Inter‐ and Intramolecular Hetero Diels‐Alder Reactions, Part XXVI. Diastereoselectivity and Kinetics of Intermolecular Hetero Diels‐Alder Reactions under High Pressure. A Significant Pressure‐Induced Increase in Stereoselectivity

The Hetero Diels-Alder reaction of the enamino ketones 5a–d and ethyl vinyl ether (2) to give the dihydropyrans 6a–d and 7a–d is studied in dichloromethane and in heptane/isodurene under high pressure up to 7 kbar at temperatures between 0.5 and 130°C. The kinetics is measured by on-line FT-IR spectroscopy. The cycloaddition shows a remarkable pressure-dependent increase of diastereoselectivity in favour of the cis adducts 6a–c with the largest effect found for the reaction of 5a and the smallest for 5c. The pressure-averaged overall activation volumes ΔV≠ in dichloromethane are determined to be between –(23.4± 1.0) and –(24.2±1.0) cm3/mol. The ΔΔV≠ values for the cycloaddition of 5a, 5b, and 5c are –(5.9±0.5), –(3.9±0.1), and –(2.4±0.2) cm3/mol, respectively, and the ΔΔH≠ values are –(8.1±1.7), –(8.7±2.7), and –(10.0±0.9 kJ/mol, respectively. Because of the favourable ΔΔV≠ and ΔΔH≠, the selectivity of the reaction of 5a to give 6a/7a can be increased from 1.67: 1.00 at 90°C and 1 bar to 13.6:1.0 at 0.5°C and 6 kbar. This example shows that a significant and synthetic useful increase of diastereoselectivity in chemical reactions is possible by applying high pressure.