Use of new and known terrecyclolic acid compounds (tri:cyclo:undecane(s))

The use of tricyclic undecane compounds of formula (I), especially the natural stereoisomers, and salts (especially the alkali(ne earth) metal salts) is claimed for combatting disorders which respond to inhibition of prolyl-oligopeptidase (POP). R1 = H, CH2OH, alkoxy, OH, haloalkyl, CHO, CHS, alkylcarbonyl, carboxyl, alkoxycarbonyl, trialkylsilyloxycarbonyl, alkylamidocarbonyl or Z-R1a; R2 = alkyl, alkoxy, oxo, alkylidene, alkylthio or alkyloxirane; or R1+R2 = a lactone ring with 5-20 ring members; R3 = oxo, thio, alkylidene or hydroxyl; or R2+R3 = a bond; Z = alkylene or a bond; R1a = O-P(O)(OH)2, P(O)(OH)2, S(O)2(OH), O-S(O)2(OH), and alkyl esters of these. (I) and their salts are new, provided that R1 is not CH2OH or COOH; R1+R2 is not a lactone ring with 5 or 6 ring members; and R3 is not O or OH.