FORMATION OF 1,2,3,4-TETRAHYDRO-2-PYRIDONES BY AZA-ANNULATION OF IMINES WITH ACRYLATE DERIVATIVES

The aza-annulation of imines with activated acrylate derivatives was studied as a means of preparing the corresponding 1,2,3,4-tetrahydro-2-pyridones. Through the use of reagents known to facilitate the formation of amide bonds from carboxylic acids, several methods of activating the acrylate species were compared. The acrylate derivatives studied were acryloyl chloride and acrylic anhydride as well as acrylic acid activated by reaction with EtO2CCl, (PhO)2P(O)N3, or MCPI. Optimum annulation was obtained with imines derived from cyclohexanone to produce octahydro-2-quinolone products. The N-isobutylimine prepared from cyclopentanone also produced selective ring annulation to efficiently produce the corresponding bicyclic product, but the reaction with the imine of n-butanal produced lower yields of cyclic product. Ring formation was relatively unaffected by substituents at the a-position of the acrylate derivative, demonstrated by the use of methacrylate, but beta-substituents hindered the annulation process and, in turn, increased the amounts of byproduct resulting from only N-acylation of the imine. Increasing the steric bulk of the imine alkyl substituent produced the opposite effect; the relative amount of N-acylation compared to complete aza-annulation was diminished as the size of the substituent was increased. Mechanistic features of the reaction are discussed in terms of product distribution and competition experiments.