Synthesis of 8-C-glucosylflavones

The syntheses of orientin, parkinsonin A, isoswertiajaponin, and parkinsoni n B, which are 8-C-beta -D-glucopyranosyl-3',4',5,7-tetrahydroxyflavone, 5- methyl orientin, 7-methyl orientin, and 5,7-dimethyl orientin, respectively , are reported herein. The C-glucosyl phloroacetophenone derivatives were o btained via a regio- and stereoselective O --> C glycosyl rearrangement. Al dol condensation of the C-glucosyl phloroacetophenone derivatives with 3,4- bisbenzyloxybenzaldehyde afforded the corresponding C-glucosylchalcones. Co nstruction of the flavone system by reaction with I-2-Me2SO followed by the elimination of the 5-benzyl protecting group in the flavone structure, yie lded an orientin derivative and a isoswertiajaponin derivative. Methylation of the orientin derivatives with dimethyl sulfate afforded the parkinsonin A derivative, the isoswertiajaponin derivative, and the parkinsonin B deri vative. Finally, hydrogenolysis of these C-glucosylflavone derivatives led to the four 8-C-glucosylflavones. The NMR spectra of these C-glucosylflavon es showed a duplication of signals corresponding to a major rotamer, along with a minor one. Based on NOESY experiments in Me2SO at ambient temperatur e, they adopted conformations in which the H-2" and H-4" protons in the glu cose moiety were oriented toward the B-ring in the flavone structure. (C) 2 001 Elsevier Science Ltd. All rights reserved.