The present invention provides compounds of formula (I):
##STR1##
wherein R1, R2, R3, R4, R5
and R6 are as defined in the specification, and pharmaceutically
acceptable salts thereof. The compounds are useful for the treatment and/or prophylaxis
of diseases which are associated with DPP-IV, such as diabetes, particularly non-insulin
dependent diabetes mellitus, and impaired glucose tolerance.