Natural products structural characterization

In the first of several natural product structure characterization reports published in the Journal of Natural Products during 2003, Schmidt et a/.163 reported the isolation and characterization of a series of tetramic acids and pyridone alkaloids, militarinones B, C, and D from the insect pathogenic fungus Poaecilomyces mi-litaris. The structure of militarinone D (82) is shown. 

Finally, in the only natural product structure characterization report during... 

In 2006, Milosavljevic and co-workers64 reported a study of the complete 4H and 13C NMR assignment of a new triterpenoid saponin, leucantho-side-A (13), from Cephalaria leucantha L. In the course of determining the structure and assigning the spectra, the authors made extensive use of the normal ensemble of 2D NMR experiments in use for the characterization of natural product structures HSQC, HMBC, DQF-COSY, TOCSY, and NOESY. The authors supplemented the aforementioned list of experiments with 2D /-resolved, DINE-(Double INEPT-Edited)-HSQC, and INADEQUATE spectra. The authors made no mention of the use of the connectivity information derived from the 1,1-ADEQUATE spectrum in the assembly of the triterpene nucleus of the molecule but reported extensive tabulations of the 1,1-ADEQUATE correlations that were used to sequence and assign the saccharide resonances of the tri- and di-saccharide sub-units, 14 and 15, respectively, linked to the triterpene nucleus. 

Several sulfated polyhydroxysterols have been recently isolated from sponges. Most of these natural products are characterized by the 2p,3a,6a-tri-0-sulfate functions together with additional alkylation in the side chain. These steroids are of interest not only because of their structures but... 

Applications of 3 mm probe technology in the characterization of natural product structures continued unabated in 1999, with a number of papers appearing in the Journal of Natural Products that employed this technology. The first of these reports also came from the author s laboratory and described the characterization of a new oxobenzylisoquinoline alkaloid, thalprezwalskiinone (49) from Thalictrum prezwalskii.112... 

The three types of PKSs described here, the enediyne PKS, the C-0 bondforming PKS, and the AT-less PKS, are only representive examples that reside outside the archetypical PKS paradigms. Continued exploration on the mechanism of polyketide biosynthesis will undoubtly uncover more unusual PKSs. These novel PKSs, in combination with the archetypical ones, will ultimately enhance the toolbox available to facilitate combinatorial biosynthesis and production of iinnatural natural products. The full realisition of the potential embodied by combinatorial biosynthesis of PKSs for natural product structural diversity, however, depends critically on the fundamental characterization of PKS structure, mechanism, and catalysis. 

The increase in sensitivity afforded by the proton-detected HMBC experiment revolutionized structure elucidation studies. The utilization of HMBC data in the characterization of alkaloid structures has been reviewed [70] and is also treated in a more general review of the application of inverse-detected methods in natural products structure elucidation [71]. Other applications of the experiment are quite... 

MS, however, offers a series of obvious advantages over NMR spectroscopy. They include superior resolution (if high-resolution MS detectors are employed), superior sensitivity, more facile and economical coupling to online separation techniques (LC and GC), analysis of samples in aU three states of matter (gas, liquid, and solid), no molecular weight restrictions, and finally the ability to trap selected ions in the gas phase prior to characterization, thereby allowing a second dimension of separation. For complex samples containing natural products as minor constituents, especially, structure elucidation by MS constitutes the only practical access to natural product structures. 

The isolation and structural characterization of plant constituents are often difficult and time-consuming. Consequently, at the present time, only a small percentage of the total number of natural plant products has been identified. Early in the development of natural product chemistry it was apparent to many botanists and chemists that plants might be characterized and classified on the basis of their chemical constituents. The distribution of chemical constituents among plant families is presented by Swain (139) and Hegnauer (66). 

A highlight in the application of RCM methodology in natural product synthesis is Hirama s total synthesis of ciguatoxin CTX3C (183) [90], including the more recent improved protective group strategy, as depicted in Scheme 34 [90b]. The structure of 183 spans more than 3 nm and is characterized by 12 six- to nine-membered trans-fused cyclic ethers and a spiroannulated terminal tetra-... 

Figure 7.23 shows the prototype diazobenzo[Z ]fluorene-based natural products kinamycin A and prekinamycin. The kinamycin A-D family were first isolated from Streptomyces murayamaensis, but the structures were incorrectly characterized as having a cyanobenzo[Z ]carbazole ring. Since the initial discovery of the kinamycins, many new analogues have been discovered from natural sources.88-92... 

Gu, J.Q., Nguyen, K.T., Gandhi, C. et al. (2007) Structural characterization of daptomycin analogues A21978C1 3(D-Asn11) produced by a recombinant Streptomyces roseosporus strain. Journal of Natural Products, 70, 233. 

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