Total synthesis and reassignment of stereochemistry of obyanamide

The total synthesis of a marine cytotoxic cyclic depsipeptide obyanamide is reported. The synthesis has led to a reassignment of the C-3 configuration in β-amino acid residue. And this revision is also supported by biological test.     

Total synthesis of erythromycin B

We report the details of the first total synthesis of erythromycin B using two different strategies for the end game. The first of these follows a classical approach in which the desosamine and cladinose residues are sequentially appended to a macrocyclic lactone, which was formed by cyclization of a seco acid derivative, to give a bis-glycosylated macrolide intermediate that is converted into erythromycin B. The second strategy features an abiotic approach in which a seco acid bearing a desosamine residue is cyclized to give a monoglycosylated macrocyclic lactone that is then transformed into erythromycin B via a sequence of steps involving refunctionalizations and a glycosylation to introduce the cladinose moiety. Attempts to prepare a bis-glycosylated seco acid by de novo synthesis were unsuccessful. The syntheses of the key seco acid intermediates feature the oxidative transformation of a furan containing C(3)-C(10) to provide a dioxabicyclo[3.3.1]nonenone that served as a template on which to create the stereocenters at C(6) and C(8). A stereoselective aldol reaction was used to establish the C(11)-C(15) segment, and a stereoselective crotylation was implemented to introduce the propionate subunit comprising C(1)-C(2).     

Solid-phase synthesis of biaryl cyclic peptides containing a histidine-tyrosine linkage

A solid-phase strategy for the synthesis of biaryl cyclic peptides containing a side-chain to side-chain His-Tyr linkage was developed. The key step was the macrocyclization of a linear peptidyl resin incorporating a 5-bromohistidine and a 3-boronotyrosine via the formation of the biaryl bond by means of a microwave-assisted Suzuki-Miyaura reaction. This method allowed direct access to biaryl cyclic peptides containing a 3- or 5-amino acid ring and bearing the histidine residue at the N- or the C-terminus, being especially conducive for analogues in which this amino acid is located at the C-terminus. This study also served to establish a strategy for the synthesis of biaryl cyclic peptides derived from the two hemispheres of the natural biaryl bicyclic peptides aciculitins.     

Nonenzymatic template-directed synthesis on oligodeoxycytidylate sequences in hairpin oligonucleotides

We have developed a novel method for studying template-directed synthesis in hairpin oligonucleotides. An unpaired segment at the 5'-terminus of the hairpin acts as an intramolecular template for the extension of the paired 3'-terminus. Products are analyzed by denaturing gel electrophoresis of [32P]-labeled hairpins. Using this system, we have studied the synthesis of oligoguanylates on an oligodeoxycytidylate template. We find that guanosine 5'-phosphoro(2-methyl)imidazolide adds efficiently to a terminal riboguanylate residue at temperatures in the range 0-37 degrees C but not at 50 degrees C. At 0 degree C, the half-time for addition of the first G residue is about 3 h, and the reaction rate is independent of pH in the range 6.5-8.0. The first addition reaction results in the formation of a predominantly 3'-5'-internucleotide bond. When the 3'-terminal riboguanylate residue is placed by a deoxyguanylate residue, the half-time for the first addition increases from about 3 to about 30 h.     

Total synthesis and structural confirmation of chlorodysinosin A

The first enantiocontrolled total synthesis of the marine sponge metabolite chlorodysinosin A is described. The structure and absolute configuration are identical to those of dysinosin A except for the presence of a novel 2S,3R-3-chloroleucine residue in the former. A concise stereocontrolled synthesis of the new chlorine-containing amino acid fragment was developed. An X-ray cocrystal structure of synthetic chlorodysinosin A with the enzyme thrombin confirms the structure and configuration assignment achieved through total synthesis. Within the aeruginosin family of natural products, chlorodysinosin A is the most potent inhibitor of the serine proteases thrombin, factor VIIa, and factor Xa, which are critical enzymes in the process leading to platelet aggregation and fibrin mesh formation in humans.     

Stereoselective synthesis and conformational analysis of pseudo-heptapeptides: Part 5

The stereoselective synthesis of pseudo-heptapeptides 8, 9 and 12, incorporating a 2,6-diamino-4-methylen-1,7-heptanedioic acid residue, was accomplished starting from chiral synthon 3, a masked unnatural dipeptide derived from an l-valine unit and (2R)-methylaspartic acid. Investigations of the conformational preference and structure of these unnatural peptides were carried out using ¹H NMR and IR spectroscopic data and a conformational analysis based on molecular dynamics (MD).     

