Initial trials of this strategy focused on the utilisation of both 2methylallyl and allyl moieties to facilitate peptide cyclisation in the presence of a Cys residue (Table ). Addition of the alanyl radical to a 2-methylallyl group would generate a tertiary radical intermediate previously shown to improve the efficiency of this chemistry; 56 however, diastereomers of the desired macrocycle will be produced due to the methyl branch of the linkage. Employing an allyl group as the radical trap would result in a single product, however, the reaction will be less effective and may not fully out-compete H-atom abstraction. Both of these strategies were initially explored. Amino acid building blocks 1 and 2 were synthesised via alkylation of Boc-Ser-OH with 1bromo-3-methylbut-3-ene and allyl bromide, respectively (SI). These building blocks were incorporated into a simple peptide sequence using solid phase peptide synthesis (SPPS) to afford Table . Exploration of peptide cyclisation via intramolecular interception of visible-light-mediated desulfurisation. Entry Peptide [Peptide] /mM [TCEP] /mM Ir(III) PC (mol%) Reaction duration (min) Isolated yield [% conversion ]