A convenient approach to cyclic enol phosphates via ring-closing metathesis

A strategy employing the second generation Grubbs catalyst in order to facilitate the generation of a variety of cyclic enol phosphates, including 2H-chromen-4-yl, 1,2-dihydroquinolin-4-yl, 2H-thiochromen-4-yl, 2H-thiochromen-4-yl 1,1-dioxide, and benzofuran-2-yl enol phosphate scaffolds is described. This work represents the first case of an olefin metathesis reaction in which one of the groups participating in the metathesis event is an enol phosphate moiety.     

Synthesis of 2,3-di(ω-hydroxyalkyl)quinolines from anilines and cyclic enols using sequential cycloaddition/aromatization reactions

A new synthetic protocol for the one-pot synthesis of 2-(4-hydroxybutyl)-3-(3-hydroxypropyl)quinolines and 3-(2-hydroxyethyl)-2-(3-hydroxypropyl)-quinolines based in the AB² imino-Diels-Alder reaction is reported. The protocol describes simple and efficient preparation of 2,3-disubstituted quinolines using a domino-sequential process and applying the chemical properties of cyclic enol ethers, such as 2,3-dihydrofuran and 3,4-dihydro-2H-pyran and diverse arylamines, through an imino-Diels-Alder process between an in situ generated 2-azadiene and another equivalent of the cyclic enol, catalyzed by BiCl₃, and followed by the aromatization (oxidation) process guided by the MnO₂.     

Pheromone synthesis—63 Synthesis of both the enantiomers of 2,3-dihydro-2- isopropyl-2,5-dimethylfuran,a sex specific compound in females of the beetle hylecoetus dermestoidesL

Both the enantiomers of 2,3-dihydro-2-isopropyl-2,5-dimethylfuran were synthesized employing the Sharpless asymmetric epoxidation reaction. The (R)-(—)-enantiomer of this cyclic enol ether was also synthesized starting from (R)-(—)-linalool.     

Enantioselective rhodium-catalyzed hydrogenation of enol carbamates in the presence of monodentate phosphines

The rhodium-catalyzed asymmetric hydrogenation of different acyclic and cyclic enol carbamates to give optically active carbamates has been examined in the presence of chiral monodentate ligands based on a 4,5-dihydro-3H-dinaphthophosphepine motif 4. The enantioselectivity is largely dependent upon the reaction conditions, the nature of substituents on the phosphorus ligand and structure of the enol carbamate. By applying the optimized reaction conditions, enantioselectivities of up to 96% ee have been achieved.     

Construction of quaternary carbon centers by a base-catalyzed enantioselective aldol reaction and related reactions of trimethoxysilyl enol ethers

The aldol reactions of trimethoxysilyl enol ethers catalyzed by lithium binaphtholate were found to be powerful tools for the construction of quaternary asymmetric carbon centers. The stereoselectivities were greatly affected by the presence of water. Trimethoxysilyl enol ether derived from a cyclic ketone, such as cyclohexanone, was used as a substrate to obtain the anti-adduct preferentially under anhydrous conditions; by contrast, the syn-adduct was preferentially obtained under aqueous conditions with high stereoselectivity. The aldol-Tishchenko reaction of a trimethoxysilyl enol ether derived from acyclic ketones proceeded to give monoacyl 1,3-diol derivatives in high enantioselectivities.     

Efficient three-component synthesis of tetrahydrothieno[3,2-f]quinolines

An efficient one-pot method for the synthesis of tetrahydrothieno[3,2-f]quinolines is described, using the imino Diels-Alder reaction between ethyl-5-aminobenzothiophene-2-carboxylate, aromatic aldehydes and cyclic enol ethers. Furthermore, hydroxyalkyl-substituted tetrahydrothieno[3,2-f]quinolines were synthesized in good yields, by performing the same imino Diels-Alder reaction in absence of aldehydes. The intramolecular aza-Diels-Alder reaction of 5-aminobenzothiophene with o-(allyloxy)benzaldehyde was also performed resulting in the fully oxidized thienoquinoline derivative.     

