Cyclopentane derivatives five-membered carbocycles

Versatile [3 + 2]-cycloaddition pathways to five-membered carbocycles involve the trimethylenemethane (= 2-methylene-propanediyl) synthon (B.M. Trost, 1986). PaIladium(0)-induced 1,3-elimination at suitable reagents generates a reactive n - -methylene-1,3-propa-nediyl complex which reacts highly diastereoselectively with electron-deficient olefins. The resulting methylenecyclopentanes are easily modified, e.g., by ozonolysis, hydroboration etc., and thus a large variety of interesting cyclopentane derivatives is accessible. 

Although a number of multistep procedures are available for the introduction of five-membered carbocycles, their direct formation in a thermal cycloaddition is rare.6 Interest in the potential application of such a three-carbon + two-carbon cyclopentane cycloaddition has been derived from the expectation that such a process could prove to be an effective complement to the four-carbon + two-carbon Diels-Alder reaction which is used extensively in the regio- and stereocontrol led preparation of functionalized six-membered carbocycles. 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

An interesting carbocyclization process was observed when alkenyl stannanes were treated with electrophilic selenenylating reagents containing a non-nucleo-philic counterion. Thus, Nicolaou showed that compound 213 reacted with AT-PSP 11 to form the intermediate 214 which then afforded the cyclopropane derivative 215 (Scheme 32) [109]. Further examples were reported by Herndon [110]. As indicated in Scheme 32, in the presence of tin tetrachloride, the stannane 216 was converted into the cyclopentane derivative 217. This cyclization reaction proved to be quite general with respect to a variety of substitution patterns but it appears to be restricted to the formation of three- and five-membered ring. 

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