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.     

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.     

Stereoselective synthesis of the C1-C10 fragment of rhizoxin D

A novel approach towards the synthesis of the C1-C10 fragment of the biologically active antimitotic agent rhizoxin D is described. The synthesis involves a stereoselective Michael addition reaction of lithium diallyl cuprate with an α,β-unsaturated six membered lactone.     

Cover Picture: Polyketide‐Chain Branching by an Enzymatic Michael Addition (Angew. Chem. Int. Ed. 27/2009)

The fungus Rhizopus microsporus …? ; shown in the micrograph on the cover picture, harbors the endosymbiotic bacteria Burkholderia rhizoxinica that produces the antimitotic polyketide macrolide rhizoxin (see structure). In their Communication on page 5001 ff. , C. Hertweck and co‐workers show that the enzyme rhizoxin polyketide synthase is uniquely capable of introducing a β branch (highlighted by the magnifying glass) by a Michael‐type addition of a malonyl unit to an acryloyl intermediate.

     

Total synthesis of (+)-dactylolide

[reaction: see text] The total synthesis of the new cytotoxic marine macrolide (+)-dactylolide (1) has been achieved in nine steps from known vinyl bromide (-)-AB. In addition, (+)-zampanolide (2) has been converted to (+)-dactylolide (1) via thermolysis.     

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.     

Stereocontrolled total synthesis of (+)-concanamycin F: the strategic use of boron-mediated aldol reactions of chiral ketones

A highly stereocontrolled total synthesis of the 18-membered macrolide (+)-concanamycin F, a potent inhibitor of vacuolar ATPases, is described that proceeds in 5.8% yield over 26 steps. The three key fragments, C1-C13 vinyl iodide, C14-C22 vinyl stannane and C23-C28 aldehyde, were efficiently constructed using asymmetric boron-mediated aldol reactions of appropriate chiral ketone building blocks. The nature of the silyl protection of the C7/C9 hydroxyls proved to be critical for achieving macrocyclisation, with TES ethers being superior to a cyclic silylene derivative. Following a Liebeskind-Stille cross-coupling reaction between the C1-C13 vinyl iodide and C14-C22 vinyl stannane fragments to assemble the (12E,14E)-diene, a modified Yamaguchi macrolactonisation delivered the requisite 18-membered macrocyclic core. This advanced intermediate was also obtained by an alternative sequence using an esterification step to connect the C1-C13 and C14-C22 fragments followed by a Pd-catalysed intramolecular Stille reaction to install the (12E,14E)-diene. Conversion of the resulting macrocyclic intermediate into a methyl ketone then enabled a highly diastereoselective Mukaiyama aldol coupling of the derived silyl enol ether with the C13-C28 aldehyde fragment to install the fully elaborated side chain, whereby subsequent global deprotection of the resulting β-hydroxyketone under suitable conditions (TASF followed by p-TsOH) afforded (+)-concanamycin F.     

Total synthesis of aigialomycin D

[reaction: see text] The first total synthesis of resorcinylic macrolide aigialomycin D was described. The resorcinylic moiety was constructed by a highly efficient Diels-Alder reaction using a disiloxydiene and a 14-membered "ynolide" as the dienophile synthesized by ring-forming olefin metathesis.     

A concise route to the C3–C23 fragment of the macrolide palmerolide A

A concise route to the C3–C23 part of the macrolide palmerolide A was developed. This part features the 7,10,11-trihydroxy sector containing the 8E-double bond as well as the 14,16-diene subunit. The stereocenter at C-7 originated from a Noyori reduction on alkynone 8. The substrate 16 containing an enyne was obtained via a Claisen rearrangement. The vicinal diol at C10,C11 was created by a Sharpless asymmetric dihydroxylation. After selective protecting group manipulations the propargylic alcohol was reduced with Red-Al to the E-alkylic alcohol 26. The conjugated diene in the fragment 40 resulted from a Stille cross-coupling reaction between the vinylstannane derived from alkyne 30 and the vinyl iodide 39. The latter could conveniently be prepared by an aldol/Wittig strategy.     

Polyene synthesis: A comparative study

As part of an effort to synthesize the polyene macrolide antibiotics, a comparison of several methods of polyene synthesis has been carried out with the finding that superior results were obtained using the Wollenberg vinyl ether method.     

Phorboxazole B synthetic studies: construction of C(1-32) and C(33-46) subtargets

The convergent syntheses of the C(1-32) and C(33-46) domains of phorboxazole B are described. An iterative cyclocondensation strategy exploited the Jacobsen hetero-Diels-Alder (HDA) reaction as a platform for the synthesis of both the C(5-9) and C(11-15) tetrahydropyran rings. The use of 2-silyloxydiene coupling partners bearing an increasing resemblance to the phorboxazole skeleton was found to lead to a reduction in diastereoselectivity, however, in the case of the C(11-15) ring. The coupling of aldehyde and 2-silyloxydiene by this route provided a C(1-32) fragment which was elaborated to the macrolide core of phorboxazole B. The synthesis of the C(33-46) domain involved a Nozaki-Kishi coupling of aldehyde 31 and vinyl iodide 39. The syntheses of 31 and 39 were highly diastereoselective: an Evans [Cu(Ph-pybox)](SbF6)2-catalysed Mukaiyama aldol reaction formed the cornerstone of the synthesis of 31 whilst a Nagao-Fujita acetate aldol reaction provided a convenient means of installing the sole stereogenic centre of 39.     

