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.     

Synthesis and establishment of stereochemistry of the unusual polyoxazole-thiazole based cyclopeptide YM-216391 isolated from Streptomyces nobilis

A concise total synthesis of the unusual oxazole-based cyclopeptide structure YM-216391, which also establishes the stereochemistry of the natural product i.e. 1, is described.     

Preparation of penta-azole containing cyclopeptides: challenges in macrocyclization

Herein is described the synthesis of several analogs of the natural product IB-01211 from concatenated azoles, via a biomimetic pathway based on cyclization-oxidation of serine containing peptides combined with the Hantzsch synthesis. The macrocyclization of rigid peptide compounds 1 and 2 to give IB-01211 and its epimer 12b was explored, and the results are compared here to those previously obtained for the macrocyclization of more flexible structures in the syntheses of YM-216391, telomestatin, and IB-01211. Lastly, the preliminary results of anti-tumor activity screening of the synthesized analogs are discussed.     

Telomestatin: formal total synthesis and cation-mediated interaction of its seco-derivatives with G-quadruplexes

The structurally unique natural product telomestatin incorporates seven oxazole rings and one sulfur-containing thiazoline in a macrocyclic arrangement. The compound is a potent inhibitor of the enzyme telomerase and therefore provides a structural framework for developing new potential therapeutic agents for cancer. An efficient formal total synthesis of telomestatin is reported in which the key steps are the use of dirhodium(II)-catalyzed reactions of diazocarbonyl compounds to generate six oxazole rings, demonstrating the power of rhodium carbene methodology in organic chemical synthesis. CD spectroscopy establishes that seco-derivatives of telomestatin are potent stabilizers of G-quadruplex structures derived from the human telomeric repeat sequence. Mass spectrometry studies, confirmed by molecular dynamics simulations, provide the first evidence that high affinity binding to terminal G-tetrads in both 1:1 and 2:1 ligand complexes is mediated through the macrocycle coordinating a monovalent cation, with selectivity for the antiparallel structure.     

Synthesis and biological activities of the tris-oxazole macrolactone analogs of mycalolides

Mycalolides are tris-oxazole macrolides isolated from the marine sponge Mycale sp., which shows cytotoxic, antifungal, and actin-depolymerizing activities. To develop an efficient synthetic route of mycalolides and to evaluate its functional mechanism of biological activities, tris-oxazole macrolactone analogs of mycalolides were synthesized through the use of ring-closing metathesis (RCM). The presence/absence of protecting groups at C3, solvent polarity, and reaction temperature significantly affected the stereoselectivity of RCM (E/Z=2.5/1.0–1.0/2.5). The 19E- and 19Z-stereoisomers both exhibited moderate cytotoxicity against tumor cells, but neither showed significant actin-depolymerizing properties or antimycotic activity against pathogenic fungi. Thus, both the side-chain (actin-binding) moiety and the macrolactone moiety were suggested to be essential for the potent biological activities of the parent molecules.     

Synthesis of erythro-alpha-amino beta-hydroxy carboxylic acid esters by diastereoselective photocycloaddition of 5-methoxyoxazoles with aldehydes

A new photoaldol route to alpha-amino-beta-hydroxy carboxylic acid esters is initiated by the photocycloaddition of aromatic or aliphatic aldehydes to 5-methoxyoxazoles. The 4-unsubstituted 5-methyloxazole 1 gave the cycloadducts 8a-f in high yields and excellent exo-diastereoselectivities. Hydrolysis of 8a-f gives the N-acetyl alpha-amino-beta-hydroxy esters 9a-f, which could be subsequently converted into the corresponding Z-didehydro alpha-amino acids 10a-f. Quartenary alpha-amino-beta-hydroxy esters 12, 14, 16, 18, and 20, which are stable against dehydration, were obtained from the 4-alkylated 5-methoxyoxazoles 2-6, in most cases highly erythro-selective due to the high degree of stereocontrol (exo) at the photocycloaddition (to give 11, 13, 15, 17, and 19) level. The relative configurations of the N-acetyl alpha-amino-beta-hydroxy esters were determined by NMR spectroscopy and comparison with chiral pool-derived compounds as well as by X-ray structure determination of the ester 23, formed by hydrolysis of the cycloadduct 22, derived from photocycloaddition of propionaldehyde to the isoleucine-derived oxazole 21.     

