Oxidative coupling. Part VII. Biogenetic type synthesis of naturally occurring xanthones

Abstract

The co-occurrence of isomeric xanthones in certain plant extracts suggests their derivation from a common hydroxylated benzophenone. ln vitro oxidation of some of these benzophenones produces xanthone mixtures corresponding to oxidative coupling occurring para and ortho or para only to an activating hydroxy-group. The oxidations can also be carried out enzymically.

Regioselective oxidative phenol couplings of 2,3′,4,6-tetrahydroxybenzophenone in cell cultures of Centaurium erythraea RAFN and Hypericum androsaemum L.

“…After elution with chloroform:ethanol (9:1, v:v) further puri®cation was performed by HPLC using a reversed phase-(RP)-8 column and methanol (17%) as a solvent. The UV spectrum of the product agreed with published data (Atkinson and Lewis 1969). (84).…”

“…The latter step either proceeds spontaneously or is catalyzed by a membrane-bound dehydratase. Enzymic benzophenone cyclizations have previously been achieved with a fungal laccase and a horseradish peroxidase; however, these enzymes lacked regioselectivity (Atkinson and Lewis 1969).…”

“…1,3,7-Trihydroxyxanthone was prepared from THBP as described by Atkinson and Lewis (1969). The chemical coupling was carried out at pH 9.0 using potassium hexacyanoferrate(III) as an oxidant.…”

Naturally occurring pentaoxygenated, hexaoxygenated and dimeric xanthones: a literature survey

“…Initially these attempted to interrelate the observed oxygen pattern of natural xanthones and correlate them with recognized oxygenation patterns. In general it is seem that ring A and attached CO group are provided by the shikimic acid pathway whereas ring B arises via the acetate-malonate polyketide route 5,16,148 . Therefore Locksley et al 100 reported the significance of Maclurin in xanthone biosynthesis and the biogenetic-type synthesis of xanthones from their benzophenone precursors 101 .…”

Synthesis and Photochromic Behavior of Novel Annelated 2H-Chromenes Derived from Hydroxy-9H-xanthen-9-ones

“…M.p. 233 ± 2368 ( [18]: 230 ± 2338). 1 H-NMR ((D 6 )Acetone): 9.15 (s, OH); 8.29 (dd, J 7.8, 1.8, HÀC(8)); 7.89 (ddd, J 8.0, 7.5, 1.8, HÀC(6)); 7.76 (dd, J 7.8, 2, HÀC(1)); 7.66 (dd, J 8.0, 1.2, HÀC(5)); 7.50 (ddd, J 7.8, 7.5, 1.2, HÀC(7)); 7.40 (dd, J 7.8, 2, HÀC(3)); 7.32 (t, J 7.8, HÀC(2)).…”