Scheme 2. (A) Synthesis of Boc-Sec(DEAMC)-OH building block 1: (a) (i) NaH, KOtBu, THF, rt, 16 h; (ii) MeI, KOtBu, THF, rt, 16 h, 63% yield over 2 steps; (b) (i) 3-diethylaminophenol, 40 mol% Y(NO₃)₃·6H₂O, 115 °C, 24 h; (ii) TFA, DCM, rt, 2 h; (iii) CBr₄, PPh₃, THF, rt, 3 h, 11% yield over 3 steps; (c) (i) NaBH₄, EtOH : THF (2 : 7 v/v), (Boc-Sec-OH)₂, rt, 30 min; (ii) 4, rt, 5 min, 83%. (B) Synthesis and deprotection of model DEAMC-protected selenopeptide 7. (a) Fmoc-SPPS (see ESI†) (b) Boc-Sec(DEAMC)-OH 1 (1.1 eq.), Oxyma (1.2 eq.), DIC (1.1 eq.), DMF, rt, 3 h. (c) TFA, iPr₃SiH, H₂O (18 : 1 : 1 v/v/v), rt, 2 h. (d) 6 M Gdn·HCl, 0.1 M Na₂HPO₄, pH 6.0, λ = 450 nm, 36 W LED irradiation, 10 min.