Cyclopentane stereoisomerism

A Ni(0)-catalyzed l,oo-hydrosilylation across the two dienyl moieties of 1,3,8,10-undecatetraene 9 proceeds regioselectively and stereoselectively and provides vie-trans-divinyl cyclopentane products 10 in modest yield (Eq. 3) [13]. The reaction shows an interesting stereoselectivity with respect to the substituent geometry both of the vinyl groups of 10a and 10b are stereoisomeric to each other, and one of the two double bonds is cis and the other is trans. 

Cyclic compounds also exist in stereoisomeric forms. Let us now study the case of cyclopentane, 1, 2 diol. It has two chiral carbons and exists in three stereoisomeric forms. 

Cyclopentane bromohydroperoxides, stereoisomerism, 313, 314 Cyclopentene, primary ozonide, 720 Cycloperoxy chlorination, 1,2-dioxolane synthesis, 238, 240... 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

The formation of the stereoisomeric products 7-2 and 7-3 is analogous to the formation of exo and endo adducts in the Diels-Alder reaction. One way to keep track of the stereochemical relationships is to note that in 7-2 the circled phenyl group and the ca o-methylene group in the cyclopentane ring are on the same side of the oxazole ring (cis), whereas in 7-3 it is the circled hydrogen that is cis to the ero-methylene group. 

Stereoisomerism of this same sort should be possible for compounds other than glycols, and for rings other than cyclopentane. Some examples of isomers that have been isolated are ... 

The selectivity for the alkene-substituted cycloadduct is also almost complete in the case of the reaction of l-methylene-2-(trimethylsilyl)cyclopropane (5) with dimethyl fumarate, which results in a 78% yield of dimethyl /ra x-4-(trimethylsilylmethylene)cyclopentane-l,2-di-carboxylate (7). Interestingly, only one of the stereoisomeric dimethyl 4-methylene-3-(trimethyl-silyl)cyclopentane-l,2-dicarboxylates [the (/ , / , / )-isomer 8] is formed selectively. ... 

The lack of free rotation around C—C bonds in disubstituted cycloalkanes leads to an extremely important kind of isomerism called stereoisomerism. Two different compounds that have the same molecular formula and the same structural formula but different spatial arrangements of atoms are called stereoisomers. For example, consider a molecule of 1,2-dimethylcyclopentane. The cyclopentane ring is drawn in I Figure 1.17 as a planar pentagon with the heavy lines indicating that two of the carbons are in front as one views... 

Cyclopentane derivatives ofifer a convenient starting point for a discussion of the stereoisomerism of cyclic compounds. For example, 1,2-dimethylcyclopentane has two chirality centers and exists in three stereoisomeric forms 5, 6, and 7 ... 

In this section, we concentrate on derivatives of cyclopentane and cyclohexane containing two stereocenters. We can analyze stereoisomerism in cyclic compounds in the same way as in acyclic compounds. 

Cisak [108-110] recently made a detailed investigation of the reduction of the three stereoisomeric pairs of compounds aldrin and isodrin, dieldrin and endrin, and o - and )3-chlordanes (see p. 151). It was found that, in the case of the first four compounds, three chlorine atoms are successively removed from the hexachloro-cyclopentene ring and replaced by hydrogen in the case of the chlordanes, the same three atoms are removed and replaced by hydrogen and two chlorine atoms in the least chlorinated cyclopentane ring are removed to form a new multiple bond. Here the reduction product from each of the isomers evidently retains the configuration of the original compound. Characteristic of these examples is the fact that the usual course for reduction of vicinal... 

When cis- and tra 5-2-(hex-4-enyl)cyclopent-2-enones were irradiated separately, a different ratio between the stereoisomeric adducts was obtained. This observation was taken as evidence that biradicals A and B, resulting from the initial formation of either a cyclopentane or a cycloheptane ring on the alkene unit, respectively, were required to explain the results. When a large t-butyl substituent was present, only the less hindered stereoisomer could be observed. 

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