We recently reported a novel strategy for the site-selective modification of peptides and proteins via visible-light-mediated desulfurative C(sp 3 )-C(sp 3 ) bond formation. Desulfurisation of Cys (and alternative non-proteinogenic thiol-containing amino acids ) can be applied post-peptide ligation as an elegant method to access a broad range of ligation junctions and facilitate chemical protein synthesis. A widely used free-radical-mediated Cys desulfurative protocol, developed by Danishefsky and co-workers, proceeds via a thiophosphoranyl radical species generated using a radical initiator (VA-044) to form a thiyl radical from the thiol sidechain of Cys in the presence of the water-soluble phosphine, tris(2carboxyethyl)phosphine hydrochloride (TCEP). Homolysis of the thiophosphoranyl C-S bond 68 produces a peptide 'alanyl' radical which, in the presence of a suitable thiol additive (e.g. glutathione), will abstract an H-atom to yield the residue, alanine (Ala), at the ligation junction. In previous work, we demonstrated desulfurisation using an iridium photocatalyst (PC) and employed alkenes to intercept the alanyl radical species, enabling installation of Lys sidechains carrying natural modifications as well as effective mimics of this modified sidechain. Due to the requirement to out-compete H-atom abstraction during this process, a large excess of the alkene is required (a minimum of 200 equivalents). By installing an appropriate alkene into a peptide containing a Cys residue, we postulated that intramolecular trapping of the radical produced upon desulfurisation may proceed preferentially to H-atom abstraction, leading to the cyclic peptide product; essentially allowing us to use an equimolar equivalent of the alkene. The chemoselectivity of this chemistry should ensure that the reaction enables efficient cyclisation of unprotected peptides in aqueous solution. If realised, this technique would be a valuable addition to the toolkit available to researchers for the production of cyclic peptides.