The polyether and macrolide antibiotics: biogenetic analysis and structural correlations

Towards better syntheses of partially methylated carbohydrates?

We give an overview of the reported synthetic strategies towards partially methylated glycosides and discuss how better protocols could stem from catalytic site-selective transformations of carbohydrates and cleaner methylation reagents.

Polycyclizations of Ketoesters: Synthesis of Complex Tricycles with up to Five Stereogenic Centers from Available Starting Materials

Here we present a polycyclization of oxotriphenylhexanoates. The polycyclization is governed by electronic effects, and three major synthetic paths have been established leading to stereochemically complex tricyclic frameworks with up to five stereogenic centers. The method is compatible with an array of functional groups, allowing pharmacophoric elements to be introduced post cyclization.

Biosynthesis of ebelactone A: isotopic tracer, advanced precursor and genetic studies reveal a thioesterase-independent cyclization to give a polyketide β-lactone

Macrocyclization of polyketides generates arrays of molecular architectures that are directly linked to biological activities. The four-membered ring in oxetanones (β-lactones) is found in a variety of bioactive polyketides (for example, lipstatin, hymeglusin and ebelactone), yet details of its molecular assembly have not been extensively elucidated. Using ebelactone as a model system, and its producer Streptomyces aburaviensis ATCC 31860, labeling with sodium [1-(13)C,(18)O2]propionate afforded ebelactone A that contains (18)O at all oxygen sites. The pattern of (13)C-(18)O bond retention defines the steps for ebelactone biosynthesis, and demonstrates that β-lactone ring formation occurs by attack of a β-hydroxy group onto the carbonyl moiety of an acyclic precursor. Reaction of ebelactone A with N-acetylcysteamine (NAC) gives the β-hydroxyacyl thioester, which cyclizes quantitatively to give ebelactone A in aqueous ethanol. The putative gene cluster encoding the polyketide synthase (PKS) for biosynthesis of 1 was also identified; notably the ebelactone PKS lacks a terminal thioesterase (TE) domain and no stand alone TE was found. Thus the formation of ebelactone is not TE dependent, supporting the hypothesis that cyclization occurs on the PKS surface in a process that is modeled by the chemical cyclization of the NAC thioester.

Epoxide-hydrolase-initiated hydrolysis/rearrangement cascade of a methylene-interrupted bis-epoxide yields chiral THF moieties without involvement of a "cyclase"

In contrast with electrophilic enzyme-catalysed cyclisations in terpenoid biosynthesis, cyclisations of tetrahydrofuran moieties found in several groups of natural products, such as annonaceous acetogenins, neurofurans and phytooxylipins, appear to proceed through a nucleophilic cascade mechanism starting from bis-epoxy fatty acid precursors. This hypothesis was verified by epoxide-hydrolase-catalysed hydrolytic ring-opening/cyclisation cascades starting from a methylene-interrupted meso-bis-epoxide model substrate, which furnished the corresponding THF products with excellent de and ee values. Molecular modelling showed that the points of enzyme attack were consistent with the stereospecificities of the enzymes, whereas the stereochemical courses of the cyclisation were solely governed by Baldwin's rules and did not invoke the involvements of a "cyclase".

Studies in marine polypropionate synthesis: total synthesis of (-)-baconipyrone C

[reaction--see text] An asymmetric total synthesis of the unusual siphonariid metabolite, (-)-baconipyrone C (3), is described. Key steps included a tin(II)-mediated aldol coupling for the preparation of the carboxylic acid 17 and two different boron-mediated aldol additions leading to alcohol 8. Ester formation using modified Yamaguchi conditions gave 24, leading on PMB deprotection to (-)-baconipyrone C.

Acid-Mediated Conversion of Methylene-Interrupted Bisepoxides to Tetrahydrofurans: A Biomimetic Transformation

The acid-mediated transformation of syn and anti methylene interrupted cis,cis and cis,trans bisepoxides to tetrahydrofurans is high yielding, and demonstrates both regioselectivity and stereoselectivity. Trans,trans methylene interrupted bisepoxides do not yield tetrahydrofurans under the same conditions.

Stereospecific acyl transfers on the erythromycin-producing polyketide synthase

During assembly of complex polyketide antibiotics like erythromycin A, molecular recognition by the multienzyme polyketide synthase controls the stereochemical outcome as each successive methylmalonyl-coenzyme A (CoA) extender unit is added. Acylation of the purified erythromycin-producing polyketide synthase has shown that all six acyltransferase domains have identical stereospecificity for their normal substrate, (2S)-methylmalonyl-CoA. In contrast, the configuration of the methyl-branched centers in the product, that are derived from (2S)-methylmalonyl-CoA, is different. Stereoselection during the chain building process must, therefore, involve additional epimerization steps.

Vitamins as effectors of monensin production by Streptomyces cinnamonensis

Vitamins added to submerged Streptomyces cinnamonensis cultures stimulated the production of monensins. Vitamins B2, B3, B5 and B12 enhanced the production by about 50%, vitamins B1 and B6 by 100%. The addition of biotin in optimal concentration resulted in more than 3-fold increase in total production.