Some uses of acyliminium ions in the synthesis of isoquinolones with potential biological activity
The preparation of a number of 2-substituted homophthalimides through the condensation of homophthalic anhydride with different arylalkyl arnines is reported. The prepared compounds were alkylated at the 4-position to generate 4-mono-, 4,4-disubstituted and 4-spirocyclic homphthalimides, the analogues of which were reported to have interesting biological activity. Regioselective reduction of the 4-substituted derivatives generated the corresponding carbinolamides. Treating the carbinolamides with mineral or Lewis acids generated N-acyliminiurn ions, which were trapped in situ by one of the following: ( 1) aromatic neucleophiles to generate analogues of the natural product berberine, (2) alkyl chain migration to generate tetrahydrophenanthridones and functionalised isoquinolones, (3) cyclopropane ring-opening to generate 4-alkylisoquinolones, (4) addition to double bond to generate cyclopentaisoquinolones and (5) benzyl or allyl elimination. The oxidation of 4-monosubstituted homophthalimides with triplet dioxygen in alkaline media was investigated, and it generated 4-hydroxyhomophthalimides and isobenzofurancarboxamides. Treating isobenzofurancarboxamides with POCI3 provided a concise route to analogues of the neuroactive naturally-occurring phthalideisoquinolines.