| Abstract
| - Several dienals were prepared and reacted in the presence of zinc metal with ethyl 2-bromomethylacrylate to provide in a Reformatsky-like reaction α-methylene lactones carrying a dienylside chain. Thermolysis of these compounds (11, 21, 29, and 37) gave in an intramolecular Diels−Alder reaction the corresponding tricyclic cycloadducts (51, 52, 53, and 54). The cycloadditionstook place with good diastereoselectivity and yields. The stereochemistry of the major isomer is inaccordance with an endo transition state for cycloadducts 51, 52, and 54. In one instance (compound58), the structure was supported by X-ray crystallography. In contrast to the other substrates, thenonatriene 29 cyclized to the exo product 53exo. The stereochemical situation could also be provenby NOESY NMR. However, the intramolecular Diels−Alder reaction did not work with furan asdienophile (compound 41) and substrate 50 featuring a densely functionalized tether connectingdiene and α-methylene lactone.
|
