4,6-Dimethyl-7-acetylumbelliforone (4,6-dimethyl-7-acetoxycoumarin) (I) is submitted to the Fries' rearrangement, obtaining the 4,6-dimethyl-8-acetylumbelliferone (II); the latter (II) is condensed with ethylbromoacetate (A) and the resulting ethyl [(4,6-dimethyl-8-acetyl-7-coumarinyl)oxy]acetate (III) is hydrolyzed by refluxing with 5% KOH aqueous methanolic solution; from this solution, acidified with HCl, the [(4,6-dimethyl-8-acetyl-7-coumarinyl)oxy]acetic acid (IV) is extracted with EtOAc and purified by crystallization (EtOAc / n-hexane). Cyclization of (IV), carried out by refluxing in Ac2O in the presence of anhydrous natrium acetate, is accompanied by an almost complete decarboxylation of (V), yielding 4,6,4'-trimethylangelicin (TMA).

4,6-Dimethyl-7-acetylumbelliforone (4,6-dimethyl-7-acetoxycoumarin) (I) is submitted to the Fries' rearrangement, obtaining the 4,6-dimethyl-8-acetylumbelliferone (II); the latter (II) is condensed with ethylbromoacetate (A) and the resulting ethyl [(4,6-dimethyl-8-acetyl-7-coumarinyl)oxy]acetate (III) is hydrolyzed by refluxing with 5% KOH aqueous methanolic solution; from this solution, acidified with HCl, the [(4,6-dimethyl-8-acetyl-7-coumarinyl)oxy]acetic acid (IV) is extracted with EtOAc and purified by crystallization (EtOAc / n-hexane). Cyclization of (IV), carried out by refluxing in Ac2O in the presence of anhydrous natrium acetate, is accompanied by an almost complete decarboxylation of (V), yielding 4,6,4'-trimethylangelicin (TMA).