Inter‐ and Intramolecular Hetero Diels‐Alder Reactions, 49 [1] Kinetics and Selectivity of the Intramolecular Hetero Diels‐Alder Reaction of 1‐Oxa‐1,3‐butadienes using a Benzylidenebarbituric Acid Derivative Effect of Pressure, Temperature, and Solvent

The intramolecular hetero Diels‐Alder reaction of the benzy‐lidenebarbituric acid derivative 1 to give the ortho and meta products 2 and 3 is studied under high pressure up to 6 kbar in various solvents. The kinetics is measured by on‐line FT‐IR spectroscopy up to 3 kbar. The cycloaddition shows a pressure‐dependent increase in regioselectivity in favour of the ortho adduct 2. The activation volumes, ΔV, are determined to be ‐(33.1· 1.2) and ‐(34.2· 1.5) cm 3 · mol −1 for the reactions in dichloromethane and tetrahydrofuran at 100°C, respectively. For the cycloaddition in toluene and acetonitrile the activation volumes are found to be ‐(13.4· 1.5) and ‐(17.0· 4.1) cm 3 · mol −1 , respectively. Contrary to the large solvent effect on the activation volume, only a minor effect on the activation volume differences, ΔΔV, is observed. Measurement of the molar volumes of 1 and the cycloadducts 2 and 3 show a strong solvent dependency.