2-Morpholinobutadienes (1) behave like typical enamines in their reactions with dimethyl acetylenedicarboxylate (2), β-nitrostyrene (3), diethyl 2-oxomalonate (4), and benzaldehyde (5). The nature of the resulting products is strongly dependent on the conformation which the 2-aminobutadienes (1) can adopt, as a function of the substituents. Only when the substituents can participate in the course of the reaction may the 2-morpholinobutadienes behave like a 4π-system in a concerted process.
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Barluenga et al. (1990) studied this question.
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