Enzimatic synthesis of aspartyl-containing dipeptides

We show that the ability of thermolysin to catalyze the coupling of aspartyl residue with other aminoacids is restricted to phenylalanine. Esterification of the two carboxylic groups of N-protected aspartic acid allows chymotrypsin and papain to catalyze the desired synthesis.     

Improving the carboxyamidomethyl ester for subtilisin A-catalysed peptide synthesis

A series of novel glycine esters was evaluated for efficiency in subtilisin A-CLEA-catalysed peptide synthesis. The reactivity of the easily accessible carboxyamidomethyl (Cam) ester was further enhanced by elongating it with an amino acid residue, thereby creating more recognition space for subtilisin A.     

Total synthesis of Met10-teixobactin

The total synthesis of cyclic depsipeptide Met₁₀-teixobactin in solution-phase is described. Teixobactin is a structurally and mechanistically novel antimicrobial peptidic natural product with very impressive activities against Gram-positive pathogens. It happens to possess an l-allo-enduracididine (End) residue as part of macrocyclic ring which is not readily accessible. In this report, we have used serine ligation strategy as the key step to prepare an analogue of teixobactin where End being replaced with a readily available amino acid methionine.     

Isolation, structure determination, and synthesis of neodysiherbaine A, a new excitatory amino acid from a marine sponge

[structure: see text] A new excitatory amino acid, neodysiherbaine A (2), was isolated as a minor constituent of the aqueous extract from the marine sponge Dysidea herbacea. The structure was deduced by spectroscopic methods and established unambiguously by the total synthesis. The present synthesis, including as a key step cross-coupling of the 6/5-bicyclic core with an amino acid residue, is useful in constructing its structural analogues.     

Direct solid-phase synthesis of quinoxaline-containing peptides

The solid-phase synthesis of a series of model dipeptides containing various 3-(quinoxalin-6-yl)alanine analogues is described. The method involves formation of a quinoxaline heterocycle by condensation between an α-dicarbonyl compound and a β-(3,4-diaminophenyl)alanine residue, immobilized on a solid support.     

Solid-phase synthesis of CD40L mimetics

The C3-symmetric molecule has been previously shown to mimic CD40 ligand (CD40L) homotrimers and to display effector functions. This molecule consists of a cyclic hexapeptide core containing the repetition of the D-Ala-L-Lys motif. The side chains of the lysine residues have been modified by appending the CD40L-derived sequence 143Lys-Gly-Tyr-Tyr146 via a 6-aminohexanoic acid residue as a spacer. The present report describes a general solid-phase synthesis approach to and related trimeric architectures. In addition, their CD40 binding properties as well as their effector functions have been evaluated.     

Practical enantioselective synthesis of beta-substituted-beta-amino esters

[reaction: see text] A practical, large-scale synthesis of a beta-amino ester 1 was developed. A chiral imine derived from (S)-phenylglycinol and 3-trimethylsilylpropanal was coupled with the Reformatsky reagent 3 with high diastereoselectivity (de > 98%) to give (SS)-4a as the major isomer. The amino alcohol residue of the coupling product 4 was oxidatively cleaved with sodium periodate in the presence of methylamine. An unusual selective oxidative cleavage of the (SS)-isomer was observed and the imine 6 was obtained with ee > 99% while the (RS)-4b isomer was not cleaved. Reaction with p-toluenesulfonic acid monohydrate allowed for the hydrolysis of the imine and the isolation of the amine as its salt. The title compound 1 was then obtained by transesterification, desilylation, and hydrochloride salt formation in a one-pot process. The method was successfully applied toward the synthesis of a wide variety of beta-amino esters.     

An easy and stereoselective synthesis of N-Boc-dolaproine via the Baylis-Hillman reaction

In this communication we report a stereoselective total synthesis of N-Boc-dolaproine (Dap), an amino acid residue of the antineoplastic pentapeptide Dolastatin 10. Our strategy is based on a Baylis-Hillman reaction between N-Boc-prolinal and methyl acrylate, followed by a diastereoselective double bond hydrogenation and hydrolysis of the ester function.     

Total synthesis of cyclic tetrapeptide FR235222, a potent immunosuppressant that inhibits mammalian histone deacetylases

[structure: see text] The total synthesis of FR235222, a potent immunosuppressant with in vivo activities, has been achieved. The key steps include assembling its (2S,9R)-2-amino-9-hydroxy-8-oxodecanoic acid residue via an olefin cross-metathesis of a methyl (R)-lactate-derived homoallyl ketone with protected allyl amino acid and constructing its trans-(2R,4S)-4-methylproline unit from protected (R)-pyroglutamic acid in seven steps.     

