Fig. 2. Synthesis and characterization of H23. (A) Synthesis of H23; reagents and conditions: (a) ethyl (S)-lactate (for (R)-5) or ethyl (R)-lactate (for (S)-5), DIAD, PPh3, toluene, 0 → 20 °C; (b) H2, Pd/C, EtOAc, 20 °C; (c) ethyl (S)-lactate (for (R,R)-7) or ethyl (R)-lactate (for (S,S)-7), DIAD, PPh3, toluene, 0 → 20 °C; (d) LiAlH4, THF, 0 → 20 °C; (e) TsCl, pyridine, CH2Cl2, 0 → 20 °C; (f) ZnCl2, SOCl2, CH2Cl2, 20 °C; (g) K2CO3, CH3CN, reflux. For clarity, the synthetic scheme only displays the structures of all (R)-derivatives; yields of all (S)-derivatives are indicated in square brackets. (B) ECD spectra of (R,R,R,R)-H23 and (S,S,S,S)-H23 (c = 3 × 10−5 M in CHCl3/CH3CN, 1 : 1, v/v). (C) Negative chemical shift region of the 1H NMR spectrum of (R,R,R,R)-H23 (400 MHz, c = 10−3 M in CDCl3/CD3CN, 1 : 1, v/v) with assignment of protons a–c. (D) Front view of the DFT-optimized structure of (R,R,R,R)-H23 at the B3LYP/6-311+G(d) level. Color code: white = hydrogen, grey = carbon, blue = nitrogen, red = oxygen.
