Enolethers, synthesis with

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

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). 

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... 

Eq. 2. Synthesis of fluorosilyl-enolethers with 3,3-dimethyl-2-butanone. 

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)... 

A1-A5) in a Michael-type reaction. These compounds then reacted with lithiated aromatic or heteroaromatic compounds (Fig. 6.2, B3-B5) or with enolethers (Fig. 6.2, B1-B2). After thermolysis and air oxidation in toluene under reflux, substituted 1,4-qui-none derivatives were formed and cleaved from the resin with 20% TFA/CH2C12. Twenty-five single compounds were synthesized in a seven-step synthesis, and the isolated yields ranged from 0% to 53 %. 

Eq. 3. Synthesis of fluorosiiyl-enolethers with 2,4-dimethyl-3-pentanone. 

The synthesis of the twistane-ketal 279 using endo-6-hydroxy-9-oxabicyclo[3.3.1]-nonan-2-one (274) or its acetate 227, resp., was nevertheless successful by applying the following route according to a procedure by Inhoffen etal. the keto-acetate 221 was treated with trimethyl orthoformate. From the reaction mixture which contained the dimethoxy-ketals 222 and 223 as well as the enolethers 224 and 225, the acetates 222 (63%) and 224 (5%) were isolated by chromatography. On base-hydrolysis they gave the alcohols 223 and 225. Pyrolysis of the ketal 222 and simultaneous distillation yielded 67% of the enolether 224. Finally by treating the dime-... 

A highly stereoselective hetero Diels-Alder reaction of achiral N-acylimines with chiral enolethers derived from 8-phenylmenthol has been applied to the synthesis of the taxol side chain as outlined below. 

A soln. of the startg. bicyclic enolether and 72-butyl nitrite in 50%-ethanol treated with acetic acid or 10%-HCl product. Y almost 100%. - Any alkyl nitrite or even Na-nitrite may be used. F. e. s. J. R. Mahajan, G. A. L. Ferreira, and H. C. Araujo, Chem. Commun. 7972, 1078 N-acetyloximinolactams s. Synthesis 1973, 313. 

Acetals. 5,5-Dimethyl-2-triethylammonio-l,3-dioxane fluoroborate (prepn. s. 995) refluxed 2 hrs. with acetylacetone in abs. methylene diloride product. Y 89%. F. e., also formation of diacetals and enolethers, s. S. Kabusz and W. Tritsdiler, Synthesis 1971, 312. 

Beckmann Rearrangement The Beckmann rearrangement of oxime constitutes another approach to the insertion of nitrogen into the C-C=0 bond. Wishka and coworkers developed an asymmetric synthesis of (25,55)-5-substituted azepane-2-carboxylate 12 based on this reaction [8]. Condensation of ketone 8 with hydroxylamine afforded oxime 9. Treatment of 9 with TsCl triggered the Beckmann rearrangement to afford the desired lactam 10 as the sole product. Compound 10 was then converted to 12 by ozonolysis of the enolether 11 (Scheme 4.3). 

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