Syntheses of Substituted 6-Hydroxyhexenoic Acids and Related Tetrahydrofuran Systems by Metallocene-Template Directed 1,4-Selective Coupling of 1,3-Butadiene with Carbonyl Compounds

Butadiene is 1,4-selectively coupled with a metal carbonyl and a ketone at the bis(η-cyclopentadienyl)zirconium template to yield the nine-membered metallacyclic products 3. Subsequent hydrolysis under oxidative conditions (pyridine N-oxide) gives the corresponding substituted 6-hydroxy-3-hexenoic acids 5. By this procedure a -CH₂CH=CHCH₂CO₂H nucleophile has thus formally been added to the ketone substrate. Esterification of 5 followed by treatment with a base converts these organic template coupling products into substituted tetrahydrofuran acetic acids. Similarly, ketones are linearly coupled with butadiene and a nitrile to yield analogously structured hydroxyalkenones 15 which undergo acid catalyzed ring closure to yield the -CH₂COR substituted tetrahydrofurans 16.