Enolether

Lewis Acid Mediated Alkylation of Silyl Enolethers- SNi like alkylations... 

A solution of 1 g of the diacetate (III) in lOOcc of n-heptane containing 2.5 ccof cyclopentanol and 50 mg of p-toluenesulfonic acid is heated under reflux for 20 hours. After cooling, a few drops of pyridine are added and the solvent is eiiminated by evaporation under vacuum. The residue is taken up with methanol to give 3-cyclopentyl enolether of 17a-ethvnyl-19-nortes-tosterone ecetate which, after recrystalllzation from methanol, melts at 182°C to 1B4°C. 

Lewis-acid catalyzed inverse electron-demand Diels-Alder reactions between conjugated carbonyl compounds and simple alkenes and enolethers also allow dihydropyranes to be prepared. SnCU-Catalyzed cycloaddition of... 

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). 

The 2-pyrones can behave as dienes or dienophiles depending on the nature of their reaction partners. 3-Carbomethoxy-2-pyrone (84) underwent inverse Diels-Alder reaction with several vinylethers under lanthanide shift reagent-catalysis [84] (Equation 3.28). The use of strong traditional Lewis acids was precluded because of the sensitivity of the cycloadducts toward decarboxylation. It is noteworthy that whereas Yb(OTf)j does not catalyze the cycloaddition of 84 with enolethers, the addition of (R)-BINOL generates a new active ytterbium catalyst which promotes the reactions with a moderate to good level of enantio selection [85]. 

Acetoxypyran-2-one (169) reacted with chiral enolethers 170 under high pressure conditions. Diastereofacial selectivities ranged from 52/48 to 88/12 depending on the nature of the dienophile [63] (Equation 5.20). 

A mixture of catalyst 110 and vinyl trimethylsilyl enolether 115 has been used in cycloisomerisation of (V-allyl-o-vinylanilines 114 and (V.A-diallyl-p-toluenesulfonamide 115 to afford the corresponding products 118 and 119, respectively (Scheme 5.30) [34]. It is believed that the active catalyst species is the ruthenium hydride NHC complex 117. 

Enolether radical cation 7, generated in a double strand, oxidizes selectively a nearby guanine base (G) in the same double strand [3, 4]. This elec-... 

Benzofuran derivatives and enolethers are offered as chemical antiozonants for light coloured rubber compounds. Benzofuran derivatives are used in CR and its blends with other rubbers and give ageing protection in addition to ozone protection. 

The enolethers give ozone protection but not ageing protection with natural rubber (NR) and polyisoprene (IR), styrene-butadiene rubber (SBR) and butadiene rubbers (BR), but are less... 

Scheme 6.24 Formation of the enolether 86 and of oligomers from 5-methyl-l,2-cyclohexadiene (81) and trapping of the diradical 84 by di-tert-butyl nitroxide, according to Bottini and co-workers.

The intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

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]. 

Oxidation of arylolefins, enolethers, or dienes yields intermolecular homocoupling products in moderate to good yield (see Sect. 13.2.1.4) however, no pronounced diastereoselectivity was observed. This is also due to the fact that the coupling sites do not tolerate substituents that would make up a prostereogenic center. Furthermore, the fairly stable cations of the dimerized radical cation solvolyze stereounselectively. The same holds for the intermolecular coupling of aromatic compounds, in... 

The intramolecular coupKng of enolethers with enolethers, styrenes, dienes, alkyl-substituted olefins, allylshanes, and vinylshanes was systematically studied by Moeller [90,198-200]. These couplings allow the smooth formation of quaternary carbon atoms in fused bicycKc rings having a cis stereochemistry (Fig. 43) [201, 202]. 

Fig. 43 Stereoselective intramolecular coupling of enolether with allylsilane [201, 202].

Scheme 9 EGA-catalyzed diastereoselective aldol reaction of acetal with an enolether.

Base i-Pr NEt ratios deteraiined by conversion to TMS enolethers ratios determined by H-NMR isolated yields (values in parenthesis determined by H-NMR). 

It is worthwhile emphasising here that the observed chemoselectivity is due to the fact that enolethers are better electrophiles than the free carbonyl groups. In fact, the ketoacetal 15, obtained by chromatography, is an equilibrium 60 40 mixture of methyl epimers at C(4) in which the desired a-epimer 5 is the predominant isomer. However, the pure compound 5. could be obtained in good yields by recrystallisation from hexane (m.p. 117-118 °C) and re-equilibration-crystallisation recycling of the remaining mixmre 15 in the mother liquor. 

In model studies directed towards the synthesis of ( )-gelsemine, 5-exo-trig cyclization of an aryl radical, derived from the vinylogous uretane 58, onto a methoxymethyl enolether resulted in partial fragmentation of the intermediate... 

The stereoselective [2+2] cycloaddition between ketenes and enolethers can be used as a key step in the construction of y-butyrolactones (Scheme 14) [45], if the resulting cyclobutanones can subsequently undergo ring enlargement by a regioselective Baeyer-Villiger oxidation. 

Using (-)-lOO [46] as a chiral auxiliary tethered to the enolether, one face of the alkene can be specifically blocked by a n-n interaction of the phenyl rest for the [2 r5+2 r ] cycloaddition with a ketene [47], resulting in the highly diastereoselective formation of the cyclobutanone 102 (Scheme 15). The observed regio- and stereoselectivity is in accord with the stereochemical predictions made on the basis of the Woodward-Hoffmann... 

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... 

Treatment of the azine (156) with enolether (157) in benzene at room temperature gave the bistriazolemethanes (Equation (59)) (158) <89JHC327,89JHC763). 

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