oup for a phenolic hydroxyl group of Tyr; X5 is either hydrogen or a protecting group for a guanidino or imidazole side chain; X6 is a protecting group for a primary amino side chain of which Xa is a subgroup that is removable without removing other protecting groups; X7 is hydrogen or a protecting group for Met; X8 is Gly-NH-[resin support], D-Ala-NH-[resin support], N(A)-[resin support], an amid