Activated Amino Acids – Structures of [α‐(Phthalimido)isobutyric] Anhydride, pentachlorophenyl α‐(tert‐Butyloxycarbonylamino)isobutanoate, and Pentachlorophenyl α‐(Benzyloxycarbonylamino)isobutanoate

Abstract [α‐(Phthalimido)isobutyric] anhydride [(Pht‐Aib)2O, 1] crystallizes in the space group P21/n with a = 10.293(4), b = 26.511(6), c = 9.192(4) Å, β = 113.9(3)°, and Z = 4 (R value for 1246 observed reflections 0.067). The anhydride moiety is significantly nonplanar and its conformation is of type a (trans‐trans), as found for monochloroacetic anhydride. – α‐(tert‐Butyloxycarbonylamino)isobutyric acid pentachlorophenyl ester (Boc‐Aib‐OPcp, 2) crystallized in the space group P21/c with a = 6.375(3), b = 32.801(6), c = 9.367(4) Å, β = 94.9(3)°, and Z = 4 (R value for 1781 observed reflections 0.0535). The urethane group adopts the cis conformation, an unusual observation for a N‐urethane‐protected Aib derivative. The plane of the pentachlorophenyl ring is perpendicular to that of the ester group. The molecules 2 form dimers via intermolecular NH…O=C(urethane) hydrogen bonds. – α‐(Benzyloxycarbonylamino)isobutyric acid pentachlorophenyl ester (Z‐Aib‐OPcp, 3) crystallizes in the space group C2/c with a = 27.841(6), b = 6.224(3), c = 24.335(6) Å, β = 106.3(3)° and Z = 8 (R value for 2375 observed reflections 0.047). In contrast to the Boc derivative 2 the activated ester Z‐Aib‐OPcp 3 has a trans urethane bond.