Epoxy polyene cyclizations

Cyclization. Epoxy polyenes undergo cyclization mediated by zeolites. 

Scheme 25 Oltra s and Cuerva s catalytic epoxy polyene cyclizations...

Polyene cycUzation of epoxy trienylsilanes 114 by use of TiCLj in CH2CI2 containing 2,6-di-tert-butylpyridine at —78°C gives a mixture of 9,10-syw and 9,10-anti bicycles 117 and 118 in comparable amounts (equation 49). The formation of six-membered ring products proceeds through a chair/chair or a chair/boat transition state (i.e. 115 and 116) with similar steric interactions. These results contrast sharply with the consistent bias favoring the chair/chair orientation in the cyclization of polyene epoxides lacking the MesSi substiment " . ... 

Epoxides can also serve as effective carbocyclization promotors, either through a polyene cyclization, as in the biomimetic epoxy-olefin cyclization of 100 in the presence of boron trifluoride etherate <99CC325>, or by a Friedel-Crafts approach, as exemplified by the cycli-alkylation of arylalkyl epoxides 102 under the influence of solid acid catalysts <99EJOC837>. 

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