Derivatization of a tris-oxazole using Pd-catalyzed coupling reactions of a 5-bromooxazole moiety

Modification at the C5 position of an oxazole ring contained in a 2,4-concatenated tris-oxazole by Pd-catalyzed coupling reactions was performed. Novel Pd-catalyzed amination and alkoxylation of a 5-bromooxazole derivative as well as Suzuki-Miyaura coupling and Migita-Stille coupling have been demonstrated. A wide variety of functional groups, including aryl, heteroaryl, primary and secondary amines, and phenol, were introduced in the 5-bromooxazole moiety in moderate to excellent yields using Pd(OAc)₂/S-PHOS or Pd(OAc)₂/X-PHOS as a catalyst.     

A two-stage iterative process for the synthesis of poly-oxazoles

[reaction: see text]. Methodology has been developed to prepare bis-oxazoles via a two-stage iterative process. The sequence begins with C(2)-chlorination of a lithiated oxazole using hexachloroethane. Generation of the C(2)-C(4)(') bond by S(N)Ar substitution with TosMIC anion, followed by conversion to the heterocycle in a one-pot reaction with glyoxylic acid monohydrate, affords the desired bis-oxazole in good yield and purity. The two-stage process allows for efficient synthesis of a tris-oxazole and the first iterative preparation of a tetra-oxazole.     

Regiospecific synthesis of 3-alkyl-4-hydroxybenzimidazoles as intermediates for an expedient approach to potent EP3 receptor antagonists

Regiospecific construction of 3-alkyl-4-hydroxybenzimidazoles is detailed. The synthetic route involves a novel O- to N-acyl transfer reaction to address the observed exclusive O-acylation of 2-amino-3-nitrophenol starting material. This efficient route provides the targeted 3-alkyl-4-hydroxybenzimidazoles in good yields, in five steps, without the use of chromatographic purification. These key intermediates were subsequently elaborated, as shown, to provide acylsulfonamide-derived potent EP₃ receptor antagonists.     

New peptide synthesis using the ozonolysate of 2-(1-phthalimido)alkyl-5-phenyloxazoles

It was proved that 2-(1-phthalimido)alkyl-5-phenyloxazoles 3 were useful synthetic intermediates for peptide synthesis, where the oxazole ring acted as not only the carboxyl protecting group but also the carboxyl activating group upon ozonolysis.     

Photochemistry and vibrational spectra of matrix-isolated methyl 4-chloro-5-phenylisoxazole-3-carboxylate

Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier (<0.3 kJ mol(-1)). In situ UV irradiation (λ > 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization.     

1,2]- or [2,3]-rearrangement of onium ylides of allyl and benzyl ethers and sulfides via in situ-generated iodonium ylides

Iodonium ylides, generated in situ with bisacetoxyiodobenzene, are converted to allyl- or benzyl-substituted oxonium or sulfonium ylides via rhodium- or copper-catalyzed carbene transfer. Except for the S-benzyl example, the resulting ylides undergo rearrangement to the corresponding 2-substituted heterocycles. This demonstrates the first use of iodonium ylides as diazoketone surrogates for the generation and rearrangement of onium ylide intermediates. This abbreviated one-step method proceeds in comparable yields relative to the corresponding two-step route employing diazoketone intermediates.     

Total synthesis of natural and unnatural lamellarins with saturated and unsaturated D-rings

Twenty-eight natural and unnatural lamellarins with either a saturated or an unsaturated D-ring were synthesized according to our developed synthetic route. The key step involved the Michael addition/ring closure (Mi-RC) of the benzyldihydroisoquinoline and alpha-nitrocinnamate derivatives, which provided the 2-carboethoxypyrrole intermediates in moderate to good yields (up to 78% yield). Subsequent hydrogenolysis/lactonization furnished lamellarins with a saturated D-ring in excellent yields (up to 93% yield). DDQ oxidation of the saturated lamellarin acetates led directly to the corresponding unsaturated analogues in 54-95% yield. In addition, only two steps in our developed strategy require column chromatography.     

Halogen-lithium exchange versus deprotonation: regioselective mono- and dilithiation of aryl benzyl sulfides. A simple approach to α,2-dilithiotoluene equivalents

Halogen-lithium exchange and deprotonation reactions between aryl benzyl sulfides and alkyllithiums were investigated. The resultant mono- and dilithiated intermediates were converted into the corresponding aldehydes and boronic, or carboxylic acids in good yields. It was found that diethyl ether stabilizes the ortho-lithiated compounds toward isomerisation to the benzylic derivatives. The process occurs easily in THF at low temperature and is a facile route to the α,2-dilithiotoluene derivative which can be transformed into a dicarboxylic acid on treatment with CO₂.     

