No abstract

The enantioselectivity increases from 4.5 to 20.4% ee in favor of the (–)‐enantiomer in the intramolecular hetero‐Diels‐Alder reaction of 1, which provides 2 and 3 when the pressure is increased from 1 to 5000 bar. A chiral Lewis acid is also employed in this reaction.

Von 4.5 auf 20.4% ee steigt die Enantioselektivität bezüglich des ()‐Enantiomers bei der intramolekularen Hetero‐Diels‐Alder‐Reaktion von 1 zu 2 und 3 bei der Erhöhung des Drucks von 1 auf 5000 bar. Bei der Reaktion wurde eine chirale Lewis‐Säure eingesetzt.

The intramolecular hetero Diels-Alder reaction of the benzylidenebarbituric 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, AV*, are determined to be -(33.1 +_ 1.2) and -(34.2 f 1.5) cm3 . rno1-I 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 f 4.1) cm3 . mol-', respectively. Contrary to the large solvent effect on the activation volume, only a minor effect on the activation volume differences, AAV*, is observed. Measurement of the molar volumes of 1 and the cycloadducts 2 and 3 show a strong solvent dependency.The application of high pressure to increase the reaction rate and selectivity has been studied for a wide variety of intermolecular Diels-Alder reactions [']. The results have been used for mechanistic interpretations by many authors. Investigations into the solvent influence on the transition state were performed by Grieger and McCabe in the early seventied3]. In particular, they elucidated the solvent effect on the Diels-Alder reaction of maleic anhydride with several dienes finding a change in activation volume with the solvent up to 5 ~m~/ r n o l [~~l . Only very few intramolecular Diels-Alder reactions have been studied under highThe reported values for the activation volume indicate a slightly smaller but still preparatively useful pressure effect on the reaction rate. In the light of this it seemed worthwhile to carry out a detailed study of an intramolecular Diels-Alder reaction over a broad pressure range and in several solvents.In this paper we present data on the kinetics and selectivity of the intramolecular hetero Diels-Alder reaction of the benzylidenebarbituric acid derivative 1 leading to the ortho and meta addition products 2 and 3 (see Scheme 1).We consider the effects of pressure, temperature, and solvent; we also present data on the molar volumes of the substrate and of the products in dichloromethane, 1 -chlorobutane, tetrahydrofuran, acetonitrile, and toluene which allow us to perform calculations of the molar volume of transition structures and of the reaction volumes.The cycloaddition has been studied by direct quantitative infrared spectroscopy under high pressure up to 3 kbar. Spectral series in the region of the C=C and C=O fundamentals were measured during each experiment at constant pressure and temperature. As the molecular surrounding is

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