The contrasting behavior of cyclic and acyclic 2-amidomethyleneresorcinols towards cyclization with acetaldehyde

The two cyclic amidomethylene resorcinols, 3a and 3b, readily undergo cyclization with acetaldehyde to the azacannabinoids 4 and 6, respectively. However, the acyclic amidomethylene resorcinol 10, under the same conditions, gives the benzodioxan 11. This contrasting behavior is rationalized in terms of conformational differences between 3a,b and 10.