 1,2- and 1,4-additions of C_{5–7, 12}-cycloalkanones and enamines derived therefrom to levoglucosenone were found.     

A practical and efficient preparation of (−)-(4aS,5R)-4,4a,5,6,7,8-hexahydro-4a,5-dimethyl-2(3H)-naphthalenone: a key intermediate in the synthesis of (−)-dehydrofukinone

A novel diastereoselective route to octalone (−)-1 has been developed. The key step involves an asymmetric Michael addition of the corresponding chiral secondary enamines derived from (S)-(−)-1-phenylethylamine and (3R)-2,3-dimethylcyclohexanone to methyl vinyl ketone. This enone was successfully transformed into the eremophilane-type sesquiterpenoid (−)-dehydrofukinone.     

Trichloroacetylation of some cyclic enamines

The pyrrolidine and morpholine enamines of cyclic ketones such as cyclohexanone and cyclopentanone were successfully diacetylated at alpha- and alpha'-positions with trichloroacetyl chloride using zinc catalyst. Morpholine enamines of the cyclic ketones gave acetylated morpholine in good yields besides the corresponding diacetylated cyclic enamines. When the same reactions were performed by using triethylamine without using zinc, monoacetylation products of the same enamines were synthesized.     

N-Haloamidines-V: Reaction of N-chloro-N'-aroyl-acetamidines and -benzamidines with enamines

The reaction between N-chloro-N'-aroyl-amidines and β,β-disubstituted enamines affords 1 - aroyl - 4 -amino - 4,5 - dihydro - imidazoles in low yields, the main reaction product being N - (2 - morpholino - 2,2 -disubstituted) - ethylidene - N' - aroyl - amidines. A similar reaction course was not observed with enamines bearing a hydrogen atom in the β-position. In this case only products derived from electrophilic chlorination on enamines were isolated.     

5-Pyrrolidin-2-yltetrazole as an asymmetric organocatalyst for the addition of ketones to nitro-olefins

The organocatalytic Michael addition of enamines derived from ketones to a range of nitro-olefins has been effected using the l-proline derived 5-pyrrolidin-2-yltetrazole.     

The stereochemical course of formic acid reduction of enamines

Formic acid reduction of enamines derived from 2-methyl- and 2-(β-cyanoethyl)-cyclohexanone has been shown to proceed with high stereoselectivity, the cis to trans ratio of the resulting cyclohexylamines being about 9. The stereochemistry of the isomers has been established by Cope-eliminations of the corresponding N-oxides and by applying the boiling point and refractive index relations of the Auwers-Skita rules.     

Imines et enamines stanniques : IV. Protolyse: synthese d'enamines secondaires; structure,isomerisation, reactivite vis a vis de l'acrylonitrile

The partial protolysis of organotin imines—enamines leads to organic secondary enamines, which we found to be stable in completely aprotic media.Their structure was determined by IR and NMR methods, and their reactivity towards acrylonitrile studied. The results show a much greater reactivity for enamines, as compared to that of the corresponding imine tautomers.     

Reactions of N-alkylazinium cations. Reaction of acridinium salts with enamines

The reaction pyridinium N-methylazinium salts and their benzo analogs — quinolinium, isoquinolinium, and acridinium ions — with enamines was investigated. Acridanyl-substituted enamines, their iminium salts, and the corresponding ketones were obtained by reaction of the N-methylacridinium salts with the enamines in dimethyl sulfoxide at room temperature. The quinolinium and isoquinolinium salts have lower activities; the N-methylpyridinium cation does not react at all under the indicated conditions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 666–669, May, 1979.     

Neuartige Merocyanine vom Typ des «Bindons»

The title compounds (new type of merocyanines) were obtained from the reaction of heterocyclic enamines with indan-1,3-dione, biindantrione, 2-(3-methoxy-inden-1-yliden)-indan-1, 3-dione, or 2-(3-chloro-inden-1-yliden)-indan-1, 3-dione respectively. The enamines were obtained from the corresponding quaternary salts and reacted in situ.     

Synthesis of 6-nitro-4-sulfanyl-1H-indole derivatives from 2,4,6-trinitrotoluene

The synthesis of 6-nitro-4-sulfanyl-1H-indoles from 2,4,6-trinitrotoluene (TNT) is described. The first step is the nucleophilic substitution of an ortho-nitro group with a thiol to give the corresponding sulfide. The latter were transformed into the corresponding enamines upon treatment with dimethylformamide dimethyl acetal (DMF DMA). The enamines were converted into the indoles applying the Batcho–Leimgruber synthetic protocol.