Heterocyclic enamines: The versatile intermediates in the synthesis of heterocyclic compounds and natural products

Heterocyclic enamines are versatile intermediates for the synthesis of heterocyclic compounds and natural products, which are not readily obtainable by other synthetic means. One of the most intriguing features of heterocyclic enamines is their ambident bisnucleophilicity; nucleophilic reaction can take place at the site of enaminic carbon and/or the secondary amino nitrogen. These unique reaction properties lead to intriguing regioselectivity in the reaction with monoelectrophiles and biselectrophiles to produce different types of heterocyclic compounds. In this article, we will briefly introduce the methods of preparing secondary and tertiary heterocyclic enamines and then review the selective reactions of secondary heterocyclic enamines with emphasis on the selective annulation reaction with biselectrophiles. In part three, we will summarize the applications of heterocyclic enamines in the syntheses of alkaloids and related pharmaceutically useful compounds. We will focus on the synthesis of pyrrolidine, piperidine, bipyrrolidine, perhydroindolizine, perhydroquinolizine, and other fused heterocyclic alkaloids. Synthesis of antibacterial and other bioactival compounds using heterocyclic enamines protocol will also be included.