Pyridinecarboxylic acids, directed

More powerful directing groups such as those based on amides and sulphonamides are successful with pyridines as with carboxylic rings, and will not be discussed separately. The enhanced acidity of pyridine ring protons makes the simple carboxylate substituent an ideal director of lithiation in pyridine systems . The pyridinecarboxylic acids 232-234 are deprotonated with BuLi and then lithiated with an excess of LiTMP all the substitution patterns are lithiated nicotinic acid 233 is lithiated in the 4-position (Scheme 113). The method provides a valuable way of introducing substituents into the picolinic, nicotinic and isonicotinic acid series. 

Directed lithiation. Unprotected pyridinecarboxylic acids are readily lithiated... 

The tetrahydroquinolylimidazolinone 4 (AC 263,754) was also found to be an active herbicide. It was anticipated that fusion of a heterocycle such as a pyran to pyridylimidazolinone 1 might also afford interesting herbicidal activity. With this background in mind, investigations directed toward the synthesis of isomeric pyranopyridine analogs were undertaken. This paper will focus on the synthesis and herbicidal activity of imidazolinyldihydropyrano[2,3-b]pyridinecarboxylic acid 5 and its derivatives. However, for comparison of the herbicidal activity, the other pyranopyridylimidazolinones such as 6, 7, and 8 will also be included. The syntheses of these compounds (i.e. 6, 7, and 8) have been reported elsewhere (8). 

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