Scheme 2. Synthesis of Dipeptide Alkyne 18 as a Diastereomeric Mixture.

See Scheme 6 for the structural definition of stereochemically pure compounds. Reagents and conditions: (a) PyBOP, DIPEA, NaBH₄, THF, 1 h; (b) Dess-Martin periodinane, CH₂Cl₂, 4 h; (c) dimethyl (1-diazo-2-oxopropyl)phosphonate, K₂CO₃, MeOH, 0 °C, 2 h, rt, overnight; (d) TFA/CH₂Cl₂ (1:1), 2 h; (e) N-Boc-3-methyl-l-phenylalanine, PyBOP, DIPEA, THF, 3 h; (f) 2,4-difluorobenzoyl chloride, NMM, CH₂Cl₂, 2.5 h; (g) NaOH, THF/MeOH (3:1), overnight.