Total synthesis of phorboxazole A via de novo oxazole formation: strategy and component assembly

The phorboxazole natural products are among the most potent inhibitors of cancer cell division, but they are essentially unavailable from natural sources at present. Laboratory syntheses based upon tri-component fragment coupling strategies have been developed that provide phorboxazole A and analogues in a reliable manner and with unprecedented efficiency. This has been orchestrated to occur via the sequential or simultaneous formation of both of the natural product's oxazole moieties from two serine-derived amides, involving oxidation-cyclodehydrations. The optimized preparation of three pre-assembled components, representing carbons 3-17, 18-30, and 31-46, has been developed. This article details the design and syntheses of these three essential building blocks. The convergent coupling approach is designed to facilitate the incorporation of structural changes within each component to generate unnatural analogues, targeting those with enhanced therapeutic potential and efficacy.     

Total synthesis of (+)-phorboxazole A, a potent cytostatic agent from the sponge Phorbas sp

A convergent total synthesis of phorboxazole A (1a), from the C(3-19), C(20-27) and C(33-46) fragments 5, 4 and 91, respectively, concentrating on stereocontrolled formation of the bonds at C(2-3), C(19-20) and C(27-28), is described. Although a coupling reaction between a macrolide ketone and the side chain substituted sulfone, at C(27-28) was not successful, a Wadsworth-Emmons olefination involving the oxane methyl ketone 4 and an oxazole produced the oxane 90 which was next coupled to 91 leading to the C(20-46) unit 100. A further coupling of 100 to 71c at C(19-20) then led to 105, ultimately, and the synthesis was completed by a macrocyclisation reaction from 105, at the C(2-3) alkene bond, followed by deprotection of 106.     

Total synthesis of phorboxazole A. 2. Assembly of subunits and completion of the synthesis

[Structure: see text] Subunits of phorboxazole A containing C1-C2, C3-C8, C9-C19, C20-C32, C33-C41, and C42-C46 were connected in a sequence that first linked C32 with C33 and then C41 with C42. A C3-C8 fragment was joined to C9-C19, and the assembled unit was then joined with the left half of 1. Closure of the macrolide was accomplished by esterification of the C24 alcohol followed by intramolecular Horner-Wadsworth-Emmons condensation to set the (E)-C2-C3 alkene.     

Studies on the synthesis of phorboxazole B: stereoselective synthesis of the C28-C46 segment

A stereoselective synthesis of C28-C46 segment of phorboxazole B is described. Key features of the synthetic route involved the use of 1,3-asymmetric induction of Mukaiyama aldol reaction to construct the stereogenic center at C35, and the employment of metalated oxazole chemistry to prepare the ketal 6.     

Total synthesis of 8-deshydroxyajudazol B

The total synthesis of a stereoisomer of 8-deshydroxyajudazol B (4), the putative biosynthetic intermediate of the ajudazols A (1) and B (2), is described. The key steps in the synthesis included an intramolecular Diels-Alder (IMDA) reaction to secure the isochromanone fragment, a novel selective acylation/O,N-shift to give a hydroxyamide which was cyclized to the oxazole and a high yielding Sonogashira coupling to form the C18-C19 bond. Partial alkyne reduction then afforded the target 4.     

Total synthesis of (-)-ulapualide A, a novel tris-oxazole macrolide from marine nudibranchs, based on some biosynthesis speculation

