T- Amino effect

Heterocycles by Ring Closure of Ortho-substituted t-Anilines (the t-Amino Effect) O. 

II. Type 1 Reactions Formation of Five-Membered Rings from t-Amino-Effect... 

A typical example of the type 1 t-amino effect is shown in Scheme 2. All the reactions in this section proceed by an initial [l,6]H-shift from the 0 position of the tertiary amino group to the Y terminus of the ortho substituent. Subsequent cyclization of the dipolar intermediate leads to 5-membered heterocyclic products. 

It is evident that although much is now known about these type I t-amino-effect reaction pathways involving C=C substituents, the synthetic applications have barely been touched. The predictable stereochemistry and remarkable specificity of the reactions await further development. 

French workers (86SC79) noted that thermolysis of a 4H-l,3-thiazine 97 took place by way of an acyclic thionoenamine 98 to give a 2H-thiazine 99, the reaction proceeding via the t-amino effect (Scheme 32). 

The first example of a surprising reaction sequence due to a t-amino effect was observed by Pinnow as early as 1895 during his attempts to prepare the acetyl derivative (3) of o-aminodimethyl-aniline (1) by prolonged reflux in acetic anhydride. He obtained 1,2-dimethylbenzimidazole (2) instead, and on the basis of analogous observations in related reactions he suggested that the formation of... 

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