Synthesis and structural elucidation of xanthonolignoids: trans-(+/-)-kielcorin B and trans-(+/-)-isokielcorin B

This paper reports the synthesis and characterization of two isomeric xanth onolignoids - trans-(+/-)-kielcorin B and trans-(+)-isokielcorin B. The syn thetic approach is based on the oxidative coupling of 2,3-dihydroxy-4-metho xyxanthone with coniferyl alcohol in the presence of silver carbonate. The structural elucidation of these compounds was achieved by extensive NMR stu dies:H-1 and C-13 NMR spectra, homonuclear correlation spectroscopy (COSY, COSYLR), heteronuclear correlation spectroscopy (HETCOR), nuclear Overhause r effect (NOE) and one-dimensional selective INEPT. The NMR spectroscopic t echniques used in this study led to an unambiguous characterisation of tran s-(+/-)-kielcorin B and trans-(+/-)-isokielcorin B, which includes the assi gnment of all proton and carbon resonances and the establishment of the sub stituents orientation and protons configurational relationship on their 1,4 -dioxane ring.