| http://www.w3.org/ns/prov#value | - compound of formula ##STR12## (wherein R.sup.a, R.sup.b, R.sup.7, B and the dotted line are as hereinbefore defined; R.sup.8 and R.sup.8a may independently represent hydrogen or a carboxyl blocking group; and X isa replaceable residue of a nucleophile, e.g. an acetoxy or dichloroacetoxy group or a halogen atom such as chlorine, bromine or iodine) or a salt thereof, with a pyridine compound of the
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