In fact, upon glutamic acid coupling and Fmoc deprotection, the Mtt strategy led to the presence of a peak in the LC-MS chromatogram with a difference of +129 m/z compared to the desired intermediate, attributable to an extra glutamic acid residue (Supplementary Figure ). This is likely due to Boc deprotection during the Mtt cleavage conditions (1% TFA in DCM). Finally, conjugate 15 was obtained using 1-(4,4-dimethyl-2,6-dioxocyclohex-1-ylidene)-3-methylbutyl (ivDde) as a protecting group, which could be cleaved in 4% (v/v) hydrazine hydrate in DMF, leading to selective labelling of one folate molecule to the peptide.