Silanes polyene cyclization

As the intermediate formed in a polyene cyclization is a carbocation, the isolated product is often found to be a mixture of closely related compounds resulting from competing modes of reaction. The products result from capture of the carbocation by solvent or other nucleophile or by deprotonation to form an alkene. Polyene cyclizations can be carried out on reactants that have structural features that facilitate transformation of the carbocation to a stable product. Allylic silanes, for example, are stabilized by desilylation.12... 

The incorporation of silyl substituents not only provides for specific reaction products but can also improve the effectiveness of polyene cyclization. For example, although cyclization of 2a gave a mixture containing at least 17 products, the allylic silane 2b gave a 79% yield of a 1 1 mixture of stereoisomers.13 This is presumably due to the enhanced reactivity and selectivity of the allylic silane. 

A spectacular example of cationic cyclization is the Johnson polyene cyclization, described in Section 10.8.A. Polyenes such as squalene are expected to assume a steroid-like conformation in the lowest energy form (sec. 1.5.E), based on the biogenetic preparation of cholesterol from squalene. In practice, treatment of polyenes with acid led to a very low yield of tri- or tetracyclic products, giving instead significant amounts of polymeric material. Diligent work over many years prevailed, however, and Johnson solved the many problems (as described in sec. 10.8.A) to make this reaction an excellent and efficient synthetic route to di-, tri-, and tetracyclic molecules. One of the later examples of polyene cyclization uses an allyl silane to quench the cyclization process. A Lewis acid was used to initiate the reaction via reaction with the acetal. Treatment of... 

Propargyltrimethylsilanes can be readily prepared from halomethyltrimethyl-silane and the 1-yne anion. They have been extensively used in organic synthesis of furanones and allenes, and like silylacetylenes, as terminator groups for biomimetic polyene cyclizations and the generation of steroids. ... 

---