Photoreactions of benzonitrile with cyclic enol ethers

Upon irradiation, cyclic enol ethers such as 1-methoxy-cyclopentene (4) mainly add across the cyano group of benzonitrile (1), under formation of 2-azabutadienes of an imidoester type. This is in agreement with the so-called Δ G-correlation which was reported earlier (ref. 5 and 6). 4-Phenyloxazole (9) is formed from 1 and 1,3-dioxole (5) probably by a similar photochemical process followed by electrocyclic ring opening and hydrolysis. The low yield of the latter photoreaction and the almost exergonic electron transfer between 5 and 1 may point to back electron transfer as the main energy wasting process. From 1 and 2,3-dihydrido-furan (2) only the ortho cycloadduct 6 has been isolated in low yields.     

Synthesis of cyclic enol ethers from alkenyl-beta-dicarbonyl compounds

In this work we describe the cyclofunctionalization of eleven differently substituted alkenyl-beta-dicarbonyl compounds, employing three electrophilic reagents, namely, iodine, p-methoxyphenyltellurium trichloride, and phenylselenenyl bromide. The reactions occur through the enolic form of the substrates, to afford the corresponding iodo-, telluro-, or selenocyclic enol ethers. Substrates bearing trisubstituted double bonds failed in reacting with the selenium and tellurium reagents. In general, beta-diketones reacted faster than beta-keto esters with the three studied electrophiles.     

A general synthesis of substituted indoles from cyclic enol ethers and enol lactones

[reaction: see text] X = CH2, C[double bond]O, R2 = H, alkyl. A general method was developed for the one-pot synthesis of highly functionalized indoles from simple, commercially available aryl hydrazines and cyclic enol ethers. Enol lactones were also used as substrates, affording substituted indole acetic acid or indole propionic acid derivatives. This procedure affords 2,3-disubstituted indoles as single regioisomers from the appropriately substituted enol ether or enol lactone. This method was highlighted in the efficient synthesis of the antimigraine drug sumitriptan and the antiinflammatory drug indomethacin.     

Phosphonyl radical addition to enol ethers. The stereoselective synthesis of cyclic ethers

Addition of diethyl phosphite or diethyl thiophosphite to enol ethers, in the presence of a radical initiator, results in the regioselective synthesis of organophosphonate or phosphonothioate derivatives, respectively, under mild conditions. This method can be applied to the stereoselective formation of substituted tetrahydrofurans and tetrahydropyrans, on cyclisation of vinyl ethers bearing unsaturated side chains.     

An expeditious and environmentally benign methodology for the synthesis of substituted indoles from cyclic enol ethers and enol lactones

A simple and environmentally friendly method is developed for the synthesis of substituted indoles from commercially available aryl hydrazines and cyclic enol ethers with Montmorillonite-K10 as a heterogeneous catalyst. The catalyst is non-toxic, inexpensive and recyclable and the process is clean, high yielding and operationally simple.     

Phenylthiomethylstannylation of silyl enol ethers and silyl dienol ethers

Silyl enol ethers (2) react with tributyl [(phenylthio) chloromethyl] stannane (1) in the presence of Zinc bromide to give β-phenylthiomethylstannyl ketones (3) in good yields. Silyl dienol ethers (4) mainly give products (5) due to γ-attack, under these conditions.     

Diastereocontrol via the phenol- and palladium(II)-catalyzed Claisen rearrangement with cyclic enol ethers

Taking the judicious choice of either 2,6-dimethylphenol or PdCl₂(RCN)₂ as the catalyst, the Claisen rearrangements with the enol ethers of cyclic ketones are shown to proceed with a high level of either anti or syn diastereoselection, respectively.     

Deracemization of silyl enol ethers

The deracemization by enantioselective protonation of silyl enol ethers was tested using 2,2-dimethyl 5-phenyl 1,3-dioxolan 4-one 1. The results obtained, especially with pantolactone as a chiral proton donor, are better than when the deracemization is carried out with the lithium enolate of 1.     

Reaction of silyl enol ethers with cyanogen chloride

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1447–1448, June, 1988.     

Sodium dithionite initiated reactions of 1-bromo-1-chloro-2,2,2-trifluoroethane with enol ethers of cyclic ketones

Sodium dithionite initiated reactions of 1-bromo-1-chloro-2,2,2-trifluoroethane (1) with methyl and trimethylsilyl ethers of cyclopentanone and cyclohexanone enols (2a-d) in a MeCN/H₂O system were investigated. 2-(2,2,2-Trifluoroethylidene)cyclopentanone (4a) and 2-(2,2,2-trifluoroethylidene)-cyclohexanone (4b), respectively, were obtained as the main products and isolated in reasonable yields. The reaction with a 1:1 mixture of 5- and 3-methyl substituted 1-methoxycyclohexenes, 2e and 2f, revealed strong influence of steric hindrance on the reaction rate; a mixture of 2-(2,2,2-trifluoroethylidene)-5-methylcyclohexanone (6) and 2-(2,2,2-trifluoroethylidene)-3-methylcyclohexanone (7) in a 9:1 ratio was formed. Ketones 4a and 4b were reduced to the corresponding alcohols 8 and 9 and the reaction of 4b with hydrazine gave an indazole derivative 10.     

Regiospecific ureidoalkylation of silyl enol ethers

Site-specific ureidoalkylation of silyl enol ethers can be achieved by their reactions with chloromethylcarbamates at ?78° under the influence of titanium tetrachloride.     

Condensation of enol silyl ethers with 2-acetoxytetrahydrofuran and -tetrahydropyrans

Trimethylsilyl trifluoromethanesulfonate catalyzes stereoselective condensation of enol silyl ethers and 2-acetoxytetrahydrofuran or -tetrahydropyran derivatives.     

Lewis acid-catalyzed nucleophilic substitutions of propargylic and allylic silyl ethers with enol silyl ethers

Nucleophilic reactions of various enol silyl ethers with carbocation species generated from propargyl silyl ethers by the action of a Lewis acid have been developed. The present method is highly useful for the introduction of allenic or enyne functionalities into the alpha-position of substituted ketones. [reaction--see text]     

Synthesis of vicinal perfluoroalkyl enol ethers

A convenient preparation of vicinal perfluoroalkyl enol ethers, by the action of alcohols on 1-bromo-1-perfluoroalkylethylenes in the presence of potassium hydroxide, is described. Some aspects of their reactivity are reported.     

A highly erythro selective aldol reaction between chromium-complexed benzaldehyde derivatives and cyclic silyl enol ethers

Reaction of chromium-complexed benzaldehydes having a trimethylsilyl group at the position with cyclic silyl enol ethers gave, after decomplexation, the products in a highly selective manner.     

Lanthanide catalysis of cycloadditions of heterodienes with enol ethers

The cycloadditions of enol ethers with α,β-unsaturated aldehydes is catalyzed by Yb(fod)₃.     

Medium-sized carbocycles by a zirconocene-catalyzed tandem formal ring expansion-magnesation reaction of alkenyl-substituted cyclic enol ethers

A new regio- and stereoselective zirconocene-catalyzed reaction for the synthesis of medium-sized rings is described. The global reaction supposes a formal ring expansion of a cyclic enol ether to give a functionalized carbocycle.