One‐pot Syntheses of 3‐Amino‐2‐pyrones and 3‐Amino‐5,6‐dihydro‐2‐pyridones from β,γ‐Unsaturated α‐Iminoesters: Application to the Synthesis of A Tricyclic Pyranoindole

Abstract

Abstract As the α‐imino ester has an ester moiety adjacent to the imino group, it is known as a highly reactive imine, and the reaction with a nucleophile leads to the umpolung N‐addition reaction predominantly by subjecting it to appropriate reaction conditions, while the general C‐addition reaction is suppressed. Utilizing N‐addition makes it possible the synthesis of α‐amino acids incorporating various kinds of nucleophiles on the nitrogen atom. Herein, an efficient method for the one‐pot synthesis of 3‐amino‐2‐pyrones utilizing tandem N‐alkylation/vinylogous aldol reaction/cyclization of β,γ‐unsaturated α‐iminoester was described, and it was found that BF 3 ⋅ OEt 2 promoted the elimination of an alkylthio group to give 2‐pyrones in good yields. The synthesis of 3‐amino‐5,6‐dihydro‐2‐pyridones was also developed using a vinylogous Mannich‐type reaction in good yields. These reactions exhibit broad generality for electrophiles and Grignard reagents. Further functionalization to the tricyclic pyranoindole derivative was achieved in 4 steps by using this tandem reaction as a key step.