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