Beschreibung:
<jats:p><jats:bold>Derivatives of Ketipic Acid, II. – Ketipic Acid Dilactone and Related Compounds</jats:bold></jats:p><jats:p>The highly reactive title compound <jats:bold>7</jats:bold> was prepared by thermal decomposition of the disguised oxalylbisketene <jats:bold>6</jats:bold>. Ring opening of <jats:bold>7</jats:bold> with nucleophiles leads to γ‐alkylidenetetronic acids <jats:bold>4</jats:bold>. α‐Substituted derivatives of <jats:bold>4a</jats:bold> give rise to unsymmetrical dilactones <jats:bold>9a–c</jats:bold> and the dibromodilactone <jats:bold>9d</jats:bold>. The alkylidenetetronic acids are stabilized in the <jats:italic>E</jats:italic> configuration by hydrogen bridges. The pairs of isomeric (<jats:italic>E</jats:italic>)‐ and (<jats:italic>Z</jats:italic>)‐alkylidenetetronic acids <jats:bold>3</jats:bold> and <jats:bold>4</jats:bold> show characteristic spectral differences.</jats:p>