Metathesis at an Implausible Site: A Formal Total Synthesis of Rhizoxin D

The new approach to the anticancer agent rhizoxin D described herein does not cohere with the conventional logic of metathesis, according to which macrocycles are best closed at a disubstituted olefinic site; rather, the trisubstituted C11?C12 alkene flanked by an allylic ‐OH group served as the pivot point for synthesis. This motif was attained in good yield and excellent selectivity by a sequence of alkyne metathesis, trans‐hydrostannation and cross coupling. Because the exact same substructure is prominently featured in numerous other natural products, the underpinning strategy, though unusual, might bear more general relevance.     

Total synthesis of bafilomycin V(1): a methanolysis product of the macrolide bafilomycin C(2)

A synthesis of bafilomycin V(1), a methanolysis product of the macrolide natural product bafilomycin C(2), is described. The route utilizes chiral nonracemic allenylzinc reagents, prepared in situ from propargylic mesylates, to access key segments of this methyl ester. The acetylenic moieties of the derived homopropargylic alcohol adducts play an important role in further elaboration of these subunits. Final assemblage of the 25-carbon chain, containing 12 stereocenters, an alpha-methoxy Z,E 1,3-dienic ester, and an additional E,E 1,3-diene, was achieved through Stille coupling of an acetylene-derived vinyl stannane and vinyl iodide of approximate equal complexity. Attempted cyclization of several C15 hydroxy acid derivatives to the 16-membered lactone bafilomycin A(1), a potent inhibitor of vacuolar ATPases, could not be achieved.     

Direct ether formation of semibenzopinacol moieties in a photopolymerization system featured constant intensity of absorbed light

In a special photopolymerization reaction system, the reactivity of semibenzopinacol moieties was examined. Structural analysis by 1D/2D NMR showed that the hydroxyl groups of the semibenzopinacol moieties after hydrogen abstraction reaction have reacted directly to form a new ether structure instead of the cleavage of a C-C bond in a photopolymerization system featured constant intensity of absorbed light when there is only vinyl acetate added. In order to clarify this new reaction, comparative experiments and several characterization methods were applied. Results revealed that poly(vinyl acetate) radicals initiated by UV irradiation or benzophenone/triethanolamine system, and UV irradiation under or above 300 nm, were two necessary factors for the formation of the new ether structure from hydroxyl groups of semibenzopinacol moieties.     

The stereoselective synthesis of novel macrolide antibacterial agents via an intramolecular 1,3-dipolar cycloaddition of azomethine ylide

An intramolecular 1,3-dipolar cycloaddition of azomethine ylide, generated in situ via the reaction of C12-glycinate derivative of macrolide with formaldehyde, provided a novel tricyclic macrolide. The high stereoselectivity of this [2+3] reaction was achieved by introducing a suitable directing group at C-6 position of macrolide.     

The first total synthesis of concanamycin f (concanolide a)

A highly stereoselective total synthesis of the macrolide antibiotic concanamycin F (1), a specific and potent inhibitor of vacuolar H(+)-ATPase, has been achieved by a convergent route involving the synthesis and coupling of its 18-membered tetraenic lactone and beta-hydroxyl hemiacetal side chain subunits. The C1-C19 18-membered lactone aldehyde 4 was synthesized through the intermolecular Stille coupling of the C5-C13 vinyl iodide 24 and the C14-C19 vinyl stannane 25, followed by construction of the C1-C4 diene and macrolactonization. Synthesis of 4 via a second convergent route including the esterification of the C1-C13 vinyl iodide 45 and the C14-C19 vinyl stannane 47 followed by the intramolecular Stille coupling was also realized. The highly stereoselective aldol coupling of 4 and the C20-C28 ethyl ketone 5 followed by desilylation provided 1 which was identical with natural concanamycin F.     

(+)-Rimocidin synthetic studies. Construction of an advanced C(1-18) polyol fragment

[structure: see text] The synthesis of an appropriately functionalized advanced C(1-18) polyol fragment of the mycosamine-glycosylated polyene macrolide, (+)-rimocidin (1), has been achieved in a highly efficient manner. Highlights of the strategy include the S(N)2/S(N)2' addition of dithiane anions to vinyl epoxides and the multicomponent linchpin union of 2-TBS-1,3-dithiane with two advanced epoxides.     

The vinyl + NO reaction: determining the products with time-resolved Fourier transform spectroscopy

We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.     

18O assisted analysis of a γ,δ-epoxyketone cyclization: synthesis of the C16-C28 fragment of ammocidin D

The C16-C28 fragment common to the cytotoxic macrolide ammocidin D has been prepared by a stereospecific 5-exo closure of a γ,δ-epoxyketone followed by a rearrangement to a pyran acetal. The reaction pathway was traced by (18)O labeling of the keto carbonyl and observation of (18)O induced (13)C shifts in the pyran acetal product. NMR data of the synthetic C16-C28 fragment compared favorably to the natural product providing support of the assigned stereochemistry.     

Synthetic study on telomerase inhibitor, D8646-2-6: synthesis of the key intermediate using Sn(OTf)2 or Sc(OTf)3 mediated aldol-type reaction and Stille coupling

The synthesis of the key intermediate (4) in the proposed route to D8646-2-6 is described. The aldol reaction of the carbohydrate-containing pyrone 7 with the aldehyde 6 was accomplished by using LiHMDS and Sc(OTf)3 or Sn(OTf)2. The stepwise dehydration reaction of the aldol adduct 14, followed by Stille coupling with vinyl stannane 5 which contained phosphonate gave the desired 4.