| prov:value
| - The method of claim 5 wherein A is selected from CO and SO2 ; wherein X is selected from oxygen atom and methylene; wherein each of R1 and R9 is a group independently selected from hydrido, methyl, ethyl, n-propyl, isopropyl, benzyl, b, b, b-trifluoroethyl, t-butyloxycarbonyl and methoxymethylcarbonyl, and wherein the nitrogen atom to which R1 and R9 are attached may be combined with oxygen to for
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