Step 5: Synthesis of Boc-L-ynylglycine
L-alkynylglycine (10 g) [prepared according to Schwyzer et al. Helv. Chim. Acta, 59, 2181 (1976)] was suspended in tetrahydrofuran (30 mL). To this suspension, water (30 mL), potassium carbonate (36.7 g) and di-tert-butyl dicarbonate (21.9 g) were added sequentially. Subsequently, additional water was added to completely dissolve the mixture and the reaction mixture was stirred at room temperature for 12 hours. Upon completion of the reaction, the organic solvent was removed by evaporation. The resulting aqueous solution was first washed with ether and then acidified with aqueous 1N citric acid to pH 3. The acidified solution was extracted with dichloromethane, and the extract was evaporated to remove the solvent to afford Boc-L-alkynylglycine (18.9 g, 97% yield), which was used in subsequent steps without further purification.
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