A mixture of andrographolide (15 g), 2,2-dimethoxy propane (20 ml) and a catalytic amount of pyridinium p-toluene sulphonate in benzene/dimethyl sulphoxide (300 ml/40 ml) was refluxed for 30 min. After completion of the reaction (checked by TLC), the contents were cooled to room temperature and treated with excess triethylamine (10 ml) to quench the remaining catalyst. The reaction mixture was diluted with benzene (200 ml) and washed with water (3×300 ml). The organic layer was separated, dried over Na2SO4 and concentrated to obtain a yellow coloured solid which on maceration with diethyl ether gave 3,19-isopropylidene andrographolide as a pale yellow product (16.5 g). m.p. 194.5° C., m/z 390.
