Crystal structure of a mixture of lupeol-acetate tautomers isolated from Hieracium plumulosum A. Kerner, Asteraceae

Abstract

Transparent single crystals obtained from the purified extract of the plant Hieracium plumulosum A. Kerner, Asteraceae were submitted to spectroscopic and X-ray studies. The spectroscopic (infra-red, mass spectrometry, 1H and 13C nuclear magnetic resonance) investigations suggested the presence of two isomers of lupeol acetate (C 32H 52O 2) with tautomeric relationship. Their mixture crystallizes in the monoclinic system with space group P2 1: a = 19.170(2) A ̊ , b = 8.128(1) A ̊ , c = 21.832(2) A ̊ , β = 90.24(1)°, V = 3401.7(6) A ̊ 3 , Z = 4 (two molecules in the asymmetric unit), D x = 1.095 g cm −3, μ(Cu K α ) = 0.488 mm −1. The structure was solved by direct methods and refined to a final R = 0.071 for 6636 reflections with I > 2 σ( I). The bond lengths around C20 ( C60) are between single and double bonds indicating the superposition of the tautomers, i.e., lup-20(29)-en-3β-ol and lup-19(20)-en-3β-ol acetates. They are distributed with different ratio in the symmetry-independent positions. In addition, the difference maps revealed that toluene solvent molecules are also captured in the lattice. They substantially hindered the refinement of atomic positions in particular for the C19 ( C59) functions.