6-PI-ELECTROCYCLIZATION OF 1-AZATRIENES TO 1,2-DIHYDROPYRIDINES

A series of alpha,beta-cis-dienals were synthesized by oxidation and t hen in situ isomerization of easily available beta-allenic alcohols. S ubsequent reaction between the alpha,beta-cis-dienals and primary amin es represents a simple and useful method for the direct preparation of 1,2-dihydropyridines. Reactivity profiles and activation parameters f or the cyclization of alpha,beta-cis-dienimines are similar to those o f the carbocyclic counterparts except the reaction is much faster. Des pite the lack of a clear stereochemical label, the disrotatory nature of the ring closure is illustrated by the greater cyclization rates of 8'c and 8'd as compared to 7'c and 7'd, respectively. The observation that sterically more hindered systems (8'c and 8'd) cyclize faster th an less hindered ones (7'c and 7'd) is a feature of 6 pi-electrocycliz ations that is unprecedented.