Synthesis of 5,6‐dimethoxyquinazolin‐2(1H)‐one derivatives was the subject of investigations leading to the preparation of title compounds 11, 13, 14 and 26. Target quinazolines 1 were synthesized in three ways; the route starting from o‐vanillin via the intermediacy of 6‐amino‐2,3‐dimethoxyacetophenone (19) was used for most of the preparative work. The unexpected formation of an acid‐labile dimer of 13 was discovered and solid state 13C nmr was used for structural assignment. The 5‐methoxy substituent in these systems shows anomalous spectral characteristics and, in one case, was cleaved in acid media to 22.
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