Synthesis of Multifunctionalized 1,2,3,4-Tetrahydropyridines, 2,3-Dihydropyridin-4(1H)-ones, and Pyridines from Tandem Reactions Initiated by [5+1] Cycloaddition of N-Formylmethyl-Substituted Enamides to Isocyanides: Mechanistic Insight and Synthetic Application

Tandem reactions for the efficient synthesis of multifunctionalized 1,2,3,4-tetrahydropyridines, 2,3-dihydropyridin-4(1H)-ones, and pyridine derivatives have been developed and reaction mechanisms have been investigated. Synthetic cascades are initiated by the Zn(OTf)2-mediated [5+1] cycloaddition of N-formylmethyl-substituted tertiary enamides to isocyanides, thus leading to the versatile heterocyclic enamino imine intermediates. Interception of the intermediates by diastereoselective reduction of imine functionality with Me4NBH(OAc)3 afforded 1,6-disubstituted trans-3-hydroxy-4-aryl- amino- or -alkylamino-1,2,3,4-tetrahydropyridines, whereas acylation of the imino group followed by acidic hydrolysis produced 1,6-disubstituted 3-acyl- oxy-2,3-dihydropyridin-4(1H)-ones. Aerobic oxidation led to the aromatization followed by intermolecular acylgroup transfer from the pyridinium nitrogen to the 3-hydroxy moiety, thereby yielding substituted 3-acyloxy-4- aminopyridines. Synthetic potentials of the resulting products have been demonstrated by expedient and highly stereoselective synthesis of cis,cis-4,5-dihydroxy-2-phenylpiperidine and trans,trans-4-amino-5-hydroxy-2-phenylpiperidine compounds, which are important in medicinal chemistry, through simple and practical reduction reactions.