Synthesis, spectroscopic and theoretical study of isoquinoline alkaloid dehydrosalsolinol base and its triacetylated derivative

Abstract

Dehydrosalsolinol and its triacetylated derivative i.e. are synthesized, isolated, and the spectra and structure are elucidated. Solid-state linear-dichroic infrared spectroscopy, 1H and 13C NMR, TGV and DTA methods are applied. The potential energy surface of both compounds are explored with DFT correlation functional B3LYP method and MP2, using the 6-311++G ∗∗ basis set. Single point calculations are performed at levels up to B3LYP/6-311++G ∗∗// MP2/6-311++G ∗∗ and MP2/6-311++G ∗∗. Two isomers of dehydrosalsolinol are located and the relative energies are determined. The planar quinoidal structure with strong intramolecular OH…O hydrogen bond and dipole moment of 7.392 D is more stable than the diphenolic form. Calculated vibrational frequencies are used to determine the type of molecular motions associated with each of the experimental IR-bands observed. IR-spectroscopic data show that dehydrosalsolinol exists in solid-state as the quinoide form in contrast to solution, where 1H NMR data indicate a keto-diphenolic equilibrium. Comparison between calculated and experimental IR-frequencies of triacetyl derivative shows the presence of two phenolic ester carbonyl and one-amide groups and an exocyclic double bond. The same conclusions are obtained by corresponding 1H NMR data.