Enolethers cyclic

Inverse electron-demand Diels-Alder reaction of (E)-2-oxo-l-phenylsulfo-nyl-3-alkenes 81 with enolethers, catalyzed by a chiral titanium-based catalyst, afforded substituted dihydro pyranes (Equation 3.27) in excellent yields and with moderate to high levels of enantioselection [81]. The enantioselectivity is dependent on the bulkiness of the Ri group of the dienophile, and the best result was obtained when Ri was an isopropyl group. Better reaction yields and enantioselectivity [82, 83] were attained in the synthesis of substituted chiral pyranes by cycloaddition of heterodienes 82 with cyclic and acyclic enolethers, catalyzed by C2-symmetric chiral Cu(II) complexes 83 (Scheme 3.16). 

Anodic regioselective acetamidosulfeny-lation of alkenes is similarly achieved by oxidation of diphenyldisulfide in acetonitrile [81]. Cyclic enamines, which are intermediates in the oxidation of cyclic N-methoxycarbonyl amines, react in aqueous acetonitrile that contains chloride ions to a-hydroxy- 8-chloro compounds via intermediate chloronium ions [82]. Enolethers undergo a regioselective azidomethoxyla-tion to yield acetals of a-azido carbonyl compounds upon electrolysis in methanol containing sodium azide [83]. The reaction proceeds possibly via addition of an anodicaUy generated azide radical. 

Anodic C, C-coupling is a very powerful tool to synthesize cyclic compounds with high regio- and stereoselectivity. It involves inter- and intramolecular coupling of arylolefins, dienes, enolethers, phenol ethers, and aromatic amines and often opens a quick entry into complex natural products in a few steps. Although the mechanism is fully established in only a few cases, it does appear to involve the coupling of two radical cations at the site of their highest radical density and is further controlled by steric constraints. This important type of reaction is reviewed in Chap. 5 and in Refs. [89, 90]. 

The stereoselective synthesis of c/.v-fused pyrano and furanobenzopyran can be achieved through the one-pot three-component reaction of o-hydroxy benzaldehyde, aromatic amines, and cyclic enolethers in the presence of catalytic amounts of Bi(OTf)3 (10 mol%) in air and the moisture-stable ionic liquid [bmim]PF6 [116]. The reaction of salicilaldehyde, aniline and 2,3-dihydrofuran furnishes the c/.v-fused furanochroman. In a similar fashion, various substituted salicilaldehydes and... 

As an extension of this work, photoinduced [2+2]-cycloadditions of 1-acetylisatin (13) with cyclic enolethers (furan, benzofuran, 2-phenylfuran, 8-methoxypsoralen), and acyclic enolethers (//-butyl vinyl ether and vinyl acetate) were investigated which afforded the spiro-oxetanes in high yields (82-96%) and with high regio- and diastereoselectivity (Sch. 4) [19]. Treatment of the furan-derived oxetane 15 with acid resulted in oxetane ring opening and yielded the 3-(furan-3-yl)indole derivative 16. 

By treatment of acetals, obtained from chiral 1,3-diols and cyclic a,p-unsaturated ketones with /BU3AI, enolethers are obtained bearing an alcohol functionality, which proved to be necessary to obtain high diastereoselectivity. ... 

Tetrahydroquinoline ring from azomethines and cyclic enolethers... 

Cyclic enolethers from carboxylic acid chlorides... 

According to mechanistic studies, the hydrogenation of the aldehyde precedes the double-bond isomerization to the cyclic enolether, which is rapidly transformed by a Lewis acid-catalyzed acetalization. In this sequence, the cationic rhodium complexes obviously catalyze four steps. The three last steps all proceed in a domino manner and a rhodium-hydride complex is the presumed catalytically active species. 

I The oxidative cycloaddition of 1,3-dicarbonyl compounds to cyclic enolethers can be conveniently mediated by tetra-n-butylammoniumperoxidisulfate as eremplified by the following reaction ... 

Triphenylmethyl chloride Bicyclic enolethers from cyclic hydroxyketones... 

Cyclic 1-alkoxy-l-sulfones from enolethers and sulfur dioxide... 

With a more substantial difference in the catalyst, even the more rigid transition states of cyclic enolethers (see 381) can be configurationally directed. 

Cinenic acid rearrangement with labeled compounds s. 12, 766 Cyclic ketones from enolethers... 

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