Diversonolic esters

In 1983 Holker, Simpson, and O Brien reported the isolation of the diversonolic esters (972, 973) (Fig. 13.15), which they obtained from Penicillium diversum, along with lichexanthone (840a) and several other known compounds (634). The authors performed a structural analysis for these new compounds based on methyl-ation, proton-NMR analysis, chelate ferric effects, and other spectroscopic techniques, which led to the assignment of the structures of these new compounds as 970 and 971. These substances appear reminiscent of hemisecalOTiic acids (ergochrome monomers), but the structures originally proposed were later found to be incorrect (see below). 

Twenty-five years later, the racemic syntheses of diversonolic esters (972, 973) were reported by Nicolaou and Li, almig with the synthesis of blennolide C (897) and diversonol (932) (see entries above) (S5). In a twist of good fortune, the use of MOM-protecting groups as an alternative to allyl groups (see diversonol synthesis) 

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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). 

In 2008, Nicolaou and Li reported the synthesis of blennolide C (897), in addition to the racemic synthesis of diversonol (932) and the diversonolic esters... 

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). 

Scheme 13.18 Nicolaou s synthesis of a- and P-diversonol esters. Reagents and conditions a) Bt2, EtsN, 90% b) DiBAL-H, 95% c) MeLi, t-BuLi then 976 d) IBX, 78% (2 steps) e) HC104(aq), 90%...

Holker JSE, O Brien E, Simpson TJ (1983) The Structures of Some Metabolites of Penicil-lium diversum a-and (3-Diversonolic esters. J Chem Soc Perkin Trans 1 1365... 

Nicolaou KC, Li A (2008) Total Syntheses and Structural Revision of a- and p-Diversonolic esters and Total Syntheses of Diversonol and Blennolide C. Angew Chem Int Ed 47 6579... 

The final step is the methoxylation of acyl-Pd species 313 to afford ester 309, which is further converted into (—[-diversonol (185), the enantiomer of the natural product. However, use of (i ,R)-Bn-BOXAX (ewt-310), in the domino process allows for the preparation of (+)-diversonol (e t-185). Recently, a similar procedure has been applied to the first enantioselective total synthesis of blennolide A (317) starting from resorcinol (314) via the intermediates 315 and 316 (Scheme 14.49)... 

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