"with R1, R3, R4, R5, L, An, m and n as defined above; Y' representing a thiol-function-protecting group; Met* representing an alkali metal or an alkaline earth metal, particularly sodium or potassium, it being understood that, when the metal is an alkaline earth metal, 2 chromophores with a thiolate function -S- can be associated with 1 metal2+; R representing a nucleofuge leaving group, for insta" . . . . .