prov:value
| - R1 is hydrogen or a thiol-protecting group; each R2 and R3 is independently hydrogen or C1-C4 alkyl; R4 is hydrogen, C1-C4 alkyl, C1-C4 alkyl or peptidyl; R5 is hydrogen or C1-C4 alkyl; R6 is hydrogen or C1-C6 alkyl optionally substituted with hydroxy, phenyl, hydroxyphenyl, indolyl, imidazolyl, mercapto, methylthio, amino, carboxyl, carbamoyl, ureido, amidino or guanidino; A is a direct bond or a
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