Heterodiene syntheses—XII : The conformational analysis of cis and trans 2-alkoxy-4-phenyl-2,3-dihydropyran[2,3-c] pyrazoles: Steric interactions and the anomeric effect

The conformational equilibria of a homogeneous series of cis and trans 2-alkoxy-4-phenyl-2,3-dihydropyran2,3-c] pyrazoles have been investigated by a graphical method, and the preferences are rationalized in terms of steric interactions between the 4 and 5-substituents and of the anomeric effect.The anomeric interaction depends upon the nature of the alkoxy group and the conformational free-energy differences correlate with the steric parameters E_s.