13C, 15N NMR and CP-MAS as well as FT-IR and PM5 studies of Schiff base of gossypol with l-phenylalanine methyl ester in solution and solid

The Schiff bases of racemic gossypol with l-phenylalanine amino acid methyl ester (GSBA) was studied by FT-IR, 13C and 15N NMR spectroscopy as well as by the PM5 semiempirical method. The spectroscopic methods have provided clear evidence that GSBA exists in the solid state and in solution as enamine-enamine tautomer. The existence of diastereoisomers is very well evidenced in the 13C NMR spectra in the solution by the double character of all carbon atoms' signals of the naphthalene rings. The structure of GSBA and the hydrogen bonds within this structure are discussed.