(Chemical Equation Presented) In the present investigation, solid-supported peptide olefins were converted quantitatively to aldehydes via the OsO ₄-NaIO₄-mediated oxidative cleavage reaction. Addition of DABCO was essential to efficiently suppress the formation of hydroxymethyl ketone side products. The generated aldehydes were used in intramolecular N-acyliminium Pictet-Spengler reactions to produce highly pure pyrroloisoquinoline derivatives. The methodology was extended to allylglycine derivatives to enable the incorporation of pyrroloisoquinoline scaffolds within peptides.