Scheme 5. Oxetane modified cyclic peptides by disulphide bond formation.^a,baReagents and conditions: (a) I₂ (3.0 equiv.), MeOH, 1 h; (b) 70% TFA/20% CH₂Cl₂/10% TIS, 2.5 h. ^bBond formed in macrocyclisation indicated in red. ^cTFA step omitted; yield in parenthesis for the cyclisation of linear peptide with C O rather than oxetane in backbone. ^dData taken from ref. 14.