(2S,4S)-N-Fmoc-4-Et-Pro-OH

The title compound, C(10)H(8)O(6), a promising raw material to obtain colorless polyimides which are applied to microelectronic and optoelectronic devices, adopts a folded conformation in which the dihedral angle between the two anhydro rings is 55.15 (8)°. The central six-membered ring assumes a conformation inter-mediate between boat and twist-boat. In the crystal, mol-ecules are linked by weak C-H⋯O inter-actions, forming a layer parallel to the bc plane.

The crystal structure of the title compoud, C(17)H(20)BrNO(3), contains three chiral centers, which all exhibit an S configuration. The C=C double bond has an E conformation. The cyclo-hexane ring is in a chair conformation. In the crystal, mol-ecules are linked by weak N-O⋯Br inter-actions [O⋯Br = 3.136 (4) Å].

In the title compound, C(14)H(14)N(2)O(4), the central six-membered ring adopts a twisted boat conformation with the phenyl substituent occupying an orthogonal position [dihedral angle = 86.88 (11)°]. In the crystal, mol-ecules are linked by carboxylic acid-carbonyl O-H⋯O and amide-carbonyl N-H⋯O hydrogen bonds, forming a three-dimensional network.