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 condi tions afforded octahydrobenzocycloheptenone derivative 5 as a mixture of th ree or four diastereomers. High diastereoselectivities in favour of the end o-product cis-5a could be obtained with TiCl4 as promoter. This selectiviti es were attributed to the formation of a seven-membered ring chelate as int ermediate. The configurational assignment of all four diastereomers of 5 wa s based on isomerisation reactions and NMR analysis.