DBU-Promoted Cyclizations of Cyclopentyl-Substituted Oxazapolyenes to Cyclopentapyridones and Hydroxypyrroles: Experimental and DFT Study

1,1-Di(alkoxycarbonyl)-4-cyclopentyl-2-azabuta-1,3-dienes react with DBU to form two types of heterocyclic products: 1 H -cyclopenta[ c ]pyrid-1-ones and 3-hydroxy-1 H -pyrroles. These previously unobserved transformations proceed through the formation of 1-azapentadienyl anion which undergoes 1,6-shift of the alkoxycarbonyl group to the cyclopentyl moiety followed by 1,6-cyclization to form the cyclopentapyridone (path a ) and 1,5-cyclization accompanied by 1,3-shift of the methoxy group followed by dialkyl carbonate elimination to afford the hydroxypyrrole (path b ). The mechanisms of the reactions were studied using DFT calculations. Pyridones can be synthesized in one pot in three steps via Rh(II)-catalyzed isomerization of 4-cyclopentylisoxazoles to 2 H -azirines and their subsequent reaction with diazomalonic esters, followed by heating of the resulting 2-azabuta-1,3-dienes with DBU.