BF3-promoted cyclization reaction of imines and salicylaldehyde with silyl enol ethers: unexpected formation of dioxaspiro compounds

Unexpected spiro cyclic products were formed from the reaction of imines and salicylaldehyde with silyl enol ethers in the presence of BF₃·OEt₂. Different kinds of dioxaspiro products were afforded depending on the nature of starting materials. Furthermore, salicylaldehyde could also react directly with several silyl enol ethers, giving three products with different spiro cyclic structure under the same reaction conditions.     

Aza-Diels-Alder reactions in ionic liquids: a facile synthesis of pyrano- and furanoquinolines

Room temperature ionic liquids are found to catalyze efficiently the three component-coupling reactions of aldehydes, amines and cyclic enol ethers such as 3,4-dihydro-2H-pyran and 2,3-dihydrofuran under mild and convenient conditions to afford the corresponding pyrano- and furanoquinolines in excellent yields with high endo-selectivity. Interestingly, 2,3-dihydrofuran afforded selectively endo-products under the similar reaction conditions.     

Phosphomolybdic acid-catalyzed efficient one-pot three-component aza-Diels-Alder reactions under solvent-free conditions: a facile synthesis of trans-fused pyrano- and furanotetrahydroquinolines

trans-Fused pyrano- and furanotetrahydroquinoline derivatives have been synthesized via a one-pot three-component aza-Diels-Alder reaction of aryl imines formed in situ from aromatic amines and arylaldehydes with cyclic enol ethers,such as 3,4-dihydro-2H-pyran and 2,3-dihydrofuran,in the presence of 1 mol % of phosphomolybdic acid under solvent-free conditions at room temperature in good to excellent yields.     

Radical cation salts induced domino reaction of anilines with enol ethers:Synthesis of 1,2,3,4-tetrahydroquinoline derivatives

A domino reaction of anilines with cyclic and acyclic enol ethers induced by catalytic amounts of TBPA^·+（5 mol%） was investigated and a series of 2,4-disubstituted-1,2,3,4-tetrahydroquinolines were synthesized.Different from cyclic enol ethers, when acyclic enol ethers were used in the reaction,they serve as surrogates of acetaldehyde,producing a series of 2-methyl-4- anilino-1,2,3,4-tetrahydroquinolines.A single electron transfer mechanism was proposed to rationalize the products formation.     

Acid anhydrides as alternatives to acid chlorides in the palladium‐catalyzed reaction with silacyclobutanes

The palladium‐catalyzed reaction of acid anhydrides with silacyclobutane gives a mixture of cyclic silyl enol ether, carboxy(propyl)silane, and 3‐(carboxysilyl)ketone. In the presence of N,N‐dicyclohexylcarbodiimido (DCC), the reaction preferentially provides a cyclic silyl enol ether in a good yield. In addition, the palladium‐catalyzed reaction of benzoic acid with silacyclobutane in the presence of two equivalents of DCC also affords a cyclic silyl enol ether in a moderate yield. Copyright © 2001 John Wiley & Sons, Ltd.     

Asymmetric Michael addition of silyl nitronates to cyclic α,β-unsaturated ketones catalyzed by chiral quaternary ammonium bifluorides: isolation and selective functionalization of enol silyl ethers of optically active γ-nitro ketones

Highly enantioselective Michael addition of silyl nitronates to cyclic α,β-unsaturated ketones has been accomplished by the utilization of N-spiro C₂-symmetric chiral quaternary ammonium bifluoride 1 as an efficient catalyst, offering a new route to the enol silyl ethers of optically active γ-nitro ketones. The synthetic utility of this transformation has been demonstrated by the diastereoselective derivatizations of the optically active enol silyl ethers to the corresponding α-substituted cyclic ketones having three consecutive stereochemically defined stereocenters.     

Chiral base-catalyzed aldol reaction of trimethoxysilyl enol ethers: effect of water as an additive on stereoselectivities

An aldol reaction of trimethoxysilyl enol ether catalyzed by lithium binaphtholate is described. The aldol reaction of trimethoxysilyl enol ether derived from cyclohexanone under anhydrous conditions predominantly afforded the anti-aldol adduct with moderate enantioselectivity, whereas the reaction under aqueous conditions predominantly resulted in the syn-adduct and the enantioselectivity of the syn-adduct was considerably improved. The best enantioselectivity was obtained in the reaction of trimethoxysilyl enol ether derived from 1-indanone with cyclohexanecarboxaldehyde (97% ee (syn)). This is the first example of an aldol reaction of trimethoxysilyl enol ether catalyzed by a chiral base.     

