prov:value
| - d of formula (IV):##STR00051## wherein R.sub.1, R.sub.2, R.sub.3, W and Z are defined as in Claim 1; R.sub.20 is C.sub.1-4alkyl or benzyl; and PG is a protecting group selected from the group: formyl, benzyl, p-methoxybenzyl, nitrobenzyl, 2,4-dimethoxybenzyl,triphenylmethyl, di-p-anisylmethyl, furylmethyl, C.sub.1-4alkoxycarbonyl, C.sub.1-4 allyloxycarbonyl, benzyloxycarbonyl, p-methoxybenzyloxyca
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