a The a series has the R stereochemistry (derived from D-arginine) and the b series has the S stereochemistry (derived from L-arginine) at the chiral center (*). Steps (yields for D-series): (a) Fmoc-NHCH2CHO, MeOH, 0 °C, 4h, then acetic acid, NaBH3CN, 30 min, 57%; (b) Thymine-1-acetic acid, 3-hydroxy-1,2,3-benzotriazine-4(3H)-one, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide, dimethylformamide, 40 °C, 12 h, 60%; (c) Pd(PPh3)4, N-ethyl aniline, tetrahydrofuran, rt, 1h, 81%.