Carboxylic esters, 3-alkoxy-2-bromo

Heterosubstituted cyclopropanes can be synthesized from appropriate olefins and car-benes. Since cyclopropane resembles olefins in its reactivity and is thus an electron-rich car bo-cycle (p. 76ft). it forms complexes with Lewis acids, e.g. TiCL, and is thereby destabilized This effect is even more pronounced in cydopropanone ketals. If one of the alcohols forming the ketal is a silanol, the ketal is stable and distillable. The O—Si-bond is cleaved by TiCl4 and a d3-reagent is formed. This reacts with a -reagents, e.g. aldehydes or ketals. Various 4-substituted carboxylic esters are available from 1-alkoxy-l-siloxycyclopropanes in this way (E. Nakamura, 1977). If one starts with l-bromo-2-methoxycyclopropanes, the bromine can be selectively substituted by lithium. Subsequent treatment of this reagent with carbonyl compounds yields (2-methoxycyclopropyl)methanols, which can be transformed to /7,y-unsaturated aldehydes (E.J. Corey, 1975B). 

The a>bromo carboxylic acid required for preparation of the a>alkoxy acid is often obtained by a malonic ester synthesis the malonic ester is treated with an alkyl halide, the alkylmalonic acid obtained by hydrolysis of the product is brominated by elemental bromine at a low temperature, and the resulting a>bromo dicarboxylic acid is decarboxylated.54 This has led to a particularly important synthesis of aldehydes which utilizes the decarbonylation of -hydroxy or a-alkoxy acids for conversion of an alkyl halide RX into the aldehyde RCHO containing one more carbon atom ... 

Like cyclohexyl systems, the bicyclo-[2.2.2]octanes are usually only prepared with an alkyl terminal group and the principal route to the central intermediate (l-alkylbicyclo[2.2.2]octane with a 4-hy-droxy-,4-methoxy- or 4-bromo-substituent) is shown in Scheme 3 [45, 46]. All three intermediates can provide the 4-alkylbi-cyclo[2.2.2]octane-l-carboxylic acids (although the hydroxy or methoxy route is more direct) and the bromo compound can be used in Friedel-Crafts alkylation of aromatic compounds. The main examples of bicyclo[2.2.2]octane systems are cyano-aryl esters (3.2, X = CN [47-49], alkyl- and alkoxy-aryl esters (3.2, X = R, OR) [48,... 

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