prov:value
| - nesulfonate, and trifluoromethanesulfonate; and R.sub.20 is C.sub.1-4alkyl or benzyl;with a compound of formula (IIIa):##STR00040## wherein Y and R.sub.9 are defined as in Claim 5; 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 allyloxycarbony
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