Effect of Solvent on the Optical Rotation of Azatryptophan Derivatives

Chirally pure enantiomers of differently protected 7-azatryptophan derivatives (R-3c, S-3c, R-3i, S-3i, R-3m, S-3m, R-3aa, and S-3aa) were synthesized, which showed solvent-dependent optical rotation. The obtained results not only exhibited changes in the values but also showed the variation in sign (- or +) with the different solvents studied. The change in optical rotation value was essentially attributed to the electron-donating property, which can be correlated to the donor number of the solvents. There are two types of hydrogen bonds, intramolecular (i.e., form within the structure) and intermolecular (i.e., form with external groups such as solvents). These hydrogen bonds are responsible for the value and sign variations, and ¹H NMR experiments were used to further characterize them. The NMR data suggested that hydrogen bond formation is occurring between the Fmoc NH group vicinal to the chiral center and donor group of the corresponding solvent.