Disclosed are t-butyl octahydrocyclopenta[c]pyrrole derivatives of formula (1a) e.g. (1S,3aR,6aS)-t-butyl 2-((S)-2-((S)-2-(benzyloxycarbonylamino)-2-cyclohexylacetamido)-3,3-dimethylbutanoyl)octahydrocyclopenta[c]pyrrole-1-carboxylate, wherein the variables are defined in the specification.