Step 5: Preparation of (R)-2-((tert-butoxycarbonyl)amino)pent-4-ynoic acid
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 resulting solution was stirred for 12 hours at room temperature. Upon completion of the reaction, the organic solvent was removed by evaporation. The remaining aqueous phase was washed with ether and then acidified with 1N aqueous citric acid to pH 3. The acidified aqueous phase was extracted with dichloromethane, and the organic phases were combined and the solvent was removed by evaporation to afford (R)-2-((tert-butoxycarbonyl)amino)pent-4-ynoic acid (18.9 g, 97% yield), and the product was ready to use in subsequent reactions without further purification.