Preparation of halomethyl-1,3,4-thiadiazoles. Conversion to 2-amino-5-(1-methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole,an important antimicrobial agent

A new route for the synthesis of 2-amino-5-(l-methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole ( 1 ) is described. This route was based upon the preparation of 2-amino-5-halomethyl-1,3,4-thiadiazoles by condensation of haloacetic acids with thiosemicarbazide. One of these intermediates, 2-acetamido-5-dichloromethyl-1,3,4-thiadiazole ( 4 ), was hydrolyzed to the corresponding 5-amino-2-carboxaldehyde 6 , which was trapped as its oxime 5 . 5-Acetamido-1,3,4-thiadiazole-2-carbonitrile ( 7 ), formed upon dehydration of 5 , was then converted into 2-amino-5-(2-imidazolyl)-1,3,4-thiadiazole ( 11 ) by a route based on the Pinner amidine synthesis. Methylation and nitration of the imidazole moiety then completed the preparation of 1.     

Furanosteroid studies. Stereoselective synthesis of the A,B,E-ring core of wortmannin

Alkyne oxazole 11c is converted in three steps, and approximately 45% overall yield, to furanolactone 21alpha having the A,B,E-ring core of the wortmannin (2) family of furanosteroids. The TiCl4-catalyzed insertion of EtO2C-CH=O between C3 and C10 in furanoacid 14d is >98% stereoselective via a pathway involving chemoselective lactonization of equilibrating aldol intermediates 23alpha,beta (dynamic kinetic resolution).     

A short and highly stereoselective synthesis of α-(2-aminothiazolyl)-C-nucleosides

A short route to novel α-(2-aminothiazolyl)-C-nucleosides has been developed. The key step was the high diastereoselective reduction of the hemiacetal intermediates using L-Selectride, which afforded the corresponding R-diols in quantitative yields. These diols were converted, after C4-C1 ring closure and protecting groups cleavage, to their corresponding free α-C-nucleosides.     

Total synthesis of (R)-telomestatin

We have achieved a total synthesis of telomestatin, and its absolute configuration was determined to be (R). Coupling of cysteine-containing trisoxazole amine and serine-containing trisoxazole carboxylic acid, followed by macrocyclization, provided a 24-membered diamide. The seventh oxazole ring was formed by a Shin's procedure via dehydroamide. Cyclodehydration of a modified (R)-cysteine-(S-(t)Bu) moiety using Kelly's method (PPh3(O)-Tf2O) with anisole furnished (R)-telomestatin, whose CD spectrum was in good agreement with that of the natural product.     

Hydroxyiminoisoquinolin-3(2H)-ones. 9. Synthesis of some oxazolo[5,4-c]-, thiazolo[5,4-c]- and 2,3-dihydro-1H-[1,4]oxazino[2,3-c]isoquinolines

4-Acetylamino-1-phenyl-1,4-dihydro-3(2H)-isoquinolinones and 4-amino-1-phenylisoquinolin-3-ol, prepared from the corresponding 4-hydroxyimino compounds, were converted into new isoquinolines containing fused oxazole, thiazole and 1,4-oxazine rings.     

Total synthesis of phorboxazole A via de novo oxazole formation: convergent total synthesis

The phorboxazoles are mixed non-ribosomal peptide synthase/polyketide synthase biosynthetic products that embody polyketide domains joined via two serine-derived oxazole moieties. Total syntheses of phorboxazole A and analogues have been developed that rely upon the convergent coupling of three fragments via biomimetically inspired de novo oxazole formation. First, the macrolide-containing domain of phorboxazole A was assembled from C3-C17 and C18-C30 building blocks via formation of the C16-C18 oxazole, followed by macrolide ring closure involving an intramolecular Still-Genarri olefination at C2-C3. Alternatively, a ring-closing metathesis process was optimized to deliver the natural product's (2Z)-acrylate with remarkable geometrical selectivity. The C31-C46 side-chain domain was then appended to the macrolide by a second serine amide-derived oxazole assembly. Minimal deprotection then afforded phorboxazole A. This generally effective strategy was then dramatically abbreviated by employing a total synthesis approach wherein both of the natural product's oxazole moieties were installed simultaneously. A key bis-amide precursor to the bis-oxazole was formed in a chemoselective one-pot, bis-amidation sequence without the use of amino or carboxyl protecting groups. Thereafter, both oxazoles were formed from the key C18 and C31 bis-N-(1-hydroxyalkan-2-yl)amide in a simultaneous fashion, involving oxidation-cyclodehydrations. This synthetic strategy provides a total synthesis of phorboxazole A in 18% yield over nine steps from C3-C17 and C18-C30 synthetic fragments. It illustrates the utility of a synthetic design to form a mixed non-ribosomal peptide synthase/polyketide synthase biosynthetic product based upon biomimetic oxazole formation initiated by amide bond formation to join synthetic building blocks.