A substituted hydropyrazino[1,2-
a]pyrimidin-6-one derivative was synthesized stereoselectively via the intramolecular
N-acyliminium ion cyclization between an amide nitrogen and an
N-acetal derived from Cbz-protected aminopropyl-phenylalaninamide in very good yields. The formation of a single diastereomer is due to the low energy chairlike conformation of its
bicyclic structure. This methodology provides a convenient tool to build internal
bicyclic peptidomimetics.