Cascade reactions stabilized carbon

Despite the toxicity of organoselenium compounds, they are routinely applied as precursors for radical reactions. This is due to the unrivaled quality of organoselenium compounds in term of stability, ease of preparation and ease of homolysis of the carbon-selenium bond. Their rich chemistry will continue to make them particularly attractive for the development of new radical processes. This is particularly true when complex systems are examined, such as radical precursors for cascade reactions. Moreover, processes where organoselenium derivatives are used in catalytic amounts will become more attractive and environmentally more friendly. 

Other Cascades Initiated by Michael Reactions Using Stabilized Carbon Nucleophiles... 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

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