Interceptive [4+1] Annulation of in Situ Generated 1,2-Diaza-1,3-dienes with Diazo Esters: Direct Access to Substituted Mono-, Bi-, and Tricyclic 4,5-Dihydropyrazoles

In situ derived acylic and cyclic 1,2-diaza-1,3dienes (DDs) were engaged in interceptive [4 + 1] annulation strategy with diazo esters (DEs). The catalytic activity of inexpensive copper(1) chloride allows the direct synthesis of mono-, hi-, and tricyclic 4,5-dihydropyrazole-5-carboxylic acid derivatives in a process that circumvents the use of an anhydrous and inert atmosphere.