Inter‐ and intramolecular hetero Diels‐Alder reactions, 52. Effect of high pressure on the kinetics and selectivity of the intramolecular hetero Diels‐Alder reaction of an 1‐oxa‐1,3‐butadiene

The intramolecular hetero Diels‐Alder reaction of the benzylidene‐isoxazolone 3 giving the adducts 4 and 5 is studied in dichlormethane under high pressure up to 5 kbar. The kinetics is measured by on‐line FT‐IR spectroscopy up to 3 kbar. The cycloaddition shows a clear pressure‐induced increase in diastereoselectivity. For the difference in activation volume ΔΔV ≠ of the reactions affording the two diastereomers, a value of −(1.6 ± 0.2) cm 3 · mol −1 is obtained. The activation volume ΔV   cis ≠is determined to be ‐(19.4 ± 0.5) cm 3 · mol −1 at 343 K. The activation enthalpy and entropy, as derived from the overall rate coefficient k measured at 1000 bar, are (62.9 ± 1.7) kJ · mol −1 (at 343 K) and ‐(135 ± 11) J · mol −1 · K −1 .