Carbon nucleophiles, ANRORC reactions

Pyridones are normally resistant to nucleophilic attack at ring carbon atoms, but the pyrones react rather readily. There is a useful correlation between the position of attack and the hardness or softness of the nucleophile, and the situation for the pyrones is summarized in Schemes 12 and 13. Representative transformations illustrating these concepts are shown in equations (45)-(51). ANRORC reactions are also very common and examples are given in equations (52)—(55). 

Reactions with strongly basic nucleophiles such as potassium amide in liquid ammonia may prove much more complex than direct substitution. 2-Chloro-4,6,7-triphenylpteridine reacts under these conditions via an S ANRORC mechanism to form 2-amino-4,6,7-triphenylpteridine and the dechlorinated analogue (78TL2021). The attack of the nucleophile exclusively at C-4 is thereby in good accord with the general observation that the presence of a chloro substituent on a carbon position adjacent to a ring nitrogen activates the position meta to the chlorine atom for amide attack. 

It should be stressed that although outcome of these reactions is identical to that of classical Sj Ar of halogens, the substitution proceeds at the carbon atom of the ring connected with hydrogen thus, it is fully justified to consider cZwe-, tele-, and ANRORC substitutions as processes of nucleophilic substitution of hydrogen Sj H. 

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