Chemistry of the Heterocyclic Pseudobasic Amino Alcohols, XXXVIII Ring‐Chain Tautomerism of Pyrimido[6,1‐ a ]isoquinolines

In basis media, 4‐substituted pyrimido[6,1‐ a ]isoquinolin‐2‐imine hydrochlorides ( 4 ) constitute an equilibrium system involving the tautomeric forms characteristic of pseudobases 5, 6, 7 and the anhydro base 9 . In aqueous solution in the pH range 7–13 the ammonium hydroxide form 5 is present. Between pH 13 and 14 the pseudobase 6 probably arises from this by covalent binding of the hydroxy group. 6 is then converted to the more stable imino type anhydrobase 9 . Ring cleavage of 4 by excess alkali or of 9 by a small amount of water gives 1‐(acylamidinomethylen)isoquinolines 7 . Compounds 7 and 9 are stable in the solid state or in aprotic solutions but revert to cation 5 in dilute protic solutions.