Tetrahydroxanthones

Vinylchromones undergo a [4+2]-cycloaddition with pyrrolidine enamines, generated in situ, leading to xanthones substituted in the C-ring. The reaction proceeds by way of a methylidene tetrahydroxanthone 76 (Scheme 52) <00JCS(P1)3732>. 

I, 2-alkyl shift provides a convenient route for the synthesis of chromones 335 (Eq. 46), tetrahydroxanthones (336, n = 1) and higher homologs (336, n = 2) (Eq. 47). Both heat and ultrasonic conditions work well to effect this alkyl shift (94SC2637). 

Chromone-3-carbaldehydes undergo Diels-Alder reactions with ort o-benzoquinodimethane 819 and deformyla-tion to yield initially benzo[b]-1,6,6a, 12a-tetrahydroxanthones 820, which are easily transformed into their corresponding benzo[ ]xanthones upon treatment with I2-DMSO (Scheme 232) <2002T105>. 

The xanthone family as found in fungi has four sub-classes, and fully aromatized, dihydroxanthones, tetrahydroxanthones, and hexahydroxanthones all occur in Nature, and can be found also in combination with other xanthone components as either homodimers or heterodimers. The numbering of the xanthone nucleus is in accordance with lUPAC recommendations. 

Tetrahydroxanthones from fungi include the blennolides (895-901), dihydroglobo-suxanthone (931) (632), diversonol (932) and the diversonolic esters (972, 973) (Fig. 13.15). Tetrahydroxanthones are the monomeric components of interesting mycotoxins such as the beticolins (1063-1070) and the secalonic acids (1034-1046) (see Sect. 13.5.3). 

Krohn and co-workers reported in 2009 the isolation of 3,4-dihydroglobosuxanthone A (931) as a result of their reinvestigation of a. Microdiplodia sp. (559). As with the diversonolic esters (A and B, 972, 973) (Fig. 13.15) this compound has the methyl ester located at the C-1 position, rather than the usual C-4a position, as foxmd for remaining monomeric and dimeric members of the tetrahydroxanthone family. This compound was demonstrated to have potent antibacterial activity against Escherichia coli. Bacillus megaterium, and Chlorellafusca (Fig. 13.11). 

The interconversion between the four pamafungin forms para- and orthooxygen coimection giving parnafungins A and B), and syn (A1 (1005) and B1 (1007), major diastereomers) and anti (A2 (1005 ) and B2 (1008), minor diastereomers), is due to a retro-oxa-Mlc/taeZ-addition, a process that is seen frequently among tetrahydroxanthones. This interconversion was blocked by meth-ylation of the C-15 alcohol, which allowed the structure of a derivative of pamafungin A1 (1005) to be established by X-ray crystallography (Fig. 13.24). 

Rugulotrosins A and B (1082 and 1083, Fig. 13.36) were reported in 2004 after being isolated from cultures of a Penicillium sp. from soil samples. Rugulotrosin A is a symmetrical 2,2 - ortho-ortho)-caap eA dimer of tetrahydroxanthone subunits,... 

Rezanaka T, Sigler K (2007) Hirtusneanoside, an Unsymmetrical Dimeric Tetrahydroxanthone from the Lichen Usnea hirta. J Nat Prod 70 1487... 

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