Total synthesis of rhizoxin D

Isolation and identification of rhizoxin analogs from Pseudomonas fluorescens Pf-5 by using a genomic mining strategy

The products synthesized from a hybrid polyketide synthase/nonribosomal peptide synthetase gene cluster in the genome of Pseudomonas fluorescens Pf-5 were identified using a genomics-guided strategy involving insertional mutagenesis and subsequent metabolite profiling. Five analogs of rhizoxin, a 16-member macrolide with antifungal, phytotoxic, and antitumor activities, were produced by Pf-5, but not by a mutant with an insertion in the gene cluster. The five rhizoxin analogs, one of which had not been described previously, were differentially toxic to two agriculturally important plant pathogens, Botrytis cinerea and Phytophthora ramorum. The rhizoxin analogs also caused swelling of rice roots, a symptom characteristic of rhizoxin itself, but were less toxic to pea and cucumber roots. Of the rhizoxin analogs produced by Pf-5, the predominant compound, WF-1360 F, and the newly described compound 22Z-WF-1360 F were most toxic against the two plant pathogens and three plant species. These rhizoxin analogs were tested against a panel of human cancer lines, and they exhibited potent but nonselective cytotoxicity. This study highlights the value of the genomic sequence of the soil bacterium P. fluorescens Pf-5 in providing leads for the discovery of novel metabolites with significant biological properties.

Antimitotic Rhizoxin Derivatives from a Cultured Bacterial Endosymbiont of the Rice Pathogenic Fungus Rhizopus microsporus

The potent antimitotic polyketide macrolide rhizoxin, the causal agent of rice seedling blight, is not produced by the fungus Rhizopus microsporus, as has been believed for over two decades, but by endosymbiotic bacteria that reside within the fungal mycelium. Here we report the successful isolation and large-scale fermentation of the bacterial endosymbiont ("Burkholderia rhizoxina") in pure culture, which resulted in a significantly elevated (10x higher) production of antimitotics. In addition to several known rhizoxin derivatives, numerous novel natural and semisynthetic variants were isolated, and their structures were fully elucidated. Cell-based assays as well as tubulin binding experiments revealed that methylated seco-rhizoxin derivatives are 1000-10000 times more active than rhizoxin and thus rank among the most potent antiproliferative agents known to date. Furthermore, more stable didesepoxy rhizoxin analogues were obtained by efficiently inhibiting a putative P-450 monooxygenase involved in macrolide tailoring.

A total synthesis of the antitumour macrolide rhizoxin D

An enantioselective synthesis of rhizoxin D, isolated from the plant pathogenic fungus Rhizopus chinensis, is described. The overall strategy is based on elaboration of the delta-lactone-substituted vinyl stannane and the phosphonate-substituted vinyl iodide, followed by their coupling to the core 16-membered macrolide via a sequential intermolecular Horner-Wadsworth-Emmons olefination, leading to (50), and by an intramolecular Stille reaction. The triene oxazole-containing side chain in rhizoxin D is then introduced using the phosphine oxide in an E-selective Horner-Wittig reaction with the macrolide aldehyde.

Stereoselective Total Synthesis of Antitumor Macrolide (+)-Rhizoxin D

[structure: see text] A convergent, stereoselective total synthesis of the macrolide antitumor agent rhizoxin D is described. (+)-DIPCl-promoted asymmetric aldol reaction, Evans-Tishchenko 1,3-diol synthesis, modified Julia coupling, and Horner-Wadsworth-Emmons reactions are featured.

Enantioselective total synthesis of the antitumor macrolide rhizoxin D

The convergent, highly enantioselective synthesis of rhizoxin D, a natural product possessing potent antitumor and antifungal bioactivity, is described. The C(1)-C(9) fragment of the molecule was synthesized utilizing a threefold pseudosymmetric intermediate ultimately derived from gamma-butyrolactone. The central core of rhizoxin D was prepared via a chiral resolution/asymmetric aldol protocol. Several methods for the generation of the polyene fragment were explored, and the side-chain was ultimately prepared from serine in six steps. The unification of the left and right wings of the molecule was achieved using a one-step olefination protocol, and the macrocyclization was carried out using a Horner-Emmons olefination at the C(2)-C(3) olefin.

Total synthesis of rhizoxin D, a potent antimitotic agent from the fungus Rhizopus chinensis

Rhizoxin D (2) was synthesized from four subunits, A, B, C, and D representing C3-C9, C10-C13, C14-C19, and C20-C27, respectively. Subunit A was prepared by cyclization of iodo acetal 21, which set the configuration at C5 of 2 through a stereoselective addition of the radical derived from dehalogenation of 21 at the beta carbon of the (Z)-alpha,beta-unsaturated ester. Aldehyde 29 was obtained from phenylthioacetal 24 and condensed with phosphorane 30, representing subunit B, in a Wittig reaction that gave the (E,E)-dienoate 31. This ester was converted to aldehyde 33 in preparation for coupling with subunit C. The latter in the form of methyl ketone 55 was obtained in six steps from propargyl alcohol. An aldol reaction of 33 with the enolate of 55 prepared with (+)-DIPCl gave the desired beta-hydroxy ketone 56 bearing a (13S)-configuration in a 17-20:1 ratio with its (13R)-diastereomer. After reduction to anti diol 57 and selective protection as TIPS ether 58, the C15 hydroxyl was esterified to give phosphonate 59. An intramolecular Wadsworth-Emmons reaction of aldehyde 62, derived from delta-lactone 60, furnished macrolactone 63, which was coupled in a Stille reaction with stannane 68 to give 2 after cleavage of the TIPS ether.

Total Synthesis of (−)-Ratjadone

[reaction in text] A convergent asymmetric synthesis of (-)-ratjadone (1) has been achieved, confirming the assignments of stereochemistry for the naturally occurring antifungal metabolite.