Intramolecular Diels-Alder reactions. 5 - Facile synthesis of an octahydrobenzocycloheptenone derivative utilising intramolecular Diels-Alder reaction

Undecatrienone 4 was prepared in a short sequence via siloxycyclopropanes 1 and 3. Intramolecular Diels-Alder reaction (IMDA) of 4 under various conditions afforded octahydrobenzocycloheptenone derivative 5 as a mixture of three or four diastereomers. High diastereoselectivities in favour of the endo-product cis-5a could be obtained with TiCl4 as promoter. This selectivities were attributed to the formation of a seven-membered ring chelate as intermediate. The configurational assignment of all four diastereomers of 5 was based on isomerisation reactions and NMR analysis.