Lehmstedt-Tanasescu reaction

The Lehmstedt-Tanasescu reaction is a method in organic chemistry for the organic synthesis of acridone derivatives (3) from a 2-nitrobenzaldehyde (1) and an arene compound (2):



The reaction is named after two chemists who devoted part of their career to research into this synthetic method, the German chemist Kurt Lehmsted and the Romanian chemist Ioan Tănăsescu. Variations to the reaction name are the Lehmsted-Tănăsescu-reaction, Lehmsted-Tănăsescu-acridone-synthesis en Lehmsted-Tanasescu-acridone-synthesis.

Reaction mechanism
In the first step of the reaction mechanism the precursor molecule 2-nitrobenzaldehyde 4 is protonated, often by sulfuric acid,  to intermediate 5, followed by an electrophilic attack to benzene (other arenes can be used as well). The resulting benzhydrol 6 cyclisizes to 7 and finally to compound 8. Treatment of this intermediate with nitrous acid (sodium nitrite en sulfuric acid) leads to the N-nitroso acridone 11 via intermediates 9 en 10. The N-nitroso group is removed by an acid in the final step. The procedure is an example of a one-pot synthesis.