Highly Strained Ring Systems

The present preparation affords high-purity nortricyclanol in good yield without the necessity of tedious purification. It illustrates a convenient way to convert olefins to alcohols and to remove olefinic impurities from alcohols. Nortricyclanol is of current interest in studies of highly strained ring systems. It is readily oxidized to nortricyclanone. ... 

Oxabicyclo[2 2 l] heptanes which polymerize readily may be considered disubstituted THF s, but due to the highly strained ring system, they are obviously a special case. 

The above study of saturated hydrocarbons illustrates how theoretical calculations may be used to extract information unattainable by experiment alone. From this study, we also conclude that caution should be exercised in applying Pascal s rule to highly strained ring systems even when no jt-electrons are present. In the next section, we shall turn our attention to molecular systems containing the more loosely attached -electrons. 

Because of the highly strained ring system, polymerization readily occurs via ring opening, yielding a linear polyester that exhibits properties suitable for use as a textile fiber or as an engineering plastic. 

Highly strained ring systems can be generated from a tandem Heck reaction sequence. This achievement attests to the value of the synthetic method. ... 

Cyclic azoalkanes continue to be of active interest because they serve as precursors to interesting diradicals and as synthons for the preparation of highly strained ring systems and sterically crowded structures. One of the most important syntheses of the azoalkanes involves the cycloaddition of TADs to a suitable substrate to give urazoles by a method mentioned in preceding parts of this review. These methods include Diels-Alder, homo Diels-Alder, and domino Diels-Alder addition, as well as the ene reaction, 1,2-cycloaddition, or other types of cycloaddition reactions. These adducts are transformed into cyclic azoalkanes by hydrolysis an oxidation. The azoalkanes are very often used for thermal or photochemical decomposition to cyclic compounds. This sequence is outlined in Scheme 79. 

The magnitude of for W coupling is usually small except in highly strained ring systems. Apparently these rigid strained arrangements somehow have a favorable geometry for the overlaps involved. 

Intramolecular insertion reactions are much more selective because the relative proximity of the various C-H bonds to the carbene center plays a major role in determining product distribution. For this reason, the reaction has synthetic utility and is especially valuable for generating derivatives of highly strained ring systems. Cyclopropanes formed by intramolecular insertion reactions are often major products from alkyl carbenes. A competing process for aliphatic carbenes is stabilization... 

Closure of the biradical leads to a four-membered ring containing oxygen, which is known as an oxetane. The overall 1,2-photocycloaddition reaction is often called the Patemo-Biichi reaction (equation 12.60). The Patemo-Biichi reaction provides synthetic entry into highly strained ring systems containing oxygen atoms, as exemplified by equations 12.61 and 12.62 (where Ar is 2-naphthyl). ... 

The reaction chemistry and further synthetic application of organic compounds are of great importance and have direct relationship to the structures. 2,6-Dia-zasemibullvalene (NSBV, 6-1) features rapid intramolecular aza-Cope rearrangement and highly strained ring system. Thus, novel reaction types and different selectivity with reactions of standard aziridines arc expected, which could be also applied to the synthetic application of functionalized heterocycles [1-6]. 

Attempts to prepare hexaphenylcyclotrisilane by reaction of 1,3-dichloro-hexaphenyltrisilane with sodium in refluxii xylene or with magnesium in tetrahydrofuran resulted only in a variety of products, including octaphenylcyclotetrasilane, decaphenylcyclopentasilane, and Compound C. No product so far isolated could be ascribed the structure of the cyclotrisilane, a highly strained ring system. 

The P-H-elimination is impeded in the norbomene polymerization by the reformation of a highly strained ring system. The catalysts required for this reaction have a non-coordinating anion and weak nitril ligands as a characteristic. Such a pallatUum complex was described 1981 by Senn (5). The modification of this catalyst by Risse et al. resulted in a homogeneous reaction in the polymerization of norbomene (4). Vinyl polymerized norbomene is an example of conformationally restricted polymers having typically very high transition temperatures. 

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