Scheme 1.
Chemical synthesis of compounds 1–4. Reagents and conditions: a) ε-caprolactone, SnOct, 130 °C; b) thioctic acid, DCC, DMAP, Et3N, CH2Cl2; c) i) TsCl, pyridine, CH2Cl2, ii) NaN3, DMF, 80 °C; d) Ph3P, THF, H2O; e) i) 4-(N’-tert-butoxycarbonyl-hydrazino)-4-oxo-butyric acid, DCC, DMAP, CH2Cl2, ii) 20% TFA, CH2Cl2.