Constrained Dipeptide Surrogates: 5- and 7-Hydroxy Indolizidin-2-one Amino Acid Synthesis from Iodolactonization of Dehydro-2,8-diamino Azelates

The constrained dipeptide surrogates 5- and 7-hydroxy indolizidin-2-one N-(Boc)amino acids have been synthesized from L-serine as a chiral educt. A linear precursor ∆⁴-unsaturated (2S,8S)-2,8-bis[N-(Boc)amino]azelic acid was prepared in five steps from L-serine. Although epoxidation and dihydroxylation pathways gave mixtures of hydroxy indolizidin-2-one diastereomers, iodolactonization of the ∆⁴-azelate stereoselectively delivered a lactone iodide from which separable (5S)- and (7S)-hydroxy indolizidin-2-one N-(Boc)amino esters were synthesized by sequences featuring intramolecular iodide displacement and lactam formation. X-ray analysis of the (7S)-hydroxy indolizidin-2-one N-(Boc)amino ester indicated that the backbone dihedral angles embedded in the bicyclic ring system resembled those of the central residues of an ideal type II’ β-turn indicating the potential for peptide mimicry.

Crystal structure of benzyl 3-oxo-2-oxa-5-azabicyclo[2.2.1]heptane-5-carboxylate

The title compound, C₁₃H₁₃NO₄ (also known as N-benzyl­oxycarbonyl-4-hy­droxy-l-proline lactone), crystallizes with two mol­ecules in the asymmetric unit. They have slightly different conformations: the fused ring systems almost overlap, but different C—O—C—C torsion angles for the central chains of −155.5 (2) and −178.6 (2)° lead to different twists for the terminal benzene ring. In the crystal, the mol­ecules are linked by C—H⋯O inter­actions, generating a three-dimensional network. The absolute structure was established based on an unchanging chiral centre in the synthesis.

Crystal structure of (1R,4R)-tert-butyl 3-oxo-2-oxa-5-azabicyclo[2.2.2]octane-5-carboxylate

In the title compound, C₁₁H₁₇NO₄, commonly known as N-tert-but­oxy­carbonyl-5-hy­droxy-d-pipecolic acid lactone, the absolute configuration is (1R,4R) due to the enantiomeric purity of the starting material which remains unchanged during the course of the reaction. In the crystal there no inter­molecular hydrogen bonds.

α-Chymotrypsin andl-acylase aided synthesis of 5-hydroxypipecolic acid via Jacobsen's hydrolytic kinetic resolution of epoxy amino acids

5-hydroxypipecolic acid synthesis from intramolecular reaction of epoxy amino acids; enantiomers separated by hydrolase and diastereomeric epoxide separated by Jacobsen's hydrolytic kinetic resolution.