Intramolecular ene reactions, III. Diastereoselective formation of cyclohexanes by intramolecular ene reactions of 1,7‐dienes

The 1,7‐dienes 3a–c with a double‐activated enophile moiety undergo thermal and zinc bromide catalyzed intramolecular ene reactions leading to trans ‐1,2‐disubstituted cyclohexanes 4a – c in up to 89% yield, highly diastereoselectively. The synthesis of cyclohexanes from 1,7‐dienes has not been feasible so far in a selective way and with good yields. Reaction of 3d, 3e , and 3f gives mixtures of intramolecular hetero Diels‐Alder adducts, ene and tandem ene products. The 1,7‐dienes 3 are obtained by Knoevenagel condensation of 7‐methyl‐6‐octenal ( 2 ) with acyclic 1,3‐dicarbonyl and analogous compounds 1 .