Homologative Enamination of Aromatic Aldehydes as a Route to β-Dialkylaminostyrenes

Enamines serve as pivotal compounds in synthetic organic chemistry. Herein, a homologative enamination of aromatic aldehydes was achieved through three steps: [3 + 2] cycloaddition with nonstabilized azomethine ylide, quaternization with various alkyl halides, and eliminative ring-cleavage of oxazolidine core with sodium hydride in dimethyl sulfoxide. The obtained dialkylstyrylamines were employed as useful intermediates, furnishing a number of phenethylamines via reduction with sodium borohydride in good yields or were acylated with trifluoroacetic anhydride. We also demonstrated that the titled enamination could be performed in two step manner applying thermal ring-cleavage of 5-aryloxazolidine with trifluoroacetic anhydride, providing N-acylaminostyrenes in moderate yields. The proposed methodology for the homologative enamination features with gram-scale approach, readily available inexpensive materials, and simple laboratory techniques.