Strained olefins synthesis

The intermediate formation of a dianthracene derivative was utilized in the synthesis of the highly strained olefin 9,9 -dehydrodianthracene,<37)... 

However, from the outset of this field, the limitations as well as the potentials of this cycloaddition were also apparent. For instance, the efficiency of this cycloaddition in an intermolecular manner was typically low unless strained olefins were used. Moreover, the use of unsymmetrical alkenes led to a mixture of the cyclopentenone regioisomers. Synthetic utility of this reaction is considerably expanded by the emergency of the intramolecular reaction. Schore introduced the first intramolecular version forming several rings simultaneously, which is now the most popular synthetic strategy in natural product synthesis because of its conceptual and operational simplicity. Additionally, the regiochemistry is no longer the problem in this variation. 

Nickel catalysts for the syntheses of cyclic compounds were first successfully utilized by Reppe, who was able to prepare cyclooctatetraene from acetylene (65). This eight-membered ring synthesis, and also the preparation of cyclic products from strained olefins (e.g., bicycloheptene and norbornadiene) and acrylonitrile, have been adequately reviewed elsewhere (7) and will therefore not be considered further. A short account of the cyclization reactions of butadiene using nickel-containing catalysts has appeared previously in this series (/). The discovery of new synthetic possibilities and a deeper understanding of the mechanism of these reactions justify a more extensive treatment. 

All of the preceding syntheses exploited the reactions of o-benzoquinones with reactive (strained) olefins. A different approach led to the successful synthesis 62)... 

The reaction is an important synthetic route to triazolines and their derivatives [5-7] such as cycUc imines or aziridines and is hence a valuable technique in the synthesis of heterocycles [8]. The reaction rate is dependent on the dipolarophile. Whereas strained olefins, such as norbornene, react readily, terminal alkenes react extremely slowly [9]. 

A new addition to the array of polymer synthesis methodologies is ring-opening metathesis polymerization (ROMP). Although the basic reaction had been known for some time, it was not until the development of new transition metal catalysts based on work by Schrock and Grubbs (Chapter 12), that the methodology became practical (Figure 13.18). Typically the reaction requires strained olefins such as norbornene so that relief of strain... 

The preparation of strained olefins continues to challenge the synthetic chemist. Although the synthesis of tetra-t-butylethylene remains an elusive goal, the related tetrakis(trimethylsilyl)ethylene has now been prepared and characterized. ... 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

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