Examples of imidazolyl-protecting groups which may be represented by R13 include: aralkyl groups in which an alkyl group having from 1 to 4 carbon atoms is substituted by at least one (preferably from 1 to 3) aryl groups, which themselves can optionally be substituted by at least one nitro group or alkoxy group having from 1 to 4 carbon atoms, for example the benzyl, p-nitrobenzyl, p-methoxybenzyl