Addition enolethers

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]. 

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. 

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. 

In the anodic olefin oxidation, the electrode material plays an important role as well. The oxidation of enolethers at platinum anodes mainly leads to the dimethoxylation product [74], while the application of graphite anodes favors the additive dimerization (Eq. 22.33) [39]. 

To accelerate the addition sequence TMSC1 is added, leading to the formation of the TMS enolether 18 at the end. 

Conjugate addition of silyl enolethers leads to the silyl enol ether of the product... 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

Substitution of acid hydrogens by silyl groups in ketones leads to the formation of silylenolethers. In competition with the synthesis of the silyl-enolethers is an addition reaction in which... 

We have shown that heterogeneous catalysis can be applied to reductive alkylation with success in reactions such as ether synthesis or N-alkylation of amides and anilines. Concerning the mechanism, several pathways are in competition depending on the structure of the substrate and of the alkylating agent. The important point is that both the product of addition (the hemiacetal or hemiaminal) and the product of elimination (imine, enamine or enolether)... 

Early synthetic efforts involved TMSOTf-catalyzed addition of the cyclohexanone-derived TMS enolether to the /V-TMS protected 1, followed by /V-deprotection to give ketone 2. While the stereocontrol at C-4 of the p-lactam ring was complete and only trans products were obtained, the stereoselection at the newly formed stereocenter was rather poor since the reaction yields a 70 30 mixture of epimers in favor of the indicated one. 

The addition of the alkyl group from the lithiated reagent to the carbonyl carbon is affected by the dimension of the alkyl group. Sterically undemanding groups favor the addition reaction and prevent the formation of the silyl-enolethers. 

Without additional reagents Mixed O-heterocyclic acetals from enolethers and quinones... 

Without additional reagents Reaction of enolethers with thionyl chloride... 

Ethyl ethoxymethylenemalonale (EMME) allowed to react at -60 to -50 in ether with phenylmagnesium bromide prepared from bromobenzene and Mg diethyl benzylidenemalonate. Y 50.5%. - Whether simple or double conjugate addition (to a satd. product) occurs appears to depend on the structures of both the organometallic compd. as well as the conjugated enolether. F. e., also addition of 2 molecules, s. D. P. Nabar and S. V. Sunthankar, Bull. Chem. Soc. Japan 42, 2991 (1969). 

Aldehydes from ketones via enolethers Synthesis with addition 1 C-atom... 

Angular polyarenes. A soln. of startg. aldehyde and cyclohexanone trimethylsilyl enolether in methylene chloride under N2 treated dropwise at —78° with excess 1 M TiCl4 in the same solvent, after stirring at that temp, for 2 h the mixture allowed to warm to room temp., stirred for an additional 48 h, then quenched with ice-water -> 1,2,3,4-tetrahydrochrysene (Y 65%), treated with 2 eqs. 2,3-dichloro-5,6-dicyanoqui-none in refluxing benzene for 3 h chrysene (Y 77%). F.e. inch phenanthrene ring and peri-fused chrysenes s. P. Di Raddo, R.G. Harvey, Tetrahedron Letters 29, 3885-6 (1988). 

Electrophilic additions, like that of HOX - 1) and hydroboration/oxidation 2), occur regio-selectively, since 0-participation directs the electrophile to the -position of the enolether moiety, for example ... 

Activated alkenes, for example, enamines or enol ethers, likewise undergo cycHzing addition to N-arylimines catalyzed by Lewis acids like BF3 or, particularly effective, Yb(OTf)3 [190]. With enolethers, the primary products of cyclization 131 are aromatized to quinolines 132 by Lewis acid-promoted ROH-elimination and dehydrogenation. 

The investigation by DFT theory, including solvent effects (molecular solvents and ILs), shows that these cycloadditions proceed by a concerted but asynchronus reaction mechanism. The lowest activation energies for concerted reactions are obtained. However, the stepwise additions have significantly lower activation energy lead to substantially less stable products. Moreover, the primary cycloadducts could never be isolated but were converted into 5-hydroxybenzofurans by subsequent extrusion of nitrous acid, hydrolysis of the silyl enolether, and elimination of methanol. Elimination of nitrous acid is calculated to have lower overall barriers than cycloaddition reactions and is strongly exothermic, thus explaining the preferred reaction pathway. 

M methanolic CrCla-soln. added dropwise at -40° under COg during 15 min. to a stirred soln. of N-chlorurethan and cyclohexene in abs. methanol until a starch-iodide paper test shows complete consumption of the N-dilorurethan, then allowed to warm to room temp. -> ethyl cis- and tmn -2-chlorocyclohexylcarba-mate. Y 99%. F. e. s. J. Lessard and J. M. Paton, Tetrah. Let. 1970, 4883 addition to enolethers cf. ibid. 1970, 4887. 

It should be noted, however, that although some heteronucleophiles such as amines and thiolates underwent clean nucleophilic substitution, treatment with sodium methylate produced enolether 248 as the major product [103]. Owing to electronic polarization, acceptor-substituted allenes should be ideal candidates for regioselective additions. Nevertheless, ketone 251 demonstrates that again the choice of the catalyst is crucial for a clean directed addition. Moreover, cycHzation reactions may make things quite complicated [104]. 

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