Inter‐ and intramolecular hetero Diels‐Alder reactions, 51. Intermolecular hetero Diels‐Alder reactions of enamino ketones. Effect of high pressure on the kinetics and diastereoselectivity

The hetero Diels‐Alder reaction of the enamino ketones 1b and 1c with ethyl vinyl ether (2) in dichloromethane to give the dihydropyrans 3b/4b and 3c/4c is studied at high pressure up to 5 kbar. The kinetics is measured by on‐line FT‐IR spectroscopy up to 3 kbar. The cycloadditions show a relatively high pressure‐dependent increase in diastereoselectivity in favor of the trans adducts 4b and 4c , respectively. The activation volumes at atmospheric pressure, ΔV   o ≠ , are determined to be −(43.3 ± 2.1) and −(43.9 ± 2.9) cm 3 mol −1 for the reactions of 1b and 1c at 100°C in dichloromethane solution. The activation enthalpies, Δ H ≠ for the two cycloadditions at 1500 bar are (64.4 ± 0.4) kJ mol −1 and (64.0 ± 0.6) kJ mol −1. The ΔΔV   o ≠values are (3.8 ± 0.3) and (4.6 ± 0.3) cm 3 mol −1 and the ΔΔ H ≠ values (1.5 ± 0.2) and (2.1 ± 0.3) kJ mol −1 , respectively.