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Syntheses of Fluorescent Dyes, 11. - Rearrangement of Alkoxymethylene- and Aminomethylenehomophthalic Anhydrides into Isocoumarins and Isoquinolinones, resp.
Wolfbeis, Otto S.
, p. 819 - 827 (2007/10/02)
Homophthalic acid (1) and its anhydride (3,4-dihydro-1H-2-benzopyran-1,3-dione, 2) react with trimethoxymethane in acetic acid anhydride to give 4-methoxymethylenehomophthalic anhydride (3a).This is converted into the enamines 4a-i by reaction with primary or secondary amines and ureas, respectively. 4b,d-i may also be obtained directly from 2 by reaction with a mixture of trimethoxymethane and the corresponding amine (or urea).In alkaline methanol 3a and similarly 4h suffer ring opening to give 5 and 6, respectively. - Treatment of 3a with aqueous sodium hydroxide and subsequent acidification furnishes by a rearrangement reaction the isocoumarin-4-carboxylic acid (7a) together with some of its decarboxylation product 7b, probably via an open ring intermediate of type A.Better yields of 7a are obtained by rearrangement performed in concentrated hydrochloric acid. - Similarly, compounds 4 rearrange to form the isoquinoline-4-carboxylic acids 8a-d. - Starting from homophthalic acid (1), these rearrangements offer a straightforward route to obtain fluorescent isocoumarin as well as isoquinolinone derivatives.
