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

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Abstract
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.

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