PURPOSE: To obtain a phthalazineacetic acid ester derivative as an intermediate of medicines by obtaining a novel intermediate and 3,4dihydro-4- oxophthlazineacetic acid from (Z)-3-oxo-l-(3H)isobenzofuranylideneacetic acid and reacting it with an acid.

CONSTITUTION: (Z)-3-oxo-1(3H)-benzofuranylideneacetic acid of formula III is reacted with hydrazine to obtain a mixture of a novel 1hydrazino-1,2,3,4- tetrahydro-4-oxophthlazineacetic acid of formula I with 3,4-dihydro-4- oxophthalazine acetic acid of formula II, and this mixture is acidified with NaOH to convert the compound of formula I into a compound of formula II, and the compound of formula II is reacted with an acid in the presence of ROH (wherein R is an alkyl) to obtain a phthalazineacetic acid ester of formula IV. The compound of formula IV is useful to produce a heterocyclic oxophthalazinylacetic acid of formula V and ester (useful as an aldose reductase inhibitor).