Total synthesis and biological mode of action of largazole: a potent class I histone deacetylase inhibitor

The efficient total synthesis of the recently described natural substance largazole (1) and its active metabolite largazole thiol (2) is described. The synthesis required eight linear steps and proceeded in 37% overall yield. It is demonstrated that largazole is a pro-drug that is activated by removal of the octanoyl residue from the 3-hydroxy-7-mercaptohept-4-enoic acid moiety to generate the active metabolite 2, which is an extraordinarily potent Class I histone deacetylase inhibitor. Synthetic largazole and 2 have been evaluated side-by-side with FK228 and SAHA for inhibition of HDACs 1, 2, 3, and 6. Largazole and largazole thiol were further assayed for cytotoxic activity against a panel of chemoresistant melanoma cell lines, and it was found that largazole is substantially more cytotoxic than largazole thiol; this difference is attributed to differences in the cell permeability of the two substances.     

Apoptolidin A: total synthesis and partially glycosylated analogues

The total synthesis of apoptolidin A is described employing an early glycosylation strategy. Strategic disconnections were chosen between C11-C12 (cross-coupling) and C19O-C1 (macrocyclization). The cis-selective glycosylation at C9-OH was achieved with the new SIBA protective group at O2/O3 of the L-glucose residue. Auxiliary substitutents at the 2-position of the 2-deoxy sugars were applied to form selectively the glycosidic linkages of the C27 disaccharide. The cross-coupling of the glycosylated northern half with the glycosylated southern half was achieved with CuI-thiophene carboxylate. The macrocyclization of a trihydroxy carboxylic acid produced the 20-membered macrolide selectively. H2SiF6 was suitable for the final deprotection of the silyl ethers and the conversion of the C21 methylketal into the hemiketal. The synthetic flexibility of the approach was proven by the synthesis of some glycovariants.     

Ribosomal synthesis of dehydroalanine-containing peptides

Dehydroalanine is a nonproteinogenic amino acid, but it is a component of a wide variety of natural products with therapeutic activities. Indeed, this alpha,beta-unsaturated residue is a highly versatile building block due to its rigidifying effect on peptide backbones and its electrophilicity which allows site-specific thiol ligations of peptides with small molecules or proteins. To harness such versatility in genetically encoded, combinatorial peptide libraries, we report a simple and robust method for the ribosomal synthesis of dehydroalanine-containing peptides. Selenalysine, a selenium-containing lysine analogue, was recruited as a masked dehydroalanine equivalent. This residue is efficiently incorporated by a reconstituted Escherichia coli translation system at high fidelity and efficiency despite the presence of low levels of lysine. Mild oxidative conditions were used to convert selenalysine into dehydroalanine post-translationally. Using this method, we demonstrate the preparation of polyunsaturated and highly decorated peptides. This report is an important step toward the preparation and selection of large libraries of protein-reactive compounds with potential use as novel drugs or as analytical tools.     

Aza-amino acid scan for rapid identification of secondary structure based on the application of N-Boc-aza(1)-dipeptides in peptide synthesis

Azapeptides, peptide analogues in which the alpha-carbon of one or more of the amino acid residues is replaced with a nitrogen atom, exhibit propensity for adopting beta-turn conformations. A general protocol for the synthesis of azapeptides without racemization on solid phase has now been developed by introducing the aza-amino acid residue as an N-Boc-aza(1)-dipeptide. This approach has been validated by the synthesis of six N-Boc-aza(1)-dipeptides and their subsequent introduction into analogues of the C-terminal peptide fragment of the human calcitonin gene-related peptide (hCGRP). By performing an aza-amino acid scan of such antagonist peptides, a set of aza-hCGRP analogues was synthesized to examine the relationship between turn secondary structure and biological activity.     

Toward beta-amino acid proteins: design, synthesis, and characterization of a fifteen kilodalton beta-peptide tetramer

A 28 residue "retro" beta-peptide has been designed to self-assemble as a tetramer, successfully recapitulating the octameric beta-peptide bundle complex Zwit-1F. Z28, the largest beta-peptide synthesized to date, was achieved in a linear, microwave-assisted synthesis in 19% isolated yield and high purity. Z28 forms the expected tetrameric complex and is more stable and cooperative in its folding than Zwit-1F. The successful design and synthesis of Z28 is an important step toward true beta-amino acid proteins.