Ring vinylcyclopropanes, 1-siloxy

Siloxy-l-vinylcyclopropanes rearrange thermally to give 1-siloxycyclopentene derivatives or may be hydrolyzed to cyclobutanones by acids (J.M. Conia, 1975 B.M. Trost, 1973 G.C. Girard, 1974). The cyclopropane ring may also be opened by base-catalyzed hydrolysis of the silyl ethers to form ethyl ketones. 

Because of the discovery of a growing number of naturally occurring substances of biological importance that contain the five-membered ring moiety 178), the synthesis of cyclopentanoid compounds is a subject of present interest. Indeed, among the various approaches recently investigated, the thermal vinylcyclopropane-cyclopentene rearrangement of readily available 1-siloxy-l-vinylcyclopropanes (vide supra, Sect. 4.1.5) constitutes an efficient three-carbon annelation process 179). 

The thermal C3 -> 5 ring expansion of 1-siloxy-l-vinylcyclopropanes occurs either from the Z or from the E isomers. It leads to cyclopentanone silyl enol ethers that are able to undergo either further regiospecific alkylation into 2,3-disubstituted cyclopenta-nones 145.209,213 dehydrosilylation into cyclopentenones The overall process constitutes an efficient three-carbon annelation process (equation 129). 

Reaction with afi-unsaturated ketones. 1-Siloxy-l-vinylcyclopropanes can be prepared easily by cyclopropanation of cisoid or labile a,/3-ethylenic ketones. It has already been shown that 1-vinylcyclopropanols undergo ring expansion to cyclobutanones on treatment with acid and to cyclopentanones on thermolysis. Two examples of this useful new cyclopropanation sequence are shown in... 

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