Heterocycle Formation through Aza-Annulation: Stereochemically Controlled Syntheses of (+/-)-5-Epitashiromine and (+/-)-Tashiromine

Article abstract of DOI:10.1021/jo00086a009

N-alkenylamines, stabilized through conjugation with an electron-withdrawing group, undergo aza-annulation with acryloyl chloride to provide a convergent route for the construction of six-membered nitrogen heterocycles.In addition to enhancing the C-alkylation process of annulation relative to the competing N-acylation process, the electron withdrawing substituent controlled the regioselectivity of alkene formation in both the intermediate enamine and in the unsaturated lactam product.A variety of functional groups, which include -COMe, -COPh, -CO2R, -CONHPh, -CN, -P(O)(OEt)2, and -SO2Ph, were used to determine the effect of the electron-withdrawing substituents upon both the annulation reaction with acryloyl chloride and the subsequent hydrogenation process.When the enamide annulation product was stabilized through conjugation with ester or amide substituents, catalytic hydrogenation of the aza-annulation product resulted in the formation of vicinal stereocenters with high cis selectivity.The utility of this methodology was demonstrated by application of the condensation/aza-annulation/hydrogenation sequence as the key for construction and stereochemical control of the indolizidine ring system of (+/-)-tashiromine.