Bioinspired polyene cyclization promoted by intermolecular chiral acetal-SnCl4 or chiral N-acetal-TiCl4: investigation of the mechanism and identification of the key intermediates.

New strategies using chiral acetal or chiral mixed-acetal in the presence of Lewis acids (SnCl4 or TiCl4) to promote polyene cyclization reaction are described. Acetal-promoted and mixed-acetal-promoted polyene cyclization products are very versatile and can easily be converted into various optically active tricyclic and tetracyclic terpenoids. One of the derivatives of the cyclization products was obtained up to 96% ee after a single recrystallization. In addition, an oxocarbenium intermediate was found to be responsible for the good asymmetric selectivity for this type of reaction.