A new, second generation, total synthesis of ulapualide A (1), whose stereochemistry was recently determined from X-ray analysis of its complex with the protein actin, is described. The synthesis is designed and based on some speculation of the biosynthetic origin of the contiguous tris-oxazole unit in ulapualide A, alongside that of the related co-metabolites that contain only two oxazole rings, e.g. 6 and 7. The mono-oxazole carboxylic acid 67b and the mono-oxazole secondary 55b alcohol which, together, contain all of the 10 asymmetric centres in the natural metabolite, were first elaborated using a combination of contemporary asymmetric synthesis protocols. Esterification of 67b with 55b under Yamaguchi conditions gave the ester 77 which was then converted into the omega-amino acid 18a following simultaneous deprotection of the t-butyl ester and the N-Boc protecting groups. Macrolactamisation of 18a, using HATU, now gave the key intermediate macrolactam 17, containing two of the three oxazole rings in ulapualide A (1). A number of procedures were used to introduce the third oxazole ring in ulapualide A from 17, including: a) cyclodehydration to the oxazoline 78a followed by oxidation using nickel peroxide leading to 76; b) dehydration to the enamide 79, followed by conversion into the methoxyoxazoline 78b, via 80, and elimination of methanol from 78b using camphorsulfonic acid. The tris-oxazole macrolide 76 was next converted into the aldehyde 82b in four straightforward steps, which was then reacted with N-methylformamide, leading to the E-alkenylformamide 83. Removal of the TBDPS protection at C3 in 83 finally gave (-)-ulapualide A, whose 1H and 13C NMR spectroscopic data were indistinguishable from those obtained for naturally derived material. It is likely that the tris-oxazole unit in ulapualide A (1) is derived in nature from a cascade of cyclodehydrations from an acylated tris-serine precursor, e.g.9, followed by oxidation of the resulting tris-oxazoline intermediate, i.e.10. It is also plausible to speculate that the biosynthesis of metabolites related to ulapualide A, e.g. the bis-oxazole 6 and the imide 7, involve cyclisations of just two of the serine units in 9. These speculations were given some credence by carrying out pertinent interconversions involving the bis-oxazole amide 24, the enamide 25, the imide 26, the oxazoline 27 and the tris-oxazole 30 as model compounds. An alternative strategy to the tris-oxazole macrolide intermediate 76 was also examined, involving preliminary synthesis of the aldehyde 73, containing a shortened (C25-C34) side chain from 67b and 47b. A Wadsworth-Emmons olefination reaction between 73 and the phosphonate ester 74 led smoothly to the E-alkene 75, but we were not able to reduce selectively the conjugated enone group in 75 to 76 without simultaneous reduction of the oxazole alkene bond, using a variety of reagents and reaction conditions.     

Total synthesis of phorboxazole a. 1. Preparation of four subunits

[Structure: see text] Four subunits of the potent antitumor agent phorboxazole A were constructed; fragments C20-C32 and C9-C19 containing tetrahydropyrans A and B, respectively, were assembled using palladium-catalyzed intramolecular alkoxycarbonylation.     

Cyclization mechanism for the synthesis of macrocyclic antibiotic lankacidin in Streptomyces rochei

The lankacidin biosynthetic gene cluster in Streptomyces rochei strain 7434AN4 was found to span 31 kb of the giant linear plasmid pSLA2-L and contain a polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) hybrid gene (lkcA), type I PKS genes, and pyrroloquinoline quinone (PQQ) biosynthetic genes (lkcK-lkcO). Feeding of PQQ to a pqq mutant restored the lankacidin production, suggesting its crucial role in an oxidation process. However, formation of the 17-membered macrocyclic ring was not catalyzed by PQQ-dependent dehydrogenase (Orf23), but was by flavin-dependent amine oxidase (LkcE). Compound LC-KA05 isolated from an lkcE disruptant was an acyclic intermediate lacking the C2-C18 linkage. These results suggested a cyclization mechanism for the synthesis of the lankacidin macrocyclic skeleton.     

A second-generation total synthesis of (+)-phorboxazole A.

A highly convergent second-generation synthesis of (+)-phorboxazole A has been achieved. Highlights of the synthetic approach include improved Petasis-Ferrier union/rearrangement conditions on a scale to assemble multigram quantities of the C(11-15) and C(22-26) cis-tetrahydropyrans inscribed with the phorboxazole architecture, a convenient method to prepare E- and Z-vinyl bromides from TMS-protected alkynes utilizing radical isomerization of Z-vinylsilanes, and a convergent late-stage Stille union to couple a fully elaborated C(1-28) macrocyclic iodide with a C(29-46) oxazole stannane side chain to establish the complete phorboxazole skeleton. The synthesis, achieved with a longest linear sequence of 24 steps, proceeded in 4.6% overall yield.     

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.     

Stereocontrolled synthesis of the C8-C22 fragment of rhizopodin

A convergent synthesis of the central C8-C22 core of the potent macrolide antibiotic rhizopodin is reported. Notable features of the stereocontrolled approach include an asymmetric reverse prenylation of an alcohol using a method of Krische, a thiazolium catalyzed transformation of an epoxyaldehyde as described by Bode, and a late-stage oxazole formation from advanced intermediates. This route demonstrates the applicability of these methodologies in complex natural product synthesis.     

