ALUMINA-CATALYZED DEHYDRATION OF METHYLHEXADIENOLS . A REINVESTIGATION

Catalytic dehydration of substituted hexadienols usually produces a mixture of substituted 1,3,5-hexatrienes and the corresponding substituted 1,3-cyclohexadienes. Alumina dehydration of l,5-heptadien-4-ol or 3-methyl-l,5-hexadien-3-ol at temperatures ranging from 250 to 350° yielded the expected methyltrienes, but the cyclohexadiene fraction consisted of double-bond isomers whose distribution proved to be temperature dependent. Thermolysis of authentic 1,3,5-heptatriene or 3-methyl-1,3,5-hexatriene over alumina or glass helices yielded similar results. The 1,3-cyclohexadienes formed at 250° can be predicted on the basis of electrocyclic ring closure of an intermediate triene having a cis configuration about the central double bond. At 350°, 1,3-cyclohexadiene mixtures, resulting from intramolecular 1,5-hydrogen shifts in the Woodward-Hoffmann product, predominate. © 1969, American Chemical Society. All rights reserved.