1-Siloxy-l-vinylcyclopropane

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). 

In the case of 1-siloxy-l-vinylcyclopropanes, treatment with base produces the corresponding ethyl ketones. For example, l-[l-(trimethylsiloxy)cyclopropyl]cyclopentene (24) can be effectively converted to 1 -(cyclopent-l-enyl)propan-l -one (25). In contrast, reaction of cyclopropane 24 with acid results in rearrangement to give cyclobutanone 26 in good yield. " However, thermolysis of 1-vinylcyclopropanol at 100 C for 10 minutes gives a mixture of 2-methylcyclo-butanone (85%) and pent-l-en-3-one (15%). °... 

Siloxy-l-vinylcyclopropanes 29 afforded conveniently, and in almost quantitative yield, cyclo-butanones 30 (R = H, Me) by treatment with a catalytic amount of aqueous hydrochloric acid in refluxing tetrahydrofuran for 1 hour. ... 

Addition of acetals to 1-siloxy-l-vinylcyclopropane 35 in the presence of trimethylsilyl triflate led stereoselectively to high yields of spirocyclobutanones 36. ... 

Cyclopentanones may also be synthesized from unsaturated ketones and diiodo-methane. The ketone is converted to the O-silyi enol, and carbene is added to the enol double bond using the Simmons-Smith reaction (see p. 74f.). Thermal rearrangement of the resulting 1-siloxy-l-vinylcyclopropane and add-catalyzed hydrolysis of the silyl enol ether leads to cyclopentanones in excellent yields (C. Girard, 1974). Very high temperatures, however, are needed, and this obviously limits the generality of this rearrangement reaction. 

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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