The effect of pressure on the diastereoselectivity of the reaction of prenal with monoalkyl ylidenemalonates catalysed by homochiral secondary amines

The dienamine-mediated formation of 6-substituted cyclohexa-1,3-dienes from the title reactants catalysed by either (S)- or (R)- prolinol at 8 kbar proceeds with different net enantioselectivities depending on the structure of RCH=C(CO₂H)CO₂Alk to give a product with the same configuration as that obtained at atmospheric pressure (if R = Me₂C=CH) or with a configuration opposite to the latter (if R = Ph); by contrast, with both dienophiles the sense of enantioselectivity does not change with pressure when (S)-α,α-diphenyl-2-pyrrolidinemethanol is used as the catalyst.