Transformation of l-phenylalanine to (S)-indoline-2-carboxylic acid without group-protection

(S)-Indoline-2-carboxylic acid was synthesized by use of a nitro amination approach with l-phenylalanine as chiral pool. The first step of the synthesis was nitration of l-phenylalanine, with urea nitrate (UN)/H₂SO₄ as nitrating reagent, to give 2,4-dinitro-l-phenylalanine in 75.7 % yield in one-pot synthesis and 69.1 % yield by step-wise nitration. Intramolecular nitro amination of 2,4-dinitro-l-phenylalanine gave (S)-6-nitro-indoline-2-carboxylic acid in 65.7 % yield and more than 99.5 % enantiomeric excess (ee). The title compound, (S)-indoline-2-carboxylic acid, was obtained in 85.9 % yield and high ee by one-pot transformation of (S)-6-nitroindoline-2-carboxylic acid. The total synthesis consisted of three operations and gave the title compound in 42 % yield and more than 99.5 % ee.