77% (dr 69:31 ), under irradiation of blue light (450 nm) using inexpensive blue LED light strips (see SI for details). The reaction progress was monitored using analytical HPLC. Excess phosphine was employed to ensure that the thiyl radical did not interfere with the reaction via thiol-ene radical addition. Promisingly, we observed complete and clean conversion of the starting peptide 3 within 10 min. As anticipated, two products were observed, each with the same mass. Purification of the material via preparative HPLC was followed by NMR analysis which determined that the two products were diastereomers of the desired cyclised peptide, isolated in an excellent combined yield of 77% (5a/5b, 53% and 24%, respectively) (Table , Entry 1). No resonances were observed corresponding to the allyl group in the 1 H NMR spectrum, which precludes the misidentification of the desulfurised starting peptide (which has the same molecular weight as the cyclised peptide) as the product.