Synthesis, solvatochromic properties, and dipole moments of Fmoc-l-alaninol
Abstract
An efficient protocol for the activation of N-(9-Fluorenylmethoxycarbonyl)-l-alanine [Fmoc-Ala-OH] employing 1-[(1-(cyano-2-ethoxy-2-oxoethylideneaminooxy)-dimethylamino-morpholinomethylene)] methanaminium hexa-fluorophosphate [COMU] and its reduction into (9H-fluoren-9-yl)methyl 1-hydroxypropan-2-ylcarbamate [Fmoc-l-alaninol] using sodium borohydride has been described. The method is trouble-free, quick and free from racemization. Fmoc-l-alaninol was characterized by 1H and 13C NMR, and mass spectral studies. The electronic absorption and fluorescence emission spectra of Fmoc-l-alaninol have been studied in solvents of different polarities, and the data were used to study the solvatochromic properties. The spectral variations were analyzed by the linear solvation energy relationship concept to visualize the nature and extent of solvent–solute interactions. Experimental values of ground-(μg) and excited-state (μe) dipole moments of Fmoc-l-alaninol were calculated by the solvatochromic shift method, and theoretical μg values were evaluated by quantum chemical calculations using Gaussian 03 and Chem3D Ultra 8.0. The higher value of excited-state dipole moment than the ground-state value indicates a substantial redistribution of π–electron densities in a more polar excited-state. Also, fluorescence emission peak undergoes a hypsochromic shift with increase in the polarity of the solvent, confirming n?→?π* transition.