Compounds having the structure of Formula (I): and pharmaceutically acceptable salts thereof, wherein X1, R1, R2, R3, R4, R5and R6are as defined in the specification; pharmaceutical compositions comprising such compounds and salts; use of such compounds and salts to treat or prevent Prolyl endopeptidase fibroblast activation protein (FAP)-mediated conditions; kits comprising such compounds and salts; and methods for manufacturing such compounds and salts.