Ferric sulfate [Fe2(SO4)3·xH2O]: an efficient heterogeneous catalyst for the synthesis of tetrahydroquinoline derivatives using Povarov reaction

Fe₂(SO₄)₃·xH₂O can be used as an efficient and reusable catalyst for the synthesis of pyrano- and furanotetrahydroquinolines via one-pot three-component Povarov reaction involving aromatic aldehydes, aromatic amines, and cyclic enol ethers. The catalyst is recyclable, economically viable, and environmentally benign. This protocol provides good yields and diastereoselectivity as well as applicability on a wide range of substrates.     

Cyclooctanone synthesis via a formal [6+2] cycloaddition reaction of a dicobalt acetylene complex

An efficient formal [6+2] cycloaddition reaction of a new six-carbon unit with enol silyl ether was developed on the basis of a dicobalt hexacarbonyl propargyl cation species. Under the influence of EtAlCl₂, 6-benzoyloxy-2-(triisopropylsilyloxy)-1-hexen-4-yne-dicobalthexacarbonyl reacted with enol triisopropylsilyl ethers to yield 7-(triisopropylsilyloxy)-3-cyclooctyn-1-one-dicobalthexacarbonyl derivatives in good yield. The reactions with cyclic enol silyl ethers as well as acyclic enol silyl ethers exhibited remarkably high diastereoselectivity.     

Cellulose-SO3H: an efficient and biodegradable solid acid for the synthesis of quinazolin-4(1H)-ones

The condensation of 2-aminobenzamide with aldehydes or ketones has been achieved using cellulosesulfonic acid under mild reaction conditions to furnish 2,3-dihydroquinazolin-4(1H)-ones in good yields with a high selectivity. The use of biodegradable solid acid catalyst, cellulosesulfonic acid makes this method quite simple, more convenient, and practical. This catalyst was also found to be very active for the synthesis of hydroxyalkylquinazolin-4-ones from cyclic enol ethers.     

Synthesis of cyclobutanones and four-membered enol ethers by using a rearrangement reaction of enol triflates

A new synthetic method of cyclobutanone derivatives and four-membered enol ethers via an intramolecular cyclization of a ketone enolate was developed. The cyclization precursors, enol triflates having a silyloxy group at the β′-position, were synthesized from the corresponding β-hydroxy ketones, which were prepared via an aldol reaction of a cycloalkanone and an aldehyde. Under the influence of TBAF, the enol triflates afforded a cyclobutanone or a four-membered enol ether through rearrangement of the trifluoromethanesulfonyl group followed by an intramolecular C- or O-alkylation reaction.     

Optimization of Grignard ring-opening of α-brominated dioxolanes for the preparation of β-hydroxy-protected vinyl ethers by a three-step one-pot procedure

Aliphatic, cyclic, and aromatic benzoate-protected γ-hydroxy enol ethers were prepared from α-brominated dioxolanes by a three-step reaction procedure and a one-pot protocol involving Grignard reagent formation, ring-opening, and final benzoate protection. Additionally, tert-butyldiphenylsilyl, acetate, and 4,4′-dimethoxytrityl were also successfully used as alternative protecting groups.     

Synthesis of α-hydroxyalkyl dehydroazepanes via catalytic enantioselective borylative migration of an enol nonaflate

A Pd-catalyzed borylative migration methodology for cyclic enol perfluorosulfonates was applied to the synthesis of the corresponding 7-membered, azepane ring system. Throughout the optimization, it was shown that the reaction is sensitive to the nitrogen protecting group as well as the type of base and solvent. The resulting cyclic allylboronate reacts stereoselectively with aldehydes for the synthesis of novel α-hydroxyalkyl dehydroazepanes in good yield and enantioselectivity over two steps. We highlight the utility of this methodology with an efficient synthesis of the de novo 7-membered ring analogue of the piperidine alkaloid β-conhydrine.     

Tandem cobalt mediated rearrangement and Pauson-Khand reaction for the synthesis of functionalized polycyclic systems

The cobalt mediated rearrangement of enol ether complex 2 furnishes cyclic ketone 6, which undergoes an intramolecular Pauson-Khand reaction allowing access to functionalized polycyclic systems.