Potassium Hydroxide → Methyl alpha-methyl-D-tryptophanate

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To a suspension of (±)-α-methyltryptophan methyl ester (79.5 mg, 0.342 mmol) in 30 mL of 50 mM sodium phosphate buffer pH 7.5 was added carboxypeptidase W (3.0 mg of solid). The resulting suspension was stirred at ambient temperature. After 6.5 h, 56% conversion was observed by HPLC (C18 column, 280 nm detection, amino acid tR =8.2 min and amino ester tR =11.1 min). The reaction was adjusted to pH 12 by the addition of 2 N KOH and then extracted with 1×15 and 1×20 mL ethyl acetate. The combined extracts were treated with brine, dried with magnesium sulfate, then concentrated in vacuo to yield (R)-α-methyltryptophan methyl ester (10.4 mg, 44.8 mmol, 13.1%). The enantiomeric excess of the unreacted amino ester was determined by chiral HPLC (Uttron ES-OVM column from Shinwa Chemical Industries, Ltd., 280 nm, (R)-enantiomer tR =13.7 min and (S)-enantiomer tR =18.8 min) to be 89.9% (r)-enantiomer.

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