Quinolactacin Biosynthesis Involves Non-Ribosomal-Peptide-Synthetase-Catalyzed Dieckmann Condensation to Form the Quinolone-γ-lactam Hybrid

Quinolactacins are novel fungal alkaloids that feature a quinolone-γ-lactam hybrid, which is a potential pharmacophore for the treatment of cancer and Alzheimer's disease. Herein, we report the identification of the quinolactacin A2 biosynthetic gene cluster and elucidate the enzymatic basis for the formation of the quinolone-γ-lactam structure. We reveal an unusual β-keto acid (N-methyl-2-aminobenzoylacetate) precursor that is derived from the primary metabolite l -kynurenine via methylation, oxidative decarboxylation, and amide hydrolysis reactions. In vitro assays reveal two single-module non-ribosomal peptide synthetases (NRPs) that incorporate the β-keto acid and l -isoleucine, followed by Dieckmann condensation, to form the quinolone-γ-lactam. Notably, the bioconversion from l -kynurenine to the β-keto acid is a unique strategy employed by nature to decouple R*-domain-containing NRPS from the polyketide synthase (PKS) machinery, expanding the paradigm for the biosynthesis of quinolone-γ-lactam natural products via Dieckmann condensation.     

Studies directed toward the synthesis of rhizopodin: stereoselective synthesis of the C1-C15 fragment

A stereoselective synthesis of the C1-C15 fragment of a G-actin binding natural macrodiolide, rhizopodin was achieved using, as key steps, highly stereoselective acetate aldol reactions to build the C1-C7 fragment, one pot oxazole synthesis and an asymmetric Keck allylation reaction to build the C8-C15 fragment and finally, a Stille reaction to couple both the fragments.     

Selective lithiation of 2-methyloxazoles. Applications to pivotal bond constructions in the phorboxazole nucleus

[formula: see text] The lithiation of 2-methyloxazoles with alkyllithium and hindered lithium amide bases generally results in the competitive formation of a mixture of 5-lithio- and 2-(lithiomethyl)oxazole isomers. Herein a synthetically useful lithiation method which allows for the selective formation of 2-(lithiomethyl)oxazole is described. Diethylamine has been found to be a kinetically competent proton source that will mediate the equilibration of the kinetically formed 5-lithiooxazole to its more stable 2-(lithiomethyl)oxazole counterpart. Application of this metalation strategy with lithium diethylamide to two important bond constructions relevant to a projected phorboxazole synthesis is presented.     

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.     

Toward the synthesis of peloruside a: fragment synthesis and coupling studies

[reaction: see text] The asymmetric synthesis of building blocks 3, 4, and 5, corresponding to C(12)-C(19), C(7)-C(11), and C(1)-C(6) segments of peloruside A, is reported, along with boron-mediated aldol coupling studies directed toward the assembly of the complete carbon skeleton of this microtubule-stabilizing macrolide.     

Concise epoxide-based synthesis of the C14-C25 bafilomycin A(1) polypropionate chain

An efficient non-aldol convergent synthesis of the C14-C25 polyketide fragment of bafilomycin A(1) was completed in 16% overall yield and 8 steps in its longest linear sequence. This synthesis highlights the formation of the key fragments using a three-step sequence of epoxide cleavage, alkyne reduction, and epoxidation developed in our laboratory; starting from suitably protected enantiomeric epoxides of trans-2,3-epoxybutanol. This chemistry represents a quick asymmetric and diastereoselective construction of the polyketide chain of bafilomycin A(1), in which every stereogenic center was constructed using solely epoxide chemistry.     

Enantioselective total synthesis of amphidinolide f

A common-intermediate approach is utilized in the total synthesis of amphidinolide?F to access both the C1-C8 and the C18-C25 portions of the macrolide. A silver-catalyzed rearrangement/cyclization was employed to construct the two tetrahydrofuran rings. A Felkin-controlled, dienyl lithium addition to an α-chiral aldehyde incorporated both the C9-C11 diene and the alcohol at C8. An umpolung sulfone alkylation/oxidative desulfurization sequence is employed to couple the two moieties.     

Total synthesis of apoptolidin: construction of enantiomerically pure fragments

A general strategy for the total synthesis of the antitumor agent apoptolidin (1) is proposed, and the chemical synthesis of the defined key building blocks (4, 5, 6, 8, and 9) in their enantiomerically pure forms is described. The projected total synthesis calls for a dithiane coupling reaction to construct the C(20)-C(21) bond, a Stille coupling reaction to form the C(11)-C(12) bond, and a Yamaguchi macrolactonization to assemble the macrolide ring, as well as two glycosidation reactions to fuse the carbohydrate units onto the molecule. First and second generation syntheses to the required fragments for apoptolidin (1) are described.     

Expedient synthesis of a stereoisomer of acremolide B

A highly straightforward strategy for the synthesis of the acremolide class of lipodepsipeptides has been developed. Synthetic highlights include a cross-metathesis to couple the C1-C7 and the C8-C12 fragments, an esterification to introduce the dipeptide unit, a macrolactamization to build the macrolide core, and two stereoselective allylations/crotylations to control all four stereogenic centers of the C1-C12 polypropionate segment.