Reactions of Phthalazinecarboxylic Acids

The various reactions of phthalazinecarboxylic acids are illustrated by the following classified examples. 

6-Ethoxycarbonyl-5,7-dimethyl-4-oxo-3,4-dihydro-l-phthalazinecarboxylic acid (14) underwent thermal decarboxylation to give ethyl 5,7-dimethyl-4-oxo-3,4-dihydro-6-phthalazinecarboxylate (15) (neat substrate, 190°C, 15 min 60%). °  

Note Although linear phthalazinecarboxylic anhydrides appear to be unrepresented, phthalazinedicarboxylic anhydrides have been made. 6,7-Phthalazinedicarboxylic acid (16) with acetic anhydride gave 6,7-phthala-zinedicarboxylic anhydride (17) (neat reactants, reflux, 2h 78%).  

3-p-Chlorophenyl-4-oxo-3,4-dihydro-l-phthalazmecarboxylic acid (18, R = OH) gave 3-p-chlorophenyl-4-oxo-3,4-dihydro-l-phthaiazinecarbonyi chloride (18, 

4-Oxo-3,4-dihydro-5-phthalazinecarboxylic acid (19, R = OH) gave 4-oxo-3,4-dihydro-5-phthalazinecarbonyl chloride (19, R = C1) (neat SOCI2, reflux, 2.5 h 88% or SOCI2, PhCl, reflux, 4h 75%) 

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