o-methoxyaryl-aldehydes (or -ketones) react in solid-phase with Meldru
m's acid under mild conditions, to give benzylidene derivatives which
are cyclized in high yield, with cold sulfuric acid to substituted 3-c
arboxycoumarins. Thermal decarboxylation, speed up by copper powder, p
rovided an easy access to numerous coumarins.