Biosynthetic origin of the carbon skeleton of simaomicin .alpha., a hexacyclic xanthone antibiotic

Kibdelones: Novel Anticancer Polyketides from a Rare Australian Actinomycete

“…This observation was further supported by a diagnostic downfield shift for the N À Me lactam carbonyl carbon atom (d C = 165.0 ppm). [5] Detailed 2D NMR spectroscopic analysis of kibdelone C (3) (see the Supporting Information) revealed correlation sequences consistent with those observed for kibdelone A (1), and supported the structure as assigned (less stereochemistry). Once again, unambiguous confirmation of the structure assigned to kibdelone C (3) was obtained on chemical interconversion with kibdelone A (1) (see below).…”

“…Complete relative stereostructures were assigned to kibdelones A-C (1-3), kibdelone B rhamnoside (5), 13-oxokibdelone A (7), and 25-methoxy-24-oxokibdelone C (8) on the basis of detailed spectroscopic analysis and chemical interconversion, as well as mechanistic and biosynthetic considerations. Under mild conditions, kibdelones B (2) and C (3) undergo a facile equilibration to kibdelones A-C (1-3), while kibdelone B rhamnoside (5) equilibrates to a mixture of kibdelone A-C rhamnosides (4)(5)(6). A plausible mechanism for this equilibration is proposed and involves air oxidation, quinone/hydroquinone redox transformations, and a choreographed sequence of keto/enol tautomerizations that aromatize ring C via a quinone methide intermediate.…”

“…The upfield 13 C NMR chemical shift for the C-1 lactam carbonyl (d C = 156.36 ppm) was consistent with it being peri to a carbonyl/quinone (compare xantholipin) [2] as opposed to the sp 2 -hybridized carbon atom of a hydroquinone (compare actinoplanones and simaomicin). [3,4,5] The 13 C NMR shift for one of the remaining unassigned carbon atoms was indicative of a quinone (d C = 183.4 ppm), and was attributed to C-3-thereby defining ring B as a p-quinone. The four structure fragments described in Figure 1 account for all but three quaternary sp 2 carbon atoms.…”

“…This issue was successfully resolved by NMR spectroscopic analysis (see the Supporting Information) of the hydroquinone 10 obtained by the in situ (NMR tube) reduction of kibdelone A (1) with sodium dithionite. [5] HRESI(+)MS analysis of kibdelone B (2) revealed a pseudo molecular ion consistent with a molecular formula (C 29 H 26 35 ClNO 10 , Dmmu = À1.5) requiring 17 DBE, and indicative of a dihydro analogue of kibdelone A (1). While it was tempting to speculate that kibdelone B (2) was the hydroquinone analogue 10 prepared by reduction of 1, this proved to not be the case, with the 1 H NMR spectroscopic data for 2 displaying only a single chelated phenolic OH attributable to 6-OH.…”

Profiling, Isolation, Chemical Characterisation and Distribution of Gentianaceae Constituents

“…The distribution of xanthones with specific oxidation patterns is important to support taxonomic classification through chemotaxonomic relationships, as will be discussed below. In ferns, lichens and fungi, the biosynthesis of xanthones is different, as xanthones in these groups stem are derived from acetate precursors (Carter et al 1989;Hill et al 1982). Xanthones appear as aglycones, O-glycosides and C-glycosides in the Gentianaceae ( Fig.…”