Intramolecular Diels-Alder reactions using alpha-methylene lactones as dienophile.
Several dienals were prepared and reacted in the presence of zinc metal with ethyl 2-bromomethylacrylate to provide in a Reformatsky-like reaction alpha-methylene lactones carrying a dienyl side 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 cycloadditions took place with good diastereoselectivity and yields. The stereochemistry of the major isomer is in accordance with an endo transition state for cycloadducts 51, 52, and 54. In one instance (compound 58), the structure was supported by X-ray crystallography. In contrast to the other substrates, the nonatriene 29 cyclized to the exo product 53exo. The stereochemical situation could also be proven by NOESY NMR. However, the intramolecular Diels-Alder reaction did not work with furan as dienophile (compound 41) and substrate 50 featuring a densely functionalized tether connecting diene and alpha-methylene lactone.
Richter
Frank
F
Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany.
Bauer
Matthias
M
Perez
Caroline
C
Maichle-Mössmer
Cäcilia
C
Maier
Martin E
ME