Acid Chloride Method [Procedure A] Triethylamine (2.42 g ≡ 3.4 ml, 24 mmol) was added in portions to a well stirred cold solution (0˚C - 5˚C) of thiophene-2-carbonyl chloride 2 [freshly prepared from thiophene-2-acrylic acid (1.85 g, 12 mmol)] and amino acid ester hydrochloride (12 mmol) in THF/DCM (1:4, 150 ml).