Fig. 1. Optimisation of dehydroalanine conversion by LCMS. (a) Peptide 6 was converted to 7 using either 3 or 4. The level of by-products 8 or 9 were monitored by LCMS when using 3 or 4 respectively. (b) The ratio of stapled by-product 8/9 produced in the conversion of cysteine to dehydroalanine in peptide 6 using either 3 or 4 in a variety of base and solvent systems. At low ratios of 3 to 6, high levels of stapled by-product 8 are observed. This is not entirely mitigated by increasing the equivalents of 3 used, with a persistent residual fraction of stapled peptide observed. The use of 4 demonstrated much reduced staple character, increasing the production of peptide 7 containing multiple dehydroalanine residues. ■ K₂CO₃, 1 : 1 H₂O/DMF; ◆ K₂CO₃, 1 : 1 H₂O/DMSO; red/black = use of 3/4 respectively.