Ketene alkylsilyl acetals

An unprecedented cyclopropanation reaction was observed during the reaction of ketene alkylsilyl acetals (191) with bromoform-diethylzinc. When monosubstituted acetals were used, cyclopropanecarboxylic esters (195) were formed by a novel C-H insertion. When disubstituted ketene acetals were used, byproducts such as a,)5-ethylenic esters (197) were also formed presumably via 196 (equation 49). This reaction provides a convenient method for the preparation of the bicyclo[3.1.0] hexane system and can be advantageously compared to the copper-catalysed intramolecular cyclization of unsaturated a-diazoketones . 

In a related reaction, addition of chloro-, chloromethyl-, chlorophenyl or chlorofluoro-carbene to ketene alkylsilyl acetals and subsequent ring-opening (MeOH-NEt3 reflux) leads to the corresponding 2-substituted-2-alkenoic esters in high yields (equation 66) ... 

Acrylonitrileundergoes a [2+2] cycloaddition reaction with ketene alkylsilyl acetals. Like cyanoketenes, cyanoacetylenes readily undergo cycloaddition reactions, and research was given an additional stimulus by their observation in the atmosphere of Titan. ... 

Enantioselective protonation. Cleavage of enol silyl ethers and ketene bis(tri-alkylsilyl) acetals by the complex leads to chiral ketones and esters. 

Analogously, alkanoic esters 15 were transformed via alkylsilyl ketene acetals 16 and their chloromethylcarbene adducts 17 (not isolated) into 2-methylalk-2-enoic esters 18 (EjZ mixtures). 

A similar rearrangement occurs with phenylhalocarbene adducts of alkylsilyl ketene acetals 16/ and with siloxycyclopropanes obtained by addition of other carbenoids. ° The method is useful in terpene synthesis. ° ° ... 

In chlorofluorocarbene adducts of alkylsilyl ketene acetals, chloride is extruded during rearrangement to give a-fluoro-a,)S-unsaturated esters,e.g. reaction of 19. ° ... 

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See also in sourсe #XX -- [ ]

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