or any carboxyl group present is optionally amidated, Rb is a hydrogen or Ra and Rb taken together are trimethylene, Rc is the side-chain of a natural ??-amino acid in which any functional group is optionally protected or any amino group present is optionally acylated or sulfonylated or any carboxyl group present is optionally amidated, Rd is a hydrogen or Rc and Rd taken together are trimethylen