Scheme 2.

Synthesis of Fmoc-cystathionine isomer 17.^a

^a Reagents and conditions: a) Fmoc-OSu, Na₂CO₃, H₂O, 1,4-dioxane; b) CCl₃C(NH)O^tBu, CH₂Cl₂, THF, 52% (two steps); c) PPh₃, CBr₄, CH₂Cl₂, 94%; d) Aloc-Cl, Na₂CO₃, H₂O, CH₃CN; e) allyl bromide, NaHCO₃, DMF, 70% (two steps); f) CF₃CO₂H, ⁱPr₃SiH, CH₂Cl₂, 85%; g) Bu₄NBr, NaHCO₃, H₂O, EtOAc, 93%; h) CF₃CO₂H, PhSiH₃, CH₂Cl₂, 98%.