Scheme 6. Synthesis of protected ^tzA (25) and ^tzisoG (27). Conditions: (a) triethyl orthoformate, acetic anhydride, 100 °C, 69% yield; (b) (i) P₂S₅, Py, 120 °C, (ii) NH₃, MeOH, 80 °C, 80% crude yield; (c) ethoxycarbonyl isothiocyanate, EDC, HMDS, MeCN, rt, 72 h, 40% yield; (d) 1 M NaOH, MeOH, 80 °C, 1 h, 90% crude yield.