A compound of formula (I) and salts and solvates thereof, wherein: R.sup.10 is a nitrogen protecting group and R.sup.11 is either OH or O--R.sup.12, wherein R.sup.12 is an oxygen protecting group, or R.sup.10 and R.sup.11 together form a double bond between N10 and C11; and R.sup.10' and R.sup.11' are selected from the same options as R.sup.10 and R.sup.11 respectively. ##STR00001##