Swor CD1, Zakharov LN, Tyler DR.
J Org Chem. 2010 Oct 15;75(20):6977-9. doi: 10.1021/jo101381r.1,8-Bis(dimethylamino)naphthalene ("Proton Sponge") and bromomaleic anhydride react quickly at room temperature, generating 3-(4,5-bis(dimethylamino)napthalen-1-yl)furan-2,5-dione (4-maleicanhydridoproton sponge or "MAPS"). MAPS is a deep purple solid that exhibits positive solvatochromism in solution. It is a weaker base than Proton Sponge. When protonated in solution, MAPS loses its color, but the color change can be reversed by deprotonation with a stronger base. MAPS thus acts as a colorimetric version of a proton sponge.