The Hetero Diels-Alder reaction of enamino ketones and ethyl vinyl ether to give dihydropyrans is studied in different solutions under pressure up to 7 kbar. The kinetics is measured via on-line FT IR spectroscopy. The cycloaddition shows a remarkable pressure dependent increase in diastereo-selectivity. For the difference in activation volumes referring to the reaction to the two diastereomers, values up to 5.9 ± 0.5 cm3/mol are observed. The ratio of cis and trans diastereomers can be improved by almost one order of magnitude by changing pressure and temperature from 1 bar and 90 °C to 6 kbar and 0.5 °C.